How do I interpret an electrocardiogram (EKG)?

How to Interpret an EKG: A Systematic Approach

Use a systematic, step-by-step method to interpret every EKG, beginning with rate and rhythm, then progressing through intervals, axis, and waveform morphology, always integrating clinical context with your findings. 1, 2

Step 1: Verify Technical Quality First

Before interpreting any EKG, check for technical problems that can lead to misdiagnosis:

• Confirm proper electrode placement, particularly precordial leads, as misplacement significantly alters findings and causes false diagnoses 1, 3
• Ensure adequate filtering settings (minimum 150 Hz high-frequency response for adults) to maintain diagnostic precision 1, 3
• Identify any artifacts from patient movement, electrical interference, or poor electrode contact 1

Step 2: Calculate Heart Rate

Use one of two methods:

• Count QRS complexes in a 6-second strip and multiply by 10 1, 2, 3
• Divide 300 by the number of large boxes between consecutive R waves 1, 2
• Normal: 60-100 bpm; bradycardia <60 bpm; tachycardia >100 bpm 1, 2, 3

Note that sinus bradycardia ≥30 bpm may be a normal variant in athletes 1

Step 3: Identify the Rhythm

• Confirm sinus rhythm by identifying a P wave before each QRS complex with consistent PR interval 1, 2
• Examine R-R intervals for regularity 2
• Note any irregularities: premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation 1, 3
• Assess P wave morphology (normally upright in leads I, II, aVF; biphasic in V1) 2

Step 4: Measure Critical Intervals

PR Interval (normal: 120-200 ms or 3-5 small squares):

• Measures AV conduction 1, 2, 3
• Prolongation suggests AV block 1

QRS Duration (normal: <120 ms or <3 small squares):

• Evaluates ventricular conduction 1, 2, 3
• Widening indicates bundle branch block or ventricular conduction delay 1, 3

QT Interval (corrected for heart rate):

• Calculate QTc using Bazett's formula 1, 2, 3
• Normal: <450 ms for men, <460 ms for women 1, 2
• Prolongation increases risk of arrhythmias 2

Step 5: Determine Electrical Axis

Use leads I and aVF for rapid quadrant determination: 2, 3

• Normal axis (-30° to +90°): Both leads I and aVF positive 1, 2, 3
• Left axis deviation (-30° to -90°): Lead I positive, aVF negative 1, 2, 3
• Right axis deviation (+90° to +180°): Lead I negative, aVF positive 1, 2, 3
• Extreme axis deviation (+180° to -90°): Both leads I and aVF negative 1

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

Step 6: Analyze Waveform Morphology

P Waves:

• Duration should be <120 ms and amplitude <2.5 mm 2
• Abnormalities suggest atrial enlargement 1, 2

QRS Complexes:

• Look for pathological Q waves (>0.04 seconds or >25% of R wave amplitude) indicating myocardial infarction 2, 3
• Assess R wave progression across precordial leads (normally increases V1 to V4, then decreases toward V6) 2
• Check voltage criteria for left ventricular hypertrophy: S in V1 + R in V5 or V6 >3.5 mV 1

ST Segments:

• ST elevation >0.1 mV in limb leads or >0.15-0.2 mV in precordial leads indicates acute injury requiring emergent reperfusion 1, 3
• ST depression may indicate ischemia 2
• Measure at the J point 2

T Waves:

• Normally upright in leads I, II, V3-V6; inverted in aVR 2
• Inversion, hyperacute changes, or flattening suggest ischemia or other pathology 1, 3
• In children >1 month, T waves are often inverted in V1, V2, and V3 1

Step 7: Identify Location of Abnormalities

When ST changes or Q waves are present, determine the affected coronary territory to guide catheterization lab activation decisions 1, 3

Step 8: Integrate Clinical Context

Never interpret the EKG in isolation—this is a critical pitfall that leads to inappropriate management and adverse outcomes: 1, 3

• The same EKG finding has different implications depending on symptoms 1, 3
• Clinical signs and symptoms alone have insufficient sensitivity (35-38%) and specificity (28-91%) without EKG and biomarkers 3
• Always compare with previous EKGs when available, as missing dynamic changes is a common error 1, 3

Step 9: Verify Computer Interpretation

Computer interpretations are helpful adjuncts but never substitutes for physician interpretation: 1, 2, 3

• Computer programs accurately measure heart rate, intervals, and axes 3
• However, interpretations of rhythm disturbances, ischemia, or infarction require careful physician over-reading 3
• Errors in computer interpretation remain common 1, 3

Critical Pitfalls to Avoid

• Do not over-rely on computer interpretation without physician verification 1, 3
• Do not fail to compare with previous EKGs when available 1, 3
• Do not interpret findings without clinical context 1, 3
• Recognize that the same EKG pattern may occur in different structural and pathophysiologic states, explaining frequent low specificity 1, 2
• Be aware that noncardiologists are more influenced by patient history than cardiologists when interpreting EKGs 1, 3

Maintaining Competency

• Formal EKG training significantly improves interpretation accuracy compared to self-study alone 4
• Continuing medical education through seminars or self-assessment programs is encouraged, especially for physicians who read EKGs infrequently 1, 3
• Residency training with Advanced Cardiac Life Support certification is sufficient for bedside interpretation in routine and emergency situations 3