What is an Electrocardiogram (EKG)?
An electrocardiogram (EKG or ECG) is a noninvasive recording of the heart's electrical activity displayed as waveforms on a standard 12-lead tracing that must be interpreted by a qualified physician to diagnose cardiac rhythm abnormalities, ischemia, infarction, conduction disturbances, and electrolyte imbalances. 1
Core Definition and Technology
The ECG records electrical signals generated by the heart through electrodes placed on the skin, producing a graphical representation of cardiac electrical activity across multiple leads. 1 A properly interpreted ECG consists of a standard 12-lead recording with or without rhythm strips, analyzed 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 always be verified by a qualified physician, as automated systems can produce significant differences in measurements and diagnostic statements depending on their technical specifications. 1
Primary Clinical Applications
The ECG serves multiple critical diagnostic functions:
It is the only practical noninvasive method for recording and analyzing cardiac rhythm abnormalities, making it indispensable for arrhythmia diagnosis and treatment. 1
It is essential for diagnosing acute coronary syndromes and initiating prompt therapy, serving as the most accurate means of diagnosing intraventricular conduction disturbances. 1
It detects life-threatening drug effects, severe metabolic disturbances, and electrolyte abnormalities (particularly potassium and calcium), often before other clinical signs appear. 1
It identifies genetically mediated electrical or structural cardiac abnormalities and can reveal asymptomatic myocardial infarction, especially in elderly patients. 1
Class I Indications (First-Line Use)
The American College of Cardiology/American Heart Association guidelines establish that ECG is the first choice for:
Patients presenting with chest pain, dizziness, or syncope, as these symptoms may predict sudden death or myocardial infarction. 1
Monitoring patients on antiarrhythmic drugs or medications with potential cardiac effects, where ECG abnormalities may be the first indicator of toxicity. 1
Preoperative assessment of patients undergoing noncardiac surgery in appropriate clinical contexts. 1
How the ECG Works
The ECG measures electrical activity through systematic analysis of specific components:
Heart rate and rhythm are assessed by examining R-R interval consistency and counting QRS complexes. 2
PR interval (normal 120-200 ms) evaluates atrioventricular conduction between atria and ventricles. 2
QRS duration (normal <120 ms) assesses ventricular conduction and identifies bundle branch blocks. 2
QT interval (corrected for heart rate) evaluates ventricular repolarization, with normal values <450 ms for men and <460 ms for women. 2
Electrical axis (normal -30° to +90°) provides information about chamber enlargement and myocardial hypertrophy. 2
P waves, QRS complexes, ST segments, and T waves are analyzed for morphology abnormalities indicating specific pathologies. 2
Critical Limitations and Pitfalls
The ECG records electrical activity only, meaning diagnoses of structural changes (myocardial infarction, hypertrophy) or pathophysiologic changes (electrolyte disturbances, drug effects) are made by inference and are subject to error. 1 This explains the frequent low specificity of ECG for determining the etiology and anatomy of cardiovascular disease. 1
The same ECG pattern may occur in patients with different structural and pathophysiologic states, requiring skilled physician integration of clinical data, comparison with previous tracings, and differential diagnosis to avoid incorrect interpretations with potentially serious medical and legal consequences. 1
Technical Considerations
Each ECG waveform has its own sensitivity and specificity, which may be influenced differently by various pathologic and pathophysiologic factors. 1
Age and gender affect normal ECG parameters, with QT intervals typically longer in women. 2
Electrode placement and filtering can significantly affect ECG appearance and interpretation, requiring attention to technical factors. 2
The standard 12-lead ECG may require additional leads (V7-V9) in certain situations, such as suspected posterior wall involvement or in pediatric patients. 1, 3
Modern ECG Systems
Digital automated ECG machines now dominate clinical practice, equipped with software that measures intervals and amplitudes, provides instantaneous interpretation, and compares tracings to previous recordings. 1 However, different automated systems may have varying technical specifications that result in significant measurement differences, reinforcing the absolute requirement for physician verification. 1