What is the proper approach to interpreting an electrocardiogram (ECG) and guiding subsequent management?

Systematic Approach to ECG Interpretation

Use a structured, step-by-step method to interpret every ECG, beginning with rate and rhythm assessment, followed by interval measurements, axis determination, and morphologic analysis of waveforms, always correlating findings with the patient's clinical presentation. 1, 2

Step 1: Calculate Heart Rate and Assess Rhythm

• Count QRS complexes in a 6-second strip and multiply by 10, or use the 300-divided-by-large-boxes method (300 divided by the number of large boxes between consecutive R waves) 1, 2
• Normal sinus rhythm is 60-100 beats per minute in adults; sinus bradycardia (<60 bpm) may be normal in athletes, while sinus tachycardia (>100 bpm) can occur with physiological stress 2
• Examine R-R intervals for regularity—consistent intervals indicate regular rhythm, while irregular patterns may suggest atrial fibrillation or other arrhythmias 1, 2
• Identify the underlying rhythm by confirming a P wave precedes each QRS complex with consistent PR interval for sinus rhythm 1, 2

Step 2: Measure Critical Intervals

• PR interval should be 120-200 ms (3-5 small squares) to assess AV conduction; prolongation suggests AV block 1, 2
• QRS duration must be <120 ms (<3 small squares) to confirm normal ventricular conduction; widening indicates bundle branch block or ventricular conduction delay 1, 2
• Calculate QTc using Bazett's formula (normal <450 ms for men, <460 ms for women)—this is critical for identifying prolonged repolarization that increases arrhythmia risk 1, 2, 3
• Measure QT interval manually in lead II or V5 using the teach-the-tangent method for accuracy 3

Step 3: Determine Electrical Axis

• Examine leads I and aVF to quickly determine axis quadrant 1, 2
• Normal axis: both leads I and aVF positive (+90° to -30°) 1, 2
• Left axis deviation: lead I positive, aVF negative (-30° to -90°) 1, 2
• Right axis deviation: lead I negative, aVF positive (+90° to +180°) 1, 2
• Extreme axis deviation: both leads I and aVF negative (+180° to -90°) 2

Step 4: Analyze P Wave Morphology

• P waves should be upright in leads I, II, and aVF, and biphasic in V1 1
• P wave duration should be <120 ms and amplitude <2.5 mm 1
• Abnormal P wave morphology suggests atrial enlargement or abnormal atrial conduction 1, 2

Step 5: Examine QRS Complex Morphology

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

Step 6: Evaluate ST Segments and T Waves

• Measure ST segments at the J point for elevation or depression 1, 2
• ST elevation >0.1 mV in limb leads or >0.15-0.2 mV in precordial leads indicates acute injury 2
• ST depression may indicate ischemia 1, 2
• T waves should be upright in leads I, II, V3-V6, inverted in aVR, and variable in III, aVL, aVF, V1, and V2 1
• T wave inversions, hyperacute T waves, or flattening suggest ischemia or other pathology 2

Step 7: Identify Additional Features

• Look for U waves, which may indicate hypokalemia or bradycardia 1
• Assess for chamber enlargement using P wave abnormalities and QRS voltage criteria 1, 2
• Identify conduction abnormalities including PR prolongation and QRS widening 1

Critical Clinical Integration

Always interpret the ECG in the context of the patient's clinical presentation—the same ECG finding has different implications depending on symptoms 2, 3

• Patients with chest pain, dyspnea, palpitations, or syncope require immediate ECG interpretation 4
• Note the location of ST-T wave abnormalities to determine affected coronary territory 2
• Computer interpretations are helpful adjuncts but never substitutes for physician interpretation—errors in automated readings are common and physician verification is mandatory 5, 1, 2, 3

Common Pitfalls to Avoid

• Electrode misplacement, particularly precordial leads, significantly alters interpretation and causes false diagnoses 2
• Inadequate filtering distorts waveforms and affects measurements 2
• Over-reliance on computer interpretation without physician verification leads to diagnostic errors 2
• Failure to compare with previous ECGs when available misses important changes 2
• Interpreting ECG findings in isolation without clinical context leads to inappropriate management 2

Special Considerations

• Age and gender affect normal parameters—QT intervals are typically longer in women 1
• Medications can alter ECG findings, particularly antiarrhythmics, psychotropics, anti-infectives, antihypertensives, and heart failure medications 1, 3
• Athletes may have normal variants including sinus bradycardia (≥30 bpm) 2
• Children older than 1 month often have inverted T waves in leads V1, V2, and V3 2
• Ensure adequate high-frequency response (minimum 150 Hz for adults) to maintain diagnostic precision 2

Maintaining Competency

• Residency training in internal medicine with Advanced Cardiac Life Support instruction is sufficient for bedside interpretation in routine and emergency situations 5
• Continuing medical education through seminars or self-assessment programs is encouraged, especially for physicians who read ECGs infrequently 2
• Noncardiologists are more influenced by patient history than cardiologists, making clinical context particularly important 5, 2
• Interpretation varies even among expert electrocardiographers, underscoring the need for ongoing education 2