How do I interpret an electrocardiogram (ECG)?

How to Interpret an ECG: A Systematic Approach

Use a systematic, step-by-step method to interpret every ECG, starting with rate and rhythm, then progressing through intervals, axis, and waveform morphology—this structured approach minimizes errors and ensures you don't miss critical findings. 1

Step 1: Calculate Heart Rate and Assess Rhythm

Calculate the heart rate first by either counting QRS complexes in a 6-second strip and multiplying by 10, or by dividing 300 by the number of large boxes between consecutive R waves. 1, 2

• Normal sinus rhythm: 60-100 beats per minute with a P wave before each QRS complex and consistent PR interval 2
• Sinus bradycardia: <60 bpm (may be normal in athletes) 2
• Sinus tachycardia: >100 bpm (may be physiological during stress) 2

Evaluate rhythm regularity by examining R-R intervals for consistency—irregular rhythms may indicate atrial fibrillation, premature beats, or other arrhythmias. 1, 2

Step 2: Measure Critical Intervals

Measure the PR interval (normal: 120-200 ms or 3-5 small squares) to assess AV conduction—prolongation suggests AV block. 1, 2

Measure QRS duration (normal: <120 ms or <3 small squares) to evaluate ventricular conduction—widening indicates bundle branch blocks or ventricular conduction delays. 1, 2

Calculate the QT interval and correct for heart rate (QTc) using Bazett's formula—normal is <450 ms for men and <460 ms for women. Prolonged QTc increases risk of dangerous arrhythmias. 1, 2

Step 3: Determine the Electrical Axis

Examine leads I and aVF to quickly determine the axis quadrant: 1, 2

• Normal axis (-30° to +90°): Positive in both leads I and aVF 1, 2
• Left axis deviation (-30° to -90°): Positive in lead I, negative in aVF 1, 2
• Right axis deviation (+90° to +180°): Negative in lead I, positive in aVF 1, 2
• Extreme axis deviation (+180° to -90°): Negative in both leads I and aVF 2

Step 4: Analyze P Waves

Examine P wave morphology (normal: upright in I, II, aVF; biphasic in V1) to assess atrial conduction. 1

Evaluate P wave duration and amplitude—normal is <120 ms and <2.5 mm. Abnormalities suggest atrial enlargement. 1

Step 5: Analyze QRS Complexes

Look for pathological Q waves (>1 mm wide and >1/3 the height of the R wave or >0.04 seconds)—these suggest prior myocardial infarction. 1, 2

Assess R wave progression across precordial leads—R wave amplitude should increase from V1 to V4, then decrease toward V6. 1

Check for voltage criteria for left ventricular hypertrophy: S wave in V1 + R wave in V5 or V6 >3.5 mV (35 mm). 2

Step 6: Evaluate ST Segments and T Waves

Examine ST segments for elevation or depression measured at the J point: 1, 2

• ST elevation (>0.1 mV in limb leads or >0.15-0.2 mV in precordial leads) with active chest pain indicates acute coronary artery occlusion requiring emergent reperfusion therapy 2, 3, 4
• ST depression in leads other than V1-V3 suggests subendocardial ischemia 3, 4
• ST depression in V1-V3 may represent posterior wall injury 3

Assess T wave morphology—normally upright in leads I, II, V3-V6; inverted in aVR; variable in III, aVL, aVF, V1, and V2. Inversions or hyperacute T waves may indicate ischemia. 1, 2

Step 7: Identify Additional Features

Look for U waves—may indicate hypokalemia, bradycardia, or medication effects. 1

Identify conduction abnormalities including PR prolongation (AV blocks) and QRS widening (bundle branch blocks). 1, 2

Critical Pitfalls to Avoid

Never rely solely on computer interpretation—errors in automated ECG analysis are common (0-94% accuracy depending on the diagnosis), with arrhythmias being the most problematic. Computer interpretations are helpful adjuncts but must always be verified by a qualified physician. 5, 2

Check for electrode misplacement, particularly precordial leads—this significantly alters ECG interpretation and leads to false diagnoses. 2

Always compare with previous ECGs when available—missing important changes is a common error that can delay critical diagnoses. 2

Interpret the ECG in clinical context—the same ECG finding has different implications depending on the patient's symptoms, age, medications, and clinical presentation. Never interpret ECG findings in isolation. 5, 2, 3, 4

Consider normal variants, especially in athletes (sinus bradycardia ≥30 bpm) and children (T wave inversions in V1-V3 after 1 month of age). 2

Account for medications and electrolytes—antiarrhythmics, psychotropics, and electrolyte abnormalities significantly affect ECG findings. 1

Understanding Sensitivity and Specificity

Recognize that the ECG records electrical activity—diagnoses of structural changes (myocardial infarction, hypertrophy) or pathophysiologic changes (electrolyte disturbances, drug effects) are made by inference and are therefore subject to error. Each waveform has its own sensitivity and specificity. 5, 2

Know that subtle changes matter—in acute coronary syndromes, changes may be subtle and only recognized when ECG recording is repeated after changes in symptom severity. 3, 4

Maintaining Competency

Competency requires understanding pathophysiology, recognizing normal and abnormal patterns, identifying technical artifacts, and applying knowledge in bedside clinical decision-making—not just completing a minimum number of interpretations. 5

Continuing medical education through seminars or self-assessment programs is essential to keep ECG interpretation skills current, especially for physicians who read ECGs infrequently. 2