Intraoperative Hemodynamic Monitoring

The combination of V5 with another lead (II, III, V4, or V6) increases the sensitivity of ischemia detection to 90%. In high-risk patients, most intraoperative ST segment changes occur in the absence of major hemodynamic lability, and maintenance of hemodynamic variables (blood pressure and heart rate) to within 20% of baseline values does not appear to reduce intraoperative ischemic events. Electrocardiograph (ECG) monitoring for myocardial ischemia augmented by continuous, computerized ECG monitors should be continued into the early postoperative period for high-risk patients. Invasive blood pressure monitoring is indicated when (1) large or abrupt hemodynamic changes are anticipated intraoperatively, (2) coexisting disease necessitates close hemodynamic management, (3) pharmacologic or mechanical intervention is anticipated or planned, or (4) noninvasive methods are impractical or impossible. Excessive variation of the systolic blood pressure (>15 mm Hg) is a strong predictor of occult hypovolemia. Overall, the risk of an ischemic complication from peripheral arterial catheterization is less than 0.1%. Central venous pressure (CVP) reflects the balance among intravascular volume, venous tone, and right ventricular function. In general, a single isolated CVP value provides little information unless it is very high or very low. Recent development of small, portable ultrasound machines has made it possible to image the great veins of the neck prior to or during placement of central venous catheters. This has been shown in multiple clinical studies as well as in a recent meta-analysis to decrease the incidence of failed catheter placement as well as the complication rate, especially in high-risk patients. Pulmonary artery catheter (PAC)-derived filling pressures are a poor index of right and left ventricular preload as they are influenced by heart rate, valvular disease, myocardial compliance, myocardial ischemia, and positive pressure ventilation. Perioperative goal-directed therapy, that is, the “optimization” of oxygen delivery and cardiac index, has been shown to reduce mortality, complications, and hospital length of stay in high-risk surgical patients. According to current American Society of Anesthesiologists (ASA) recommendations, the use of PAC is considered necessary only in high-risk patients undergoing high-risk surgery, and under the condition that the practice environment is favorable (i.e., knowledgeable and experienced physicians and intensive care unit nurses). Mixed venous oxygen saturation depends on the balance between oxygen delivery and consumption, and might be one of the more telling indices for judging the adequacy of the patient’s hemodynamic status. Thermodilution is the standard method for measuring cardiac output; it is subject, however, to intrinsic variability, effect of intracardiac shunts and right-sided valvular regurgitation, and influence of other factors affecting instantaneous blood temperature. Two newer cardiac output measurement methods that obviate the need for a PAC include the transpulmonary arterial thermodilution and the lithium dilution. Both seem to offer accuracy similar to the traditional thermodilution method. Several methods exist for continuous measurement of cardiac output. The first method employs a modified PAC with a blood-warming filament. It does not produce true real-time continuous measurements, but rather a trend of recent measurements over a couple of minutes. Esophageal Doppler cardiac output (CO) measurement employs a probe inserted into the lower esophagus to measure descending aorta blood flow. It tends to underestimate CO measured by other methods, although accurately tracking changes occurring over time. Pulse contour methods derive continuous beat-to-beat CO from the arterial pressure waveform. In ventilated patients, they also allow measurement of stroke volume variation, an index of the adequacy of cardiac preload. Despite its classification as category II for use in routine coronary artery bypass graft (CABG) surgery, transesophageal echocardiography (TEE) in high-risk patients seems to be associated with better outcome. Transesophageal echocardiography has become invaluable in planning for mitral valve repair and reconstruction with excellent agreement between mechanism and location between TEE and surgical findings. Following valve repair or replacement, TEE is especially useful in immediate evaluation of paravalvular leaks, leafl et motion, and other forms of valvular dysfunction. Although TEE has been reported to be more predictive than ECG in identifying patients who suffer perioperative myocardial infarction, concordance between TEE and ECG-detected ischemia is poor. Direct complications of TEE are the result of mechanical trauma to the upper gastrointestinal tract and oropharynx, and include abrasion, laceration, and perforation.