Principles of surgery

PART I
Basic Considerations

CHAPTER 13
Physiologic Monitoring of the Surgical Patient

BASIC SCIENCE QUESTIONS

1. Critical oxygen delivery (transition from supply-independent to supply-dependent oxygen delivery) occurs when oxygen delivery falls below

A. 1.5 mL/kg

B. 2.0 mL/kg

C. 4.5 mL/kg

D. 6.0 mL/kg

Answer: C

Under normal conditions when the supply of O₂ is plentiful, aerobic metabolism is determined by factors other than the availability of O₂. These factors include the hormonal milieu and mechanical workload of contractile tissue. However, in pathologic circumstances when O₂ availability is inadequate, O₂ utilization (O₂) becomes dependent upon O₂ delivery (O₂). The relationship of O₂ to O₂over a broad range of O₂ values is commonly represented as two intersecting straight lines. In the region of higher O₂ values, the slope of the line is approximately equal to zero, indicating that O₂ is largely independent of O₂. In contrast, in the region of low O₂ values, the slope of the line is nonzero and positive, indicating that O₂ is supply dependent. The region where the two lines intersect is called the point of critical O₂ delivery (O_2crit), and represents the transition from supply-independent to supply-dependent O₂ uptake. Below this critical threshold of O₂ delivery (approximately 4.5 mL/kg per minute), increased O₂ extraction cannot compensate for the delivery deficit; hence, O₂ consumption begins to decrease. (See Schwartz 9^th ed., p 344.)

2. Preload is determined by

A. End systolic volume

B. End systolic pressure

C. End diastolic volume

D. End diastolic pressure

Answer: C

Starlings law of the heart states that the force of muscle contraction depends on the initial length of the cardiac fibers. Using terminology that derives from early experiments using isolated cardiac muscle preparations, preload is the stretch of ventricular myocardial tissue just before the next contraction. Preload is determined by end-diastolic volume (EDV). For the right ventricle, central venous pressure (CVP) approximates right ventricular (RV) end-diastolic pressure (EDP). For the left ventricle, pulmonary artery occlusion pressure (PAOP), which is measured by transiently inflating a balloon at the end of a pressure monitoring catheter positioned in a small branch of the pulmonary artery, approximates left ventricular EDP. The presence of atrioventricular valvular stenosis will alter this relationship. (See Schwartz 9^th ed., p 346.)

3. In measuring cardiac output with a Swan-Ganz catheter, a rapid bolus of saline is used. What is the best temperature for that bolus?

A. 4° centigrade

B. 15° centigrade

C. 30° centigrade

D. Room temperature

Answer: D

Determination of cardiac output by the thermodilution method is generally quite accurate, although it tends to systematically overestimate QT at low values. Changes in blood temperature and QT during the respiratory cycle can influence the measurement. Therefore, results generally should be recorded as the mean of two or three determinations obtained at random points in the respiratory cycle. Using cold injectate widens the difference between TB and TI and thereby increases signal-to-noise ratio. Nevertheless, most authorities recommend using room temperature injectate (normal saline or 5% dextrose in water) to minimize errors resulting from warming of the fluid as it transferred from its reservoir to a syringe for injection. (See Schwartz 9^th ed., p 348.)

4. Which of the following will result in a subnormal SO₂?

A. Fluid overload

B. Increased PaO₂

C. Anemia

D. Hypothermia

Answer: C

SO₂ is a function of O₂ (i.e., metabolic rate), QT, SaO₂, and Hgb. Accordingly, subnormal values of SO₂ can be caused by a decrease in Q_T (due, for example, to heart failure or hypovolemia), a decrease in SaO₂ (due, for example, to intrinsic pulmonary disease), a decrease in Hgb (i.e., anemia), or an increase in metabolic rate (due, for example, to seizures or fever). (See Schwartz 9^th ed., p 348.)

CLINICAL QUESTIONS

1. In direct measurement of blood pressure, what effect will overdamping have on the mean arterial pressure (MAP)?

A. The MAP will be artificially high

B. The MAP will be artificially low

C. There is no effect on the MAP reading

D. The MAP cannot be calculated if the system is over-damped

Answer: C

The fidelity of the catheter-tubing-transducer system is determined by numerous factors, including the compliance of the tubing, the surface area of the transducer diaphragm, and the compliance of the diaphragm. If the system is underdamped, then the inertia of the system, which is a function of the mass of the fluid in the tubing and the mass of the diaphragm, causes overshoot of the points of maximum positive and negative displacement of the diaphragm during systole and diastole, respectively. Thus in an underdamped system, systolic pressure will be overestimated and diastolic pressure will be underestimated. In an overdamped system, displacement of the diaphragm fails to track the rapidly changing pressure waveform, and systolic pressure will be underestimated and diastolic pressure will be overestimated. It is important to note that even in an underdamped or overdamped system, mean pressure will be accurately recorded, provided the system has been properly calibrated. For these reasons, when using direct measurement of intra-arterial pressure to monitor patients, clinicians should make clinical decisions based on the measured mean arterial blood pressure. (See Schwartz 9^th ed., p 345.)

2. Addition of which of the following leads to continuous EKG monitoring will improve the ability to detect myocardial ischemia?

A. Left arm

B. Right arm

C. V₁

D. V₄

Answer: D

A standard 3-lead ECG is obtained by placing electrodes that correspond to the left arm (LA), right arm (RA), and left leg (LL). The limb leads are defined as lead I (LA-RA), lead II (LL-RA), and lead III (LL-LA).

Additional information can be obtained from a 12-lead ECG, which is essential for patients with potential myocardial ischemia or to rule out cardiac complications in other acutely ill patients. Continuous monitoring of the 12-lead ECG now is available and is proving to be beneficial in certain patient populations. In a study of 185 vascular surgical patients, continuous 12-lead ECG monitoring was able to detect transient myocardial ischemic episodes in 20.5% of the patients. This study demonstrated that the precordial lead V₄, which is not routinely monitored on a standard 3-lead ECG, is the most sensitive for detecting perioperative ischemia and infarction. To detect 95% of the ischemic episodes, two or more precordial leads were necessary. Thus, continuous 12-lead ECG monitoring may provide greater sensitivity than 3-lead ECG for the detection of perioperative myocardial ischemia, and is likely to become standard for monitoring high-risk surgical patients. (See Schwartz 9^th ed., p 346.)

3. The width of a blood pressure cuff should be what percentage of the circumference of the patient’s arm?

A. 30%

B. 40%

C. 50%

D. 60%

Answer: B

Both manual and automated means for the noninvasive determination of blood pressure use an inflatable cuff to increase pressure around an extremity. If the cuff is too narrow (relative to the extremity), the measured pressure will be artifactually elevated. Therefore, the width of the cuff should be approximately 40% of its circumference. (See Schwartz 9^th ed., p 344.)

4. In critically ill postoperative patients, monitoring with a pulmonary artery catheter

A. Decreases mortality

B. Decreases the rate of postoperative myocardial infarction

C. Decreases ICU length of stay

D. None of the above

Answer: D

(See Schwartz 9^th ed., p 349, and Table 13-1.)

TABLE 13-1 Summary of randomized, prospective clinical trials comparing pulmonary artery catheter with central venous pressure monitoring

5. Which of the following is most likely to accurately estimate cardiac output?

A. Sternal notch ultrasonography

B. Transesophageal ultrasonography

C. Impedence cardiography

D. Partial carbon dioxide rebreathing

Answer: D

Partial carbon dioxide (CO₂) rebreathing uses the Fick principle to estimate Q_T noninvasively. By intermittently altering the dead space within the ventilator circuit via a rebreathing valve, changes in CO₂ production (VCO₂) and end-tidal CO₂ (ETCO₂) are used to determine cardiac output using a modified Fick equation (Q_T = ΔVCO₂/ΔETCO₂). Commercially available devices use this Fick principle to calculate Q_T using intermittent partial CO₂ rebreathing through a disposable rebreathing loop. These devices consist of a CO₂ sensor based on infrared light absorption, an airflow sensor, and a pulse oximeter. Changes in intrapulmonary shunt and hemodynamic instability impair the accuracy of Q_T estimated by partial CO₂ rebreathing. Continuous in-line pulse oximetry and inspired fraction of inspired O₂ (FiO₂) are used to estimate shunt fraction to correct Q_T.

Two approaches have been developed for using Doppler ultrasonography to estimate Q_T. The first approach uses an ultrasonic transducer, which is manually positioned in the suprasternal notch and focused on the root of the aorta. Aortic cross-sectional area can be estimated using a nomogram, which factors in age, height, and weight, back-calculated if an independent measure of Q_T is available, or by using two-dimensional transthoracic or transesophageal ultrasonography. Although this approach is completely noninvasive, it requires a highly skilled operator to obtain meaningful results, and is labor intensive. Moreover, unless Q_T measured using thermodilution is used to back-calculate aortic diameter, accuracy using the suprasternal notch approach is not acceptable.

The impedance to flow of alternating electrical current in regions of the body is commonly called bioimpedance. In the thorax, changes in the volume and velocity of blood in the thoracic aorta lead to detectable changes in bioimpedance. The first derivative of the oscillating component of thoracic bioimpedance (dZ/dt) is linearly related to aortic blood flow. On the basis of this relationship, empirically derived formulas have been developed to estimate SV, and subsequently Q_T, noninvasively. This methodology is called impedance cardiography. The approach is attractive because it is noninvasive, provides a continuous readout of Q_T, and does not require extensive training for use. Despite these advantages, studies suggest that measurements of Q_T obtained by impedance cardiography are not sufficiently reliable to be used for clinical decision making and have poor correlation with standard methods such as thermodilution and ventricular angiography. (See Schwartz 9^th ed., p 351.)

6. Which of the following could cause a simultaneous increase in peak airway pressure AND plateau airway pressure?

A. Intrinsic PEEP

B. Endotracheal tube plugging

C. Bronchospasm

D. Insufficient expiratory time

Answer: A

If both P_peak and P_plateau are increased (and tidal volume is not excessive), then the problem is a decrease in the compliance in the lung/chest wall unit. Common causes of this problem include pneumothorax, hemothorax, lobar atelectasis, pulmonary edema, pneumonia, acute respiratory distress syndrome (ARDS), active contraction of the chest wall or diaphragmatic muscles, abdominal distention, and intrinsic PEEP, such as occurs in patients with bronchospasm and insufficient expiratory times. When P_peak is increased but P_plateau is relatively normal, the primary problem is an increase in airway resistance, such as occurs with bronchospasm, use of a small-caliber endotracheal tube, or kinking or obstruction of the endotracheal tube. (See Schwartz 9^th ed., p 354.)

7. A high level of methemoglobin will result in

A. A falsely high pulse oximetry reading

B. A falsely low pulse oximetry reading

C. A pulse oximetry reading of 85%

D. None of the above—there is no effect on the pulse oximetry reading

Answer: C

Under normal circumstances, the contributions of carboxyhemoglobin and methemoglobin are minimal. However, if carboxyhemoglobin levels are elevated, the pulse oximeter will incorrectly interpret carboxyhemoglobin as oxyhemoglobin and the arterial saturation displayed will be falsely elevated. When the concentration of methemoglobin is markedly increased, the SaO₂ will be displayed as 85%, regardless of the true arterial saturation. The accuracy of pulse oximetry begins to decline at SaO₂ values less than 92%, and tends to be unreliable for values less than 85%. (See Schwartz 9^th ed., p 354.)

8. Which of the following is the approximate PaCO₂ for a healthy individual with an end-tidal CO₂ (PETCO₂) of 25?

A. 20

B. 25

C. 30

D. None of the above—there is no accurate correlation between PaCO₂ and end-tidal CO₂

Answer: C

In healthy subjects, PETCO₂ is about 1 to 5 mmHg less than PaCO₂. Thus, PETCO₂ can be used to estimate PaCO₂ without the need for blood gas determination. However, changes in PETCO₂ may not correlate with changes in PaCO₂ during a number of pathologic conditions. (See Schwartz 9^th ed., p 354.)

9. Which of the following is considered confirmatory of the diagnosis of abdominal compartment syndrome?

A. Bladder pressure >15 mmHg

B. Bladder pressure >25 mmHg

C. Bladder pressure >35 mmHg

D. Bladder pressure >45 mmHg

Answer: B

The triad of oliguria, elevated peak airway pressures, and elevated intra-abdominal pressure is known as the abdominal compartment syndrome (ACS). This syndrome, first described in patients after repair of ruptured abdominal aortic aneurysm, is associated with interstitial edema of the abdominal organs, resulting in elevated intra-abdominal pressure. When intra-abdominal pressure exceeds venous or capillary pressures, perfusion of the kidneys and other intra-abdominal viscera is impaired. Oliguria is a cardinal sign. Although the diagnosis of ACS is a clinical one, measuring intra-abdominal pressure is useful to confirm the diagnosis. Ideally, a catheter inserted into the peritoneal cavity could measure intra-abdominal pressure to substantiate the diagnosis. In practice, transurethral bladder pressure measurement reflects intra-abdominal pressure and is most often used to confirm the presence of ACS. After instilling 50 to 100 mL of sterile saline into the bladder via a Foley catheter, the tubing is connected to a transducing system to measure bladder pressure. Most authorities recommend that a bladder pressure greater than 20 to 25 mmHg confirms the diagnosis of ACS. (See Schwartz 9^th ed., p 355.)

10. ICP monitoring is indicated for a patient with

A. An abnormal CT of the brain and a Glasgow Coma Scale score of 10 after traumatic brain injury

B. A normal CT of the brain after traumatic brain injury, systolic BP 90, and age >40

C. A normal CT of the brain and hepatic failure with coma

D. A normal CT of the brain and coma after anoxia

Answer: B

Monitoring of ICP is currently recommended in patients with severe traumatic brain injury (TBI), defined as a Glasgow Coma Scale (GCS) score ≤8 with an abnormal CT scan, and in patients with severe TBI and a normal CT scan if two or more of the following are present: age greater than 40 years, unilateral or bilateral motor posturing, or systolic blood pressure less than 90 mmHg. ICP monitoring also is indicated in patients with acute subarachnoid hemorrhage with coma or neurologic deterioration, intracranial hemorrhage with intraventricular blood, ischemic middle cerebral artery stroke, fulminant hepatic failure with coma and cerebral edema on CT scan, and global cerebral ischemia or anoxia with cerebral edema on CT scan. (See Schwartz 9^th ed., p 355.)

11. Titration of sedation in the ICU can be improved by monitoring with

A. Continuous EEG

B. Bispectral index (BIS)

C. Serum levels of the drug(s) being used

D. Near infrared spectrometry

Answer: B

A recent advance in EEG monitoring is the use of the bispectral index (BIS) to titrate the level of sedative medications. Although sedative drugs usually are titrated to the clinical neurologic examination, the BIS device has been used in the operating room to continuously monitor the depth of anesthesia. The BIS is an empiric measurement statistically derived from a database of more than 5000 EEGs. The BIS is derived from bifrontal EEG recordings and analyzed for burst suppression ratio, relative alpha:beta ratio, and bicoherence. Using a multivariate regression model, a linear numeric index (BIS) is calculated, ranging from 0 (isoelectric EEG) to 100 (fully awake). Its use has been associated with lower consumption of anesthetics during surgery and earlier awakening and faster recovery from anesthesia. The BIS also has been validated as a useful approach for monitoring the level of sedation for ICU patients, using the revised Sedation-Agitation Scale as a gold standard. (See Schwartz 9^th ed., p 355.)