How do you interpret an electrocardiogram (ECG)?

How to Read an ECG: A Systematic Approach

Use a standardized, step-by-step method to interpret every ECG, always verifying computer-generated interpretations yourself, as physician overreading is essential for accurate diagnosis. 1, 2

Step 1: Calculate Heart Rate

• Count the number of QRS complexes in a 6-second strip and multiply by 10, or use the formula 300 divided by the number of large boxes between consecutive R waves 2, 3
• Normal heart rate is 60-100 beats per minute in adults; rates below 60 indicate bradycardia and above 100 indicate tachycardia 3

Step 2: Assess Rhythm Regularity

• Examine R-R intervals for consistency to determine if the rhythm is regular or irregular 2
• Identify the underlying rhythm by looking for a P wave before each QRS complex with consistent PR interval, which characterizes normal sinus rhythm 3
• Note any irregularities such as premature beats, pauses, or completely irregular patterns that might suggest atrial fibrillation 3

Step 3: Measure Critical Intervals

• PR interval: Measure from the beginning of the P wave to the beginning of the QRS complex; normal is 120-200 ms (3-5 small squares) to assess AV conduction 2, 3
• QRS duration: Measure the width of the QRS complex; normal is <120 ms (<3 small squares) to evaluate ventricular conduction 2, 3
• QT interval: Calculate and correct for heart rate using Bazett's formula (QTc); normal is <450 ms for men and <460 ms for women to assess repolarization 2, 3

Step 4: Determine Electrical Axis

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

Step 5: Analyze P Wave Morphology

• P waves should be upright in leads I, II, and aVF; biphasic in V1 2
• Normal P wave duration is <120 ms and amplitude <2.5 mm 2
• Abnormal P waves suggest atrial enlargement or conduction abnormalities 2, 3

Step 6: Examine QRS Complex Morphology

• Look for pathologic Q waves (>1 mm wide and >1/3 the height of the R wave or >0.04 seconds), which suggest myocardial infarction 2, 3
• Assess R wave progression across precordial leads V1 to V6; R wave amplitude should increase from V1 to V4, then decrease toward V6 2
• Check for voltage criteria suggesting ventricular hypertrophy: S wave in V1 + R wave in V5 or V6 >3.5 mV indicates left ventricular hypertrophy 3

Step 7: Evaluate ST Segments

• Measure ST segments at the J point (junction between QRS and ST segment) 2
• ST elevation >0.1 mV in limb leads or >0.15-0.2 mV in precordial leads may indicate acute myocardial injury requiring emergent reperfusion therapy 3, 4
• ST depression in leads other than V1-V3 usually indicates subendocardial ischemia 4
• Note the location of ST changes to determine the affected coronary territory 3

Step 8: Analyze T Wave Morphology

• T waves are normally upright in leads I, II, and V3-V6; inverted in aVR; and variable in III, aVL, aVF, V1, and V2 2
• T wave inversions, hyperacute T waves, or flattening may indicate ischemia or other pathology 3
• The T wave represents transmural dispersion of repolarization in the ventricles 5

Step 9: Look for Additional Features

• Check for U waves, which may indicate hypokalemia, bradycardia, or other conditions 2
• Identify any conduction abnormalities such as AV blocks, bundle branch blocks, or pre-excitation patterns 3

Critical Pitfalls to Avoid

• Never rely solely on computer interpretation without physician verification, as errors remain common and physician overreading is required 1, 3
• Avoid electrode misplacement, particularly precordial leads, which can significantly alter interpretation and lead to false diagnoses 3
• Always compare with previous ECGs when available to identify important changes 3
• Interpret in clinical context: The same ECG finding may have different implications depending on the patient's symptoms and presentation 3, 4
• Ensure adequate filtering settings and recording quality, as inadequate high-frequency response (minimum 150 Hz for adults) results in systematic underestimation of signal amplitude and smoothing of important features like Q waves 3

Special Considerations

• Age and gender affect normal parameters: QT intervals are typically longer in women, and T wave inversions in V1-V3 are common in children over 1 month 2, 3
• Certain medications (antiarrhythmics, psychotropics) can affect ECG findings 2
• Athletes may have normal variants including sinus bradycardia ≥30 beats/min 3