How to Read an ECG Easily
Use a systematic, step-by-step approach that evaluates rate, rhythm, axis, intervals, and waveform morphology in a consistent sequence to ensure accurate interpretation and avoid missing critical findings. 1
Systematic Approach to ECG Interpretation
The American Heart Association and American College of Cardiology recommend following a structured sequence when analyzing any ECG 2. This methodical approach includes five core steps: signal acquisition assessment, rhythm analysis, interval measurements, axis determination, and waveform morphology evaluation 2.
Step 1: Calculate Heart Rate
- Count the number of large squares (5 mm) between consecutive R waves and divide 300 by this number for a quick rate calculation 1
- Alternatively, count QRS complexes in a 6-second strip (30 large squares at 25 mm/s) and multiply by 10 for irregular rhythms 1
- Normal heart rate ranges from 60-100 beats per minute 1
Step 2: Assess Rhythm Regularity
- Examine R-R intervals across the entire tracing for consistency to determine if the rhythm is regular or irregular 1
- Identify the underlying rhythm by analyzing P wave morphology and its relationship to QRS complexes 1
- Determine if the rhythm is sinus (P waves upright in leads I, II, aVF with consistent PR intervals), atrial, junctional, or ventricular in origin 1
Step 3: Measure Critical Intervals
- PR interval (normal 120-200 ms or 3-5 small squares): Measure from the beginning of the P wave to the beginning of the QRS complex to assess AV conduction 1
- QRS duration (normal <120 ms or <3 small squares): Measure the widest QRS complex to evaluate ventricular conduction 1
- QT interval: Measure from QRS onset to T wave end, then correct for heart rate (QTc normal <450 ms for men, <460 ms for women) 1
Step 4: Determine Electrical Axis
- Use the quick quadrant method by examining leads I and aVF simultaneously 1:
- Normal axis ranges from -30° to +90° 1
Step 5: Analyze Waveform Morphology
P Waves:
- Should be upright in leads I, II, and aVF; biphasic in V1 1
- Duration <120 ms and amplitude <2.5 mm 1
- Abnormalities suggest atrial enlargement or conduction issues 1
QRS Complexes:
- Look for pathologic Q waves (>1 mm wide and >1/3 the height of the R wave), which suggest prior myocardial infarction 1
- Assess R wave progression across precordial leads V1-V6, with amplitude normally increasing from V1 to V4 then decreasing toward V6 1
- Evaluate for signs of ventricular hypertrophy using voltage criteria 1
ST Segments:
- Measure at the J point (junction of QRS and ST segment) for elevation or depression 1
- ST elevation may indicate acute myocardial injury; ST depression suggests ischemia 1
T Waves:
- Normally upright in leads I, II, V3-V6; inverted in aVR; variable in III, aVL, aVF, V1, V2 1
- Examine for inversion, flattening, or peaked morphology 1
Critical Pitfalls to Avoid
- Never rely solely on computerized ECG interpretations—they require verification by a qualified physician as they can generate erroneous information 1, 3
- Consider patient-specific factors: age and gender affect normal parameters (QT intervals are longer in women) 1
- Account for medications that alter ECG findings, particularly antiarrhythmics and psychotropics 1
- Ensure proper electrode placement and filtering, as technical factors significantly affect ECG appearance and can lead to misinterpretation 1, 4
- The ECG records electrical activity, so diagnoses of structural changes (like myocardial infarction or hypertrophy) are made by inference and subject to error 2
Technical Considerations
The standard 12-lead ECG contains only 8 independent pieces of information: 2 measured limb lead potential differences (from which 4 remaining limb leads are mathematically derived) and 6 independent precordial leads 2. Modern digital electrocardiographs record these simultaneously, allowing precise temporal alignment of waveforms with maximum misalignment of no more than 10 ms 2. This simultaneous acquisition enables spatial-temporal insights critical for accurate diagnosis 2.