Handbook of Clinical Anesthesia

Chapter 27

Standard Monitoring Techniques

Monitoring represents the process by which anesthesiologists recognize and evaluate potential physiologic problems by identifying prognostic trends in patients in a timely manner (Greenberg SB, Murphy GS, Vender JS: Standard monitoring techniques. In Clinical Anesthesia. Edited by Barash PG, Cullen BF, Stoelting RK, Cahalan MK, Stock MC. Philadelphia: Lippincott Williams & Wilkins, 2009, pp 695–714). Effective monitoring decreases the potential for poor outcomes that may occur after anesthesia by identifying derangements before they result in serious or irreversible injury. Monitoring devices increase the specificity and precision of clinical judgments. Standards for Basic Anesthesia Monitoring have been adopted by the American Society of Anesthesiologists (ASA).

1. Inspiratory and Expired Gas Monitoring

The concentration of oxygen in the anesthetic circuit must be measured. Manufacturers of gas machines place oxygen (O₂) sensors on the inspired limb of the anesthesia circuit to ensure that hypoxic gas mixtures are not delivered to patients. Monitoring inspired O₂ concentration does not guarantee the adequacy of arterial oxygenation.

1. Monitoring of Expired Gases
2. Carbon Dioxide
3. Monitoring of expiratory CO₂(end-tidal CO₂ or P_ETCO₂) has evolved as an important physiologic and safety procedure for identifying placement of the endotracheal tube (this does not confirm placement above the carina) and for assessing variables such as

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ventilation (PaCO₂), rebreathing, cardiac output, distribution of blood flow, and metabolic activity.

2. Capnometryis the measurement and numeric representation of the CO₂ concentration (in millimeters of mercury [mm Hg]).
3. A capnogramis a continuous concentration–time display of the CO₂ concentration (divided into four distinct phases) sampled at the patient's airway during ventilation (Fig. 27-1).
4. Capnographyis the continuous monitoring of the patient's capnogram.
5. The end-tidal CO₂concentration provides a clinical estimate of the PaCO₂, assuming ventilation and perfusion in the lungs are appropriately matched (normal gradient, 5–10 mm Hg) and no sampling errors occur during measurement. (Sidestream analyzers may dilute a patient's tidal breath with fresh gas, especially when tidal volume is small, as in young patients. Loose connections and system leaks also dilute end-tidal CO₂.)
6. Dead space(ventilation without perfusion) and a resulting increase in the difference between the PaCO₂ and the end-tidal CO₂ (dead space gases containing little or no CO₂greatly dilute the end-tidal CO₂ concentration) may reflect

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hypoperfusion states, chronic obstructive pulmonary disease, and embolic phenomena (thrombus, air).

2. Shunt(perfusion without ventilation) causes minimal changes in the gradient between PaCO₂ and end-tidal CO₂.
3. Capnography has decreased the potential for unrecognized accidental esophageal intubation.
4. Because the esophageal or gastric gas concentration is primarily composed of inspired gas, it should contain exceedingly small amounts of CO₂. After an accidental esophageal intubation, the first one or two “breaths” may contain some CO₂, but the concentration should approach zero after four or five “breaths.”
5. A continuous stable CO₂waveform ensures the presence of alveolar ventilation (tube in the trachea) but does not necessarily indicate that the endotracheal tube is properly positioned above the carina.
6. Common causes of gradual increases or decreases in end-tidal CO₂reflect changes in CO₂ production or changes in CO₂ elimination (Table 27-1).

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7. A sudden decrease in end-tidal CO₂to near zero requires a rapid assessment of possible causes (Table 27-2).
8. The adequacy of cardiopulmonary resuscitation can be assessed by capnography, as reflected by a reappearance or an increase in end-tidal CO₂with restoration of pulmonary blood flow.
9. The size and shape of the capnogram waveform may be informative (Table 27-3).
10. Multiple Expired Gas Analysis.Many critical events may be detected by analysis of respiratory and anesthetic gases (Table 27-4). Nitrogen (N₂) monitoring provides quantification of washout during preoxygenation. A sudden increase in the N₂ concentration in the exhaled gas indicates either introduction of air from leaks in the anesthesia delivery system or venous air embolism.

III. Arterial Oxygenation Monitoring

1. Pulse Oximetry
2. Measurement of the peripheral O₂saturation of hemoglobin (SpO₂) on a continual basis is the standard of care for measuring oxygenation during anesthesia and in the postanesthesia care unit.

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2. Overwhelming evidence supports the capability of pulse oximetry for detecting desaturation before it is clinically apparent.
3. No definitive data demonstrate a decrease in morbidity and mortality associated with the use of pulse oximetry.
4. The absence of a pulsatile waveform limits the ability of a pulse oximeter to calculate the SpO₂.
5. A relationship exists between hemoglobin saturation and O₂tension (in mm Hg) as depicted by the oxyhemoglobin dissociation curve (Fig. 27-2).
6. The SpO₂measured by pulse oximetry is not the same as the arterial O₂ saturation (SaO₂) measured by a laboratory co-oximeter. In clinical circumstances when other hemoglobin moieties (methemoglobin, carboxyhemoglobin) are present in low

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concentrations, the SpO₂ value is higher than the SaO₂ reported by the blood gas laboratory.

7. Many factors may influence the accuracy or ability of a pulse oximeter to calculate SpO₂(Table 27-5).
8. Blood Pressure Monitoring

Intraoperative measurements and recordings of arterial blood pressure (at least every 5 minutes) are important indicators of the adequacy of circulation.

1. Indirect Measurement of Arterial Blood Pressure
2. The simplest method of blood pressure determination estimates systolic blood pressure by palpating the return of the arterial pulse or Doppler sounds while an occluding cuff is deflated.
3. Auscultationof Korotkoff sounds (which result from turbulent flow within an artery in response to the mechanical deformation from the blood pressure cuff) is a common method of blood pressure measurement.
4. Systolic blood pressure is considered to be equivalent to the appearance of the first Korotkoff sound, and disappearance of the sounds or a muffled tone is considered to be equivalent to the diastolic

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blood pressure. Mean arterial pressure (MAP) is calculated as the diastolic blood pressure plus one third of the pulse pressure (systolic blood pressure minus diastolic blood pressure).

1. The detection of sound changes is subjective, requires pulsatile flow (unreliable during low flow), and is prone to mechanical errors (Table 27-6).

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3. Automated oscillometryhas replaced auscultatory and palpatory techniques for routine intraoperative blood pressure monitoring.
4. Oscillometry accurately measures systolic blood pressure, diastolic blood pressure, and MAP (discrepancy with centrally placed arterial line <5 mm Hg).
5. A variety of cuff sizes makes it possible to use oscillometry in patients of all ages.
6. Problems with Noninvasive Blood Pressure Monitoring.Complications may accompany repeated inflations of automatically cycled blood pressure cuffs placed on the upper extremity (Table 27-7).
7. Invasive Measurement of Vascular (Arterial Blood) Pressure
8. Indwelling arterial cannulation not only offers anesthesiologists the opportunity to monitor beat-to-beat changes in arterial blood pressure but also provides vascular access for arterial blood sampling.
9. Intra-arterial measurement of blood pressure is subject to many sources of error based on the physical properties of fluid motion and the performance of the catheter–transducer–amplification system used to sense, process, and display the pressure pulse wave. Ideally, the catheter and tubing are stiff, the volume of fluid in the connecting tubing is small, the number of stopcocks is limited, and the connecting tubing length is not excessive.

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3. Because many therapeutic decisions are based on changes in arterial blood pressure, it is imperative that anesthesiologists understand the physical limitations imposed by fluid-filled pressure transducer systems.
4. In clinical practice, underdamped catheter–transducer systems tend to overestimate systolic blood pressure by 15 to 30 mm Hg and to amplify artifact (“catheter whip”).
5. Air bubbles cause overdamping and underestimation of systolic blood pressure.
6. MAP is accurately measured even in the presence of overdamping or underdamping.
7. In clinical practice, it is sufficient to calibrate the transducer to atmospheric pressure, usually with the transducer located at the level of the right atrium.
8. Arterial Cannulation
9. The radial artery remains the most popular site for cannulation because of its accessibility and the presence of a collateral blood supply (Fig. 27-3).
10. The prognostic value of the Allen test in assessing the adequacy of the ulnar collateral circulation has not been confirmed.

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1. Complications of Invasive Arterial Monitoring(Table 27-8)
2. Traumatic cannulation has been associated with median nerve dysfunction, hematoma formation, and thrombosis.
3. Abnormal radial artery blood flow after the removal of an arterial catheter (a nontapered 20- to 22-gauge Teflon catheter is recommended) occurs frequently (presumably because of radial artery thrombosis), with normalization of blood flow usually occurring in 3 to 7 days.
4. Direct arterial pressure monitoring requires constant vigilance and correlation of the measured blood pressure with other clinical parameters before therapeutic interventions are initiated.

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5. Sudden increases or decreases in blood pressure may represent a hydrostatic error when the position of the transducer is not adjusted after changes in the position of the operating room table.
6. A sudden decrease in blood pressure may be caused by a damped tracing from a partially occluded or kinked arterial catheter.
7. Before initiating therapy based on a change in blood pressure, the calibration of the transducer system and the patency of the arterial cannula should be verified.
8. Central Venous and Pulmonary Artery Monitoring

The right internal jugular vein is the most common site for cannulation (Table 27-9 and Figs. 27-4, 27-5 and 27-6). Ultrasound-guided placement of the right internal jugular vein venous catheter may decrease complications and improve first attempt success rates.

1. Central Venous Pressure Monitoring
2. Central venous pressure is essentially equivalent to right atrial pressure, and the normal waveform consists of three peaks (a, c, and v waves) and two descents (x, y) (Table 27-10 and Fig. 27-7).
3. The possibility of venous air embolism is decreased by positioning the patient in a head-down position during placement or removal of the central venous catheter.
4. Central venous catheter placement is an important source of nosocomial infection and sepsis, emphasizing the importance of sterile technique during catheter placement and of the application of appropriate dressings (Table 27-11).
5. Pulmonary Artery Monitoring
6. Indicationsfor placement of a pulmonary artery catheter are broadly defined. (Intracardiac pressures, thermodilution cardiac output, and mixed venous O₂ saturations should be measured, and derived hemodynamic indices should be calculated.) The measured and derived information is used to help define the clinical problem, monitor the progression of hemodynamic dysfunction, and guide the response to therapy.

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1. Pulmonary artery catheter monitoring may decrease perioperative complications if its use is tailored to the clinical condition of the patient as it changes with time.
2. The ASA has developed Practice Guidelines for Pulmonary Artery Catheterization.
3. Correct placement of a pulmonary artery catheter is most often guided by observing changes in vascular waveforms (Fig. 27-8).

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3. Pulmonary capillary wedge pressure is used to indirectly assess left ventricular end-diastolic volume by reflecting changes in left ventricular end-diastolic pressure. Right-sided filling pressures often are poor indicators of left ventricular filling, either as absolute numbers or in terms of direction of change in response to therapy.
4. Factors Affecting the Accuracy of Pulmonary Artery Catheter Data(Table 27-12)

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1. Complications of Pulmonary Artery Catheter Monitoring(Tables 27-13 and 27-14)
2. Mixed Venous Oximetry
3. Advances in fiberoptic technology have led to the development of pulmonary artery catheters that can continuously measure mixed venousO₂ saturation (SvO₂).
4. SvO₂varies directly with cardiac output, hemoglobin concentration, and SaO₂ and inversely with minute O₂ consumption.
5. When all other variables are constant, SvO₂reflects corresponding changes in cardiac output.
6. The normal SvO₂is 75%, and anaerobic metabolism occurs when SvO₂ is below 30%.

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1. Central Venous Oxygen and Its Relation to Mixed Venous Oxygen
2. Central venous O₂(ScvO₂) represents O₂ extraction from the upper body and brain but may serve as a surrogate measure of SvO₂.
3. ScvO₂may not reflect SvO₂ in the presence of septic shock.

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1. Clinical Benefits of Pulmonary Artery Monitoring.Perioperative outcomes have been reported to be improved, worsened, or unchanged by pulmonary artery catheter usage. Several recent articles have illustrated insufficient

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evidence for an outcome benefit from the use of a pulmonary artery catheter.

1. Temperature Monitoring
2. The potential for accidental heat loss or the risk of triggering malignant hyperthermia requires the continued observation of temperature changes.
3. Perioperative hypothermiacommonly results from anesthetic-induced inhibition of thermoregulation as well as a cold ambient environment in the operating room and heat loss owing to surgical exposure of tissues.
4. Anesthetized patients often behave like poikilotherms until the core temperature approaches a new set point for thermoregulation.
5. Patients at greatest risk for perioperative hypothermia include elderly patients, burn patients, neonates, and patients with spinal cord injuries.

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1. Perioperative hyperthermiaoccurs rarely, and potential explanations other than malignant hyperthermia include exposure to endogenous pyrogens, increases in metabolic rate secondary to thyrotoxicosis or pheochromocytoma, and anticholinergic blockade of sweating.
2. Central temperature is customarily measured using temperature probes placed in the nasopharynx, esophagus, blood (pulmonary artery catheter), bladder, or rectum.
3. During routine noncardiac surgery, temperature differences between these sites are small.
4. During and after cardiopulmonary bypass or deliberate hypothermia, gradients between these sites are predictable.
5. During cooling in anesthetized patients, changes in rectal temperature often lag behind changes in central (core) temperature.
6. During rewarming, probe locations residing in regions of high blood flow often reflect blood temperature rather than central temperature, emphasizing that the adequacy of rewarming is best judged by measuring temperature at more than one location.

Editors: Barash, Paul G.; Cullen, Bruce F.; Stoelting, Robert K.; Cahalan, Michael K.; Stock, M. Christine

Title: Handbook of Clinical Anesthesia, 6th Edition

Copyright ©2009 Lippincott Williams & Wilkins

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