How do you interpret an electrocardiogram (EKG)?

How to Read an EKG: A Systematic Approach

A systematic approach to EKG interpretation is essential for accurate diagnosis of cardiac conditions, with proper analysis following a structured methodology to avoid missing critical information.

Basic Principles of EKG Interpretation

The electrocardiogram (EKG) is a fundamental diagnostic tool that provides critical information about cardiac electrical activity. A systematic approach ensures comprehensive evaluation and accurate diagnosis.

Step 1: Rate and Rhythm Assessment

• Calculate the heart rate by counting the number of QRS complexes in a 6-second strip and multiplying by 10, or by using the formula 300 divided by the number of large boxes between consecutive R waves 1
• Identify the underlying rhythm - sinus rhythm is characterized by a P wave before each QRS complex with consistent PR interval 1
• Normal sinus rhythm has a rate between 60-100 beats per minute in adults, while sinus bradycardia (<60 bpm) and sinus tachycardia (>100 bpm) may be normal variants in athletes or during physiological stress 1
• Note any irregularities in rhythm, such as premature beats, pauses, or completely irregular patterns that might suggest atrial fibrillation 1

Step 2: Intervals and Conduction Analysis

• Measure the PR interval (normal: 120-200 ms) to assess AV conduction 1
• Evaluate QRS duration (normal: <120 ms) to identify ventricular conduction delays 1
• Calculate the QT interval corrected for heart rate (QTc) using Bazett's formula, with normal values being <450 ms for men and <460 ms for women 1
• Identify any conduction abnormalities such as AV blocks, bundle branch blocks, or pre-excitation 1

Step 3: Axis Determination

• Determine the electrical axis by examining leads I and aVF:
  • Normal axis: +90° to -30° (positive in both leads I and aVF)
  • Left axis deviation: -30° to -90° (positive in lead I, negative in aVF)
  • Right axis deviation: +90° to +180° (negative in lead I, positive in aVF)
  • Extreme axis deviation: +180° to -90° (negative in both leads I and aVF) 1

Step 4: Chamber Enlargement Assessment

• Look for voltage criteria for ventricular hypertrophy:
  • Left ventricular hypertrophy: S in V1 + R in V5 or V6 >3.5 mV
  • Right ventricular hypertrophy: R in V1 + S in V5 or V6 >1.1 mV 1
• Assess for atrial enlargement patterns 1

Step 5: Ischemia, Injury, and Infarction Evaluation

• Examine for ST-segment deviations:
  • ST elevation (>0.1 mV in limb leads or >0.15-0.2 mV in precordial leads) may indicate acute injury 1
  • ST depression (≥0.05 mV) may suggest ischemia 1
• Look for T-wave abnormalities (inversion, hyperacute, flattening) 1
• Identify pathological Q waves (>0.04 seconds or >25% of the R wave amplitude) suggesting myocardial infarction 1
• Note the location of abnormalities to determine the affected coronary territory 1

Special Considerations

Normal Variants vs. Pathological Findings

• Some findings may be normal variants, especially in athletes:
  • Sinus bradycardia (≥30 beats/min)
  • Sinus arrhythmia (respiratory variation in heart rate)
  • First-degree AV block (PR interval 200-400 ms)
  • Incomplete right bundle branch block
  • Early repolarization
  • Isolated QRS voltage criteria for ventricular hypertrophy 1

Age and Population-Specific Considerations

• In children older than 1 month, T waves are often inverted in leads V1, V2, and V3 1
• In adolescents (12-20 years), T waves may be slightly inverted in aVF and V1 1
• In Black athletes, J-point elevation with convex ST-segment elevation followed by T-wave inversion in leads V2-V4 may be a normal variant 1
• Juvenile T-wave pattern (T-wave inversion in V2-V3) is normal in athletes under 16 years 1

Common Pitfalls to Avoid

• Misplacement of electrodes, particularly precordial leads, can significantly alter EKG interpretation and lead to false diagnoses 1
• Inadequate filtering settings can distort waveforms and affect measurements 1
• Over-reliance on computer interpretation without physician verification can lead to diagnostic errors, as computer accuracy varies widely (0-94%) depending on the abnormality 1
• Failure to compare with previous EKGs when available can miss important changes 1
• Interpreting EKG findings in isolation without considering clinical context may lead to inappropriate management decisions 1

Practical Approach for Beginners

• The RIRI approach (Rate, Intervals, Rhythm, Ischemia/Infarction) offers a reliable and systematic method for EKG interpretation 2
• Regular practice with varied EKGs is essential for developing proficiency 1
• Formal training significantly improves interpretation accuracy compared to self-learning from books 3
• When in doubt, particularly for 12-lead EKG interpretation, consultation with more experienced clinicians is recommended 3

Remember that while computer analysis can be helpful, it should never replace interpretation by a qualified physician, as errors in computer interpretation remain common 1. Proper EKG interpretation requires a combination of knowledge, skill, and clinical experience to effectively correlate findings with the patient's clinical presentation.