Electrocardiography (ECG) is an essential tool in clinical practice, used for diagnosing arrhythmias, myocardial infarctions, electrolyte imbalances, and various cardiac conditions. Despite its ubiquity, ECG interpretation requires a structured approach to ensure accurate diagnosis and appropriate clinical management.
This article provides a detailed yet practical guide for medical professionals, breaking down ECG interpretation into a stepwise approach. It includes key considerations for rhythm analysis, conduction abnormalities, ischemic changes, and electrolyte disturbances, with reference to relevant guidelines.
Further Reading: NICE Guidelines on ECG Interpretation
2. Basic Principles of ECG
An ECG records the electrical activity of the heart as it propagates through the conduction system. The standard 12-lead ECG provides information from multiple perspectives, allowing assessment of different regions of the heart.
A. ECG Leads and Their Views
| Lead | View of the Heart |
|---|---|
| Limb Leads | |
| I | Lateral (left) |
| II | Inferior |
| III | Inferior |
| aVR | Right atrium |
| aVL | Lateral (left) |
| aVF | Inferior |
| Precordial Leads | |
| V1-V2 | Septal |
| V3-V4 | Anterior |
| V5-V6 | Lateral |
Each lead provides a different perspective, aiding in localizing myocardial infarctions and conduction defects.
Recommended Resource: ECG Lead Placement and Interpretation (BMJ)
3. Stepwise Approach to ECG Interpretation
A. Verify Patient Information
Always start by confirming the patient’s name, date, and time of ECG acquisition. Context is crucial, as ECG changes must be interpreted in the clinical setting.
B. Assess the Heart Rate
- Regular Rhythm → 300 ÷ number of large squares between R waves
- Irregular Rhythm → Count QRS complexes in a 10-second strip and multiply by 6
| Heart Rate (bpm) | Clinical Significance |
|---|---|
| < 60 bpm | Bradycardia (e.g., sinus bradycardia, heart block) |
| 60-100 bpm | Normal |
| > 100 bpm | Tachycardia (e.g., sinus tachycardia, AF, VT) |
C. Determine Heart Rhythm
- Regular rhythm → Equal R-R intervals (sinus rhythm, AV nodal reentrant tachycardia)
- Irregularly irregular rhythm → Suggestive of atrial fibrillation (AF)
Example:
- Atrial fibrillation → Absence of P waves, irregularly irregular rhythm
- Atrial flutter → Sawtooth P waves, atrial rate ~300 bpm
More on AF: ESC Guidelines on Atrial Fibrillation
D. P Waves and PR Interval (Atrial Activity & Conduction)
- P Wave Present? → Suggests sinus rhythm.
- P Wave Morphology? →
- Peaked P waves → Right atrial enlargement (P pulmonale)
- Broad P waves → Left atrial enlargement (P mitrale)
- PR Interval (Normal = 0.12-0.20s)
- Prolonged → First-degree AV block
- Progressively increasing PR → Dropped QRS → Mobitz Type I (Wenckebach)
- Fixed PR → Dropped QRS → Mobitz Type II
- P waves dissociated from QRS → Complete heart block
Clinical Pearl: Short PR interval (<0.12s) with delta wave suggests Wolff-Parkinson-White syndrome.
Reference: British Heart Foundation: Understanding Heart Blocks
E. QRS Complex (Ventricular Depolarization)
- Normal Duration: 0.06 – 0.10s
- Narrow QRS (<0.12s) → Supraventricular origin
- Wide QRS (>0.12s) → Ventricular origin or bundle branch block
| Finding | Interpretation |
|---|---|
| Left Bundle Branch Block (LBBB) | Wide QRS, broad R wave in V5/V6, deep S in V1 |
| Right Bundle Branch Block (RBBB) | Wide QRS, RSR’ pattern in V1, broad S in I, V6 |
| Ventricular Tachycardia | Wide QRS, no P waves, AV dissociation |
| Hyperkalemia | Peaked T waves, wide QRS, sine wave pattern |
More on Wide Complex Tachycardias: AHA Guidelines
F. ST Segment and T Waves (Ischemia and Infarction)
- ST Elevation → STEMI (localize infarction)
- ST Depression → Ischemia, NSTEMI
- T Wave Inversion → Myocardial ischemia, pericarditis, LVH
- Hyperacute T Waves → Early STEMI
- U Waves → Hypokalemia
ECG Localization of Myocardial Infarction (STEMI)
| Infarct Location | Leads Affected | Artery Involved |
|---|---|---|
| Anterior Wall | V3, V4 | LAD |
| Inferior Wall | II, III, aVF | RCA |
| Lateral Wall | I, aVL, V5, V6 | LCx |
| Posterior Wall | ST depression in V1-V3 | RCA or LCx |
STEMI vs NSTEMI Differences: ACC Guidelines
4. Special Considerations
A. Electrolyte Abnormalities
| Electrolyte Disturbance | ECG Changes |
|---|---|
| Hyperkalemia | Peaked T waves, wide QRS, absent P waves |
| Hypokalemia | U waves, prolonged QT |
| Hypercalcemia | Short QT |
| Hypocalcemia | Prolonged QT |
B. Drug-Induced ECG Changes
- Digoxin Toxicity: ST depression (scooped) + atrial tachycardia with AV block
- QT Prolongation: Torsades de Pointes risk (caused by drugs like antipsychotics, macrolides)
Full List of QT-Prolonging Medications: CredibleMeds Database
5. Conclusion
ECG interpretation requires a structured approach, integrating clinical context with waveform analysis. Mastery of rhythm assessment, conduction abnormalities, and ischemic changes ensures accurate diagnosis and patient management. Regular practice with real-world ECGs is essential to refine interpretation skills.
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