What are the steps to interpret an electrocardiogram (ECG)?

Systematic Approach to ECG Interpretation

A systematic approach to ECG interpretation should follow specific steps that assess rate, rhythm, intervals, axis, hypertrophy, and ischemic changes to ensure accurate diagnosis and appropriate clinical management. 1

Step 1: Assess Heart Rate and Rhythm

• Calculate heart rate by counting the number of QRS complexes in a 6-second strip and multiplying by 10, or using the formula 300 divided by the number of large boxes between consecutive R waves 1
• Identify the underlying rhythm by confirming if there is a P wave before each QRS complex with consistent PR interval (sinus rhythm) 1
• Note if the rate is normal (60-100 bpm), bradycardic (<60 bpm), or tachycardic (>100 bpm) 1
• Assess rhythm regularity, noting any premature beats, pauses, or completely irregular patterns that might suggest atrial fibrillation 1

Step 2: Evaluate Intervals and Conduction

• 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 (normal: <450 ms for men, <460 ms for women) 1
• Identify any conduction abnormalities such as AV blocks, bundle branch blocks, or pre-excitation 1

Step 3: Determine Electrical Axis

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

Step 4: Assess Chamber Enlargement

• Look for voltage criteria for left ventricular hypertrophy: S in V1 + R in V5 or V6 >3.5 mV 1
• Assess for atrial enlargement patterns 1
• Consider other hypertrophy patterns that may suggest specific cardiomyopathies 2

Step 5: Evaluate for Ischemia, Injury, and Infarction

• 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
• 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

Important Considerations

• Compare with previous ECGs when available to identify significant changes 1
• Consider the clinical context when interpreting findings 3
• Be aware that computer interpretation should be considered a helpful adjunct but not a substitute for physician interpretation 4
• Remember that certain populations may have normal variants (e.g., athletes commonly have sinus bradycardia) 1

Common Pitfalls to Avoid

• Electrode misplacement, particularly precordial leads, which can significantly alter ECG interpretation 1
• Inadequate filtering settings that can distort waveforms and affect measurements 1
• Over-reliance on computer interpretation without physician verification 4, 1
• Failure to compare with previous ECGs when available 1
• Interpreting ECG findings in isolation without considering clinical context 1, 3

By following this systematic approach, clinicians can ensure thorough and accurate interpretation of ECGs, leading to appropriate clinical decision-making and improved patient outcomes.