Diagnosis of Ischemia

Perioperative myocardial ischemia is predictive of adverse cardiac outcomes^[78] (see Chapter 32 and Chapter 50 ). Factors that predispose to the development of perioperative ischemia include the presence of preexisting coronary artery disease and perioperative events that affect the myocardial oxygen balance. Perioperative clinical studies have found a high incidence of electrocardiographic evidence of ischemia (20% to 80%) in patients with coronary artery disease who are undergoing cardiac or noncardiac surgery.^{[79] [80]} The incidence of perioperative myocardial infarction in patients with coronary artery disease with or

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It has also become evident that a significant number of patients suffer from asymptomatic or "silent" ischemia.^[83] Silent ischemia is manifested by characteristic electrocardiographic signs of ischemia in the absence of angina and is not necessarily associated with changes in hemodynamics or heart rate. Among patients with chronic stable angina who have ST-segment depression during exercise, ambulatory electrocardiographic monitoring during daily life identifies transient ambulant ischemic episodes in approximately 40% to 50% of patients. In these patients, silent ischemic episodes account for about 75% of all ambulant ischemic episodes.^[84]

The electrocardiographic changes occurring during myocardial ischemia are often characteristic and are detected with careful electrocardiographic monitoring. Although the electrocardiographic criteria for ischemia were established in patients undergoing exercise stress testing, they may also be applied to anesthetized patients ( Table 34-7 ). These criteria are (1) horizontal or downsloping ST-segment depression of 0.1 mV, (2) ST-segment elevation of 0.1 mV in a non-Q wave lead, and (3) slowly upsloping ST-segment depression of 0.2 mV (all measured from 60 to 80 msec after the J point) ( Fig. 34-15 ).^[85] Okin and coworkers^[86] studied the relation of the time after the J point at which ST depression is measured to the magnitude of ST-segment depression during peak exercise. These investigators found that a positive exercise ECG (0.1 mV or more of additional horizontal or downsloping ST depression at the end-exercise point) had a specificity of 96% for coronary artery disease when ST-segment depression was measured at the J point or J + 60 msec. There was no difference in sensitivity of electrocardiographic criteria at the J point or at J + 60 msec. However, at J + 60 msec,

TABLE 34-7 -- Electrocardiographic criteria for ischemia in anesthetized patients

Upsloping ST segment: 2-mm depression, 80 msec after J point

Horizontal ST segment: 1-mm depression, 60–80 msec after J point

Downsloping ST segment: >1 mm from top of curve to PQ junction

ST elevation

T-wave inversion

Figure 34-15 Assessment of ST-segment abnormality in the diagnosis of ischemia. (From Ellestad MH: Stress Testing: Principles and Practice. Philadelphia, FA Davis, 1975.)

In a perioperative study, investigators monitored 12 lead electrocardiographic changes larger than 0.2 mV from baseline in a single lead or more than 0.1 mV in two contiguous leads 60 msec after the J point to identify perioperative myocardial ischemia. These changes also had to last longer than 10 minutes.^[90] Troponins were used as markers for myocardial infarction. In this study, leads V₃ and V₄ were identified as most sensitive for myocardial ischemia detection, with lead V₅ trailing close behind.^[90] The sensitivity for detecting postoperative infarction was 100% for either combination of leads V₃ + V₅ or V₄ + V₅ . When only a single precordial lead is available for perioperative ischemia monitoring, as is frequently encountered, the most isoelectric lead of V₃ , V₄ , or V₅ should be selected. Lead V₄ rather than V₅ detects ischemia earlier, is more sensitive, and shows a greater ST-segment deviation.^[90] Lead V₄ appears to be more sensitive and appropriate for detection of prolonged postoperative ischemia and infarction. For patients with acute coronary syndromes (e.g., those with atherosclerotic plaque disruption), the recommendation is to monitor limb lead III and leads V₃ and V₅ as the most sensitive combination for ischemia detection.^[91] Given the debate, the combination of an inferior limb lead along with lead V₄ or V₅ is prudent for ischemia detection along with good clinical care such as

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It is commonly believed that monitoring for intraoperative myocardial ischemia is unnecessary in neonates. Whereas electrocardiographic lead systems for adults are concerned with the detection of ischemia and arrhythmias, neonatal electrocardiographic monitoring has focused on arrhythmia recognition alone. Results of some studies, however, suggest that the neonatal heart is more susceptible to ischemia than the adult heart.^[93] These studies have demonstrated the importance of calibrated electrocardiographic monitoring in neonates with congenital heart disease (see Chapter 51 ).

Figure 34-16 Digitalis effect on ST segments and T waves (i.e., ectopic atrial rhythm, 2:1 atrioventricular [AV] block, type 1 second-degree AV block, left ventricular hypertrophy with strain). (Adapted from Marriott HJL: Practical Electrocardiography, 7th ed. Baltimore, Williams & Wilkins, 1983.)

Although ST-segment analysis provides sensitive information about myocardial ischemia, it should be remembered that underlying electrocardiographic abnormalities hinder the analysis in about 10% of patients. These abnormalities include hypokalemia, digitalis administration, LBBB, Wolff-Parkinson-White syndrome, left ventricular hypertrophy with strain, and acute pericarditis ( Fig. 34-16 ). In these patients, other diagnostic modalities, such as transesophageal echocardiography, should be considered.