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Now I have comprehensive information from multiple authoritative textbooks. Here is a thorough guide to basic ECG interpretation:
ECG (Electrocardiogram) - Basic Interpretation
What is an ECG?
An ECG records the heart's electrical activity from 12 vantage points (leads) derived from 10 electrodes - 6 on the chest wall and 4 on the limbs. Each lead looks at the same electrical impulse from a different angle. A positive deflection is recorded when the electrical wave front moves toward the exploring electrode.
The standard 12-lead ECG comprises:
- 6 limb leads: I, II, III (Einthoven's bipolar) + aVR, aVL, aVF (Goldberger's augmented unipolar) - called frontal plane leads
- 6 precordial (chest) leads: V1, V2, V3, V4, V5, V6 - called precordial leads
The ECG Waves and What They Mean
| Wave / Segment | Electrical Event |
|---|---|
| P wave | Atrial depolarization (atria contracting) |
| PR interval | Conduction from atria to ventricles (through AV node) |
| QRS complex | Ventricular depolarization (ventricles contracting) |
| ST segment | Early ventricular repolarization |
| T wave | Ventricular repolarization (ventricles recovering) |
| U wave | Possibly Purkinje fiber repolarization (not always seen) |
| J point | End of QRS, beginning of ST segment |
Note: Atrial repolarization is buried under the QRS complex and is not visible on a normal ECG.
Normal Intervals (from Harrison's / Goldman's textbooks)
| Parameter | Normal Range |
|---|---|
| Heart Rate | 50-100 beats per minute |
| P wave duration | < 0.12 sec (120 ms) |
| PR interval | 0.09-0.20 sec (3-5 small boxes) |
| QRS duration | 0.075-0.11 sec (< 3 small boxes) |
| QTc | Males: 390-450 ms / Females: 390-460 ms |
| QRS axis | -30° to +90° |
The ECG paper: Standard speed is 25 mm/sec. Each small box = 0.04 sec (40 ms). Each large box = 0.20 sec (200 ms).
Systematic Approach to Reading an ECG
Harrison's Principles recommends analyzing 14 parameters in every ECG:
1. Calibration and Technical Features
Check lead placement, paper speed (25 mm/sec standard), and rule out artifacts. Standard calibration = 10 mm = 1 mV.
2. Heart Rate
- Method 1 (Regular rhythm): Count large boxes between two R waves → divide 300 by that number (e.g., 4 boxes = 75 bpm)
- Method 2 (Any rhythm): Count QRS complexes in a 10-second strip × 6
- Formula: HR = 60,000 / RR interval (in ms)
3. Rhythm
- Is there a P wave before every QRS?
- Is every P wave followed by a QRS?
- Are the intervals regular?
- Normal sinus rhythm: regular P waves at 60-100 bpm, P before every QRS, PR 0.12-0.20 sec
4. P Wave Morphology
- Should be upright in leads I, II, aVF; inverted in aVR
- Duration < 120 ms, height < 2.5 mm
- Biphasic P in V1 is normal; wide notched P suggests left atrial enlargement; peaked P (>2.5 mm) in II suggests right atrial enlargement
5. PR Interval
- 0.12-0.20 sec (3-5 small boxes)
- Short PR (< 0.12): Pre-excitation (WPW), junctional rhythm
- Long PR (> 0.20): First-degree AV block
6. QRS Complex
- Duration < 0.12 sec (3 small boxes)
- Wide QRS (> 0.12): Bundle branch block, ventricular rhythm, or aberrant conduction
- Q waves: normal if < 0.04 sec wide and < 25% of R wave height; pathological Q waves suggest old MI
7. QRS Axis
Determined by looking at leads I (0°) and aVF (90°):
| Lead I | aVF | Axis |
|---|---|---|
| Positive | Positive | Normal (-30° to +90°) |
| Positive | Negative | Left axis deviation (LAD) |
| Negative | Positive | Right axis deviation (RAD) |
| Negative | Negative | Extreme axis deviation |
- LAD causes: LVH, left anterior fascicular block, inferior MI
- RAD causes: RVH, pulmonary hypertension, PE, left posterior fascicular block
8. Precordial R-Wave Progression
- R wave should grow progressively from V1 to V5-V6
- Transition (R = S) normally occurs at V3-V4
- Poor R-wave progression: anterior MI, LBBB, LVH
9. QRS Voltages
- LVH: S in V1 + R in V5 or V6 > 35 mm (Sokolow-Lyon criteria)
- RVH: R > S in V1, right axis deviation
10. ST Segment
- Should be isoelectric (flat, at baseline)
- ST elevation: transmural ischemia/injury, pericarditis, Brugada
- ST depression: subendocardial ischemia, digoxin effect
11. T Waves
- Should be upright in I, II, V3-V6; inverted in aVR; variable in V1-V2
- T-wave inversion: ischemia, ventricular hypertrophy, PE (V1-V4), RBBB
- Tall peaked T waves: hyperkalemia, early MI
12. QT / QTc Interval
- Rate-corrected QTc: use Bazett's formula - QTc = QT / √RR
- Prolonged QTc: risk of Torsades de Pointes (drugs, electrolyte disturbances, congenital syndromes)
13. Abnormal Q Waves
- Pathological Q: > 1 small box wide (40 ms), depth > 25% of R wave
- Indicate prior myocardial infarction in the corresponding territory
14. U Waves
- Small deflection after T wave, best seen in V2-V3
- Prominent U waves: hypokalemia, bradycardia, LVH
Lead Territories (for localizing pathology)
| Leads | Territory | Artery |
|---|---|---|
| II, III, aVF | Inferior wall | RCA |
| I, aVL, V5-V6 | Lateral wall | LCx |
| V1-V4 | Anterior wall | LAD |
| V1-V2 (reciprocal) | Posterior wall | RCA or LCx |
Key Takeaway
As Harrison's states: "Many mistakes in ECG interpretation are errors of omission. Therefore, a systematic approach is essential." Always compare to previous ECGs whenever available, and always interpret findings in the context of the patient's clinical status.
Sources: Harrison's Principles of Internal Medicine 22E; Goldman's Cecil Medicine; Guyton & Hall Medical Physiology; Tintinalli's Emergency Medicine
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