What are the protocols and interpretations for an electrocardiogram (ECG)?

ECG Protocols and Interpretation

The American Heart Association and American College of Cardiology mandate a systematic, stepwise approach to ECG interpretation that prioritizes rate, rhythm, axis, intervals, and waveform morphology analysis, with physician verification of all computerized interpretations being absolutely essential for accurate diagnosis. 1

Systematic Interpretation Protocol

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 60-100 beats per minute with a P wave before each QRS complex and consistent PR intervals 3
• Evaluate rhythm regularity by examining R-R interval consistency across the tracing 2

Step 2: Interval Measurements

• PR interval: Normal is 120-200 ms (3-5 small squares); prolongation indicates AV conduction delay 2, 3
• QRS duration: Normal is <120 ms (<3 small squares); widening suggests ventricular conduction abnormalities 2, 3
• QTc (corrected QT): Normal is <450 ms for men, <460 ms for women; use Bazett's formula for heart rate correction 2, 3

Step 3: Axis Determination

• Examine leads I and aVF to quickly determine axis quadrant 2
• Normal axis: -30° to +90° (positive deflections in both leads I and aVF) 2, 3
• Left axis deviation: -30° to -90° (positive in lead I, negative in aVF) 2, 3
• Right axis deviation: +90° to +180° (negative in lead I, positive in aVF) 3

Step 4: Waveform Morphology Analysis

• P waves: Should be upright in leads I, II, aVF; biphasic in V1; duration <120 ms and amplitude <2.5 mm 2
• QRS complexes: Assess for pathologic Q waves (>1 mm wide and >1/3 the R wave height), which suggest myocardial infarction 2
• R wave progression: Should increase from V1 to V4, then decrease toward V6 2
• ST segments: Measure at the J point for elevation (>0.1 mV in limb leads, >0.15-0.2 mV in precordial leads) or depression indicating ischemia 2, 3
• T waves: Normally upright in leads I, II, V3-V6; inverted in aVR; variable in III, aVL, aVF, V1, V2 2

Critical Technical Standards

Recording Requirements

• Minimum high-frequency response must be 150 Hz for adults/adolescents and 250 Hz for children to maintain diagnostic precision 3
• Digital electrocardiographs are superior to analog systems for measurement reproducibility through accurate template creation 3
• Proper lead placement is essential; precordial lead misplacement is a common source of false diagnoses 3

Interpretation Standards

• Computerized interpretations must always be verified by a qualified physician—computer interpretation alone is not recognized as properly interpreted 2, 3
• Compare current ECG with previous tracings when available to identify important changes 3
• Interpret in clinical context—the same ECG finding has different implications depending on patient symptoms 3, 4, 5

Common Pitfalls to Avoid

• Electrode misplacement, particularly precordial leads, significantly alters interpretation and causes false diagnoses 3
• Inadequate filtering distorts waveforms and affects measurements 3
• Over-reliance on computer algorithms without physician verification leads to diagnostic errors 3
• Ignoring clinical context may result in inappropriate management decisions 3
• Failure to recognize technical artifacts including noise, baseline wander, and muscle tremor 4

Special Populations and Considerations

• Age and gender variations: QT intervals are typically longer in women; children often have inverted T waves in V1-V3 after 1 month of age 2, 3
• Athletes: Sinus bradycardia ≥30 bpm is a normal variant 3
• Medication effects: Antiarrhythmics and psychotropics can significantly alter ECG findings 2
• Electrolyte abnormalities: ECG can detect potassium and calcium disturbances 1

Clinical Applications

The ECG is indispensable for diagnosing acute coronary syndromes and initiating prompt therapy, serves as the most accurate method for diagnosing intraventricular conduction disturbances and arrhythmias, and can detect genetically mediated electrical or structural cardiac abnormalities 1

Quality Assurance

• Continuing medical education through seminars or self-assessment programs is essential for maintaining interpretation skills, especially for physicians who read ECGs infrequently 3
• Systematic approach following a specific routine ensures accurate interpretation 4, 5
• Consultation availability for complex or uncertain cases improves diagnostic accuracy 4