Quick EKG Interpretation: A Systematic Approach
Use a systematic, stepwise algorithm to rapidly interpret ECGs, focusing first on rate, rhythm, axis, intervals, and then morphology of P waves, QRS complexes, and ST-T segments to avoid missing critical findings. 1
Step-by-Step Interpretation Algorithm
1. Rate Assessment
- Calculate heart rate by counting QRS complexes
- Normal: 60-100 bpm
- Bradycardia: <60 bpm; Tachycardia: >100 bpm 2
2. Rhythm Analysis
- Determine if QRS complexes are present and regular 2
- Identify P waves and their relationship to QRS complexes 2
- Are P waves present before each QRS?
- Is there a consistent PR interval?
- Are there more P waves than QRS complexes? 2
- Assess QRS width: narrow (<120 ms) versus wide (≥120 ms) 2
3. Axis Determination
- Evaluate QRS axis in frontal plane using leads I and aVF
- Normal axis: -30° to +90°
- Use limb leads to determine if axis deviation is present 3
4. Interval Measurements
- PR interval: Normal 120-200 ms
- QRS duration: Normal <120 ms
- QT interval: Correct for heart rate (QTc); normal <450 ms (men), <460 ms (women) 3
5. ST Segment Evaluation (Critical for Acute Ischemia)
ST Elevation Thresholds 3:
- Men ≥40 years: ≥0.2 mV (2 mm) in V2-V3; ≥0.1 mV (1 mm) in all other leads
- Men <40 years: ≥0.25 mV (2.5 mm) in V2-V3; ≥0.1 mV in all other leads
- Women (all ages): ≥0.15 mV (1.5 mm) in V2-V3; ≥0.1 mV in all other leads
ST Depression Thresholds 3:
- ≥0.05 mV (0.5 mm) in V2-V3
- ≥0.1 mV (1 mm) in all other leads
Critical Distinction: ST elevation must be present in 2 or more anatomically contiguous leads 3
6. Differentiate ST Elevation Causes 3
Three primary causes of ST elevation exist:
- Early repolarization (benign variant): J-point elevation with rapidly upsloping ST segment, steep downsloping in V1-V2 3, 4
- Acute ischemia/infarction: Horizontal or downsloping ST segment, reciprocal ST depression in opposite leads 3, 4
- Pericarditis: Widespread concave upward ST elevation with PR depression in multiple leads (I, II, aVL, aVF, V3-V6) and PR elevation in aVR 5
Key morphology distinction: Normal early repolarization shows upsloping ST segments, while ischemia shows horizontal or downsloping ST segments 3
7. T Wave Assessment
- Normal T waves: Upright in I, II, V3-V6; inverted in aVR 3
- Abnormal T wave inversion: Particularly significant in lateral leads V5-V6 3
- T wave changes may indicate primary repolarization abnormalities (ischemia, electrolyte disturbances, drugs) or secondary changes (bundle branch blocks, hypertrophy) 3
8. Anatomic Localization of Ischemia 3
Identify contiguous lead groups (use Cabrera format when possible) 3:
- Anterior wall: V3-V6 (mid/distal LAD occlusion)
- Extensive anterior/anterobasal: I, aVL, V1-V6 with reciprocal depression in II, III, aVF (proximal LAD occlusion) 3
- Inferior wall: II, III, aVF (RCA or LCx occlusion)
- Right ventricular involvement: ST elevation in V4R with inferior changes (proximal RCA occlusion) 3
- Posterior wall: ST depression in V1-V3 (reciprocal of posterior ST elevation) 3
9. Special Considerations
Pacemaker Detection 6:
- Modern bipolar pacemaker spikes are often too small (<0.5 ms duration) to detect on standard ECG 6
- Do not rely solely on visible spikes; consider clinical context 6
Secondary Repolarization Changes 3:
- Bundle branch blocks cause ST-T changes directed opposite to terminal QRS forces
- These do not indicate primary myocardial pathology 3
Common Pitfalls to Avoid
- Computerized interpretation errors: Algorithms can provide erroneous information; always verify manually 1
- Missing reciprocal changes: ST elevation should have reciprocal depression in opposite leads in acute MI 3, 4
- Ignoring age/gender/race variations: Normal ST elevation varies significantly by demographics 3
- Overlooking right-sided leads: Always obtain V4R in inferior MI to detect RV involvement 3
- Confusing early repolarization with STEMI: Assess ST segment morphology (upsloping vs. horizontal) 3, 4