What is the approach to interpreting electrocardiogram (ECG) findings?

Systematic Approach to ECG Interpretation

Interpret every ECG using a standardized, sequential method that evaluates rate, rhythm, axis, intervals, and waveform morphology—always correlating findings with clinical context and never relying solely on computer interpretation. 1, 2

Step 1: Assess Technical Quality First

• Verify proper electrode placement and adequate signal quality before interpretation, as misplaced electrodes (particularly precordial leads) can significantly alter findings and lead to false diagnoses 1
• Identify technical artifacts from muscle tremor, electrical interference, or baseline wander that may distort waveforms 1, 2
• Ensure minimum high-frequency response of 150 Hz for adults to maintain diagnostic precision 1

Step 2: Calculate Heart Rate

• Count the number of large boxes between consecutive R waves and divide 300 by this number, or count QRS complexes in a 6-second strip and multiply by 10 1, 2
• Normal sinus rhythm: 60-100 beats per minute in adults 1
• Sinus bradycardia (<60 bpm) may be normal in athletes; sinus tachycardia (>100 bpm) may occur with physiological stress 1

Step 3: Determine Rhythm

• Identify the underlying rhythm by examining P wave morphology and its relationship to QRS complexes 1, 2
• Sinus rhythm requires: P wave before each QRS with consistent PR interval, upright P waves in leads I, II, aVF 1, 2
• Examine R-R intervals for regularity—completely irregular patterns suggest atrial fibrillation 1
• Note any premature beats, pauses, or other rhythm disturbances 1

Step 4: Measure Intervals

• PR interval (normal: 120-200 ms or 3-5 small squares): Prolongation indicates AV conduction delay 1, 2
• QRS duration (normal: <120 ms or <3 small squares): Widening suggests ventricular conduction abnormalities or bundle branch blocks 1, 2
• QT interval corrected for heart rate (QTc): Use Bazett's formula; normal <450 ms for men, <460 ms for women 1, 2

Step 5: Determine Electrical Axis

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

Step 6: Analyze Waveform Morphology

P Waves

• Normal P waves: <120 ms duration, <2.5 mm amplitude, upright in I/II/aVF, biphasic in V1 2
• Abnormal P waves suggest atrial enlargement or conduction abnormalities 2

QRS Complexes

• Look for pathological Q waves: >1 mm wide (>40 ms) and >25-33% of R wave amplitude, suggesting prior myocardial infarction 1, 2
• Assess R wave progression: Should increase from V1 to V4, then decrease toward V6 2
• Voltage criteria for left ventricular hypertrophy: S wave in V1 + R wave in V5 or V6 >3.5 mV (35 mm) 1

ST Segments and T Waves

• ST elevation (>0.1 mV in limb leads or >0.15-0.2 mV in precordial leads): Indicates acute injury or infarction 1, 2
• ST depression: May indicate ischemia or reciprocal changes 2
• T wave abnormalities: Inversion, hyperacute changes, or flattening suggest ischemia or other pathology 1, 2
• Note the location of abnormalities to determine affected coronary territory 1

Step 7: Integrate Clinical Context

Never interpret the ECG in isolation—the same finding has different implications depending on symptoms, age, medications, and prior ECGs. 3, 1

• Compare with previous ECGs when available to identify new changes 1
• Consider age and gender effects (QT intervals typically longer in women) 2
• Account for medications that affect ECG (antiarrhythmics, psychotropics) 2
• In athletes, recognize normal variants including sinus bradycardia (≥30 bpm), increased QRS voltages, and early repolarization patterns 3

Step 8: Verify Computer Interpretation

Computer analyses are useful adjuncts but should never replace physician interpretation, as computer errors remain common (0-94% accuracy for some diagnoses, particularly arrhythmias) 3, 1

• Physician overreading is mandatory for all computer-generated interpretations 3, 1
• Computers are particularly problematic for rhythm diagnoses 3
• GPs are effective at correcting false-positive computer diagnoses but must avoid reclassifying true-positives as false-negatives 4

Critical Pitfalls to Avoid

• Do not miss electrode misplacement, especially precordial leads—this is the most common source of false diagnoses 1
• Do not over-rely on pattern recognition alone—understand the underlying pathophysiology 5
• Pay special attention to computer diagnoses of ST-segment deviation, T-wave inversion, or Q-waves, as these are frequently misclassified 4
• Do not interpret findings without clinical correlation—this leads to inappropriate management decisions 1
• Recognize that interpretation varies even among expert electrocardiographers, so maintain humility and seek consultation when uncertain 3

Special Population Considerations

Athletes

• Distinguish physiological training-related changes (Group 1) from pathological findings (Group 2) 3
• Group 1 (normal): Sinus bradycardia, increased QRS voltages, early repolarization, incomplete RBBB 3
• Group 2 (abnormal): T-wave inversions, ST depression, pathological Q-waves, complete LBBB, long QT, Brugada pattern—require cardiology referral 3
• Use ST₄₀/ST₈₀ ratio >1 to distinguish benign early repolarization from Brugada pattern 3

Pediatric Patients

• T waves often inverted in V1-V3 in children older than 1 month 1
• Higher frequency response (250 Hz) required for accurate interpretation 1