What are the basics of electrocardiography (ECG)?

ECG Basics: Understanding the Fundamentals of Electrocardiography

The electrocardiogram (ECG) is the only practical, noninvasive method of recording and analyzing cardiac electrical activity, providing essential information for the diagnosis and therapy of various cardiac disorders, particularly arrhythmias. 1

Definition and Purpose of ECG

An ECG is a standard 12-lead recording that captures the electrical activity of the heart, reflecting differences in transmembrane voltages in myocardial cells during depolarization and repolarization within each cardiac cycle. 1 It serves multiple purposes:

• Acts as an independent marker of myocardial disease
• Reflects electrophysiologic, anatomic, metabolic, and hemodynamic alterations
• Provides essential information for diagnosis and therapy of cardiac disorders
• Serves as the gold standard for non-invasive diagnosis and treatment of arrhythmias 1

Components of the ECG Waveform

The ECG consists of several key components that must be systematically analyzed:

• P wave: Represents atrial depolarization
• PR interval: Measures conduction time from atria to ventricles (normal: 0.12-0.20 seconds)
• QRS complex: Represents ventricular depolarization (normal: ≤0.10 seconds)
• T wave: Marks the final phase of ventricular repolarization and represents transmural dispersion of repolarization
• QT interval: Measures total ventricular depolarization and repolarization time (normal: ≤0.44 seconds in men, ≤0.46 seconds in women)
• U wave: May represent delayed repolarization of the Purkinje system 2, 3

ECG Signal Processing and Analysis

Modern ECG interpretation involves several steps:

1. Signal acquisition and filtering: Capturing electrical signals from the body surface
2. Data transformation: Preparing data for processing, including complex identification
3. Waveform recognition: Identifying onset and offset of diagnostic waves
4. Feature extraction: Measuring amplitudes and intervals
5. Diagnostic classification: Interpreting findings based on established criteria 1

Systematic ECG Interpretation Approach

The American College of Cardiology recommends a systematic analysis that includes:

• Rhythm assessment: Identifying the underlying cardiac rhythm
• Rate calculation: Determining heart rate
• Interval measurement: Analyzing PR, QRS, and QT intervals
• Axis determination: Evaluating the electrical axis
• Wave morphology analysis: Examining the shape and characteristics of P waves, QRS complexes, and T waves 2

Clinical Applications of ECG

ECGs are particularly valuable in the following clinical scenarios:

• Patients presenting with chest pain, dizziness, or syncope
• Symptoms that may predict sudden death or myocardial infarction
• Detection of life-threatening drug side effects
• Identification of severe metabolic or electrolyte disturbances
• Detection of "asymptomatic" myocardial infarction, especially in elderly patients 1

Common Pitfalls in ECG Interpretation

To avoid misinterpretation, be aware of these common errors:

• Interpreting ECGs in isolation without clinical context
• Confusing artifacts with real arrhythmias
• Failing to compare with previous ECGs when available
• Using excessive filtering that distorts diagnostic characteristics
• Incorrectly interpreting normal variants as pathological
• Relying exclusively on automated interpretations without physician verification 2

Importance of Qualified Interpretation

All ECGs require interpretation by a qualified physician because:

• Computer programs can provide accurate information on rate, intervals, and axes, but all computer ECG interpretations must be verified by a qualified physician
• The same ECG pattern may appear in different structural and pathophysiologic states
• Incorrect ECG diagnosis can have significant undesirable medical and legal consequences 1

Understanding ECG Limitations

When interpreting ECGs, remember:

• The ECG is a record of electrical activity, not structural changes
• Diagnoses of structural changes (myocardial infarction, hypertrophy) or pathophysiologic changes (electrolyte disturbance, drug effects) are made by inference
• The sensitivity and specificity of ECG findings vary depending on the clinical question
• While sensitivity and specificity for arrhythmias are consistently high, they vary considerably for myocardial disorders depending on cause, size, and location 1

By understanding these fundamental principles of electrocardiography, clinicians can better utilize this essential diagnostic tool to improve patient outcomes in cardiovascular care.