What is the proper approach to interpreting an electrocardiogram (ECG) and managing subsequent findings?

Systematic Approach to ECG Interpretation

Use a structured, sequential method to interpret every ECG, beginning with rate and rhythm assessment, followed by interval measurements, axis determination, and waveform morphology analysis—this systematic approach prevents missed diagnoses and ensures comprehensive evaluation. 1, 2

Step 1: Rate and Rhythm Assessment

• Calculate heart rate by counting QRS complexes in a 6-second strip and multiplying by 10, or use the 300-divided-by-large-boxes method between consecutive R waves 2, 3
• Normal sinus rhythm requires a rate of 60-100 bpm in adults, with P waves preceding each QRS complex at consistent PR intervals 2, 4
• Evaluate R-R interval regularity—consistent spacing indicates regular rhythm, while irregular spacing suggests arrhythmias like atrial fibrillation 2, 3
• Identify the underlying rhythm origin (sinus, atrial, junctional, or ventricular) by examining P wave morphology and its relationship to QRS complexes 2

Step 2: Interval Measurements

• Measure PR interval (normal: 120-200 ms or 3-5 small squares) to assess atrioventricular conduction—prolongation indicates AV block 2, 4
• Assess QRS duration (normal: <120 ms or <3 small squares)—widening suggests bundle branch blocks or ventricular conduction delays 2, 4
• Calculate QTc using Bazett's formula, with normal values <450 ms for men and <460 ms for women—prolongation increases risk of torsades de pointes 2, 3, 4
• Use the teach-the-tangent method in lead II or V5 for manual QT measurement to ensure accuracy 4

Step 3: Axis Determination

• Examine leads I and aVF simultaneously—normal axis shows positive deflections in both leads (between -30° and +90°) 2, 3
• Left axis deviation: positive in lead I, negative in aVF (-30° to -90°), suggests left anterior fascicular block or left ventricular hypertrophy 2, 3
• Right axis deviation: negative in lead I, positive in aVF (+90° to +180°), indicates right ventricular hypertrophy or left posterior fascicular block 2, 3
• Extreme axis deviation: negative in both leads I and aVF (+180° to -90°), suggests severe conduction abnormalities 3

Step 4: P Wave Analysis

• Normal P waves are upright in leads I, II, and aVF, with duration <120 ms and amplitude <2.5 mm 2
• P waves should be biphasic in V1 (initial positive deflection, then negative) in normal sinus rhythm 2
• Abnormal P wave morphology suggests atrial enlargement—peaked P waves indicate right atrial enlargement, while broad notched P waves suggest left atrial enlargement 2, 3

Step 5: QRS Complex Evaluation

• Assess R wave progression across precordial leads—amplitude should increase from V1 to V4, then decrease toward V6 2
• Identify pathologic Q waves (>1 mm wide and >1/3 the height of R wave, or >0.04 seconds duration)—these indicate prior myocardial infarction 2, 3
• Apply voltage criteria for left ventricular hypertrophy: S wave in V1 + R wave in V5 or V6 >3.5 mV (Sokolow-Lyon criterion) 3, 4
• Note that QRS voltage criteria decline with age and vary by population, reducing sensitivity in older patients 4

Step 6: ST Segment and T Wave Assessment

• Measure ST segments at the J point—elevation >0.1 mV in limb leads or >0.15-0.2 mV in precordial leads indicates acute injury requiring emergent reperfusion 2, 3, 5
• ST depression in leads other than V1-V3 suggests subendocardial ischemia from subocclusion, distal embolization, or supply-demand mismatch 5
• ST depression in V1-V3 may represent posterior wall ST elevation myocardial infarction—obtain posterior leads 5
• 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, flattening, or hyperacute appearance suggest ischemia, but must be interpreted with clinical context 3, 5

Step 7: Additional Features

• Look for U waves (small deflections after T waves)—prominent U waves suggest hypokalemia or bradycardia 2
• Identify conduction abnormalities including PR prolongation (first-degree AV block) and QRS widening (bundle branch blocks) 2, 3
• Note the location of ST-T abnormalities to determine affected coronary territory—anterior (V1-V4), lateral (I, aVL, V5-V6), or inferior (II, III, aVF) 3

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, and noncardiologists are particularly influenced by patient history. 1, 3

• In patients with active chest pain, ST elevation indicates acute transmural ischemia requiring immediate catheterization laboratory activation 5
• Compare with previous ECGs when available—new changes are more significant than chronic findings 1, 3
• Serial ECGs are warranted if patients develop syncope, new chest pain patterns, dyspnea, extreme fatigue, or palpitations despite previous normal ECG 4

Computer Interpretation Limitations

• Computer-generated interpretations serve only as adjuncts—physician verification is mandatory for all ECGs 1, 2, 3, 4
• Computers show less accuracy than trained physicians and frequently generate errors, particularly with artifact or unusual patterns 1, 3
• Computer axis calculations and interval measurements must be verified manually, especially when clinical suspicion conflicts with automated interpretation 2, 3

Common Pitfalls to Avoid

• Electrode misplacement, particularly precordial leads, causes false diagnoses—verify proper placement when ECG appears inconsistent with clinical picture 3
• Inadequate high-frequency response (minimum 150 Hz for adults) causes systematic underestimation of signal amplitude and smoothing of Q waves 4
• Technical artifacts from muscle tremor, electrical interference, or poor skin contact can mimic arrhythmias—ensure clean baseline before interpretation 3, 4
• Over-reliance on voltage criteria for left ventricular hypertrophy without considering age, body habitus, and clinical context leads to false positives 4
• Failure to recognize normal variants in athletes (sinus bradycardia ≥30 bpm, early repolarization) may prompt unnecessary testing 3

Competency Maintenance

• Residency training in internal medicine with Advanced Cardiac Life Support instruction provides sufficient baseline competency for routine ECG interpretation 1
• Continuing medical education through seminars or self-assessment programs maintains interpretation skills, particularly for physicians reading ECGs infrequently 3
• Interpretation variability exists even among expert electrocardiographers—when uncertain, seek cardiology consultation for complex or high-stakes cases 1, 3