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Here is a complete, systematic guide to ECG interpretation, drawn from Harrison's Principles of Internal Medicine, Medical Physiology, and Goldman-Cecil Medicine.
How to Read an ECG - A Systematic Approach
The ECG Paper
ECG paper has a grid of 1-mm small boxes and 5-mm large boxes:
- Horizontal axis (time): 0.04 s per small box, 0.2 s per large box
- Vertical axis (voltage): 0.1 mV per mm
- 5 large boxes = 1.0 second
Normal 12-lead ECG from Medical Physiology (all 12 leads shown):
Normal 12-lead ECG from Goldman-Cecil Medicine:
The Systematic Approach (Harrison's 14-Point Checklist)
Always use the same order every time - most errors in ECG reading are errors of omission.
| Step | Parameter | What to assess |
|---|---|---|
| 1 | Standardization / Technical | 1 mV calibration pulse, paper speed, lead placement, artifacts |
| 2 | Rhythm | Regular vs. irregular; sinus vs. non-sinus |
| 3 | Heart rate | Quick method below |
| 4 | PR interval / AV conduction | 0.12-0.20 s (3-5 small boxes) |
| 5 | QRS interval | < 0.12 s (< 3 small boxes) |
| 6 | QT / QTc interval | Varies with rate; corrected QTc < 440-460 ms |
| 7 | Mean QRS axis | Normal: -30° to +90° |
| 8 | P waves | Morphology, duration, axis |
| 9 | QRS voltages | Amplitude in limb and precordial leads |
| 10 | Precordial R-wave progression | R wave grows from V1 to V5/V6 |
| 11 | Abnormal Q waves | >0.04 s wide or >25% of R wave height |
| 12 | ST segments | Elevation or depression from J point |
| 13 | T waves | Inversion, peaking, flattening |
| 14 | U waves | Small deflection after T wave |
Step-by-Step Breakdown
1. Heart Rate
Quick method (most common): Count the number of large boxes between two consecutive R waves, then use:
Rate = 300 ÷ (number of large boxes in R-R interval)
| Large boxes | Rate (bpm) |
|---|---|
| 1 | 300 |
| 2 | 150 |
| 3 | 100 |
| 4 | 75 |
| 5 | 60 |
| 6 | 50 |
- Normal: 60-100 bpm
- Bradycardia: < 60 bpm
- Tachycardia: > 100 bpm
2. Rhythm
Ask three questions (Medical Physiology):
- Where is the pacemaker? - Should be the SA node (normal sinus rhythm = upright P before every QRS in lead II)
- What is the conduction path? - SA node → AV node → His-Purkinje → ventricles
- Is it regular? - Compare R-R intervals; they should be equal
Normal sinus rhythm = regular rate 60-100, P wave before every QRS, upright P in I and II, inverted in aVR.
3. P Wave
- Represents atrial depolarization
- Normal: upright in leads I, II, aVF, V4-V6; inverted in aVR
- Duration < 0.12 s; amplitude < 2.5 mm
- Biphasic in V1 is normal
4. PR Interval
- Measured from start of P to start of QRS
- Normal: 0.12-0.20 s (3-5 small boxes)
- Short PR (< 0.12 s): pre-excitation (WPW), junctional rhythm
- Long PR (> 0.20 s): 1st degree AV block
5. QRS Complex
- Represents ventricular depolarization
- Normal duration: < 0.12 s (< 3 small boxes)
- Narrow QRS (< 0.10 s): supraventricular origin
- Wide QRS (≥ 0.12 s): bundle branch block, ventricular rhythm, hyperkalemia, pacing
- Capital letters (Q, R, S) = large amplitude; lowercase (q, r, s) = small amplitude
6. QT / QTc Interval
- Measured from start of QRS to end of T wave
- Represents total ventricular action potential duration
- Shortens at higher heart rates - always correct for rate (Bazett's formula: QTc = QT ÷ √RR)
- Normal QTc: < 440 ms (men), < 460 ms (women)
- Prolonged QTc: risk of torsades de pointes
7. QRS Axis
- Normal axis: -30° to +90° in the frontal plane
- Quick method: check leads I and aVF
- Both positive → normal axis (~0° to +90°)
- I positive, aVF negative → possible left axis deviation
- I negative, aVF positive → right axis deviation
- Both negative → extreme axis deviation ("northwest")
| Axis | Degrees | Cause |
|---|---|---|
| Normal | -30° to +90° | - |
| Left axis deviation | -30° to -90° | LBBB, left anterior fascicular block, inferior MI |
| Right axis deviation | +90° to +180° | RBBB, RVH, left posterior fascicular block, lateral MI |
8. ST Segment
- From end of QRS (J point) to start of T wave
- Normally isoelectric (at baseline)
- ST elevation: transmural ischemia/injury, pericarditis, early repolarization
- ST depression: subendocardial ischemia, reciprocal changes, digoxin effect
- In subendocardial ischemia, the ST vector points inward → ST depression in overlying leads
- In epicardial/transmural injury, the ST vector points outward → ST elevation in overlying leads
9. T Wave
- Represents ventricular repolarization
- Normally upright in I, II, V3-V6; inverted in aVR
- T-wave inversion: ischemia, hypertrophy, bundle branch block, PE
- Peaked T waves: hyperkalemia, early ischemia (hyperacute T waves)
10. U Wave
- Small deflection after the T wave, same polarity as T
- Most prominent in V2-V3
- Prominent U waves: hypokalemia, bradycardia
- Inverted U waves: ischemia, hypertension
Lead Groups to Assess Together
Examine ST segments and T waves in regional groupings (Goldman-Cecil):
| Region | Leads | Coronary artery |
|---|---|---|
| Inferior | II, III, aVF | RCA |
| Anterior | V1-V4 | LAD |
| Lateral | I, aVL, V5, V6 | LCx |
| Posterior | V7-V9 (or reciprocal V1-V2) | RCA or LCx |
| Septal | V1-V2 | LAD (septal branch) |
Key Normal Values Summary
| Parameter | Normal value |
|---|---|
| Heart rate | 60-100 bpm |
| PR interval | 0.12-0.20 s |
| QRS duration | < 0.12 s |
| QTc | < 440 ms (men), < 460 ms (women) |
| QRS axis | -30° to +90° |
| P wave amplitude | < 2.5 mm |
| P wave duration | < 0.12 s |
Sources: Harrison's Principles of Internal Medicine 22E, p. 1592; Medical Physiology (Boron & Boulpaep), pp. 708-817; Goldman-Cecil Medicine, pp. 701-736.
Would you like to go deeper into any specific area - arrhythmias, bundle branch blocks, MI patterns, axis deviation, or something else?
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