What is an electrocardiogram (EKG) and what is its purpose in diagnosing heart conditions?

What is an Electrocardiogram (EKG) and Its Purpose

An electrocardiogram (EKG/ECG) is a standard 12-lead recording of the heart's electrical activity that must be interpreted by a qualified physician and serves as the only practical, noninvasive method for diagnosing cardiac arrhythmias while also detecting myocardial disease, ischemia, infarction, and life-threatening metabolic or drug-induced abnormalities. 1

Definition and Technical Requirements

A properly performed EKG consists of a standard 12-lead recording with or without rhythm strips or additional leads, interpreted by a physician qualified in ECG interpretation and incorporated into the patient's medical record. 1 Computer-generated interpretations alone are not recognized as properly interpreted ECGs and must be verified by a qualified physician, particularly for rhythm disturbances, ischemia, or infarction. 1

Primary Clinical Purposes

The EKG serves multiple critical diagnostic functions:

Arrhythmia Detection and Management

• The EKG is unequaled as a noninvasive method for diagnosing and treating cardiac arrhythmias, serving as the only practical method for recording and analyzing rhythm abnormalities. 1
• It provides real-time detection of life-threatening arrhythmias requiring immediate intervention. 2

Myocardial Disease Detection

• The EKG serves as an independent marker of myocardial disease and can detect "asymptomatic" myocardial infarction, particularly in elderly patients. 1
• It reflects electrophysiologic, anatomic, metabolic, and hemodynamic alterations in the heart. 1
• Deep symmetrical T-wave inversions in anterior leads may indicate critical proximal left anterior descending artery stenosis (Wellens syndrome), representing a pre-infarction state requiring urgent intervention. 2

Drug Toxicity and Metabolic Disturbances

• Electrocardiographic abnormalities may be the first indicators of life-threatening drug side effects or severe metabolic and electrolyte disturbances. 1, 3
• QT prolongation can signal risk for torsades de pointes and sudden cardiac death. 3

Class I Indications (First-Line Use)

The ACC/AHA designates EKG as the first choice for patients presenting with:

• Chest pain 1
• Dizziness 1, 3
• Syncope 1
• Symptoms predictive of sudden death or myocardial infarction 1

For dizziness specifically, a 12-lead EKG should be obtained in all patients regardless of whether cardiac disease is suspected, as it may reveal asymptomatic but life-threatening conditions. 3

Critical Diagnostic Capabilities

Structural Heart Disease

The EKG can identify:

• Myocardial infarction (acute or prior) through Q waves and ST-segment changes 1
• Ventricular hypertrophy 1, 3
• Cardiomyopathy patterns 2

Conduction Abnormalities

• Atrioventricular blocks 3
• Bundle branch blocks that may progress to complete heart block 3
• Bradycardia and chronotropic incompetence 3

Ischemia and Infarction

• A 12-lead ECG should be acquired and interpreted as soon as possible at first medical contact to facilitate early diagnosis and triage. 2
• Serial ECGs should be performed if the initial ECG is equivocal or doesn't support clinical suspicion of myocardial ischemia. 2
• Extending to leads V7-V9 may identify posterior wall involvement in patients with non-diagnostic initial ECGs. 2

Important Limitations and Pitfalls

Diagnostic Accuracy Considerations

• The EKG records electrical activity, so diagnoses of structural changes (infarction, hypertrophy) or pathophysiologic changes (electrolyte disturbances, drug effects) are made by inference and are subject to error. 1
• Each waveform has its own sensitivity and specificity, influenced differently by various pathologic factors. 1
• The same ECG pattern may occur in patients with different structural and pathophysiologic states, explaining the frequent low specificity for determining etiology and anatomy. 1

Clinical Integration Required

• A skilled physician must integrate clinical data, provide differential diagnosis, and compare with previous tracings before achieving proper diagnostic accuracy. 1
• Incorrect ECG diagnosis can have significant undesirable medical and legal consequences. 1
• Approximately 5% of patients with normal ECGs discharged from emergency departments are ultimately found to have acute MI or unstable angina, emphasizing the need for serial ECGs and clinical correlation. 2

Common Confounding Patterns

• Wellens pattern can be confused with T-wave inversions from cerebrovascular accidents, cardiomyopathy, or other non-ischemic causes. 2
• Pericarditis should be considered as a differential diagnosis for T-wave inversion in patients with chest pain. 2

Prognostic Value

An abnormal initial EKG predicts adverse outcomes and increased all-cause mortality at 1 year in patients presenting with dizziness or near-syncope. 3 The prognostic value of an abnormal EKG outweighs its modest diagnostic yield (approximately 5% specific diagnosis rate), as it identifies high-risk patients requiring urgent specialist evaluation. 3

Cost-Effectiveness

The EKG is relatively inexpensive compared to other cardiovascular procedures, and its low cost justifies use even when serious abnormalities are rare, as it can reveal potentially life-threatening but treatable disorders. 1, 3