Monitoring for Myocardial Ischemia

Three methods are used to detect myocardial ischemia during the perioperative period: surface ECG, transesophageal

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Figure 52-7 Heart rate and ST segment trends the evening before surgery from an ambulatory electrocardiographic monitor for a patient scheduled for femoropopliteal bypass. Notice the "mirror image" heart rate-related ST segment depression occurring at subtachycardic heart rates (about 85 beats/min). (Adapted from Frank SM, Beattie C, Christopherson R, et al: Perioperative rate-related silent myocardial ischemia and postoperative death. J Clin Anesth 2:326–331, 1990.)

Electrocardiographic monitoring for ischemia is the least expensive method and requires the least training. Subendocardial ischemia is the most common type of ischemia in the perioperative setting and is manifested by ST segment depression on the ECG.^[145] Transmural ischemia is much less common and is usually accompanied by ST segment elevation in the leads facing the injury, with reciprocal ST segment depression in one or more of the other leads. A variety of independent measures of ischemia, including abnormalities of myocardial perfusion,^[146] regional left ventricular dysfunction, ^[147] and pulmonary and left ventricular pressure changes,^[148] have been correlated with ST segment changes that meet the following criteria (1 × 1 × 1 rule) ^[149] : horizontal or downward sloping ST segment depression, 60 to 80 msec after the J point of at least 1 mm from the isoelectric baseline; duration of at least 1 minute; and separation from other discrete episodes by at least 1 minute of normal baseline.

Electrocardiographic monitoring should be performed in the diagnostic mode (0.05-Hz low-frequency cutoff) rather than the monitoring mode (0.5-Hz cutoff). ^[149] The increased filtering in the monitoring mode can create artifactual ischemic changes as a tradeoff for decreased baseline wandering. Electrocardiographic morphology should always be assessed from a printed hard copy because monitors do not accurately show electrocardiographic morphology. ^[150] Electrocardiographic changes consistent with ischemia are difficult to detect in patients with right bundle branch blocks, left ventricular hypertrophy with a strain pattern, or atrial fibrillation.^[149] Ischemia monitoring by ECG is not possible in patients with left bundle branch block or pacemaker dependency.^[149] In one large study, ^[80] 15% of vascular surgery patients had electrocardiographic abnormalities that precluded ischemia monitoring. During the intraoperative period, London and colleagues^[151] studied noncardiac surgery patients and concluded that although V₅ was the single most sensitive lead for detecting intraoperative ischemic changes (75% of episodes were evident in this lead), the sensitivity was increased to only 80% by monitoring leads II and V₅ together. With a three-lead system (leads II, V₄ , and V₅ ) 96% of ischemic changes were detected. During the postoperative period, routine intensive care unit surveillance has a low sensitivity for

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Over the past decade, computerized ST segment analysis has become one of the most valuable tools for ischemia monitoring. There is, however, no computerized system that is reliable enough to use without the interpretation of a trained clinician who can differentiate true ischemic changes from artifact. Computers are often fooled by baseline ST segment depression from abnormalities such as left ventricular hypertrophy and strain patterns. These systems are not sophisticated enough to analyze morphology (i.e., slope of the ST segment), and they rely primarily on the degree of change from an isoelectric baseline. Nonetheless, the most reliable way to monitor for ischemia is to follow the trend for ST segment elevation or depression over time by use of a computer algorithm. Computerized ST segment analysis, when used properly, is one of the most significant advances in perioperative monitoring.

Two-dimensional TEE has emerged over the past decade to become one of the most sensitive monitors of cardiac function. Evidence suggests that segmental wall motion abnormalities occur earlier than electrocardiographic changes at the onset of ischemia.^[153] The echocardiographic changes during ischemia are characterized by decreased ventricular wall thickening during systole, but the clinician needs to be relatively well trained to recognize such changes. Another drawback is the significant cost of the echocardiographic equipment. Although it is possible to perform TEE in awake or sedated patients, the procedure is often not well tolerated, making this technique less desirable during regional anesthesia or during the postoperative period.

Detection of ischemia is possible by use of a pulmonary artery catheter, but this technique is a distant third choice when compared with other methods (i.e., TEE or ECG). Episodes of ischemia are manifested as an abnormal pulmonary capillary wedge pressure waveform or an absolute increase in mean pulmonary capillary wedge pressure of 10 mm Hg or greater.^[154] This technique has low feasibility because, ideally, the balloon-tipped catheter needs to be constantly or frequently inflated to detect such changes.