How do you interpret an electrocardiogram (EKG) in an adult patient with a potential history of cardiovascular disease?

How to Interpret an EKG

EKG interpretation requires a systematic, stepwise approach that always integrates clinical context, as the same EKG pattern can occur in different pathophysiologic states and lead to serious diagnostic errors when interpreted in isolation. 1

Systematic Interpretation Framework

Step 1: Verify Technical Quality

• Check electrode placement and signal quality before beginning interpretation, as misplaced precordial leads significantly alter findings and cause false diagnoses 2
• Ensure minimum high-frequency response of 150 Hz for adults and 250 Hz for children to maintain diagnostic precision 2
• Identify any baseline wander, electrical interference, or artifacts that may affect interpretation 2
• Never accept computer-generated interpretations without physician verification—automated systems produce frequent errors and are not recognized as properly interpreted EKGs 1, 2

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
• Normal sinus rhythm: 60-100 bpm; bradycardia <60 bpm; tachycardia >100 bpm 2
• Note that sinus bradycardia ≥30 bpm may be a normal variant in athletes 2

Step 3: Identify the Rhythm

• Confirm sinus rhythm by identifying a P wave before each QRS complex with consistent PR interval 2
• The EKG has consistently high sensitivity and specificity for arrhythmias and is the only practical method for recording cardiac rhythm abnormalities 1
• Note any irregularities such as premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation 2

Step 4: Measure Intervals

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

Step 5: Determine Electrical Axis

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

Step 6: Assess for Chamber Enlargement

• Left ventricular hypertrophy: S in V1 + R in V5 or V6 >3.5 mV (Sokolow-Lyon criterion) 2
• Recognize that QRS voltage criteria decline with age and vary by population 2
• Look for atrial enlargement patterns 2

Step 7: Evaluate for Ischemia, Injury, and Infarction

• ST elevation >0.1 mV in limb leads or >0.15-0.2 mV in precordial leads indicates acute injury requiring emergent evaluation 2, 3
• Pathological Q waves (>0.04 seconds or >25% of R wave amplitude) suggest myocardial infarction 2
• Assess T-wave abnormalities including inversion, hyperacute changes, or flattening 2
• Note the location of abnormalities to determine the affected coronary territory 2

Critical Clinical Integration

The EKG must always be interpreted in conjunction with the patient's clinical presentation, as diagnoses of structural or pathophysiologic changes are made by inference and subject to error 1. For example:

• ST segment and T wave changes are the most sensitive but least specific EKG abnormalities 1
• Moderate T wave inversion predicts 21% annual mortality when associated with heart disease versus only 3% without heart disease 1
• Clinical history influences interpretation accuracy: correct history improves accuracy by 4-12%, while misleading history reduces accuracy by 5-25% depending on expertise level 4

Common Pitfalls to Avoid

• Do not equate abnormal T waves solely with ischemia—the specificity of T wave abnormalities for any single cause is low 1
• Do not over-rely on computer interpretation—errors remain common and physician verification is mandatory 1, 2
• Do not fail to compare with previous EKGs when available—this misses important dynamic changes 2, 3
• Do not interpret findings without clinical context—this leads to inappropriate management decisions and serious iatrogenic heart disease 1, 2
• Avoid inadequate filtering settings, which distort waveforms and affect measurements 2

Maintaining Competency

• Interpretation of 800 EKGs within a 3-year training period under experienced faculty supervision is recommended for competence 1
• The EKGs should reflect a wide variety of clinical situations and abnormalities 1
• Continuing medical education through seminars or self-assessment programs is essential for physicians who read EKGs infrequently to maintain current skills 2, 3
• Recognition that interpretation varies greatly even among expert electrocardiographers underscores the need for ongoing education 2