What is the step-by-step approach to reading an Electrocardiogram (ECG)?

Systematic Approach to ECG Interpretation

Follow a structured, sequential method when reading an ECG, beginning with rate and rhythm assessment, followed by interval measurements, axis determination, and systematic waveform analysis to ensure no critical findings are missed. 1

Step 1: Calculate Heart Rate

• Determine heart rate by counting the number of large squares (5 mm) between consecutive R waves (300 divided by number of large squares), or count QRS complexes in a 6-second strip and multiply by 10 for irregular rhythms 1
• Normal heart rate is 60-100 beats per minute 1

Step 2: Assess Rhythm Regularity

• Examine R-R intervals for consistency by measuring the distance between consecutive R waves across the entire tracing 1
• Identify the underlying rhythm (sinus, atrial, junctional, or ventricular) based on P wave morphology and its relationship to QRS complexes 1

Step 3: Measure Critical Intervals

• PR interval: Measure from the beginning of the P wave to the beginning of the QRS complex (normal 120-200 ms or 3-5 small squares) to assess AV conduction 1
• QRS duration: Measure the width of the QRS complex (normal <120 ms or <3 small squares) to evaluate ventricular conduction 1
• QT interval: Measure from the beginning of the QRS to the end of the T wave and correct for heart rate (QTc normal <450 ms for men, <460 ms for women) 1

Step 4: Determine Electrical Axis

• Examine leads I and aVF to quickly determine the axis quadrant: if both are positive, the axis is normal (0° to +90°); if lead I is positive and aVF is negative, there is left axis deviation (-30° to -90°); if lead I is negative and aVF is positive, there is right axis deviation (+90° to +180°) 1
• Normal axis ranges from -30° to +90° 1

Step 5: Analyze P Wave Morphology

• Evaluate P wave characteristics: should be upright in leads I, II, and aVF; biphasic in V1; duration <120 ms; amplitude <2.5 mm 1
• Abnormal P waves suggest atrial enlargement or abnormal atrial conduction 1

Step 6: Examine QRS Complex Morphology

• Look for pathologic Q waves (>1 mm wide and >1/3 the height of the R wave in the same lead), which suggest prior myocardial infarction 1
• Assess R wave progression across precordial leads (V1-V6), with R wave amplitude normally increasing from V1 to V4, then decreasing toward V6 1
• Evaluate for signs of ventricular hypertrophy using voltage criteria 1

Step 7: Evaluate ST Segments

• Measure ST segment deviation at the J point (junction between QRS and ST segment), looking for elevation or depression that may indicate ischemia, injury, or infarction 1
• ST elevation >1 mm in two contiguous leads suggests acute myocardial injury 1

Step 8: Analyze T Wave Morphology

• T waves should normally be upright in leads I, II, and V3-V6; inverted in aVR; and variable in III, aVL, aVF, V1, and V2 1
• T wave inversions in unexpected leads may indicate ischemia or other pathology 1

Step 9: Look for Additional Features

• Check for U waves (small deflections after T waves), which may indicate hypokalemia or bradycardia 1
• Assess for signs of chamber enlargement using P wave abnormalities and QRS voltage criteria 1
• Identify conduction abnormalities such as bundle branch blocks (QRS >120 ms with specific morphology patterns) 1

Step 10: Verify and Contextualize

• Always verify computerized ECG interpretations, as automated algorithms can produce erroneous information, particularly for rhythm disturbances, ischemia, or infarction 2, 3
• Consider patient-specific factors including age, gender (QT intervals are longer in women), medications (antiarrhythmics, psychotropics can alter findings), and clinical presentation 1
• Compare with previous ECGs to identify new or evolving changes 2

Common Pitfalls to Avoid

• Inadequate filtering can distort high-frequency components (Q waves, notches) and low-frequency components (ST segments), affecting diagnostic accuracy 2
• Poor electrode placement and skin preparation create artifacts that may mimic pathology 4
• Relying solely on computer interpretation without physician over-reading can lead to missed diagnoses with significant medical and legal consequences 2