How do you interpret an electrocardiogram (ECG) in a patient with a history of cardiac disease, such as coronary artery disease, heart failure, or valvular disease?

ECG Interpretation in Patients with Cardiac Disease

Interpret the ECG using a systematic, stepwise approach that evaluates rate, rhythm, intervals, axis, and waveform morphology—always correlating findings with the patient's clinical presentation, as computer interpretation alone is unreliable and must be verified by a qualified physician. 1

Systematic Interpretation Framework

Step 1: Assess Technical Quality First

• Verify proper electrode placement and signal quality before interpretation, as misplaced electrodes (especially precordial leads) can produce false diagnoses 2
• Check for artifacts from baseline wander, electrical interference, or muscle tremor that may distort waveforms 2
• Ensure adequate high-frequency response (minimum 150 Hz for adults) to avoid systematic underestimation of signal amplitude and smoothing of critical features like Q waves 2
• Never accept computer-generated interpretations without physician verification—automated systems produce frequent errors, particularly with arrhythmias 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 2, 3
• Normal sinus rhythm: 60-100 beats per minute in adults 2
• Recognize that sinus bradycardia (<60 bpm) may be normal in athletes, while sinus tachycardia (>100 bpm) may occur with physiological stress 2

Step 3: Identify the Underlying Rhythm

• Confirm sinus rhythm by identifying a P wave before each QRS complex with consistent PR interval 2
• Examine R-R intervals for regularity—completely irregular patterns suggest atrial fibrillation 2, 3
• Note any premature beats, pauses, or other rhythm irregularities 2

Step 4: Measure Critical Intervals

• PR interval (normal: 120-200 ms or 3-5 small squares): Prolongation indicates AV conduction delay; first-degree AV block if >200 ms 2, 3
• QRS duration (normal: <120 ms or <3 small squares): Widening suggests ventricular conduction delays or bundle branch blocks 2, 3
• QTc interval: Calculate using Bazett's formula; normal <450 ms for men, <460 ms for women—prolongation increases risk of torsades de pointes 2, 3

Step 5: Determine Electrical Axis

• Examine leads I and aVF to quickly determine the quadrant 2, 3
• Normal axis: -30° to +90° (positive in both leads I and aVF) 2
• Left axis deviation: -30° to -90° (positive in lead I, negative in aVF)—suggests left anterior fascicular block or left ventricular hypertrophy 2
• Right axis deviation: +90° to +180° (negative in lead I, positive in aVF)—suggests right ventricular hypertrophy or left posterior fascicular block 2

Step 6: Evaluate for Chamber Enlargement

• Left ventricular hypertrophy: Apply Sokolow-Lyon criteria (S in V1 + R in V5 or V6 >3.5 mV) or Cornell voltage criteria 2, 3
• Recognize that voltage criteria decline with age and vary by population 3
• Assess P wave morphology for atrial enlargement patterns 2

Step 7: Identify Ischemia, Injury, or Infarction

• ST-segment elevation (>0.1 mV in limb leads or >0.15-0.2 mV in precordial leads): Indicates acute injury requiring emergent reperfusion therapy 2, 4
• ST-segment depression: Up-sloping depression with positive T waves suggests severe LAD obstruction; widespread depression with ST elevation in aVR indicates diffuse subendocardial ischemia from severe multivessel disease 4
• Pathological Q waves (>0.04 seconds or >25% of R wave amplitude): Suggest prior myocardial infarction 2, 3
• T-wave abnormalities: Inversion, hyperacute changes, or flattening may indicate ischemia 2
• Note the location of abnormalities to determine affected coronary territory 2

Critical Clinical Context Integration

Always Correlate with Patient Presentation

• The same ECG finding has different implications depending on symptoms—chest pain with ST elevation requires immediate catheterization, while asymptomatic ST elevation may represent early repolarization or pericarditis 1, 2
• In patients with known coronary artery disease, compare with previous ECGs to identify new changes that may indicate acute events 2
• For heart failure patients, serial ECGs help assess progression of chamber enlargement and response to therapy 3
• In valvular disease, ECG findings of chamber enlargement correlate with hemodynamic severity 1

Recognize Population-Specific Variations

• Athletes commonly have sinus bradycardia (≥30 bpm), early repolarization, and voltage criteria for LVH without pathology 2
• Children often have T wave inversion in V1-V3 after 1 month of age 2
• Women typically have longer QT intervals than men 3
• Elderly patients show declining QRS voltage with age 3

Common Pitfalls and How to Avoid Them

Technical Errors

• Electrode misplacement is the most common source of false diagnoses—verify proper positioning, especially for precordial leads 2
• Inadequate filtering can distort waveforms and affect measurements 2
• Poor electrode contact causes baseline wander that mimics ST-segment changes 2

Interpretation Errors

• Over-reliance on computer interpretation: Automated systems correctly classify only 0-94% of disorders, with arrhythmias being most problematic 1
• Interpreting ECG in isolation: Always integrate with clinical context—failure to do so leads to inappropriate management decisions 2
• Missing comparison with previous ECGs: Serial comparison is essential to detect important changes 2
• Ignoring subtle changes: Repeat ECG recording when symptoms change in severity, as changes may be subtle initially 4

Clinical Decision-Making Errors

• Not all ST elevation represents acute MI—consider pericarditis, early repolarization, ventricular aneurysm, and Brugada syndrome 4
• Acute transmural ischemia from abrupt coronary occlusion may occur without ST elevation meeting guideline thresholds 4
• Widespread ST depression with ST elevation in aVR during chest pain warrants urgent angiography even without classic STEMI criteria 4

Maintaining Competency

Training Requirements

• Initial competency requires interpretation of 500 ECGs under supervision, with knowledge of pathophysiology, pattern recognition skills, and clinical application 1
• Maintain competency by reading 100 ECGs yearly 1
• Continuing medical education through seminars or self-assessment programs is essential, especially for physicians who read ECGs infrequently 2, 5

Quality Assurance

• Recognize that interpretation varies greatly even among expert electrocardiographers—interobserver variability is substantial 1, 2
• Major interpretation errors occur in 4-33% of readings, though adverse patient outcomes from these errors are rare (<1%) 1
• Periodic self-study and continuing education ensure skills remain current 2