Electrocardiograms (ECGs) are essential tools for diagnosing cardiac conditions, but their squiggly lines can feel overwhelming. This guide breaks down ECG interpretation into a systematic, human-friendly approach—no jargon, just clarity. Let’s dive in.
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### **Step 1: Confirm Patient & Context**
Always start with the basics:
- **Patient details**: Age, gender, and medical history (e.g., hypertension, diabetes).
- **Symptoms**: Chest pain, palpitations, or shortness of breath?
- **Medications**: Beta-blockers or digoxin? These affect ECG results.
*Why it matters*: A 20-year-old athlete’s sinus bradycardia is normal; in a 60-year-old with dizziness, it’s a red flag.
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### **Step 2: Calculate the Heart Rate**
**Method 1**: *The Big Square Rule*
- Each big square = 0.2 seconds.
- Count the number of big squares between two R waves.
- **Rate ≈ 300 ÷ number of squares**.
- Example: 3 squares between R waves → 100 bpm.
**Method 2**: *The 10-Second Rule*
- Count all QRS complexes in a 10-second strip (usually 25 large squares) × 6.
*Normal range*: 60–100 bpm.
- **Bradycardia**: <60 bpm.
- **Tachycardia**: >100 bpm.
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### **Step 3: Assess Rhythm**
**Key question**: Is the rhythm regular?
- Use calipers or paper to measure R-R intervals. Irregularity suggests:
- **Atrial fibrillation** (no P waves, erratic rhythm).
- **Premature beats** (early QRS complexes).
**Check P waves**:
- **Sinus rhythm**: P before every QRS, upright in lead II.
- **No P waves?** Think atrial fibrillation/flutter or junctional rhythm.
**PR interval**:
- Normal: 0.12–0.20 seconds.
- **Prolonged (>0.2s)**: 1st-degree heart block.
- **Variable**: Wenckebach (Mobitz I).
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### **Step 4: Determine the Cardiac Axis**
The axis reflects the heart’s electrical direction. Use leads I and aVF:
- **Normal axis**: QRS positive in I and aVF.
- **Left axis deviation** (QRS positive in I, negative in aVF): LV hypertrophy or LBBB.
- **Right axis deviation** (QRS negative in I, positive in aVF): RV hypertrophy or pulmonary embolism.
*Quick tip*: If lead II’s QRS is predominantly positive, the axis is likely normal.
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### **Step 5: Analyze Waves & Intervals**
**P wave**:
- **Normal**: <0.12s, upright in II.
- **Peaked P waves**: Right atrial enlargement (“P pulmonale”).
- **Notched P waves**: Left atrial enlargement (“P mitrale”).
**QRS complex**:
- **Normal**: 0.06–0.10s.
- **Widened (>0.12s)**: Bundle branch block or hyperkalemia.
- **Pathologic Q waves**: >1 small square wide/deep → possible old MI.
**ST segment**:
- **Elevation**: STEMI (concave vs. convex matters), pericarditis (saddle-shaped).
- **Depression**: Ischemia, digoxin effect.
**T wave**:
- **Inverted**: Ischemia, LV strain.
- **Peaked**: Hyperkalemia.
**QT interval**:
- Corrected QT (QTc) = QT ÷ √RR.
- **Prolonged (>0.47s)**: Risk of torsades de pointes.
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### **Step 6: Look for Chamber Enlargement**
**Left ventricular hypertrophy (LVH)**:
- Sokolov-Lyon criteria: S in V1 + R in V5/V6 >35 mm.
**Right ventricular hypertrophy (RVH)**:
- Tall R in V1 (R > S), right axis deviation.
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### **Step 7: Spot Emergencies**
- **STEMI**: ST elevation in ≥2 contiguous leads (e.g., II, III, aVF for inferior MI).
- **Hyperkalemia**: Peaked T waves → widened QRS → sine wave → arrest.
- **VTach**: Wide QRS, no P waves, rate >120 bpm.
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### **Step 8: Don’t Be Fooled by Artifacts**
- **Tremor**: Jagged baseline.
- **Loose lead**: Flat lines in one lead.
- **Wandering baseline**: Patient movement.
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### **Putting It All Together**
1. **Rate**: 80 bpm.
2. **Rhythm**: Regular, sinus (P before QRS).
3. **Axis**: Normal.
4. **Intervals**: PR 0.16s, QRS 0.08s.
5. **ST/T waves**: Normal.
6. **Conclusion**: Normal ECG.
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### **Final Tips**
- **Practice**: Analyze ECGs daily—use apps or textbooks.
- **Context is king**: An abnormal ECG in an asymptomatic patient may not need urgent action.
- **When in doubt, ask**: ECGs are team sport.
By mastering this stepwise approach, you’ll transform ECG interpretation from chaos to clarity. Remember: Every squiggle tells a story. 🫀
*Got an ECG? Take a breath, grab a ruler, and let the heart speak.*
