How do I interpret an electrocardiogram (EKG)?

How to Interpret an EKG: A Systematic Approach

Use a systematic, stepwise method to interpret every EKG, 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, 2

Step 1: Verify Technical Quality First

Before interpreting any EKG, check for technical problems that could lead you astray:

• Confirm proper electrode placement, particularly the precordial leads, as misplacement significantly alters findings and causes false diagnoses 2, 3
• Ensure adequate filtering settings (minimum 150 Hz high-frequency response for adults) to maintain diagnostic precision 2, 3
• Look for artifacts from muscle tremor, electrical interference, or patient movement that could mimic pathology 4

Step 2: Calculate Heart Rate

• Count the number of 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 1, 2, 3
• Normal sinus rhythm: 60-100 bpm; bradycardia <60 bpm; tachycardia >100 bpm 2, 3

Step 3: Identify the Rhythm

• Confirm sinus rhythm by finding a P wave before each QRS complex with a consistent PR interval 1, 2, 3
• Examine R-R intervals for regularity—consistent spacing indicates regular rhythm 1
• Note any irregularities: premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation 2, 3

Step 4: Measure Intervals

• PR interval (normal: 120-200 ms or 3-5 small squares) to assess AV conduction 1, 2, 3
• QRS duration (normal: <120 ms or <3 small squares) to identify ventricular conduction delays or bundle branch blocks 1, 2, 3
• QTc (corrected QT interval) using Bazett's formula: normal <450 ms for men, <460 ms for women 1, 2, 3

Step 5: Determine the Electrical Axis

• Examine leads I and aVF to quickly determine the quadrant 1, 2, 3:
  • Normal axis: both leads I and aVF positive (+90° to -30°) 1, 2, 3
  • Left axis deviation: lead I positive, aVF negative (-30° to -90°) 1, 2, 3
  • Right axis deviation: lead I negative, aVF positive (+90° to +180°) 2, 3
  • Extreme axis deviation: both negative (+180° to -90°) 2

Step 6: Analyze Waveform Morphology

P Waves

• Normal P waves are upright in leads I, II, aVF and biphasic in V1 1
• Duration should be <120 ms and amplitude <2.5 mm 1

QRS Complexes

• Look for pathological Q waves (>0.04 seconds or >25% of R wave amplitude) suggesting prior myocardial infarction 1, 2, 3
• Assess R wave progression across precordial leads—amplitude should increase from V1 to V4, then decrease toward V6 1
• Check voltage criteria for left ventricular hypertrophy: S in V1 + R in V5 or V6 >3.5 mV (Sokolow-Lyon criterion) 2

ST Segments

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

T Waves

• Normal T waves are upright in leads I, II, V3-V6, inverted in aVR, and variable in III, aVL, aVF, V1, V2 1
• Look for inversion, hyperacute changes, or flattening that may indicate ischemia 2, 3

Step 7: Identify the Location of Abnormalities

When you find ST changes or Q waves, determine the affected coronary territory 2, 3:

• Inferior: leads II, III, aVF (right coronary artery)
• Lateral: leads I, aVL, V5-V6 (circumflex artery)
• Anterior: leads V1-V4 (left anterior descending artery)

Critical Pitfalls to Avoid

• Never rely solely on computer interpretation—errors remain common, particularly with arrhythmias, and computers should only serve as adjuncts to physician interpretation 4, 2, 3
• Always interpret the EKG in clinical context—the same finding has different implications depending on symptoms 4, 3, 5, 6
• Compare with previous EKGs when available to identify dynamic changes 2, 3
• Recognize confounding patterns like left bundle branch block, ventricular paced rhythm, and left ventricular hypertrophy that reduce the EKG's ability to detect acute ischemic changes 7
• Remember that interpretation varies even among expert electrocardiographers, so maintain humility and seek consultation when uncertain 4

Special Considerations

• Age and gender affect normal parameters—QT intervals are typically longer in women, and voltage criteria decline with age 1, 2
• Certain medications alter EKG findings, including antiarrhythmics and psychotropics 1
• Athletes may have normal variants such as sinus bradycardia ≥30 bpm 2

Maintaining Your Skills

• Read at least 100 EKGs yearly to maintain competency, as recommended by the ACC/AHA 4
• Engage in continuing medical education through seminars or self-assessment programs, especially if you read EKGs infrequently 4, 3
• Residency training with Advanced Cardiac Life Support certification provides sufficient foundation for bedside interpretation in routine and emergency situations 4, 3