Limitations and Uses of the ECG

Chapter 23 Limitations and Uses of the ECG


Throughout this book the clinical uses of the ECG have been stressed. This review chapter underscores some important limitations of the ECG, reemphasizes its utility, and discusses some common pitfalls in its interpretation 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. 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:



• Prior MI


• Acute MI∗


• Severe coronary artery disease


• Significant LVH


• Significant right ventricular hypertrophy (RVH)


• Intermittent arrhythmias such as paroxysmal atrial fibrillation (AF), paroxysmal supraventricular tachycardia (PSVT), ventricular tachycardia (VT), and bradycardias


• Acute pulmonary embolism


• Pericarditis, acute or chronic



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:



Myocardial Infarction



BOX 23-1 ECG as a Clue to Acute Life-Threatening Conditions without Primary Heart or Lung Disease




• Cerebrovascular accident (especially intracranial bleed)


• Drug toxicity


• Tricyclic antidepressant overdose, digitalis excess, etc.

• Electrolyte disorders


• Hypokalemia

• Hyperkalemia

• Hypocalcemia

• Hypercalcemia

• Endocrine disorders


• Hypothyroidism

• Hyperthyroidism

• Hypothermia


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., V4-6R) 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.



Pulmonary Embolism


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).



Pericardial Tamponade


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).



Aortic Valve Disease


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



Mitral Valve Disease


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


image

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

Related posts:

  1. Digitalis Toxicity
  2. Electrical Axis and Axis Deviation
  3. Atrioventricular Conduction Abnormalities: Delays, Blocks, and Dissociation Syndromes
  4. Ventricular Arrhythmias

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Jun 11, 2016 | Posted by in CARDIOLOGY | Comments Off on Limitations and Uses of the ECG

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