How do you interpret an electrocardiogram (EKG)?

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How to Interpret an EKG

A systematic approach to EKG interpretation is essential for accurate diagnosis, with all EKGs requiring verification by a qualified physician regardless of computer interpretation, as incorrect readings can significantly impact patient morbidity and mortality. 1

Step-by-Step Approach to EKG Interpretation

1. Technical Assessment

  • Verify proper lead placement and recording quality
  • Check calibration (standard: 10 mm/mV, 25 mm/s)
  • Ensure adequate bandwidth (high-frequency cutoff ≥150 Hz for adults/adolescents, ≥250 Hz for children) 1

2. Rate Assessment

  • Count the number of QRS complexes in a 6-second strip and multiply by 10
  • Alternative method: Count the number of large boxes between consecutive R waves and divide 300 by this number
  • Normal sinus rhythm: 60-100 beats/min
  • Sinus bradycardia: <60 beats/min
  • Sinus tachycardia: >100 beats/min 2

3. Rhythm Analysis

  • Determine if the rhythm is regular or irregular
  • Assess P waves: Are they present before each QRS? Are they uniform in morphology?
  • Measure PR interval (normal: 120-200 ms)
  • Look for patterns of irregularity (e.g., grouped beating, progressive lengthening) 2

4. Axis Determination

  • Look at leads I and aVF:
    • Normal axis: Positive in both leads (0° to +90°)
    • Left axis deviation: Positive in I, negative in aVF (-30° to -90°)
    • Right axis deviation: Negative in I, positive in aVF (+90° to +180°)
    • Extreme right axis deviation: Negative in both leads (-90° to -180°) 2

5. Intervals Assessment

  • PR interval: 120-200 ms (measured from beginning of P wave to beginning of QRS)
  • QRS duration: <120 ms (measured from beginning to end of QRS complex)
  • QT interval: Varies with heart rate, use corrected QT (QTc)
    • Normal QTc: <450 ms in men, <460 ms in women
    • Measure in lead II or V5 using the teach-the-tangent method 2

6. Morphology Analysis

  • P waves: Normal duration <120 ms, amplitude <2.5 mm
  • QRS complex: Look for pathological Q waves (>40 ms or >25% of R wave amplitude)
  • ST segment: Assess for elevation or depression (>1 mm in limb leads or >2 mm in precordial leads is significant)
  • T waves: Direction, amplitude, and symmetry 2

7. Hypertrophy Assessment

  • Left ventricular hypertrophy:
    • Sokolow-Lyon criteria: S in V1 + R in V5 or V6 >35 mm
    • Cornell criteria: R in aVL + S in V3 >28 mm (men) or >20 mm (women)
  • Right ventricular hypertrophy: R in V1 >7 mm or R/S ratio in V1 >1 2

8. Ischemia/Infarction Evaluation

  • Look for ST elevation or depression
  • Assess for pathological Q waves
  • T wave inversions
  • Determine location based on lead groups:
    • Anterior: V1-V4
    • Lateral: I, aVL, V5-V6
    • Inferior: II, III, aVF
    • Posterior: Tall R waves and ST depression in V1-V2 2

Normal vs. Abnormal Findings

Normal Findings

  • Sinus rhythm with rate 60-100 bpm
  • Normal PR interval (120-200 ms)
  • Normal QRS duration (<120 ms)
  • Normal QT interval
  • In athletes: sinus bradycardia, early repolarization, increased QRS voltage, and incomplete RBBB may be normal variants 2

Common Abnormal Findings

  • Arrhythmias: Atrial fibrillation, atrial flutter, ventricular tachycardia
  • Conduction abnormalities: AV blocks, bundle branch blocks
  • Ischemia/infarction: ST elevation/depression, pathological Q waves, T wave inversions
  • Chamber enlargement: P mitrale, P pulmonale, ventricular hypertrophy 2

Common Pitfalls in EKG Interpretation

  1. Electrode Misplacement: Especially V1/V2 placed too high can simulate anterior infarction 1
  2. Computer Algorithm Errors: Error rates of 40.7% for acute myocardial infarction and 75% for high-degree AV blocks 1
  3. Failure to Compare with Previous EKGs: Important to detect significant changes 2
  4. Overlooking Clinical Context: EKG findings must be interpreted in light of the patient's clinical situation 1
  5. Misidentification of Normal Variants: Especially in athletes and young individuals 2

Special Considerations

Athletes

Normal variants in athletes include:

  • Sinus bradycardia (≥30 beats/min)
  • Sinus arrhythmia
  • First-degree AV block (PR interval 200-400 ms)
  • Mobitz Type I second-degree AV block
  • Incomplete RBBB
  • Early repolarization
  • Isolated QRS voltage criteria for ventricular hypertrophy 2

Pediatric Patients

  • Higher heart rates are normal
  • Shorter PR intervals
  • Right ventricular dominance in newborns and infants
  • May require adjustment of amplitude criteria when using higher sampling rates 2

By following this systematic approach to EKG interpretation, clinicians can improve accuracy and reduce the risk of missing critical findings that could impact patient outcomes.

References

1
Guideline

ECG Interpretation Guidelines

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2025

2
Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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