Myocardial Infarction and Ischemia, II: Non–ST Segment Elevation and Non–Q Wave Syndromes

Chapter 9 Myocardial Infarction and Ischemia, II Non–ST Segment Elevation and Non–Q Wave Syndromes

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Myocardial infarction (MI) may be associated with abnormal Q waves associated with the typical progression of ST segment elevation myocardial infarction (STEMI) and T wave inversions described in Chapter 8. In many cases, however, myocardial ischemia (with or without actual infarction) may be limited primarily to the subendocardium (inner layer) of the ventricle, often associated with ST segment depressions, rather than primary ST elevations.

Subendocardial Ischemia

How can subendocardial ischemia occur without transmural ischemia or infarction? The subendocardium is particularly vulnerable to ischemia because it is most distant from the coronary blood supply and closest to the high pressure of the ventricular cavity. This inner layer of the ventricle can become ischemic while the outer layer (epicardium) remains normally perfused with blood.

The most common ECG change with subendocardial ischemia is ST segment depression (Fig. 9-1). The ST depression may be limited to the anterior leads (I, aVL, and V1 to V6) or to the inferior leads (II, III, and aVF), or it may be seen more diffusely in both groups of leads. As shown in Figure 9-1, the ST segment depression with subendocardial ischemia has a characteristic squared-off shape. (ST segment elevation is usually seen in lead aVR.)

Recall from Chapter 8 that acute transmural ischemia produces ST segment elevation, a current of injury pattern. This results from epicardial injury. With pure subendocardial ischemia, just the opposite occurs; that is, the ECG shows ST segment depression (except in lead aVR, which often shows ST elevation).

In summary, myocardial ischemia involving primarily the subendocardium usually produces ST segment depression, whereas acute ischemia involving the epicardium usually produces ST elevation. This difference in the direction of the injury current vector is depicted in Figure 9-2.

Exercise (Stress) Testing and Coronary Artery Disease

Many patients with coronary artery disease have a normal ECG while at rest. During exercise, however, ischemic changes may appear because of the extra oxygen requirements imposed on the heart by exertion. To assist in diagnosing coronary artery disease, the cardiologist can record the ECG while the patient is being exercised under controlled conditions. Stress electrocardiography is usually performed while the patient walks on a treadmill or pedals a bicycle ergometer. The test is stopped when the patient develops angina, fatigue, or diagnostic ST changes or when the heart rate reaches 85% to 90% of a maximum predetermined rate predicted from the patient’s age. This approach is known as submaximal testing.

Figure 9-4A is the normal resting ECG of a patient, whereas Figure 9-4B shows the marked ST depressions recorded while the patient was exercising. The appearance of ST segment depressions constitutes a positive (abnormal) result. Most cardiologists accept horizontal or downward ST depressions of at least 1 mm or more, lasting at least 0.08 sec (two small boxes) as a positive (abnormal) test result (see Fig. 9-4B). ST depressions of less than 1 mm (or depressions of only the J point) with a rapid upward sloping of the ST segment are considered a negative (normal) test response (Fig. 9-5).

The finding of prominent ischemic ST changes, with or without symptoms, occurring at a low level of activity is particularly ominous. Sometimes, these changes will be associated with a drop in blood pressure. These findings may indicate severe three-vessel coronary disease and sometimes high-grade obstruction of the left main coronary artery.

Exercise electrocardiography is often helpful in diagnosing coronary artery disease. However, like all tests, it may give both false-positive and false-negative results. For example, up to 10% of men without evidence of coronary obstructions and an even higher percentage of normal women may have false-positive exercise tests.

False-positive tests (defined here as ST depressions without obstructive coronary disease) can also be seen in patients who are taking digitalis and in patients who have hypokalemia, left ventricular hypertrophy (LVH), ventricular conduction disturbances, or a Wolff-Parkinson-White pattern (see Chapter 12).

False-negative tests can occur despite the presence of significant underlying coronary artery disease. Therefore, a normal (“negative”) exercise test does not exclude coronary artery disease. The diagnostic accuracy of exercise tests may be increased in selected patients by simultaneous imaging studies, using echocardiography or nuclear medicine scans. Pharmacologic stress testing, an important related topic, lies outside the scope of this text.

In summary, subendocardial ischemia, such as occurs with typical angina pectoris (and as may be elicited with stress testing), often produces ST segment depressions in multiple leads.

Jun 11, 2016 | Posted by in CARDIOLOGY | Comments Off on Myocardial Infarction and Ischemia, II: Non–ST Segment Elevation and Non–Q Wave Syndromes
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