Physical Address

304 North Cardinal St.
Dorchester Center, MA 02124

Limitations and Uses of the ECG


We have focused most of our attention on the major clinical uses of the electrocardiogram (ECG). This review and overview chapter (1) underscores some important limitations of the ECG, (2) reemphasizes its utility, and (3) discusses some common pitfalls to help clinicians avoid preventable errors.

Important Limitations of the ECG

The diagnostic accuracy of any test is determined by the percentages of false-positive and false-negative results it generates. 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 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. Prominent precordial voltage may occur in the absence of left ventricular hypertrophy (LVH) (see Chapter 7 ); Q waves may occur as a normal variant and are not necessarily abnormal (see 9, 10 ). 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. Some patients with acute MI may not show diagnostic ST-T changes, and patients with severe coronary artery disease may not show ST segment depressions during stress testing (see 9, 10 ). Furthermore, although the ECG may be strongly suggestive of ventricular hypertrophy or other chamber enlargement, cardiomyopathy, or pericardial effusion, echocardiogram is a gold standard for these structural abnormalities.

Clinicians need to be aware of these and other major diagnostic limitations. The following are some important problems that cannot be excluded simply because the 12-lead ECG (only 10 sec of data) is normal or shows only nondiagnostic abnormalities:

  • Prior MI

  • Acute MI a

    a The pattern of acute MI (ST elevation or non-ST elevation) may also be masked in patients with left bundle branch block (LBBB), Wolff–Parkinson–White (WPW) preexcitation patterns, or ventricular pacing.

  • Severe coronary artery disease

  • LVH

  • Right ventricular hypertrophy (RVH)

  • Intermittent major arrhythmias such as paroxysmal atrial fibrillation (AF), paroxysmal supraventricular tachycardia (PSVT), ventricular tachycardia (VT), and bradycardias, including complete atrioventricular (AV) block

  • Acute pulmonary embolism or chronic pulmonary thromboembolic disease

  • Pericarditis, acute or chronic

  • Arrhythmogenic right ventricular cardiomyopathy

  • Hypertrophic cardiomyopathy

  • Wolff–Parkinson–White (WPW) pattern (may be intermittent)

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 is an essential aid in the evaluation and management of general medical problems such as life-threatening electrolyte disorders ( Box 24.1 ). Some particular areas in which the ECG may be helpful are described here.

Box 24.1
ECG as a Clue to Acute Life-Threatening Conditions without Structural Heart or Lung Disease

  • Electrolyte disorders: especially hyperkalemia, hypokalemia, hypercalcemia, and hypocalcemia

  • Noncardiac drug toxicities: tricyclic antidepressants; multiple agents causing QT prolongation

  • Cerebrovascular catastrophes (especially subarachnoid hemorrhage and other intracranial bleeds)

  • Thyroid disorders: hypo- and hyperthyroidism

  • Hypothermia

Myocardial Infarction (MI)

The ECG is pivotal in diagnosing ST elevation MI (STEMI). However, in the weeks and months after an acute MI, these changes may become less apparent and in some cases may even disappear.

ST segment elevation in right chest precordial leads (e.g., V 3 R to V 6 R and occasionally V 1 and V 2 ) in a patient with acute inferior infarction points to associated right ventricular ischemia or infarction (see Chapter 9 ).

Persistent ST elevations several weeks after a Q-wave MI should suggest a ventricular aneurysm.

The pattern of acute STEMI can be exactly mimicked by takotsubo (stress) cardiomyopathy and other conditions ( Chapter 9 ).

Acute Pulmonary Embolism

A new S 1 Q 3 T 3 pattern or right bundle branch block (RBBB) pattern, particularly in association with sinus tachycardia and tall P waves, should suggest the possibility of acute right heart overload (acute cor pulmonale) resulting from acute pulmonary embolism (see Chapter 12 ). However, this constellation of findings is not specific and may occur with other causes, including acute pneumonitis or severe asthma. Also the sensitivity of the ECG is limited in pulmonary embolism—acute or chronic. The ECG may only show sinus tachycardia or be entirely unremarkable.

Pericardial Tamponade

Low QRS voltage in a patient with elevated central venous pressure (distended neck veins) and sinus tachycardia suggests possible pericardial tamponade. The triad of sinus tachycardia with electrical alternans and relatively low voltage is virtually diagnostic of pericardial effusion with tamponade (see Chapter 12 ).

Aortic Valve Disease

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

You're Reading a Preview

Become a Clinical Tree membership for Full access and enjoy Unlimited articles

Become membership

If you are a member. Log in here

Related Posts