How do you interpret an electrocardiogram (ECG)?

How to Read an ECG: A Systematic Approach

Physician overreading and confirmation of computer-generated ECG interpretations is essential, as computer interpretation alone is not recognized as a properly interpreted ECG. 1, 2

Step 1: Assess Technical Quality and Rate

Begin by evaluating the recording quality and identifying any technical artifacts that may affect interpretation. 3

Calculate the heart rate using one of two methods: 2, 3

• Count the number of large boxes between consecutive R waves and divide 300 by this number
• Count QRS complexes in a 6-second strip and multiply by 10
• Normal rate: 60-100 beats per minute in adults 2

Step 2: Identify the Rhythm

Determine if the rhythm is regular or irregular by examining R-R interval consistency. 3

Identify the underlying rhythm by assessing: 3

• P wave presence before each QRS complex
• Consistent PR interval (indicates sinus rhythm)
• P wave morphology (normal: upright in I, II, aVF; biphasic in V1) 3

Note any irregularities such as premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation. 2

Step 3: Measure Critical Intervals

PR Interval (normal: 120-200 ms or 3-5 small squares): Assess AV conduction 2, 3

QRS Duration (normal: <120 ms or <3 small squares): Evaluate ventricular conduction 2, 3

QT Interval: Calculate QTc using Bazett's formula 2, 3

• Normal: <450 ms for men, <460 ms for women
• Prolonged QTc with enhanced transmural dispersion of repolarization predisposes to torsade de pointes 4

Step 4: Determine the Electrical Axis

Use leads I and aVF to quickly determine the axis quadrant: 2, 3

• Normal axis (-30° to +90°): 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

Axis deviation serves as an independent marker of myocardial disease and reflects anatomic alterations in the heart's electrical conduction system. 3

Step 5: Analyze Waveform Morphology

P Waves: 3

• Duration: <120 ms
• Amplitude: <2.5 mm
• Assess for atrial enlargement patterns 2

QRS Complexes: 3

• Look for pathological Q waves (>0.04 seconds or >25% of R wave amplitude) suggesting myocardial infarction
• Assess R wave progression across precordial leads (normally increases V1 to V4, then decreases toward V6)

T Waves: 3

• Normal: upright in leads I, II, V3-V6; inverted in aVR
• Inversion, hyperacute changes, or flattening may indicate ischemia 2

U Waves: 3

• May indicate hypokalemia, bradycardia, or other conditions
• Pathological U waves result from electrical interaction among ventricular layers during phase 3 repolarization 4

Step 6: Evaluate for Chamber Enlargement

Left Ventricular Hypertrophy: 2, 3

• Sokolow-Lyon criterion: S in V1 + R in V5 or V6 >3.5 mV
• Cornell voltage: S in V3 + R in aVL
• Note that QRS voltage criteria decline with age and vary by population 3

Atrial Enlargement: Look for characteristic P wave abnormalities 3

Step 7: Assess for Ischemia, Injury, and Infarction

ST Segment Analysis (measured at J point): 2, 3, 5

• ST elevation: >0.1 mV in limb leads or >0.15-0.2 mV in precordial leads indicates acute injury
• ST depression: In leads other than V1-V3 indicates subendocardial ischemia
• ST elevation with active symptoms usually indicates acute occlusion requiring emergent reperfusion 5

Identify the coronary territory affected by noting the location of abnormalities. 2

Critical Technical Considerations

Avoid these common pitfalls: 2, 3

• Electrode misplacement, particularly precordial leads, can significantly alter interpretation and lead to false diagnoses
• Inadequate filtering (minimum 150 Hz for adults, 250 Hz for children) results in systematic underestimation of signal amplitude and smoothing of important features like Q waves 2, 3
• Over-reliance on computer interpretation without physician verification—errors remain common 2, 3
• Failure to compare with previous ECGs when available can miss important changes 2
• Interpreting ECG findings in isolation without clinical context may lead to inappropriate management decisions 2

Essential Clinical Integration

Always interpret the ECG in the context of the patient's clinical presentation, as the same finding may have different implications depending on symptoms. 3, 5 The ECG should be read along with clinical assessment, and subsequent ECGs should be obtained if there are changes in symptom quality or severity. 5

Consider special populations: 3

• Age and gender affect normal parameters (QT intervals typically longer in women)
• Athletes may have normal variants like sinus bradycardia (≥30 beats/min) 2
• Children often have inverted T waves in V1, V2, and V3 after 1 month of age 2
• Certain medications (antiarrhythmics, psychotropics) can affect ECG findings 3

Maintaining Competency

Continuing medical education through seminars or self-assessment programs is essential to keep ECG interpretation skills current, especially for physicians who read ECGs infrequently. 2, 3 Interpretation varies greatly even among expert electrocardiographers, underscoring the need for ongoing education. 2