How do you interpret an electrocardiogram (ECG)?

How to Interpret an ECG: A Systematic Approach

Interpret every ECG using a standardized, sequential method that evaluates rate, rhythm, axis, intervals, and waveform morphology—this systematic approach is essential for accurate diagnosis and must always be performed by a physician, as computer interpretations require verification and cannot substitute for clinical judgment. 1, 2

Step 1: Verify Technical Quality First

Before beginning interpretation, confirm the ECG is technically adequate:

• Check electrode placement and filtering settings, as misplaced precordial leads can significantly alter findings and lead to false diagnoses 3, 4
• Ensure minimum high-frequency response of 150 Hz for adults to maintain diagnostic precision—inadequate frequency response causes systematic underestimation of signal amplitude and smoothing of critical features like Q waves 3
• Identify any technical artifacts (muscle tremor, baseline wander, electrical interference) that may affect interpretation 3

Step 2: Calculate Heart Rate

• Count QRS complexes in a 6-second strip and multiply by 10, or use 300 divided by the number of large boxes between consecutive R waves 2, 3, 4
• Normal sinus rhythm: 60-100 bpm; bradycardia <60 bpm; tachycardia >100 bpm 3, 4

Step 3: Identify the Rhythm

• Confirm sinus rhythm by identifying a P wave before each QRS complex with consistent PR interval 2, 3, 4
• Evaluate R-R interval regularity by examining consistency between consecutive QRS complexes 2
• Note any irregularities such as premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation 3, 4

Step 4: Measure Critical Intervals

• PR interval (normal: 120-200 ms or 3-5 small squares) to assess AV conduction 2, 3, 4
• QRS duration (normal: <120 ms or <3 small squares) to identify ventricular conduction delays or bundle branch blocks 2, 3, 4
• QT interval corrected for heart rate (QTc) using Bazett's formula—normal <450 ms for men, <460 ms for women 2, 3, 4

Step 5: Determine Electrical Axis

• Examine leads I and aVF to quickly determine the quadrant: normal axis when both are positive (+90° to -30°) 2, 3, 4
• Left axis deviation: lead I positive, aVF negative (-30° to -90°) 2, 3
• Right axis deviation: lead I negative, aVF positive (+90° to +180°) 2, 3
• Extreme axis deviation: both leads I and aVF negative (+180° to -90°) 3

The cardiac axis provides essential information about chamber enlargement, myocardial hypertrophy, and conduction defects that directly impact patient morbidity and mortality 2

Step 6: Analyze Waveform Morphology

P Waves

• Examine P wave morphology: normal P waves are upright in leads I, II, aVF and biphasic in V1 2
• Measure P wave duration and amplitude: normal <120 ms and <2.5 mm 2
• Abnormalities suggest atrial enlargement 3

QRS Complexes

• Assess R wave progression across precordial leads: R wave amplitude should increase from V1 to V4, then decrease toward V6 2
• Identify pathological Q waves (>1 mm wide and >1/3 the height of the R wave, or >0.04 seconds or >25% of R wave amplitude) suggesting myocardial infarction 2, 3, 4
• Apply voltage criteria for left ventricular hypertrophy: Sokolow-Lyon criterion (S in V1 + R in V5 or V6 >3.5 mV) or Cornell voltage (S in V3 + R in aVL) 3

ST Segments

• Examine ST segments for elevation or depression measured at the J point 2, 3
• ST elevation >0.1 mV in limb leads or >0.15-0.2 mV in precordial leads may indicate acute injury requiring emergent intervention 3, 4, 5
• ST depression in leads other than V1-V3 usually indicates subendocardial ischemia 5

T Waves

• Assess T wave morphology: normally upright in leads I, II, V3-V6, inverted in aVR, and variable in III, aVL, aVF, V1, and V2 2
• Look for T wave abnormalities including inversion, hyperacute changes, or flattening that may indicate ischemia 3, 4

Additional Features

• Look for U waves, which may indicate hypokalemia, bradycardia, or other conditions 2

Step 7: Localize Abnormalities

Note the location of ST-segment or T-wave abnormalities to determine the affected coronary territory, which is critical for catheterization lab activation decisions in acute settings 3, 4, 5

Critical Clinical Integration

Never interpret the ECG in isolation—always integrate with clinical presentation, as the same ECG pattern may occur in different structural and pathophysiologic states, explaining the frequent low specificity for determining disease etiology 2, 3, 4

• In emergency settings with chest pain, the ECG has 76% sensitivity and 88% specificity for acute cardiac ischemia, and 68% sensitivity with 97% specificity for acute MI 4
• Clinical signs and symptoms alone lack sufficient sensitivity (35-38%) and specificity (28-91%) to rule in or rule out acute coronary syndrome without ECG and biomarkers 4

Computer Interpretation: Use With Caution

Computer-generated interpretations must always be verified by a qualified physician—they are helpful adjuncts but not substitutes for physician interpretation 1, 2, 3, 4

• Computer programs provide accurate heart rate, intervals, and axes, but interpretations of rhythm disturbances, ischemia, or infarction require careful physician over-reading 4
• Measurements made by different automated ECG systems from reference data can vary enough to alter diagnostic interpretation 1

Special Considerations

• Age and gender affect normal ECG parameters: QT intervals are typically longer in women, and QRS voltage criteria decline with age 2, 3
• Certain medications can affect ECG findings, particularly antiarrhythmics and psychotropics 2
• Consider normal variants in athletes, such as sinus bradycardia ≥30 bpm 3
• In children older than 1 month, T waves are often inverted in leads V1, V2, and V3 3

Common Pitfalls to Avoid

• Do not over-rely on computer interpretation without physician verification, as errors remain common 3, 4
• Do not fail to compare with previous ECGs when available, as this can miss important dynamic changes 3, 4
• Do not interpret findings without clinical context, which may lead to inappropriate management decisions and adverse outcomes 3, 4
• Recognize that interpretation varies greatly even among expert electrocardiographers, underscoring the need for ongoing education 3

Maintaining Competency

Continuing medical education through seminars or self-assessment programs is essential to keep ECG interpretation skills current, especially for physicians who read ECGs infrequently 3, 4