A Practical Approach to Cardiac Anesthesia (Practical Approach Series) 5th Ed.

4 Monitoring the Cardiac Surgical Patient

Mark A. Gerhardt and Katarzyna M. Walosik-Arenall

KEY POINTS

 1. For cardiac anesthesia, a five-electrode surface ECG monitor should be used in the diagnostic mode, with a frequency response of 0.05 to 100 Hz.

 2. After a rapid pressure change (performed by flushing the pressure line and known as the “pop test”), an underdamped system will continue to oscillate for a prolonged time. In terms of pressure monitoring, this translates to an overestimation of systolic BP and an underestimation of diastolic BP. An overdamped system will not oscillate at all but will settle to baseline slowly, thus underestimating systolic and overestimating diastolic pressures. A critically damped system will settle to baseline after only one or two oscillations and will reproduce systolic pressures accurately.

 3. Air within a catheter or transducer causes most pressure monitoring errors.

 4. The radial artery pressure may be significantly lower than the aortic pressure at the completion of cardiopulmonary bypass (CPB) and for 5 to 30 min following CPB.

 5. Standards in the United States and the United Kingdom suggest that ultrasound guidance is the preferred technique for internal jugular catheter placement.

 6. If subclavian vein cannulation is unsuccessful on one side, an attempt on the contralateral side is contraindicated without first obtaining a chest x-ray film. Bilateral pneumothoraces can be lethal.

 7. The current consensus is that pulmonary artery catheter (PAC) placement may have benefit in high-risk patients, or those with special indications. However, in routine coronary artery bypass patients, the PAC has little, if any, benefit.

 8. In cardiac surgical patients with ascending aortic atheroma identified by epiaortic scanning, modification of the surgical technique and neuroprotective strategies have been reported to reduce the incidence of neurologic complications from ~60% to 0%.

 9. Monitoring temperature at one core site and one shell site is recommended. Nasal temperature is recommended for core temperature, and bladder or rectal temperature for shell temperature.

10. Though low serum calcium may affect myocardial pumping function, administration of calcium during potential neural ischemia or reperfusion may likely worsen the outcome and should be avoided.

PATIENTS PRESENTING FOR CARDIAC SURGERY require extensive monitoring because of (a) severe, often unstable cardiovascular disease and hemodynamics, (b) coexisting multisystemic diseases, (c) the abnormal physiologic conditions associated with CPB, and (d) special considerations for minimally invasive cardiac surgery. Monitoring techniques have been developed to provide early warning of conditions that may lead to potentially life-threatening complications. Current trends in monitoring include size reduction, minimally invasive device development, and advanced display technology [1].

I. Cardiovascular monitors

   A. Electrocardiogram. The intraoperative use of the electrocardiogram (ECG) facilitates the intraoperative diagnosis of dysrhythmias, myocardial ischemia, and cardiac electrical silence during cardioplegic arrest. A five-lead system, including a V5 lead, is preferable for cardiac surgical patients. Use of five electrodes (one lead on each extremity and one precordial lead) allows the simultaneous recording of the six standard frontal limb leads as well as one precordial unipolar lead.

     1. Indications

        a. Diagnosis of dysrhythmias

        b. Diagnosis of ischemia (see Chapter 11 “Anesthetic Management of Myocardial Revascularization”). In the anesthetized patient, the detection of ischemia by ECG becomes more important because the usual symptom, angina, cannot be elicited.

        c. Diagnosis of conduction defects

        d. Diagnosis of electrolyte disturbances

        e. Monitor effect of cardioplegia during aortic cross-clamp

     2. Techniques

        a. The three-electrode system. This system utilizes electrodes only on the right arm, left arm, and left leg. The potential difference between two of the electrodes is recorded, whereas the third electrode serves as a ground. Three ECG leads (I, II, III) can be examined.

            The three-lead system has been expanded to include the augmented leads. It identifies one of the three leads as the exploring electrode and couples the remaining two at a central terminal with zero potential. This creates leads in three more axes (aVR, aVL, aVF) in the frontal plane (Fig. 4.1). Leads II, III, and aVF are most useful for monitoring the inferior wall, and leads I and aVL for the lateral wall. Several additional leads have been developed for specific indications (Table 4.1).

Table 4.1 Bipolar and augmented leads for use with three electrodes

Figure 4.1 The six frontal plane axes that are available from three leads (right arm, left arm, and left leg) are shown. I, II, and III are bipolar leads, meaning that the potential between two electrodes (one positive, one negative) is monitored. The augmented leads (aVR, aVL,and aVF) are unipolar leads; one lead is the exploring electrode (the positive terminal), and the other two are connected and set at zero potential (indifferent, or neutral). The potential difference is then the absolute difference between the exploring and zero terminals. Connecting the two indifferent leads together produces the augmented lead axes that are between the bipolar lead axes. A sample electrical vector is shown (heavy arrow), with its projections to the six frontal axes. The direction of the electrical vector, then, is dictated by the direction of the deflection seen on the axis of each particular lead of the surface ECG. (From Thys DM, Kaplan JA. The ECG in Anesthesia and Critical Care. New York, NY: Churchill Livingstone; 1987:5, with permission.)

        b. The five-electrode system. All limb leads act as a common ground for the precordial unipolar lead. The unipolar lead usually is placed in the V5 position, along the anterior axillary line in the fifth intercostal space, to best monitor the left ventricle (LV). The precordial lead can also be placed on the right precordium to monitor the right ventricle (RV; V4R lead).

           (1) Advantages. With the addition of only two electrodes to the ECG system, seven different leads can be monitored simultaneously. More important, all but the posterior wall of the myocardium can be monitored for ischemia. In patients with coronary artery disease, the unipolar V5 lead is the best single lead in diagnosing myocardial ischemia [2]; moreover, 90% of ischemic episodes will be detected by ECG if leads II and V5 are analyzed simultaneously. Therefore, a correctly placed V5 lead in conjunction with limb leads should enhance the diagnosis of the vast majority of intraoperative ischemic events. Multiple ECG leads are also useful in the diagnosis of atrial and ventricular dysrhythmias.

           (2) Disadvantage. The V5 electrode does not interfere with the operative field for a median sternotomy, although it will interfere with a left thoracotomy incision. All leads should be protected with waterproof tape, as surgical preparation solutions will loosen electrode patches and interfere with the electrical signal.

        c. Semi-Invasive ECG. Semi-invasive leads are not routinely used but specific application may provide valuable information.

           (1) Esophageal. Esophageal leads can be incorporated into the esophageal stethoscope. The esophageal lead is sensitive for the detection of posterior wall ischemia and the diagnosis of atrial dysrhythmias.

           (2) Endotracheal. ECG leads have been incorporated into the endotracheal tube and may be useful in pediatric cardiac patients for the diagnosis of atrial dysrhythmias.

           (3) Epicardial electrodes. Cardiac surgeons routinely place ventricular and/or atrial epicardial pacing wires at the conclusion of CPB prior to sternal closure. In addition to AV pacing, these pacing wires can be utilized to record atrial and/or ventricular epicardial ECGs. These leads are most useful in the postoperative diagnosis of complex conduction problems and dysrhythmias.

     3. Computer-assisted ECG interpretation. Computer programs for online analysis of dysrhythmias and ischemia are currently widely available with a 60% to 78% sensitivity in detecting ischemia compared with the Holter monitor [3]. Typically, the current ECG signal is displayed along with a graph showing the trend (e.g., ST depression) over a recent time period, usually the past 30 min.

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     4. Recommendations for ECG monitoring. It is recommended that for cardiac anesthesia, a five-electrode surface ECG monitor be used in the diagnostic mode, with a frequency response of 0.05 to 100 Hz. Ideally, this monitor should display at least two leads simultaneously evaluating dysrhythmias and ischemia of two different areas of myocardium supplied by two different coronary arteries. Typically, leads V5and II are monitored. None of the standard ECG leads can detect posterior wall ischemia or RV ischemia.

   B. Noninvasive blood pressure monitors

     1. Indications in the cardiac patient. Noninvasive methods for measuring blood pressure (BP) are not adequate for monitoring hemodynamic parameters during a cardiac surgical procedure and should only be used until an arterial catheter is placed. Noninvasive devices will not function when pulsatile flow is absent including during cardiopulmonary bypass or when continuous-flow left ventricular assist devices (LVAD) are used.

   C. Intravascular pressure measurements. Invasive monitors via intravascular catheters are required to safely administer a cardiac anesthetic. Arterial pressure can be measured by placing a catheter in a peripheral artery or femoral artery. Central venous access is obtained to measure the central venous pressure (CVP) and serve as conduit for PAC placement to measure intracardiac pressures. The components of a system of intravascular pressure measurement are the intravascular catheter, fluid-filled connector tubing, a transducer, and an electronic analyzer and display system.

     1. Characteristics of a pressure waveform. Pressure waves in the cardiovascular system can be characterized as complex periodic sine waves. These complex waves are a summation of a series of simple sine waves of differing amplitudes and frequencies, which represent the natural harmonics of a fundamental frequency. The first harmonic, or fundamental frequency, is equal to the heart rate (Fig. 4.2), and the first 10 harmonics of the fundamental frequency will contribute significantly to the waveform.

Figure 4.2 A: Generation of the harmonic waveforms from the fundamental frequency (heart rate) by Fourier analysis. B: The first six harmonics are shown. The addition of the six harmonics reproduces an actual BP wave. The first six harmonics are superimposed, showing a likeness to, but not a faithful reproduction of, the original wave. The first 10 harmonics of a pressure wave must be sensed by a catheter–transducer system, if that system is to provide an accurate reproduction of the wave. (From Welch JP, D’Ambra MN. Hemodynamic monitoring. In: Kofke WA, Levy JH, eds. Postoperative Critical Care Procedures of the Massachusetts General Hospital. Boston, MA: Little, Brown and Company; 1986:146, with permission.)

     2. Properties of a monitoring system

        a. Frequency response (or amplitude ratio) is the ratio of the measured amplitude versus the input amplitude of a signal at a specific frequency. The frequency response should be constant over the desired range of input frequencies—that is, the signal is not distorted (amplified or attenuated). The ideal amplitude ratio is close to 1. The signal frequency range of an intravascular pressure wave response is determined by the heart rate. For example, if a patient’s heart rate is 120 beats/min, the fundamental frequency is 2 Hz. Because the first 10 harmonics contribute to the arterial waveform, frequencies up to 20 Hz will contribute to the morphology of an arterial waveform at this heart rate.

        b. Natural frequency (or resonant frequency), a property of all matter, refers to the frequency at which a monitoring system itself resonates and amplifies the signal. The natural frequency ( fn) of a monitoring system is directly proportional to the catheter lumen diameter (D), inversely proportional to the square root of three parameters: The tubing connection length (L), the system compliance (V/P), and the density of fluid contained in the system (δ ). This is expressed as follows:

            To increase fn and thereby reduce distortion, it is imperative that a pressure-sensing system be composed of short, low-compliance tubing of reasonable diameter, filled with a low-density fluid (such as normal saline).

            Ideally, the natural frequency of the measuring system should be at least 10 times the fundamental frequency to reproduce the first 10 harmonics of the pressure wave without distortion. In clinical practice, the natural frequency of most measuring systems is in the range from 10 to 20 Hz. If the input frequency is close to the system’s natural frequency (which is usually the case in clinical situations), the system’s response will be amplified (Fig. 4.3). Therefore, these systems require the correct amount of damping to minimize distortion.

Figure 4.3 Pressure recording from a pressure generator simulator, which emits a sine wave at increasing frequencies (horizontal axis). The frequency response (ratio of signal amplitudeOUT to signal amplitudeIN) is plotted on the vertical axis for a typical catheter–transducer system. The useful band width (range of frequency producing a “flat” response) and the amplification of the signal in the frequency range near the natural frequency of the system are shown. (From Welch JP, D’Ambra MN. Hemodynamic monitoring. In: Kofke WA, Levy JH, eds. Postoperative Critical Care Procedures of the Massachusetts General Hospital. Boston, MA: Little, Brown and Company; 1986:148, with permission.)

        c. The damping coefficient reflects the rate of dissipation of the energy of a pressure wave. Figure 4.4 shows the relationship among frequency response, natural frequency, and damping coefficient.

Figure 4.4 Amplitude ratio (or frequency response) on the vertical axis is plotted as a function of the input frequency as a percentage of the natural frequency (rather than as absolute values). In the undamped or underdamped system, the signal output is amplified in the region of the natural frequency of the transducer system; in the overdamped system, a reduction in amplitude ratio for most input frequencies is seen. This plot exhibits several important points: (i) If a catheter–transducer system has a high natural frequency, less damping will be required to produce a flat response in the clinically relevant range of input frequencies (10 to 30 Hz). (ii) For systems with a natural frequency in the clinically relevant range (usual case), a level of  “critical” (optimal) damping exists that will maintain a flat frequency response. D, damping coefficient (From Grossman W. Cardiac Catheterization. 3rd ed. Philadelphia, PA: Lea & Febiger; 1985:122, with permission.)

            When a pressure-monitoring system with a certain natural frequency duplicates a complex waveform with any one of the first 10 harmonics close to the natural frequency of the system, amplification will result if correct damping of the catheter–transducer unit is not performed. The problem is compounded when the heart rate is fast (as in a child or a patient with a rapid atrial rhythm), which increases the demands of the system by increasing the input frequency (Fig. 4.5). Correct damping of a pressure-monitoring system should not affect the natural frequency of the system.

Figure 4.5 Comparison of three catheter–transducer systems with the same natural frequency (15 Hz) under different conditions of heart rate. Pressures are displayed as systolic–diastolic (mean). The reference BP for all panels is 100/50. A: A critically damped system (ζ = 0.6) provides an accurate reproduction until higher heart rates (greater than 150) are reached. B: An underdamped system (ζ = 0.2) shows distortion at lower rates, leading to overestimation of systolic and underestimation of diastolic pressures. C: An overdamped system (ζ = 0.8) demonstrates underestimation of systolic pressure and overestimation of diastolic pressure. Also note that diastolic and mean pressures are affected less by the inadequate monitoring systems. fn, natural frequency; ζ, damping coefficient.

            Both the natural frequency and the damping coefficient of a system can be estimated using an adaptation of the square wave method known as the “pop” test. The natural frequency is estimated by measuring the time period of one oscillation as the system settles to baseline after a high-pressure flush. The damping coefficient is calculated by measuring the amplitude ratio of two successive peaks (Fig. 4.6).

Figure 4.6 The “pop” test allows one to derive fn and ζ of a catheter–transducer system. The test should be done with the catheter in the artery in order to test the system in its entirety, as all components contribute to the harmonics of the system. The test involves a rapid flush (with the high-pressure flush system used commonly), followed by a sudden release. This produces a rapid decrease from the flush bag pressure and, owing to the inertia of the system, an overshoot of the baseline. The subsequent oscillations about the baseline are used to calculate fn and ζ. For example, the arterial pulse at the far left of the figure is followed by a fast flush and sudden release. The resulting oscillations have a definite period, or cycle, measured in millimeters. The natural frequency fn is the paper speed divided by this period, expressed in cycles per second, or Hertz. If the period were 2 mm and the paper speed 25 mm/s, fn = 12.5 Hz. For determining fn, a faster paper speed will give better reliability. The ratio of the amplitude of one induced resonant wave to the next, D2/D1, is used to calculate damping coefficients (right column). A damping coefficient of 0.2 to 0.4 describes an underdamped system, 0.4 to 0.6 an optimally damped system, and 0.6 to 0.8 an overdamped system. (From Bedford RF. Invasive blood pressure monitoring. In: Blitt CD, ed. Monitoring in Anesthesia and Critical Care Medicine. New York, NY: Churchill Livingstone; 1985:59, with permission.)

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            After a rapid pressure change (performed by flushing the pressure line), an underdamped system will continue to oscillate for a prolonged time. In terms of pressure monitoring, this translates to an overestimation of systolic BP and an underestimation of diastolic BP. An overdamped system will not oscillate at all but will settle to baseline slowly, thus underestimating systolic and overestimating diastolic pressures. A critically damped system will settle to baseline after only one or two oscillations and will reproduce systolic pressures accurately. An optimally or critically damped system will exhibit a constant (or flat) frequency response in the range of frequencies up to the fn of the system (Fig. 4.4). If a given system does not meet this criterion, components should be checked, especially for air, or the system replaced. Even an optimally damped system will begin to distort the waveform at higher heart rates because the 10th harmonic exceeds the system’s natural frequency (Fig. 4.5).

     3. Strain gauges. The pressure-monitoring transducer can be described as a variable-resistance transducer. A critical part of the transducer is the diaphragm, which acts to link the fluid wave to the electrical input. When the diaphragm of a transducer is distorted by a change in pressure, voltages are altered across the variable resistor of a Wheatstone bridge contained in the transducer. This in turn produces a change in current, which is electronically converted and displayed.

     4. Sources of error in intravascular pressure measurement

        a. Low-frequency transducer response. Low-frequency response refers to a low- frequency range over which the ratio of output-to-input amplitude is constant (i.e., no distortion). If the natural frequency of the system is low, its frequency response will also be low. Most transducer systems used in clinical anesthesia can be described as underdamped systems with a low natural frequency. Thus, any condition that further decreases fn response should be avoided. Air within a catheter–transducer system causes most monitoring errors. Because of its compressibility, air not only decreases the response of the system but also leads to overdamping of the system. Therefore, the myth that an air bubble placed in the pressure tubing decreases artifact by increasing the damping coefficient is incorrect. A second common cause of diminution of frequency response is the formation of a partially obstructing clot in the catheter.

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        b. Catheter whip. Catheter “whip” is a phenomenon in which the motion of the catheter tip itself produces a noticeable pressure swing. This artifact usually is not observed with peripheral arterial catheters but is more common with PAC or LV catheters.

        c. Resonance in peripheral vessels. The systolic pressure measured in a radial arterial catheter may be up to 20 to 50 mm Hg higher than the aortic pressure due to decreased peripheral arterial elastance and wave summation (Fig. 4.7).

Figure 4.7 Change of pulse pressure in different arteries. The central aortic waveform is more rounded and has a definite dicrotic notch. The dorsalis pedis and, to a lesser extent, the femoral artery show a delay in pulse transmission, sharper initial upstrokes (and thus higher systolic pressure), and slurring (femoral) and loss (dorsalis) of the dicrotic notch. The dicrotic notch is better maintained in the upper-extremity pressure wave (not shown). The small second “hump” in the dorsalis wave probably is due to a reflected wave from the arterial–arteriolar impedance mismatching. (From Welch JP, D’Ambra MN. Hemodynamic monitoring. In: Kofke WA, Levy JH, eds. Postoperative Critical Care Procedures of the Massachusetts General Hospital. Boston, MA: Little, Brown and Company; 1986:144, with permission.)

        d. Changes in electrical properties of the transducer. Electrical balance, or electrical zero, refers to the adjustment of the Wheatstone bridge within the transducer so that zero current flows to the detector at zero pressure. Transducers should be electronically balanced periodically during a procedure because the zero point may drift, for instance, if the room temperature changes. The pressure waveform morphology may not change with baseline drift of a transducer.

        e. Transducer position errors. By convention, the reference position for hemodynamic monitoring during cardiac surgery is the right atrium (RA). With the patient supine, the RA lies at the level of the midaxillary line. Once its zero level has been established, the transducer must be maintained at the same level as the RA. If the transducer position changes, falsely high or low-pressure values will result.This can be significant especially when monitoring CVP, PA pressure, or pulmonary capillary wedge pressure (PCWP) where the observed change is a greater percentage of the measured value. For example, if a patient has a mean arterial pressure of 100 mm Hg and a of 10 mm Hg, a 5 mm Hg offset due to transducer position would be displayed as a 5% or 50% change in the arterial or CVP pressures, respectively.

   D. Arterial catheterization

     1. Indications. Arterial catheterization (“art line”) has become the standard in the monitoring of the cardiac surgical patient. Indications for arterial pressure monitoring are as follows:

        a. Small or rapid changes in arterial perfusion pressure may increase patient risk requiring beat-to-beat assessment. Cardiac surgical patients frequently have critical coronary artery disease and/or valvular heart disease. Cardiac surgical patients are at high risk of becoming hemodynamically unstable in the perioperative period.

        b. Wide variation in BP or intravascular volume is anticipated.

        c. Frequent blood sampling, especially arterial blood gas (ABG) analysis, is required.

        d. Assessment of BP cannot be performed by other methods. CPB and continuous flow LVAD (nonpulsatile flow), dysrhythmia, or marked obesity require arterial pressure catheters.

     2. Sites of cannulation. Several sites can be used for cannulation of the arterial tree:

        a. Radial artery. The radial artery is the most commonly utilized site because catheter insertion is convenient and the radial artery provides a reasonably accurate estimation of the true aortic pressure.

           (1) Technique. Table 4.2 summarizes the steps used for radial arterial cannulation. One technique, transfixing the radial artery for catheter insertion, is shown in Figure 4.8. Ultrasound-guided radial artery cannulation [4] can be especially beneficial when difficulty placing the catheter via the traditional method is encountered. Table 4.3 and Figure 4.9 describe ultrasound guidance for arterial cannulation. Ultrasound guidance may be the method of choice with improved first-pass success when difficulty such as low flow states (shock), or nonpulsatile flow (extracorporeal membrane oxygenation [ECMO], LVAD, cardiac arrest), or nonpalpable pulses (secondary to peripheral edema, hematoma, arterial vasospasm) are encountered.

Table 4.2 Steps for arterial catheter placement

Table 4.3 Steps for ultrasound-guided arterial catheterization

Figure 4.8 One technique used for radial artery cannulation. The needle–catheter unit is advanced through the artery, as shown in the upper drawing. The lower drawing shows the needle removed and the catheter withdrawn until pulsatile flow is obtained (indicating that the catheter tip is in the lumen). The catheter then is advanced into the artery. (Redrawn from Freis ES. Vascular cannulation. In: Kofke WA, Levy JH, eds. Postoperative Critical Care Procedures of the Massachusetts General Hospital. Boston, MA: Little, Brown and Company; 1986:137, with permission.)

Figure 4.9 Ultrasound-guided radial artery cannulation. A: Color flow Doppler demonstrates patent flow in the radial artery. B: After flow is confirmed, a catheter is advanced with ultrasound guidance into the radial artery. (From Mikhael R. Radial Artery Cannulation (In-Plane Approach). 2010. Graphic. MSH Obstetric Anesthesia Ultrasound Group. http://oba.mikhaels.org/currentconcepts/ultrasound/module.php?section = vascular&name = multimedia)

           (2) Contraindications. Inadequate collateral flow to the hand is a relative contraindication to the use of a radial artery catheter. Very few ischemic complications have been reported from arterial catheterization even in patients with a positive Allen’s test indicating limited collateral ulnar artery flow [5].

           (3) Radial artery harvest. If a coronary bypass that will utilize the radial artery as a free graft is planned, an alternative site must be used. Usually the radial artery to be harvested is known preoperatively, and the contralateral radial artery can be used.

        b. Femoral artery. The femoral artery offers two advantages over the radial site: Assessment of central arterial pressure and appropriate access should placement of an intra-aortic balloon pump become necessary during the surgical procedure. Placement of a femoral artery catheter as an additional catheter site should be considered in any patient in whom difficulty in weaning from CPB is expected (e.g., those with markedly depressed ejection fraction, severe wall-motion abnormalities, or significant coronary disease).

           (1) Technique. The femoral artery is entered most easily using a Seldinger technique after sterile preparation and draping. The femoral artery typically lies at the midpoint between the pubic tubercle and the anterior superior iliac spine. These bone landmarks can be used to guide identification of the femoral pulse in difficult cases.

           (2) Contraindications. Cannulation of the femoral artery should be avoided in patients with prior vascular surgery involving the femoral arteries or a skin infection of the groin.

        c. Aortic root. Aortic root cannulation is an option when the chest is open and difficulties are encountered in obtaining a reliable BP. Pressure tubing can be handed to the anesthesiologist from the sterile field after a needle or catheter is inserted in the aortic root by the surgeon.

        d. Axillary artery. The axillary artery, like the femoral artery, provides the anesthesiologist with a superficial, large artery that has good access to the central arterial tree.

           (1) Technique. The axillary artery is most easily entered using a Seldinger technique.

           (2) Contraindications. The increased risk of cerebral embolus of air or debris must be recognized. Flushing the arterial line must be performed with caution and low pressure.

        e. Brachial artery. The brachial artery is an easily accessible artery located medially in the antecubital fossa.

           (1) Brachial artery cannulation is similar to that described for the radial artery. The elbow must be immobilized with a long arm board for stability.

           (2) Contraindications. Concern about compromised flow distal to catheter placement has limited its use at many institutions. Brachial catheterization is a secondary option or is not utilized in non-heparinized surgical procedures. A purported benefit has been the elimination of the pressure discrepancy seen occasionally with radial arterial lines in the immediate postbypass period (see Section I.D.4.e).

        f. Ulnar artery. The ulnar artery can be used in those rare circumstances when the radial artery cannot be entered easily.

        g. Dorsalis pedis and posterior tibialis arteries. In general, the distal location increases distortion of the arterial wave (Fig. 4.7). The dorsalis pedis artery is technically easier to cannulate.

     3. Interpretation of arterial tracings. The arterial pressure waveform contains a great deal of hemodynamic information.

        a. Heart rate and rhythm. The heart rate can be determined from the arterial pressure wave. This is especially helpful if the patient is being paced or if electrocautery is being used. In the presence of numerous atrial or ventricular ectopic beats, the arterial trace can provide useful information on the hemodynamic consequences of these dysrhythmias (i.e., if an ectopic beat is perfusing).

        b. Pulse pressure. The difference between the systolic and diastolic pressures provides useful information about fluid status and valvular competence. Pericardial tamponade and hypovolemia are accompanied by a narrow pulse pressure on the arterial waveform. An increase in pulse pressure may be a sign of worsening aortic valvular insufficiency or hypovolemia.

        c. Respiratory variation and volume status. Hypovolemia is suggested by a decrease in arterial systolic pressure with positive-pressure ventilation (pulsus paradox). Positive intrathoracic pressure impedes venous return to the heart with a more pronounced effect in the hypovolemic patient. Because this finding is not uniformly seen in patients with hypovolemia, correlation with other findings can help make the diagnosis.

        d. Qualitative estimates of hemodynamic indices. Inferences can be made regarding contractility, stroke volume, and vascular resistance from the arterial waveform. Contractility can be grossly judged by the rate of pressure rise during systole, keeping in mind that heart rate, preload, and afterload can affect this parameter. Stroke volume can be estimated from the area under the aortic pressure wave from the onset of systole to the dicrotic notch. Finally, the position of the dicrotic notch correlates with the systemic resistance. A notch appearing high on the downslope of the pressure trace suggests a high vascular resistance, whereas a low resistance tends to cause a dicrotic notch that is lower on the diastolic portion of the pressure trace. These elements are incorporated into the algorithms of pulse pressure analysis monitors for non-invasive cardiac output (CO).

     4. Complications of arterial catheterization

        a. Ischemia. The incidence of ischemic complications after radial artery cannulation is low. A classic study demonstrated that although abnormal flow patterns were present in up to 25% of patients between 1 and 7 days after radial artery catheterization, there were no adverse signs of ischemia with these findings [5].

        b. Thrombosis. Although the incidence of thrombosis from radial artery catheterization is high, studies have not demonstrated adverse sequelae. Recanalization of the radial artery occurs in a majority of cases. Patients with increased risk include those with diabetes or severe peripheral vascular disease.

        c. Infection. With proper sterile technique, the risk of infection from cannulation of the radial artery should be minimal. In a series of 1,700 cases, no catheter site was overtly infected [5].

        d. Bleeding. Although transfixing the artery will put a hole in the posterior wall, the layers of the muscular media will seal the puncture. In the patient with a bleeding diathesis, however, there is a greater tendency to bleed. Unlike central venous catheters, arterial catheters are not heparin bonded and thus have increased risk of thrombus development.

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        e. False lowering of radial artery pressure immediately after CPB. The radial artery pressure may be significantly lower than the aortic pressure at the completion of CPB. Forearm vasodilation secondary to rewarming may lead to arteriovenous AV shunting, resulting in a steal phenomenon 5 to 30 min or longer in duration. Alternatively, the inaccuracy of radial pressure may be due to hypovolemia and vasoconstriction. If suspicion arises that a peripheral arterial trace is dampened following CPB, a direct pressure measurement should be obtained from a central site.

     5. Recommendations for BP monitoring. In high-risk patients, invasive arterial pressure monitoring should commence prior to induction. Under most circumstances, the radial artery pressure measurement will be sufficient and accurate before and after CPB. In the patient with poor LV function, addition of a femoral arterial catheter before CPB may be warranted. Certain surgical procedures, for example, a thoracoabdominal aortic aneurysm repaired with left heart partial bypass, require both an upper and lower extremity arterial catheters. If an internal mammary artery (IMA) is dissected, retraction of the chest wall and compression of the subclavian artery can dampen or obliterate the radial artery traces. The surgeon should be informed for possible change in retractor position. A dampened radial pressure during IMA harvest may also be associated with a brachial plexus injury.

   E. Central venous pressure. CVP measures RA pressure and is affected by circulating blood volume, venous tone, and RV function.

      1. Indications

        a. Monitoring. Monitoring of CVP is indicated for all cardiac surgical patients.

        b. Fluid and drug therapy. CVP can be used to infuse fluid or blood products; as a port for administering vasoactive drugs; and for postoperative hyperalimentation.

        c. Special uses. One may elect to place a CVP catheter, with delayed PAC placement in selected patients. Placement of a PAC can be difficult in patients with numerous congenital cardiac disorders, in those with anatomic distortion of the right-sided venous circulation, or in those requiring surgical procedures of the right heart or implantation of a right heart mechanical assist device.

     2. Techniques. There are numerous routes by which a catheter can be placed in the central circulation.

        a. Internal jugular. The internal jugular vein (IJV) is the most common access route for the cardiac anesthesiologist.

           (1) Techniques. Cannulation of the IJV is relatively safe and convenient and various approaches exist for its cannulation (Table 4.4Fig. 4.10). The process of cannulation, regardless of the approach, involves the steps outlined in Table 4.5.

Table 4.4 Sites for internal jugular cannulation

Table 4.5 Steps for right internal jugular cannulation

Figure 4.10 Two methods for internal jugular cannulation. A: Anterior approach. B: Central approach (see text for further details). (Redrawn from Freis ES. Vascular cannulation. In: Kofke WA, Levy JH, eds. Postoperative Critical Care Procedures of the Massachusetts General Hospital. Boston, MA: Little, Brown and Company; 1986:130, with permission.)

           (2) Contraindications and recommendations. Relative contraindications for internal jugular central line placement include the following:

             (a) Presence of carotid disease.

             (b) Recent cannulation of the IJV (with the concomitant risk of thrombosis).

             (c) Contralateral diaphragmatic dysfunction.

             (d) Thyromegaly or prior neck surgery.

   In these cases, the IJV on the contralateral side should be considered. It should be remembered that the thoracic duct lies in close proximity to the left IJV and that laceration of the left brachiocephalic vein or superior vena cava by the catheter is more likely with the left-sided approach. This risk is due to the more acute angle between the left internal jugular and innominate veins.

           (3) Sonographic guidance. Ultrasound guidance of IJV cannulation [6] has quickly gained acceptance as several studies have suggested improved success rates and decreased complications with its use. There is debate whether ultrasound guidance should become the standard of care [7,8]. The Agency for Healthcare Research and Quality in the United States and the National Institute for Clinical Excellence in the United Kingdom suggest that the preferred method for internal jugular catheter placement is ultrasound guidance, particularly for inexperienced operators. Several commercially available hand-held ultrasound units are currently available. Alternatively, a hand-held probe from the Transesophageal echocardiography (TEE) machine can be utilized for ultrasonography. When anatomic landmark techniques prove to be difficult, ultrasound guidance becomes an invaluable tool. It can facilitate access in patients with thrombosis, hematoma formation, or vessel atrophy from multiple prior cannulations. Ultrasound-guided internal jugular cannulation (Table 4.6) requires the ability to correctly distinguish the easily compressible jugular vein from the carotid artery (CA), as shown in Figure 4.11.

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Table 4.6 Steps for ultrasound-guided cannulation of internal jugular

Figure 4.11 Ultrasound-guided internal jugular catheterization. The top picture demonstrates the two-dimensional examination showing the IJV lateral to the CA. The bottom picture with the transducer oriented caudad demonstrates the IJV with blue flow and CA with red flow. (From Barash P, Cullen B, eds. Clinical Anesthesia. Philadelphia, PA: Lippincott Williams & Wilkins; 2009:747.)

        b. External jugular. The external jugular vein courses superficially across the sternocleidomastoid muscle to join the subclavian vein close to the junction of the internal jugular and subclavian veins. Its course is more tortuous, and the presence of valves makes central line placement more difficult. The placement of rigid central catheters (e.g., PAC introducer) via the external jugular increases the risk of vessel trauma and is not recommended. Pliable central catheters and short catheters can be used to acquire intravenous access.

        c. Subclavian. The subclavian vein is readily accessible and thus has been popular for use during cardiopulmonary resuscitation.

           (1) Techniques. The patient is placed in a head-down position to distend the vein and decrease risk of air embolism. Optimal positioning can be obtained by placing a roll vertically under the patient’s spine to anatomically retract the clavicle.

           (2) Advantages. The main advantage to subclavian vein cannulation is its relative ease and the stability of the catheter during long-term cannulation.

           (3) Disadvantages

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             (a) Subclavian vein cannulation carries the highest rate of pneumothorax of any approach. If subclavian vein cannulation is unsuccessful on one side, an attempt on the contralateral side is contraindicated without first obtaining a chest x-ray film. Bilateral pneumothoraces can be lethal. Subclavian vein placement for cardiac surgery can be associated with compression of the central line during sternal retraction.

             (b) The subclavian artery is entered easily.

             (c) In a left-sided cannulation, the thoracic duct may be lacerated.

             (d) The right subclavian approach may make threading the PAC into the RA difficult because an acute angle must be negotiated by the catheter in order to enter the innominate vein. The left subclavian approach is recommended as the first option for PAC placement.

        d. Arm vein

           (1) Techniques. Central access can be obtained through the veins of the antecubital fossa (“long-arm CVP”). This has a limited role in most cardiac surgical procedures.

     3. Complications. The site-specific complications of CVP catheter insertion are listed in Table 4.7. The most severe complications of CVP insertion usually are preventable.

Table 4.7 Complications of central venous cannulation for each of four cannulation sites

     4. Interpretation

        a. Normal waves. The normal CVP trace contains three positive deflections, termed the A, C, and V waves, and two negative deflections termed the X and Y descents (Fig. 4.12).

Figure 4.12 Relationship between ECG (top) and CVP (bottom). The normal CVP trace contains three positive deflections, known as the A, C, and V waves, and two negative deflections, the X and Y descents. The A wave occurs in conjunction with the P wave on the ECG and represents atrial contraction. The C wave occurs in conjunction with the QRS wave and represents the bulging of the tricuspid valve into the RA with right ventricular contraction. The X descent occurs next as the tricuspid valve is pulled downward during the latter stages of ventricular systole. The final positive wave, the V wave, occurs after the T wave on the ECG and represents right atrial filling before opening of the tricuspid valve. The Y descent occurs after the V wave when the tricuspid valve opens and the atrium empties into the ventricle. (Modified from Reich DL, Moskowitz DM, Kaplan JA. Hemodynamic monitoring. In: Kaplan JA, ed. Cardiac Anesthesia. 4th ed. Philadelphia, PA: WB Saunders; 1999:330.)

        b. Abnormal waves. A common abnormality in the CVP trace occurs in the presence of AV dissociation, when RA contraction occurs against a closed tricuspid valve. This produces a large “cannon A wave” that is virtually diagnostic. Abnormal V waves can occur with tricuspid valve insufficiency, in which retrograde flow through the incompetent valve produces an increase in RA pressure during systole.

        c. RV function. CVP offers direct measurement of RV filling pressure.

        d. LV filling pressures. The CVP can be used to estimate LV filling pressures. Parameters that distort this estimate include LV dysfunction, decreased LV compliance (i.e., ischemia), pulmonary hypertension, or mitral valvular disease. In patients with coronary artery disease but good ventricular function (ejection fraction greater than 40% and no regional wall-motion abnormalities), CVP correlates well with PCWP. However, because RV is a thinner walled chamber, the compliance of RV is higher than that of LV. Therefore, for any given preload, CVP will be lower than either PA diastolic pressure or PCWP. Although the absolute number has not been shown to correlate with preload conditions and stroke volume, evaluating trends over time and cyclic changes during mechanical ventilation can help guide fluid therapy.

   F. PA catheter

     1. Parameters measured

        a. PA pressure reflects RV function, pulmonary vascular resistance (PVR), and left atrial (LA) filling pressure.

        b. PCWP is a more direct estimate of LA filling pressure. With the balloon inflated and “wedged” in a distal branch PA, a valveless hydrostatic column exists between the distal port and the LA at end-diastole (Fig. 4.13).

Figure 4.13 The cardiopulmonary circulation. Top: Valveless conduit from PA to LV is depicted with the mitral valve open at end-diastole. Bottom: Typical pressure waveforms corresponding to each chamber or vessel, with horizontal lines drawn at values for right ventricular and left ventricular preload. LA, left atrium; LVEDP, LV end-diastolic pressure; LVEDV, LV end-diastolic volume; PC, pulmonary capillary bed; PV, pulmonary vein; RA, right atrium; RV, right ventricle. Note that during diastole, the pulmonic valve is closed, which explains why the PA diastolic pressure, LA pressure, and LVEDP (equal to 12 mm Hg) are greater than the RA pressure and RV end-diastolic pressure (not labeled, but equal to 5 mm Hg). (From Tuman KJ, Carroll GC, Ivankovich AD. Pitfalls in interpretation of pulmonary artery catheter data. J Cardiothorac Anesth. 1989;3:626, with permission.)

        c. CVP. A sampling port of the PAC is located in the RA and allows measurement of the CVP.

        d. CO. A thermistor located at the tip of the PAC allows measurement of the output of the RV by the thermodilution technique. In the absence of intracardiac shunts, this measurement equals LV output.

        e. Blood temperature. The thermistor can provide a constant measurement of blood temperature, which is an accurate reflection of core temperature.

        f. Derived parameters. Several indices of ventricular performance and cardiovascular status can be derived from the parameters measured by PAC. Their formulas, physiologic significance, and normal values are listed in Table 4.8.

Table 4.8 Derived hemodynamic indices

        g. Mixed venous oxygen saturation. Oximetric PAC can measure real-time PA venous blood oxygen saturation providing information on end-organ oxygen utilization.

        h. RV performance. New PAC technology allows for improved assessment of RV function distinct from LV dysfunction.

     2. Indications for PA catheterization. There is no consensus among cardiac anesthesiologists regarding PAC use. PAC guidelines have been published [9]. In some institutions, cardiac surgery with CPB represents a universal indication for PA pressure monitoring in adults; other institutions rarely use PAC [10,11]. In the late 1990s several observational studies, randomized control trials, and meta-analyses did not show positive outcome benefits with the use of the PAC. Between 1994 and 2004, PAC use decreased 65% in medical intensive care units (ICUs) and 63% in surgical ICUs. Proponents of the PAC suggest that timing of catheter placement, patient selection, interpretation of PAC data, and early appropriate intervention are required for this monitor to meaningfully affect patient outcome. Decreased mortality has been reported in high-risk surgical patients in which PACs were inserted preoperatively and interventions were protocol driven. In elective cardiac surgical patients, Polanen reported in 2000 that PAC protocols reduce hospital and ICU length of stay [12]. Therefore, the current consensus appears to be that PACs may have benefit in high-risk patients or those with special indications. However, in routine patients they have limited, if any, benefit.

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          Particular indications for PACs are listed in Table 4.9. Differentiation of left versus right ventricular function and assessment of intracardiac filling pressures during cardioplegia administration (enhanced myocardial protection) are two indications that cannot be performed with CVP alone. Discordance in right and left heart function occurs with variable frequency where pressures measured on the right side (i.e., CVP) do not adequately reflect those on the left side [13].

Table 4.9 Indications for using the pulmonary artery catheter in cardiac surgery

        a. Assessing volume status. In many patients with differences in RV and LV function, volume status is difficult to determine because of the large disparity between right (CVP) and left (PCWP) ventricular filling pressures. Myocardial ischemia, LV dysfunction, and positive pressure ventilation can exacerbate these differences.

        b. Diagnosing RV failure. The RV is a thin-walled, highly compliant chamber that can fail during cardiac surgery either because of a primary disease process (inferior myocardial infarction), inadequate myocardial protection, or intracoronary air (predilection for the right coronary artery) as a result of the surgical procedure. RV failure presents as an increase in CVP, a decrease in the CVP to mean PA gradient, and a low CO.

        c. Diagnosing LV failure. Knowledge of PA and wedge pressures can aid in the diagnosis of left-sided heart failure if other causes (ischemia, mitral valve disease) are eliminated. TEE can aid in correlating the clinical paradigm to PAC measurements. Simultaneous readings of high PA pressures and wedge pressure in the presence of systemic hypotension and low CO are hallmarks of LV failure.

        d. Diagnosing pulmonary hypertension. Note that with normal PVR, the PA diastolic and wedge pressure agree closely with one another. This relationship is lost with pulmonary hypertension.

        e. Assessing valvular disease

           (1) Tricuspid and pulmonary valve stenosis can be diagnosed by means of a PAC by measuring pressure gradients across these valves, although in adults TEE is the primary diagnostic modality for these lesions.

           (2) Mitral valvular disease is reflected in the PA and wedge pressure waveform morphology. Mitral insufficiency appears as an abnormal V wave and an increase in pulmonary venous pressure from the regurgitant flow into the LA. V waves can also appear in other conditions, including myocardial ischemia, ventricular pacing, and presence of a ventricular septal defect depending on the compliance of the LA. In patients with chronic mitral valve insufficiency, the LA has a high compliance and a large regurgitant volume will not always result in a dramatic V wave.

        f. Early diagnosis of ischemia. PAC, ECG, and TEE can assist detection of myocardial ischemia. Significant ischemia (transmural or subendocardial) is often associated with a decrease in ventricular compliance, which is reflected in either an increase in PA pressure or an increase in PCWP. In addition, the development of pathologic V waves may occur secondary to ischemia of the papillary muscle (Fig. 4.14).

Figure 4.14 V waves secondary to severe mitral regurgitation. The tall systolic v wave in the PA wedge pressure (PAWP) trace also distorts the PA tracing, thereby giving it a bifid appearance. LVED pressure is best estimated by measuring PAWP at the time of the ECG R wave before the onset of the regurgitant v wave. (From Mark JB. Atlas of Cardiovascular Monitoring. New York, NY: Churchill Livingstone; 1998, Figure 17–11.)

     3. Contraindications for PAC Placement

        a. Significant tricuspid/pulmonary valvular pathology: Tricuspid/pulmonary stenosis, endocarditis, or mechanical prosthetic valve replacement.

        b. Presence of a right-sided mass (tumor/thrombosis) that would cause a pulmonary or paradoxical embolization if dislodged.

        c. Left bundle branch block (LBBB): LBBB is a relative contraindication. The incidence of transient right bundle branch block (RBBB) during PAC placement is ~5%. In a patient with LBBB, this can result in complete heart block when floating the PAC through the right ventricular outflow track. Therefore, external pacing should be immediately available for these patients.

     4. Interpretation of PA pressure data

        a. Effects of ventilation. The effects of ventilation on PA pressure readings can be significant in the low-pressure system of the right-sided circulation because airway or transpleural pressure is transmitted to the pulmonary vasculature.

           (1) When a patient breathes spontaneously, the negative intrapleural pressure that results from inspiration can be transmitted to the intravascular pressure. Thus, low or even “negative” PA diastolic, wedge, and CVPs may occur with spontaneous ventilation.

           (2) Positive airway pressures are transmitted to the vasculature during positive- pressure ventilation, leading to elevations in pulmonary pressures. Mean airway pressure is the parameter which correlates most closely with the changes in PA and CVP pressure measurements.

           (3) The established convention is to evaluate pressures at end-expiration. The digital monitor numerical readout may give incorrect information because these numbers reflect the absolute highest (systolic), lowest (diastolic), and mean (area under pressure curve) values for several seconds, which may include one or more breaths. Damping is not accounted for by the monitor. Thus, inspection and interpretation of the waveform data is required to correctly evaluate the clinical scenario.

        b. Location of catheter tip. PA pressure measurements depend on where the tip of the catheter resides in the pulmonary vascular tree. In areas of the lung that are well ventilated but poorly perfused (West zone I), the readings will be more affected by the changes in airway pressure. Likewise, even when the tip is in a good location in the middle or lower lung fields, large amounts of positive end-expiratory pressure (>10 mm Hg) will affect PA values.

     5. Timing of placement. A debate exists regarding whether PAC insertion before induction is indicated in adult patients with good LV function. The discomfort that might be associated with placement needs to be balanced with acquisition of hemodynamic data. In appropriately sedated patients, placement of a PAC is not associated with any significant hemodynamic changes. The hemodynamic data collected in the catheterization laboratory may not accurately reflect the current hemodynamic status, especially if the patient had episodes of myocardial ischemia during the catheterization or may be experiencing ischemia when entering the operating room.

     6. Types of PACs. A variety of PACs are currently available for clinical use. The standard thermodilution catheter has a PA port for pressure monitoring and a thermistor for CO measurements at its tip, an RA port for CVP monitoring and for injection of cold saline 30 cm from the tip, and a lumen for inflation of the balloon. In addition, PACs are available that provide the following:

        a. Venous infusion port (VIP). VIP PACs have a third port 1 cm proximal to the CVP (31 cm from the tip) for infusion of drugs and fluids.

        b. Pacing. Pacing PACs have the capacity to provide intracardiac pacing. Pacing PACs are seldom used for cardiac procedures because usually patients already have a temporary pacing wire for symptomatic bradycardia prior to anesthetic care. Epicardial pacing wires are routinely placed by the surgeon intraoperatively for postoperative bradycardia.

           (1) Pacing PACs have a separate lumen terminating 19 cm from the catheter tip. When the catheter tip lies in the PA with a normal-sized heart, this port is positioned in the RV. A separate sterile, prepackaged pacing wire can be placed through this port to contact the RV endocardium for RV pacing.

           (2) PACs with thermodilution and atrial or AV pacing with two separate bipolar pacing probes have been shown to provide stable pacing before and after CPB.

        c. Mixed venous oxygen saturation (SvO2). Special fiberoptic PACs can be used to monitor SvO2 continuously. The normal SvO2 is 75%, with a 5% to 10% increase or decrease considered significant. Decreased oxygen delivery or increased oxygen utilization result in a decreased SvO2. Four mechanisms can result in a significant decrease in SvO2:

           (1) Decrease in CO

           (2) Decreased hemoglobin concentration

           (3) Decrease in arterial oxygen saturation (SaO2)

           (4) Increased O2 utilization.

            Changes in SvO2 usually precede hemodynamic changes by a significant period of time, making this a useful clinical adjunct to other monitors. Some cardiac anesthesiologists advocate SvO2monitoring for off-pump coronary artery bypass (OPCAB) procedures and any patient with severe LV dysfunction and/or valve disease.

        d. Ejection fraction catheter. PACs with faster thermistor response times can be used to determine RV ejection fraction in addition to the CO. The thermistor responds rapidly enough that the exponential decay that normally results from a thermodilution CO (see Section I.H.1.b [2]) has end-diastolic “plateaus” with each cardiac cycle. From the differences in temperature of each succeeding plateau, the residual fraction of blood left in the RV after each contraction is calculated, as is RV stroke volume, end- diastolic volume, and end-systolic volume. Monitoring these parameters can be helpful in patients with RV dysfunction secondary to pulmonary hypertension, infarction, or reactive pulmonary disease.

        e. Continuous CO. PACs that use low power thermal filaments to impart small temperature changes to RV blood have been developed (Intellicath, Baxter Edwards; and Opti-Q, Abbott Critical Care Systems, Mountain View, CA, USA). Fast-response thermistors in the PA allow for semicontinuous (every 30 to 60 s) CO determinations.

     7. Techniques of insertion. General guidelines are outlined in Table 4.10. The introducer is placed in a manner similar to that described for CVP insertion. However, special care should be observed with PAC placement, noting especially the following points:

Table 4.10 Steps for pulmonary artery catheter insertion

        a. Sedation. Because the patient is under a large drape for a longer period of time, he or she should be asked questions periodically to check for oversedation. A clear drape allows visual inspection of the patient’s color and may produce a less suffocating feeling.

        b. ECG monitoring during placement. It is essential to monitor the ECG during placement of the catheter because dysrhythmias are the most common complication associated with PAC insertion.

        c. Pulse oximetry. Pulse oximetry gives an audible signal of rhythm and may alert the physician to an abnormal rhythm.

        d. Preferred approach. The right IJV approach offers the most direct route to the RA and thus results in the highest rate of successful PA catheterization. The left subclavian route is next most effective.

        e. Balloon inflation. Air should be used for balloon inflation. If any suspicion exists about balloon competency, the PAC should be removed and the balloon inspected directly to avoid iatrogenic air embolism.

        f. Waveform. A vast majority of cardiac anesthesiologists use waveform analysis to guide placement of the PAC tip. TEE or the use of fluoroscopy can aid placement in some situations. Representative waveforms are shown in Figure 4.15.

Figure 4.15 Pressure waves that will be encountered as a PAC is inserted into the wedged position from the right IJV. Distances on the catheter correspond to insertion distances read at the diaphragm of the introducer and are approximate. Actual distances may vary by +5 cm. PACs should not be advanced more than 60 cm from this approach because this increases the risk of PA rupture or catheter knotting. CVP, central venous pressure; PCW, pulmonary capillary wedge; RA, right atrium; RV, right ventricle; Thermo, thermistor connection for CO determination.

     8. Complications. Complications [14] can be divided into vascular access, PAC placement/manipulation, and monitoring problems.

        a. Vascular Access. See Table 4.7 for complications of central venous cannulation.

        b. PAC placement/manipulation.

           (1) Cardiac Arrhythmias: Reported incidence ranges from 12.5% to 70%. PVCs are the most common arrhythmia. Fortunately, most arrhythmia resolve with either catheter withdrawal or with advancement of the catheter tip from the RV into the PA. There appears to be a higher incidence when the patient is positioned in the Trendelenburg position versus right-tilt position.

           (2) Mechanical Damage: Catheter knotting and entanglement of cardiac structures, although rare, can occur. Damage to intracardiac structures such as valves, chordae, and even RV perforation has been reported. The presence of IVC filters, indwelling catheters, and pacemakers can increase the risk of such complications. The incidence of knotting is estimated at 0.03% and this complication can be decreased with careful attention to depth of insertion and expected waveforms. To reduce the risk of knotting, a catheter should be withdrawn if the RV waveform is still present 20 cm after its initial appearance or when the absolute depth of 60 cm is reached without a PA tracing.

           (3) PA Rupture: This is a rare complication with an incidence of 0.03% to 0.2%. Risk factors include pulmonary hypertension, age greater than 60, hyperinflation of the balloon, improper (distal) catheter positioning, and coagulopathy. During CPB, distal migration of the catheter tip may occur, thus some advocate pulling the PAC back a few centimeters prior to initiating bypass.

           (4) Thrombosis: Although thrombus formation on PACs has been noted at 24 hr, the incidence of thrombogenicity substantially increases by 72 hrs.

           (5) Pulmonary Infarction: It can occur as a complication of continuous distal, wedging from catheter migration, or embolization of previously formed thrombus.

           (6) Infection: The incidence of bacteremia and blood stream infection related to PAC is 1.3% to 2.3%. Additionally, the PAC can contribute to endothelial damage of the tricuspid and pulmonary valves leading to endocarditis.

           (7) Other: Balloon rupture, heparin-induced thrombocytopenia (HIT) secondary to heparin-coated catheters, anaphylaxis from latex (balloon) allergy, and hepatic venous placement have all been included in PAC-related complications.

        c. Monitoring Complications.

           (1) Errors in equipment and data acquisition: Examples include inappropriate pressure transducer leveling and over/underdamping of pressure system.

           (2) Misinterpretation or misapplication of data: Misinterpretation can occur when not considering ventilation modes, ventricular compliance changes, or intrinsic cardiac/pulmonary pathologies.

           (3) Expense.

     9. Conclusions. PACs provide a wealth of information about the right and left sides of the circulation. For this reason, they are used for every cardiac surgical procedure in some institutions because their benefits are perceived to outweigh the risks. Studies that show low morbidity rates with PAC use support this viewpoint. In other institutions, however, clinicians are more selective about which patients require PACs, because use of PA monitoring has not been demonstrated to incontrovertibly improve outcomes of cardiac surgery. The widespread application of TEE may make intraoperative PAC data less useful except for SvO2 [15]. Additionally, non-invasive CO monitoring may also replace some uses of the PAC, although these monitors have technologic obstacles to solve prior to routine use (without invasive monitoring) during cardiac surgery.

   G. LA pressure. In some patients (especially pediatric), direct LA pressure can be measured after surgical insertion of an LA catheter via the LA appendage in the open chest. LA catheters also are used in corrective surgery for congenital lesions when PAC insertion is not possible. The LA pressure tracing is comparable to the CVP tracing, with A, C, and V waves occurring at identical points in the cardiac cycle. LA catheters are used to monitor valvular function (after mitral valve replacement or mitral valvuloplasty) or to monitor LV filling pressures, whether or not a PAC is available. LA pressure measured directly is more accurate than that measured with a PAC because the effects of airway pressure on the pulmonary vasculature are removed. However, LA pressure does not necessarily reflect LV end-diastolic pressure (LVEDP) in the presence of mitral valvular disease. Air should be meticulously removed from LA flush systems to avoid catastrophic air emboli.

   H. Cardiac output

     1. Methods

        a. Thermodilution with cold injectate. This method is the most commonly utilized CO technique because of its ease of use and ability to repeat measurements over time. The indicator is an aliquot of saline (typically 10 mL, which is at a lower temperature than the temperature of blood) injected into the RA. The change in temperature produced by injection of this indicator is measured in the PA by a thermistor and is integrated over time to generate a value for RV output, which is equal to systemic CO if no intracardiac shunts are present. This method requires no withdrawal of blood and no arterial line, uses an inexpensive indicator, and is not greatly affected by recirculation. The thermodilution method underestimates the CO with right-side valvular lesions. Thermodilution remains accurate for forward LV CO for mitral and aortic valve lesions.

        b. Continuous thermodilution. A thermal filament in the catheter heats blood ~15 to 25 cm before its tip, thus generating a PA temperature change that is measured via a distal thermistor. The input and output signals are correlated to generate CO values.

        c. RV ejection fraction. Improved preload estimates might be obtained with this type of PAC [16].

     2. Assumptions and errors. Specific errors in CO determination are mentioned below:

        a. Thermodilution method

           (1) Volume of injectate. Because the output computer will base its calculations on a specific volume, an injectate volume less than that for which the computer is set will cause a falsely high value of CO, and vice versa.

           (2) Temperature of injectate. If the injectate temperature parameter is incorrect, errors can occur. For example, an increase of 1°C will cause a 3% overestimation of CO. The controversy over iced versus room-temperature injectate centers around the concept that a larger difference between the injectate temperature and blood temperature should increase the accuracy of the CO determination. Studies have not supported this hypothesis, and the extra inconvenience of keeping syringes on ice, together with the increased risk of infection (nonsterile water surrounding the Luer tip), make the iced saline method a less attractive alternative.

           (3) Shunts. Intracardiac shunts will cause erroneous values for thermodilution CO values. This technique should not be used if a communication exists between the pulmonary and systemic circulations. A shunt should always be suspected when thermodilution CO values do not fit the clinical findings.

           (4) Timing with the respiratory cycle. As much as a 10% difference in CO will result, depending on when injection occurs during the respiratory cycle. These changes are most likely due to actual changes in pulmonary blood flow during respiration.

           (5) Catheter position. The tip of the pulmonary catheter must be in the PA and must not be “wedged”; otherwise, nonsensical curves are obtained.

     3. Minimally Invasive CO Monitoring: The desire to assess cardiac function and adequate tissue perfusion in critically ill patients is traditionally accomplished using the PAC. The controversy about its invasive nature and potential harm has provoked the development of less invasive CO monitoring devices [17,18]. Just as the PAC has its nuances, these devices have their own sets of limitations that must be considered.

        a. Accuracy and precision. Accuracy refers to the capability of a measurement to reflect the true CO. This means that a measurement is compared to a “gold standard” method. Given the wide spread use of the PAC, the thermodilution method is the practical “gold” standard to which new non-invasive CO monitors are compared. However, it is important to take into account that the inherent error for thermodilution measurements of CO are in the 10% to 20% range. Precision indicates the reproducibility of a measurement and refers to the variability between determinations. For the thermodilution method, studies of precision have involved probability analyses of large numbers of CO determinations. Using this approach, it was found that with two injections, there was only a 50% chance that the numbers obtained were within 5% of the true CO. If three injections yield results that are within 10% of one another, there is a 90% probability that the average value is within 10% of the true CO.

            Given that the practical gold standard carries with it some inaccuracies, new methods based on it will also hold their own similar inherent error. Table 4.11 shows the relative accuracy and precision of invasive and non-invasive techniques to measure CO.

Table 4.11 Accuracy and precision of cardiac output measurements

        b. Methods: Minimally invasive CO monitors can be classified into one of the four main groups: Pulse pressure analysis, pulsed Doppler technologies, applications of Fick’s principle using partial CO2rebreathing, and bioimpedance/bioreactance.

           (1) Pulse pressure analysis: Monitors based on that principle stroke volume can be tracked continuously by analysis of the arterial waveform. These monitors require an optimal arterial waveform, thus arrhythmias, intra-aortic balloon pumps, left ventricular assist devices, and even properties of the arterial line monitoring systems (such as over/under damping) can alter accuracy of the CO measurement. Three common pulse contour analysis devices are compared in Table 4.12.

Table 4.12 CO monitors utilizing pulse wave analysis

           (2) Doppler devices: CO can be measured using the change in frequency of an ultrasonic beam as it measures blood flow velocity. To achieve accurate measurements, at least three conditions must be met: (i) The cross-sectional area of the vessel must be known; (ii) the ultrasound beam must be directed parallel to the flow of blood; (iii) the beam direction cannot move to any great degree between measurements. Clinical use of this technique is associated with reduced accuracy and precision.

            Two methods that use ultrasound are as follows:

             (a) Transtracheal. Flow in the ascending aorta is determined with a transducer bonded to the distal portion of the endotracheal tube, designed to ensure contact of the transducer with the wall of the trachea. This method is not yet fully validated in humans, and a study of cardiac patients reported poor correlation when compared to thermodilution.

             (b) Transesophageal. Several esophageal Doppler probes are available which are smaller than conventional TEE probes. CO is obtained by multiplying the cross-sectional area of the aorta by the blood flow velocity. Flow in the aorta is measured using a transducer placed in the esophagus. Aortic cross-sectional area is provided from a nomogram or measured by M-mode echocardiography.

             (c) TEE. In addition to the Doppler technique mentioned above, TEE utilizes Simpson’s rule in which the LV is divided into a series of disks to estimate CO without the use of Doppler. End-diastolic and end-systolic dimensions measured by echocardiography are converted to volumes, allowing stroke volume and CO to be determined. Given the size of the monitor and probe, this technique can provide intermittent CO, but is not ideal for continuous CO measurement desired in ICU settings.

Summary: The PAC still remains the practical gold standard for evaluating CO. It also provides true mixed venous saturation and pulmonary pressures that cannot be obtained from non-invasive devices. Invasive hemodynamic monitoring remains the standard in the operating theater. However, select stable cardiac patients may be candidates in the ICU or stepdown units postoperatively for minimally invasive device monitoring.

   I. Echocardiography

     1. TEE. Echocardiography, and especially TEE, has gained widespread use in the cardiac operating room. Indications for intraoperative TEE are listed in Table 4.13. For a complete discussion of TEE, see Chapter 5—“Transesophageal Echocardiogroahy”.

Table 4.13 Indications for intraoperative transesophageal echocardiography TEE

     2. Epiaortic scanning. The importance of aortic atheromas, especially in the ascending aorta and/or aortic arch, in association with poor neurologic outcomes has long been recognized. Aortic atheromas with a mobile component present the greatest risk. The introduction and use of TEE to detect aortic atheroma [19] was a significant improvement over surgical palpation. However, TEE had significant limitations particularly in the detection of disease near the typical aortic cannulation site (distal ascending aorta, proximal aortic arch) because the airway structures interfere with the TEE signal. Epiaortic scanning [20,21] is a highly sensitive and specific monitoring modality to detect atheroma in the thoracic aorta including regions where TEE evaluation is not possible. In cardiac surgical patients with identified atheroma, modification of the surgical technique and neuroprotective strategies has been reported to reduce neurologic complication from ~60% to 0% [22].

8

II. Pulmonary system

   A. Pulse oximetry

     1. Indications

        a. Preoperative uses. Document baseline O2 saturation, assess the need for supplemental O2 after premedication is administered, and provide an audible alarm for dysrhythmias.

        b. Assessment of oxygenation intraoperatively.

        c. Assessment of perfusion. The pulse oximeter utilizes plethysmography as a part of its basic operation. Thus, adequacy of perfusion is likely when the oximeter shows a saturation reading.

        d. Pulse rate. In the patient with a dysrhythmia, not every beat will lead to adequate perfusion, and this will be sensed by the pulse oximeter. An audible monitor of the heart rhythm is available.

     2. Advantages and disadvantages are listed in Table 4.14. Of note, cardiac surgery was found to be an independent predictor of pulse oximetry failure in one study that detected a better than 9% intraoperative oximetry failure rate of at least a 10-min period [23].

Table 4.14 Pulse oximetry in cardiac procedures

   B. Capnography

     1. Indications

        a. All patients. End-tidal capnography offers evidence of endotracheal intubation, ventilation, and perfusion.

        b. Patients with lung disease. Capnography assesses the severity of small airway obstruction and assists adjustment of ventilatory parameters (i.e., increasing the I:E ratio).

        c. Patients with pulmonary hypertension or reactive pulmonary vasculature. Hypercarbia results in increased PA pressures and may worsen RV function. Factors contributing to elevated arterial CO2include insufflation of CO2 during endoscopic vein harvest, hypoventilation during IMA takedown, and temporary holding ventilation for surgical manipulation.

III. Temperature.

Cardiac anesthesia is unique in that therapeutic hypothermia is utilized frequently and aggressively in many cases. Distribution of thermal energy can be manipulated, via CPB, more rapidly and extensively than any other anesthetic case. Unique to cardiac anesthesia are circulatory arrest procedures with cooling to 17°C. Temperature monitoring for cardiac anesthesia has unique considerations. This section will introduce these topics without detailed discussion of thermal issues common to all anesthesiologists [24].

   A. Indications

     1. Assessment of cooling and rewarming

     2. Diagnosing hazardous hypothermia or hyperthermia. Below 32°C, the myocardium is irritable and subject to complex arrhythmias, especially ventricular tachycardia and fibrillation. The risk of dysrhythmia is particularly high in pediatric patients. Hypothermia inhibits coagulation thus increasing bleeding/transfusion risk. Likewise, significant enzyme desaturation and cell damage can occur with temperatures greater than or equal to 41°C.

   B. Sites of measurement. Numerous possible sites exist to measure temperature. These sites can be grouped into the core or the shell.

     1. Core temperature

        a. General considerations. The core temperature represents the temperature of the vital organs. The term core temperature used here is perhaps a misnomer because gradients exist even within this vessel-rich group during rapid changes in blood temperature.

        b. PAC thermistor. This is the best estimate of the core temperature when pulmonary blood flow is present (i.e., before and after CPB).

        c. Nasopharyngeal temperature. Nasopharyngeal temperature provides an accurate reflection of brain temperature during CPB. The probe should be inserted into the nasopharynx to a distance equivalent to the distance from the naris to the tip of the earlobe. Nasopharyngeal temperature should be monitored in all hypothermic circulatory arrest procedures and CPB cases with hypothermia requiring rewarming.

        d. Tympanic membrane temperature. Temperature at this site reflects brain temperature, and may provide an alternative to nasopharyngeal temperature.

        e. Bladder temperature. This modality has been used to measure core temperature, although it may be inaccurate in instances when renal blood flow and urine production are decreased.

        f. Esophageal temperature. Because the esophagus is a mediastinal structure, it will be greatly affected by the temperature of the blood returning from the extracorporeal pump and should NOT be used routinely for cases involving CPB.

        g. CPB arterial line temperature. This is the temperature of the heat exchanger (i.e., the lowest temperature during active cooling and the highest temperature during active rewarming). During either of these phases, a gradient always exists between the arterial line temperature and any other temperature.

        h. CPB venous line temperature. This is the “return” temperature to the oxygenator and probably best reflects core temperature during CPB when no active warming or cooling is occurring.

     2. Shell

        a. General. The shell compartment represents the majority of the body (muscle, fat, bone), which receives a smaller proportion of the blood flow, thus acting as an energy sink that can significantly affect temperature fluxes. Shell temperature lags behind core temperature during cooling and rewarming. At the point of bypass separation, the core temperature will be significantly higher than shell temperature. The final equilibrium temperature with thermal redistribution probably will be closer to the shell temperature than the core temperature measured initially.

        b. Rectal temperature. Although traditionally thought of as a core temperature, during CPB procedures the rectal temperature most accurately reflects muscle mass temperature. If the tip of the probe rests in stool, a significant lag will exist with changing temperatures.

        c. Skin temperature. Skin temperature is affected by local factors (warming blanket) and is rarely utilized in cardiac surgery.

   C. Risks of temperature monitoring

Epistaxis with nasopharyngeal temperature monitoring. Most cardiac anesthesiologists consider the benefits of neurologic protection (measured by brain temperature) to outweigh bleeding risks. If the nasal mucosa is traumatized during probe placement, epistaxis can result, especially when the patient has been given heparin.

   D. Recommendations for temperature monitoring. Monitoring temperature at two sites is recommended: A core site and a shell site. Arterial and venous line temperatures are available directly from the CPB apparatus. Nasal temperature monitoring is recommended for core temperature because it will most rapidly reflect the changes in the arterial blood temperature. Nasal temperature monitoring is recommended for circulatory arrest cases to document brain temperature. Bladder or rectal temperature monitoring is simple to establish and is recommended for monitoring shell temperature.

9

IV. Renal function

   A. Indications for monitoring

     1. Increased incidence of renal failure after CPB. Acute renal failure is a recognized complication of CPB, occurring in 2.5% to 31% of cases. Acute renal failure is related to the preoperative renal function as well as to the presence of coexisting disease. The nonpulsatile renal blood flow during CPB has been speculated as a contributing mechanism, although continuous-flow LVAD has not been associated with excessive renal failure.

     2. Use of diuretics in CPB prime. Mannitol is used routinely during CPB for two reasons:

        a. Hemolysis occurs during CPB, and serum hemoglobin levels rise. Urine output should be maintained to avoid damage to renal tubules.

        b. Deliberate hemodilution is induced with the onset of hypothermic CPB. Maintenance of good urine output during and after CPB allows removal of excess free water.

   B. Urinary catheter. This monitor is the single most important monitor of renal function during surgical cases involving CPB. Establishing a urinary catheter should be a priority in emergencies.

     1. Anuria on bypass. It is not uncommon for oliguria or anuria while the patient is on CPB. Hypothermia and the reduction of arterial flow will cause a diminution in renal function. Therefore, anuria should not usually be treated aggressively with additional diuretic therapy, especially while the patient is hypothermic.

        a. How much urine is adequate? After CPB, adequacy of urine output depends on several factors, all of which should be optimized:

           (1) Volume status

           (2) CO

           (3) Hemoglobin concentration

           (4) Amount of surgical bleeding

   C. Electrolytes. Serum electrolytes, especially potassium and magnesium, should be checked toward the end of CPB and after bypass. In the vast majority of patients with adequate renal function, potassium and magnesium concentrations will decline during CPB (secondary to mannitol and improved perfusion). Replacement of potassium has to account for cardioplegia which contains potassium and hyperglycemia. A low serum ionized calcium level may be the cause of diminished pump function. The timing of the calcium therapy may affect neurologic outcome; administration during periods of neural ischemia and/or reperfusion may worsen the outcome. Many cardiac anesthesiologists will not administer calcium until at least 15 to 20 min following acceptable perfusion (e.g., after aortic cross-clamp removal).

10

V. Neurologic function

   A. General considerations. The cardiac surgical patient is at increased risk of having an adverse neurologic event during surgery because of CPB (core cooling, alterations in blood flow) and because of the potential to introduce emboli (air, atheromatous material, thrombus). Neurologic risk assessment in cardiac surgical patients [25] and risk factor modification are extremely important for the anesthesiologists. Advances in processing capability have made available new devices to monitor neurologic function during surgery [2628]. Monitoring the central nervous system is done for three primary reasons: (i) to diagnose cerebral ischemia, (ii) to assess the depth of anesthesia and prevent intraoperative awareness, and (iii) to assess the effectiveness of medications given for brain or spinal cord protection. The indications and types of neurologic monitors are outlined briefly here; for in depth applications, see Chapter 22.

   B. Indications for monitoring neurologic function

      1. Associated carotid disease

      2. Diagnosis of embolic phenomenon

      3. Diagnosis of aortic cannula malposition

      4. Diagnosis of inadequate arterial flow on CPB

      5. Confirmation of adequate cooling

      6. Hypothermic circulatory arrest, in an adult or a child (see Chapters 22 and 23)

   C. Monitors of CNS electrical activity

     1. Electroencephalogram (see Chapter 22). The electroencephalogram (EEG) measures the electrical currents generated by the postsynaptic potentials in the pyramidal cell layer of the cerebral cortex. The basic principle of clinical EEG monitoring is that cerebral ischemia causes slowing of the electrical activity of the brain, as well as a decrease in signal amplitude. An experienced electroencephalographer can interpret raw EEG data from four or eight channels, but would be hard pressed to also administer an anesthetic and monitor other organ systems.

     2. Processed EEG. To increase its intraoperative utility, the EEG data are processed by fast Fourier analysis into a single power versus time spectral array that is more easily interpreted. Examples of power spectrum analysis include compressed spectral array, density spectral array, and bispectral index (BIS). The BIS monitor analyzes the phase relationships between different frequency components over time, and the result is reduced via a proprietary method to a single number scaled between 0 (electrical silence) and 100 (alert wakefulness). The role of BIS monitoring in cardiac surgery is in evolution [29]. The BIS may be a useful indicator of the depth of anesthesia for many procedures, but its significance in cardiac anesthesia has not been determined. Studies of BIS values as a predictor of anesthetic depth during intravenous anesthesia (narcotic plus benzodiazepine) are conflicting. One study found a positive correlation between the BIS and arousal or hemodynamic responses [30], whereas another study found no such correlation between the BIS value and plasma concentrations of fentanyl and midazolam [31]. The BIS monitor may have added benefit in the management of cardiac anesthesia. During hypothermic circulatory arrest, the BIS monitor should be isoelectric (BIS of zero). Many cardiac anesthesiologists monitor the BIS during cooling and to observe the effect of supplemental intravenous anesthetic (historically thiopental) administered for neuroprotection. Evidence supporting BIS data as a monitor of neurologic function in patients at risk for hypoxic or ischemic brain injury continues to accumulate [32]. Abnormally low BIS scores and prolonged low BIS score may be associated with poor neurologic outcomes.

     3. Evoked potentials

        a. Somatosensory evoked potentials (SSEPs). SSEPs can be used to monitor the integrity of the spinal cord. It is most useful in operations such as surgery for a thoracic aneurysm, in which the blood flow to the spinal cord may be compromised. A stimulus is applied to a peripheral nerve (usually the tibial nerve), and the resultant brainstem and brain activity is quantified. Specific uses are discussed further in Chapter 23.

        b. Visual evoked response and brainstem audio evoked responses. These techniques do not have routine clinical application in cardiac surgical procedures and are not discussed here.

        c. Motor evoked potentials (MEPs). MEPs are useful to monitor the spinal cord during surgery of the descending aorta and are discussed in more detail in Chapter 23.

   D. Monitors of cerebral metabolic function

     1. Jugular bulb venous oximetry. Measuring the oxygen saturation of the cerebral jugular bulb with a fiberoptic catheter [33] is analogous to measuring the mixed venous oxygen saturation in the PA. Cerebral oxygen consumption is the product of cerebral blood flow (CBF) and the oxygen extraction by the brain. If CBF decreases, oxygen extraction would increase and the jugular O2 saturation would decrease. Owing to a significant interpatient variability, trend monitoring yields more information than individual measurements.

     2. Near-infrared spectroscopy (NIRS). NIRS is a noninvasive method to monitor cerebral metabolic function [33]. A near-infrared light is emitted from a scalp sensor and penetrates the scalp, skull, cerebrospinal fluid, and brain. The light is reflected by tissue but differentially absorbed by hemoglobin-containing moieties. The amount of absorption correlates with the oxygenation state of the hemoglobin in the tissue. Again, trend monitoring is possible because NIRS data are updated continuously. Currently, the role of NIRS cerebral oximetry application during cardiac surgery is controversial. Improved outcomes have not been conclusively demonstrated and the cost benefit is unclear.

   E. Monitors of CNS hemodynamics

     1. Transcranial Doppler ultrasonography (TCD). The Doppler technology in TCD monitoring is similar to that used in echocardiography. One difference is the signal attenuation caused by the skull, which is a source of artifact. Nonetheless, this technology is very useful in detecting emboli in the cerebral circulation.

VI. Electrical safety in the cardiac operating room

   A. Electrical hazards. The major hazards can be classified as macroshock, microshock, and thermal burns. Some general electrical terms are listed in Table 4.15. Our discussion will focus on microshock hazard.

Table 4.15 Electrical terms

     1. Macroshock. Macroshock is an uncommon occurrence in the operating room because of (i) isolation transformers, (ii) isolation of electrical equipment, (iii) patient isolation from ground, (iv) proper grounding of equipment, and (v) line isolation monitoring.

     2. Microshock. The term microshock applies to very small amounts of current (i.e., 50 to 100 mA). Significant morbidity has resulted from currents as low as 20 μA and often takes the form of cardiac dysrhythmias. Standard operating room isolation transformers provide no protection against currents of this magnitude. Microshock cannot occur unless the skin resistance has been bypassed. Because cardiac patients often have indwelling catheters that lead directly to the heart, as well as intracavitary or epicardial pacemaker wires, these patients are more susceptible to microshock hazard from these low-resistance pathways.

     3. Burns. Electricity is a form of energy, and dissipation of energy takes the form of heat. Burns usually are a complication secondary to use of the electrocautery unit. Improper patient grounding is the usual cause.

   B. Determinants of electrical hazard

     1. Current density. The disruption of physiologic function produced by a given electrical current is inversely proportional to the area over which this current is applied. In the case of a small-bore central venous catheter in the RA, the amount of current needed to produce a significant arrhythmia is small.

     2. Current duration. Cardiac muscle can recover quickly from a direct current (DC) that is applied for only microseconds but may become depolarized if the current is applied for 1 or more seconds.

     3. Type of current. DC is the unidirectional, nonoscillating current that results when a constant voltage is applied across a resistor. When DC passes through skeletal or cardiac muscle, a sustained contraction can result. Alternating current (AC) is the current that changes polarity at a specified rate. It is the rate, or frequency, of the change in polarity that determines the magnitude of the hazard. Low-frequency ACs, such as the standard 60 Hz, can cause significant tetanic contraction of skeletal muscles as well as ventricular fibrillation with small currents.

     4. Skin resistance. Resistance across intact skin can be as large as 1 million Ω when the skin is dry. This amount can drop by a factor of 1,000 when skin is wet. A centrally placed fluid-filled catheter that punctures the skin lowers resistance to 500 Ω and places the patient at a higher risk of sustaining a microshock injury.

     5. Current threshold (AC). Current threshold is a term used by some to quantify that amount of current at 60 Hz needed to cause ventricular fibrillation. Several studies have determined the value to be anywhere between 50 and 1,000 μA (microshock range). One group found that as the catheter size decreases (resulting in an increased current density), the total amount of current needed to produce fibrillation also decreases.

   C. Results of microshock

     1. Ventricular fibrillation. In humans, the current threshold is estimated to be approximately 10 to 1,000 mA.

     2. Dysrhythmias. Rhythm disturbances may occur before reaching currents needed to produce ventricular fibrillation. It is important to note that these rhythm disturbances can cause severe hypotension and mortality similar to that associated with ventricular fibrillation. Pump failure produced by rhythm disturbances occurs at approximately half the current threshold needed to produce ventricular fibrillation.

   D. Mechanisms of microshock. Electrical hazard is possible when there is a path by which electrical current can flow from an electronic device through a patient to ground and there is some fault in the electrical grounding of the apparatus. These conditions must be present simultaneously in order for a shock hazard to exist. If the circuit includes an intracardiac monitor such as a CVP line, the leakage current can be transmitted directly to the heart, creating a microshock hazard.

   E. Prevention of electrical hazard. Traditionally, the risk of electrical misadventure in the operating room was reduced with line isolation monitors separating the OR environment from the main power source. In most hospitals with up-to-date equipment, every device has its own safety monitor. The line isolation monitor may be obsolete. Many new or refurbished operating rooms do not even have line isolation monitors. Thus, microshock and burns are the focus of current electrical safety protocols.

     1. Macroshock. The combined use of isolation transformers and line isolation monitors has historically provided some protection against macroshock.

     2. Microshock. Avoidance of the conditions that lead to an AC path will prevent microshock hazard. Good equipment design and maintenance are essential. Equipment that incorporates infrared coupling between patient connections and the internal circuitry will effectively eliminate any possible current flow through a patient. Isolation of the monitoring equipment–patient connection from the internal circuitry by an isolation transformer reduces the risk of microshock hazard. In addition, any equipment that bypasses skin resistance (central fluid-filled catheters, epicardial pacemakers) should be handled with care to avoid any leakage current reaching the patient. (For example, the caregiver could touch a faulty piece of equipment and then touch a CVP catheter simultaneously.)

      3. There is no substitute for vigilance in preventing electrical hazards (macroshock or microshock). The situations in which measuring or recording systems display excess noise in the form of humming or drifting in the baseline may represent a problem with the electronic circuitry. In older hospitals where the line-isolation monitor is used, an alarm should trigger immediate identification and removal of the offending piece of equipment.

VII. Point-of-care clinical testing.

Improved patient outcomes and resource utilization can be acquired via point-of-care testing. Point-of-care testing is defined as the clinical tests that are performed at the patient’s bedside (i.e., in the operating room) or immediately adjacent to the patient rather than sending specimens to a central laboratory facility. Many cardiac surgical suites now utilize point-of-care testing for routine clinical tests.

   A. Coagulation. Cardiac surgery patients, especially with CPB, are universally anticoagulated with heparin. The vast majority of patients receive protamine to ameliorate the heparin effects. CPB results in platelet dysfunction, decreased platelet number, and dilution of coagulation factors. Monitoring coagulation status is critical. Detailed discussion of coagulation management during cardiac surgery is described in Chapter 19(Coagulation/Anticoagulation).

     1. Activated clotting time (ACT). Monitoring of adequate heparin effect is most often accomplished by measuring the ACT. For CPB, prolongation of the ACT to greater than 400 s is usually deemed adequate. OPCAB procedures sometimes use “partial heparinization” with an ACT target of about 300 s (although many cardiac anesthesiologists administer the full heparin dose in case emergent conversion to CPB is required).

     2. Thromboelastography (TEG). TEG provides unique advantages over ACT and traditional coagulation parameters [34] because it provides functional information on platelets, clotting factors, and fibrinolytic processes. The introduction of small, reliable devices has spawned a renewed interest and utilization by cardiac anesthesiologists.

   B. Glucose. Glucose control during cardiac surgery improves patient outcomes [30]. Many cardiac anesthesiologists treat any glucose measurement greater than 200 mg/dL. An insulin infusion is indicated for treatment. Blood glucose should be measured every 30 to 60 min and when otherwise indicated.

   C. Arterial blood gas (ABG). Frequent ABG measurement is required for cardiac surgical procedures, especially during CPB. Point-of-care ABG analysis simultaneously measures serum electrolytes (potassium, sodium, ionized calcium) and hematocrit.

VIII. Additional Resources.

The World Wide Web provides an abundance of resources to gain further knowledge about monitoring devices. Such resources not only include recently published studies and practice guidelines, but also online simulations for the less experienced (Table 4.16 and Table 4.17).

Table 4.16 Internet simulators

Table 4.17 Internet resources

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