What is the step‑by‑step method for interpreting a 12‑lead ECG (electrocardiogram) in an adult patient?

Understanding 12-Lead ECG Interpretation

A systematic, stepwise approach to 12-lead ECG interpretation should follow a structured sequence: assess rate and rhythm, measure intervals (PR, QRS, QT), determine axis, evaluate for chamber enlargement/hypertrophy, identify conduction abnormalities, and detect ischemia or infarction patterns. 1

Core Principles of ECG Signal Interpretation

The 12-lead ECG records potential differences between prescribed body surface sites during the cardiac cycle, reflecting transmembrane voltage changes in myocardial cells during depolarization and repolarization. 1 Understanding that the standard 12-lead ECG actually contains only 8 independent pieces of information is crucial: 2 measured potential differences from which 4 limb leads are mathematically derived, plus 6 independent precordial leads. 1

Lead Organization and Spatial Relationships

• Frontal plane leads (I, II, III, aVR, aVL, aVF) provide views of cardiac electrical activity in the vertical plane, though only 2 are truly independent measurements. 1
• Precordial leads (V1-V6) each provide uniquely measured, independent information about the horizontal plane. 1
• The augmented limb leads are mathematically derived and should not be described as "unipolar"—they are redundant but retained because multiple views facilitate clinical interpretation. 1

Systematic Interpretation Algorithm

Step 1: Rhythm and Rate Assessment

First, classify whether the ventricular rate is regular or irregular. 1

• Irregular ventricular rate suggests atrial fibrillation, multifocal atrial tachycardia, or atrial flutter with variable AV conduction. 1
• Regular rhythm may represent atrial tachycardia with 1:1 conduction or SVT involving the AV node. 1
• When AF presents with rapid ventricular response, irregularity becomes less detectable and can be misdiagnosed as regular SVT—a critical pitfall. 1

Step 2: Interval Measurements

Measure PR, QRS, and QT intervals systematically using standardized methodology. 1

The automated ECG processing follows these steps: 1

• Signal acquisition with appropriate filtering
• Data transformation (finding complexes, classifying as dominant vs. ectopic)
• Waveform recognition (identifying onset and offset of diagnostic waves)
• Feature extraction (measuring amplitudes and intervals)
• Diagnostic classification

Critical technical consideration: Inappropriate filtering distorts measurements—low-pass filtering <250 Hz reduces R-wave amplitude used to estimate ventricular mass, while high-pass filtering >0.05 Hz can introduce artifactual ST-segment deviation. 1

Step 3: Axis Determination

Evaluate the cardiac axis in the frontal plane using leads I and aVF, recognizing that lead vectors have both direction and strength dependent on body geometry and tissue impedances. 1

Step 4: Wide vs. Narrow Complex Differentiation

For wide-complex tachycardia (QRS >120 ms), immediately distinguish ventricular tachycardia from SVT with aberrant conduction. 1

Diagnostic features of VT (life-threatening if misdiagnosed): 1

• AV dissociation with ventricular rate exceeding atrial rate (diagnostic)
• Fusion complexes (diagnostic)
• Concordance of precordial QRS complexes (all positive or negative)
• Apply Brugada criteria examining precordial QRS morphology
• Apply Vereckei algorithm examining lead aVR

Conduction abnormalities causing wide QRS: 1

• Rate-related aberrant conduction
• Pre-existing bundle branch block
• Accessory pathway with pre-excitation

Step 5: P-Wave Analysis in Tachycardia

When atrial rate exceeds ventricular rate, atrial flutter or atrial tachycardia is present. 1

For regular SVT with 1:1 AV relationship, assess RP interval: 1

• Short RP (P wave closer to prior QRS): typical AVNRT or orthodromic AVRT
  • AVNRT: pseudo S-wave in inferior leads, pseudo R' in V1 at QRS terminus
  • AVRT: P wave visible in early ST-T segment
• Long RP (P wave closer to subsequent QRS): atypical AVNRT or permanent junctional reciprocating tachycardia

Step 6: Chamber Enlargement and Hypertrophy

Systematically evaluate for ventricular hypertrophy and atrial enlargement using standardized voltage and morphology criteria, recognizing that precordial lead misplacement (especially V1-V2 in 2nd vs. 4th intercostal space) distorts R-wave progression and confuses diagnostic criteria. 1

Step 7: Ischemia and Infarction Detection

Compare tachycardia ECG with sinus rhythm ECG—QRS complexes identical to sinus rhythm are consistent with SVT rather than VT. 1

Evaluate ST-segment and T-wave changes, recognizing that serial ECG changes should be assessed using one consistent 12-lead method to avoid interpretation errors from methodological variation. 2

Critical Technical Pitfalls to Avoid

Electrode misplacement is a major source of error: 1

• Inadvertent lead reversals
• V1-V2 placement in 2nd rather than 4th intercostal space
• V5-V6 placement below horizontal extension of V4
• Results in distorted R-wave progression simulating anteroseptal infarction

Simultaneous lead acquisition is essential for precise temporal alignment of waveforms from different leads, providing spatial-temporal insights with diagnostic value. 1 Modern digital electrocardiographs record 8 independent channels simultaneously, with alignment precision within 10 ms. 1

Physician overreading of computer-based ECG interpretations remains mandatory despite improving sensitivity and specificity of automated systems. 1