Systematic ECG Interpretation: A Comprehensive Approach
A systematic approach to ECG interpretation is essential for accurate diagnosis of cardiac conditions, requiring careful analysis of rate, rhythm, axis, intervals, and waveform morphology. 1
Step-by-Step ECG Interpretation Method
1. Rate and Rhythm Assessment
- Calculate heart rate by counting the number of large squares between consecutive R waves or by counting QRS complexes in a 6-second strip and multiplying by 10 1, 2
- Evaluate rhythm regularity by examining R-R intervals for consistency 1
- Identify the underlying rhythm (sinus, atrial, junctional, ventricular) based on P wave morphology and relationship to QRS complexes 1, 2
2. Intervals and Conduction Analysis
- Measure PR interval (normal: 120-200 ms or 3-5 small squares) to assess AV conduction 1, 2
- Evaluate QRS duration (normal: <120 ms or <3 small squares) to assess ventricular conduction 1
- Calculate QT interval and correct for heart rate (QTc, normal: <450 ms for men, <460 ms for women) 1, 2
- Identify any conduction abnormalities such as AV blocks, bundle branch blocks, or pre-excitation 2
3. Axis Determination
- Examine leads I and aVF to quickly determine the quadrant of the axis 1
- Normal axis is between -30° and +90° 1
- Left axis deviation: -30° to -90° (positive in lead I, negative in aVF) 2
- Right axis deviation: +90° to +180° (negative in lead I, positive in aVF) 2
4. Waveform Analysis
- Examine P wave morphology (normal: upright in I, II, aVF; biphasic in V1) 1
- Analyze QRS complex morphology for pathologic Q waves (>1 mm wide and >1/3 the height of the R wave) 1
- Assess R wave progression across precordial leads 1
- Evaluate ST segments for elevation or depression 1
- Assess T wave morphology (normally upright in leads I, II, V3-V6) 1
5. Ischemia and Infarction Evaluation
- Look for ST-segment deviations (elevation >0.1 mV in limb leads or >0.15-0.2 mV in precordial leads) 2
- Identify pathological Q waves suggesting myocardial infarction 2
- Note the location of abnormalities to determine the affected coronary territory 2
Special Considerations and Pitfalls
Technical Factors
- Evaluate the quality of the ECG recording and identify any technical artifacts 2
- Consider electrode placement issues which can significantly alter ECG interpretation 2
- The American College of Cardiology recommends a minimum high-frequency response of 150 Hz for adults and 250 Hz for children to maintain diagnostic precision 2
Clinical Context
- Always interpret the ECG in the context of the patient's clinical presentation 2
- Age and gender affect normal ECG parameters (e.g., QT intervals are typically longer in women) 1
- Certain medications can affect ECG findings (e.g., antiarrhythmics, psychotropics) 1
Computer Interpretation
- Computer analyses of ECGs are useful adjuncts but should not replace physician interpretation 3
- Computer ECG interpretation errors are still common, with accuracy varying from 0% to 94% depending on the condition 3
- Physician verification of computerized ECG interpretations is required 2
Maintaining Competency
- Continuing medical education through seminars or self-assessment programs is encouraged to maintain ECG interpretation skills 2
- Physician experience improves accuracy, with a 6% increase in odds of accurate interpretation for every 5 years of experience 4
- There is significant physician disagreement in interpreting complex ECGs, with interreader agreement (kappa) as low as 0.33 in some studies 4
Alternative Approaches
- The RIRI approach (Rate, Intervals, Rhythm, Ischemia/Infarction) offers a fast, reliable method that can be used by healthcare practitioners of all experience levels 5
- Artificial intelligence and deep learning models are emerging as potential tools to assist with ECG interpretation, though physician oversight remains essential 6