Reciprocal ST-segment changes (such as ST-segment depressions in the anterior precordial leads in the setting of an inferior wall AMI) predict a larger infarct distribution, an increased severity of underlying coronary artery disease, more severe pump failure, a higher likelihood of cardiovascular complications, and increased mortality. In general, the more elevated the ST segments and the more ST segments that are elevated, the more extensive the injury.
The ECG can also be used to predict the infarct-related vessel. Inferior wall myocardial infarctions can result from occlusion of the left circumflex artery or the right coronary artery. In the setting of an inferior wall AMI, ST-segment elevation in at least one lateral lead (V 5, V6, or aVL) with an isoelectric or elevated ST segment in lead I is strongly suggestive of a left circumflex lesion. The presence of ST-segment elevation in lead III greater than that in lead II predicts a right coronary artery occlusion. When accompanied by either ST-segment elevation in V1 or aVR, it predicts a proximal right coronary artery lesion with accompanying right ventricular infarction. Reciprocal anterior ST-segment depressions in V! through V4 are equally prevalent in both right coronary and left circumflex inferior wall AMI.
The main utility of the ECG is to detect AMI. The standard 12-lead ECG is less helpful in the detection of other acute coronary syndromes (stable angina or unstable angina). One widely used classification breaks down the ECG into six categories:
• nonspecific ST-segment or T-segment wave changes;
• abnormal but not diagnostic of ischemia or infarction;
• ischemia, strain, or infarction known to be old;
• ischemia, strain or infarction not known to be old;
• probable myocardial infarction.
This classification and others like it have been used to show that patients with more significant ECG abnormalities are more likely to have AMI, unstable angina, and serious cardiovascular complications. On the other hand, even the patients with normal or nonspecific ECGs have a 1 to 3 percent incidence of AMI and a 4 to 23 percent incidence of unstable angina. Patients with nondiagnostic ECGs or with ischemia that is not known to be old have a 4 to 7 percent incidence of AMI and a 21 to 48 percent incidence of unstable angina. Demonstration of new ischemia increases the risk of AMI 25 to 73 percent and the unstable angina risk to 14 to 43 percent.3 The standard 12-lead ECG is useful for cardiovascular risk stratification of patients with acute coronary syndromes. It can be used in conjunction with clinical history and cardiac markers to determine admission location for such patients.
Novel approaches to electrocardiography have been proposed in the last decade. A continuous 12-lead ECG monitor has been developed that records a new 12-lead ECG every 20 s. When the ST-segment baseline is altered, an alarm is raised and a copy of the new ECG is automatically shared or printed. This type of technology might be useful for monitoring patients who present with non-AMI acute coronary syndromes for ECG evidence of injury.4 Because of costs, concerns regarding labile ST-segment and T-wave changes from hyperventilation or patient movement and a lack of ED-based prospective studies, continuous-12 lead ECG monitoring has not been recommended for routine use.4
ECGs with 15, 18, and 22 leads have been studied.47 The addition of V4R, V8, and V9, increased the sensitivity without a loss of specificity for the detection of
ST-segment elevation.7 The addition of V4R through V6 R and V7 through V9 as posterior leads led to increased sensitivity, but at the cost of decreased specificity. ECGs with 22 leads and body-surface mapping have not been sufficiently studied. The report from the National Heart Attack Alert Program recommends the use of standard ECGs with right-sided leads in the setting of inferior wall infarction. 4
There are several clinical conditions where ECG interpretation is difficult ( Iable 47:6). It has been shown that in the setting of paced rhythms and left bundle branch blocks, acute myocardial ischemia can be identified. In the setting of a left bundle branch block, the presence of ST-segment elevation 31 mm and concordant with the QRS complex or ST segment depression 31 mm in leads V!, V2, or V3 suggests acute myocardial infarction.8 ST-segment elevation 35 mm and discordant with the QRS complex increases the likelihood of AMI but has poor specificity.
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