Important Limitations of the ECG

The diagnostic accuracy of any test is determined by the percentages of false-positive and false-negative results. The sensitivity of a test is a measure of the percentage of patients with a particular abnormality that can be identified by an abnormal test result. For example, a test with 100% sensitivity has no false-negative results. The more false-negative results, the less sensitive is the test. The specificity of a test is a measure of the percentage of false-positive results. The more false-positive test results, the less specific is the test.

Like most clinical tests the ECG yields both false-positive and false-negative results, as previously defined. A false-positive result is exemplified by an apparently abnormal ECG in a normal subject. For example, prominent precordial voltage may occur in the absence of left ventricular hypertrophy (LVH) (see Chapter 6). Furthermore, Q waves may occur as a normal variant and therefore do not always indicate heart disease (see Chapters 8 and 9). In other cases, Q waves may be abnormal (e.g., due to hypertrophic cardiomyopathy) but lead to a mistaken diagnosis of myocardial infarction (MI).

False-negative results, on the other hand, occur when the ECG fails to show evidence of some cardiac abnormality. For example, some patients with acute MI may not show diagnostic ST-T changes, and patients with severe coronary artery disease may not show diagnostic ST segment depressions during stress testing (see Chapter 9).

As just noted, both the sensitivity and specificity of the ECG in diagnosing a variety of conditions, including MI, are limited. Clinicians need to be aware of these diagnostic limitations. The following are some important problems that cannot be excluded simply because the ECG is normal or shows only nondiagnostic abnormalities:

Utility of the ECG in Special Settings

Although the ECG has definite limitations, it often helps in the diagnosis of specific cardiac conditions and sometimes aids in the evaluation and management of general medical problems such as life-threatening electrolyte disorders (Box 23-1). Some particular areas in which the ECG may be helpful are described here:

Most patients with acute MI show diagnostic ECG changes (i.e., new Q waves or ST segment elevations, hyperacute T waves, ST depressions, or T wave inversions). However, in the weeks and months after an acute MI these changes may become less apparent and in some cases may disappear.

ST segment elevation in right chest precordial leads (e.g., V_4-6R) in a patient with acute inferior infarction indicates associated right ventricular ischemia or infarction (see Chapter 8).

Persistent ST elevations several weeks after an MI should suggest a ventricular aneurysm.

A new S₁Q₃T₃ pattern or right bundle branch block (RBBB) pattern, particularly in association with sinus tachycardia, should suggest the possibility of acute cor pulmonale resulting from, for example, pulmonary embolism (see Chapter 11).

Low QRS voltage in a patient with elevated central venous pressure (distended neck veins) and sinus tachycardia suggests possible pericardial tamponade. Sinus tachycardia with electrical alternans is virtually diagnostic of pericardial effusion with tamponade (see Chapter 11).

LVH is seen in most patients with severe aortic stenosis or severe aortic regurgitation.

ECG signs of left atrial enlargement (abnormality) with concomitant RVH strongly suggest mitral stenosis (Fig. 23-1).

Figure 23-1 This ECG from a 45-year-old woman with severe mitral stenosis (due to rheumatic heart disease) shows multiple abnormalities. The rhythm is sinus tachycardia. Right axis deviation and a tall R wave in lead V₁ indicate right ventricular hypertrophy. The prominent biphasic P wave in lead V₁

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