How do I interpret this electrocardiogram (ECG) for a patient with potential cardiovascular disease, hypertension, or diabetes, considering their age, sex, medical history, and symptoms?

How to Interpret an ECG: A Systematic Approach

ECG interpretation requires a qualified physician using a systematic method that integrates technical assessment, pattern recognition, and clinical context—computer interpretation alone is never sufficient. 1, 2

Essential Prerequisites Before Interpretation

• Verify proper electrode placement before beginning interpretation, as misplaced precordial leads significantly alter findings and lead to false diagnoses 3, 4
• Confirm adequate technical settings: minimum high-frequency response of 150 Hz for adults (250 Hz for children) to maintain diagnostic precision 2, 4
• Check signal quality for artifacts, electrical interference, and baseline stability that could compromise interpretation 4

Step-by-Step Systematic Interpretation Framework

1. Calculate Heart Rate

• Count QRS complexes in a 6-second strip and multiply by 10, or use 300 divided by the number of large boxes between consecutive R waves 3, 4
• Normal sinus rhythm: 60-100 beats per minute 2, 4
• Bradycardia: <60 bpm; Tachycardia: >100 bpm 3, 4

2. Identify the Underlying Rhythm

• Confirm sinus rhythm by verifying a P wave before each QRS complex with consistent PR interval 3, 4
• Note any irregularities: premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation 3, 4

3. Measure Critical Intervals

• PR interval (normal: 120-200 ms or 3-5 small boxes) to assess AV conduction 2, 3
• QRS duration (normal: <120 ms or <3 small boxes) to identify ventricular conduction delays or bundle branch blocks 2, 3
• QTc interval using Bazett's formula: normal <450 ms for men, <460 ms for women 2, 3

4. Determine Electrical Axis

• Use leads I and aVF for quick axis determination 3, 4:
  • Normal axis (+90° to -30°): both leads I and aVF positive
  • Left axis deviation (-30° to -90°): lead I positive, aVF negative
  • Right axis deviation (+90° to +180°): lead I negative, aVF positive

5. Assess for Ischemia, Injury, and 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 3, 4
• Pathological Q waves (>0.04 seconds or >25% of R wave amplitude) suggest prior myocardial infarction 3, 4
• T-wave abnormalities including inversion, hyperacute changes, or flattening 3, 4
• Note the location of abnormalities to determine affected coronary territory for catheterization lab activation decisions 3, 4

6. Evaluate for Chamber Enlargement

• Left ventricular hypertrophy: S in V1 + R in V5 or V6 >3.5 mV (Sokolow-Lyon criterion) 4
• Assess for atrial enlargement patterns 4

Critical Integration with Clinical Context

Never interpret the ECG in isolation—the same ECG pattern may occur in different pathophysiologic states and have vastly different implications depending on symptoms 3, 5. Clinical signs and symptoms alone lack sufficient sensitivity (35-38%) and specificity (28-91%) to rule in or rule out acute coronary syndromes without ECG and biomarkers 3.

• In patients with suspected acute coronary syndrome: obtain ECG as soon as possible after first patient contact, particularly with chest pain, syncope, or symptoms predictive of sudden death 3
• Repeat ECGs are essential when symptoms change in quality or severity, as serial ECGs combined with cardiac biomarkers improve diagnostic accuracy 3, 5
• Compare with previous ECGs when available, as this can identify important dynamic changes 3, 4

Computer-Assisted Interpretation: Use with Caution

Computer interpretations must always be verified by a qualified physician—they are helpful adjuncts but never substitutes for physician interpretation 2, 3, 4. Computer programs provide accurate heart rate, intervals, and axes, but interpretations of rhythm disturbances, ischemia, or infarction require careful physician over-reading 3.

Common Pitfalls and How to Avoid Them

• Do not over-rely on computer interpretation without physician verification, as errors remain common 2, 3, 4
• Do not fail to compare with previous ECGs when available, which can miss important dynamic changes 3, 4
• Do not interpret findings without clinical context, which may lead to inappropriate management decisions and adverse outcomes 3, 4
• Recognize that noncardiologists are more influenced by patient history than cardiologists when interpreting ECGs, making systematic approach even more critical 1, 3
• Avoid inadequate filtering settings, which can distort waveforms and affect measurements 4

Competency and Ongoing Education

Residency training with Advanced Cardiac Life Support certification is sufficient for bedside interpretation in routine and emergency situations 1, 3. However, continuing medical education through seminars or self-assessment programs is strongly encouraged for physicians who read ECGs infrequently to maintain current skills 1, 2, 3.

Prior clinical exposure to ECG diagnoses significantly improves diagnostic accuracy (odds ratio 1.46-1.58), emphasizing that ECG training should not rely on experiential learning alone but be supplemented by formal methods of instruction 6.