Ecg
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ECG normal electrocardiogram waveform labeled PQRST
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ECG systematic interpretation steps rate rhythm axis intervals
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Excellent — I now have comprehensive content from Goldman-Cecil Medicine and Harrison's. Here's a thorough ECG basics & interpretation guide:
ECG Basics & Interpretation
What the ECG Records
The ECG captures the electrical activity of the heart from multiple viewpoints (leads) simultaneously. Paper records at 25 mm/sec — so on the grid:
- 1 small box = 1 mm = 0.04 sec (40 ms)
- 1 large box = 5 mm = 0.20 sec (200 ms)
- Voltage: 10 mm (2 large boxes) = 1 mV
The Waveforms
| Waveform | What it represents |
|---|---|
| P wave | Atrial muscular depolarization |
| PR interval | Conduction through atria → AV node → His-Purkinje |
| QRS complex | Ventricular muscle depolarization |
| ST segment | Early ventricular repolarization (isoelectric normally) |
| T wave | Ventricular repolarization |
| U wave | Possibly Purkinje fiber repolarization (small, after T wave) |
| QT interval | Total ventricular electrical systole (QRS onset → T wave end) |
Sinoatrial nodal depolarization is not visible on the surface ECG. — Goldman-Cecil Medicine
Normal Intervals
| Parameter | Normal Range |
|---|---|
| Heart rate | 50–100 bpm |
| P wave duration | < 120 ms (< 3 small boxes) |
| PR interval | 90–200 ms (0.09–0.20 sec) |
| QRS duration | 75–110 ms (< 3 small boxes) |
| QTc (males) | 390–450 ms |
| QTc (females) | 390–460 ms |
| QRS axis | −30° to +90° |
Source: Goldman-Cecil Medicine, Table 42-1
QRS Nomenclature
- Capital letters (Q, R, S) = deflections ≥ 5 mm (0.5 mV)
- Lowercase (q, r, s) = deflections < 5 mm
- Q/q = initial negative deflection
- R/r = any positive deflection
- S/s = negative deflection after an R wave
- QS = entirely negative complex (no positive deflection at all)
Calculating Heart Rate
Regular rhythm: Count large boxes between two consecutive R waves:
- 1 box = 300 bpm
- 2 boxes = 150 bpm
- 3 boxes = 100 bpm
- 4 boxes = 75 bpm
- 5 boxes = 60 bpm
- 6 boxes = 50 bpm
Irregular rhythm (e.g., AF): Count QRS complexes in the 10-second strip × 6.
The 12 Leads
Limb leads (frontal plane):
- Standard: I, II, III
- Augmented: aVR, aVL, aVF
Precordial leads (horizontal plane):
- V1 → V6 (right to left across chest)
Regional groupings for ischemia assessment:
| Territory | Leads |
|---|---|
| Inferior | II, III, aVF |
| Anterior/septal | V1–V4 |
| Lateral | I, aVL, V5–V6 |
| Posterior | V1–V2 (reciprocal changes) |
Systematic Approach to ECG Interpretation
Harrison's recommends analyzing 14 parameters every time:
- Standardization & technical quality — calibration, lead placement, artifacts
- Rhythm — sinus vs. non-sinus, regular vs. irregular
- Heart rate
- PR interval — AV conduction
- QRS duration — bundle branch block if > 120 ms
- QT / QTc — corrected with Bazett's formula: QTc = QT ÷ √RR
- Mean QRS axis — normal: −30° to +90°
- P wave morphology — duration, amplitude, axis
- QRS voltages — high (hypertrophy) vs. low
- Precordial R-wave progression — transition zone normally V3–V4
- Abnormal Q waves — pathological if ≥ 40 ms wide or ≥ 25% of R wave height
- ST segments — elevation or depression
- T waves — inversion, tall/peaked
- U waves
"Many mistakes in ECG interpretation are errors of omission. Therefore, a systematic approach is essential." — Harrison's Principles of Internal Medicine, 22nd ed.
Key Interval Abnormalities at a Glance
| Finding | Implication |
|---|---|
| PR > 200 ms | 1st-degree AV block (delayed AV conduction) |
| Short PR + delta wave | WPW (ventricular pre-excitation) |
| QRS > 120 ms | Bundle branch block or ventricular rhythm |
| QTc > 450 ms (M) / 460 ms (F) | Risk of torsades de pointes |
| ST elevation | STEMI, pericarditis, Brugada, early repolarization |
| ST depression | Ischemia, NSTEMI, digoxin effect |
QRS Axis
- Normal: −30° to +90°
- Left axis deviation (LAD): −30° to −90° → think LBBB, left anterior fascicular block, inferior MI
- Right axis deviation (RAD): +90° to +180° → think RVH, RBBB, lateral MI, PE
- Quick tip: If QRS is positive in lead I and negative in aVF → LAD. Positive in both → normal. Negative in I and positive in aVF → RAD.
Normal Sinus Rhythm Criteria
- P wave before every QRS
- P wave upright in leads I, II; inverted in aVR
- PR interval 120–200 ms
- Regular rate 60–100 bpm
- Narrow QRS (< 120 ms) unless aberrant conduction
Want me to go deeper on any specific part — arrhythmia recognition, axis calculation, bundle branch blocks, ischemia patterns, or QTc prolongation?
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