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I now have comprehensive content from Goldman-Cecil Medicine and Guyton & Hall for a full ECG overview. Let me compile this into a thorough response.
ECG — Electrocardiogram: A Complete Overview
What Is the ECG?
The electrocardiogram (ECG/EKG) is a recording of the electrical activity of the heart from electrodes placed on the body surface. It captures depolarization and repolarization of cardiac muscle, providing a window into cardiac rhythm, conduction, structure, and ischemia.
The Cardiac Conduction System
Before reading an ECG, understand what generates the signals:
- SA node (high lateral right atrium near SVC) — dominant pacemaker, spontaneously depolarizes at the highest rate
- Bachmann bundle — speeds the wave to the left atrium
- AV node — the only electrical connection between atria and ventricles; physiologic conduction delay here
- Bundle of His → Right & Left bundle branches → Purkinje fibers — rapid spread to ventricular muscle
Goldman-Cecil Medicine, p. 421
ECG Waves and What They Represent
| Wave/Segment | Represents |
|---|---|
| P wave | Atrial muscular depolarization |
| PR interval | Conduction through atrial muscle, AV node, and His-Purkinje system |
| QRS complex | Ventricular muscular depolarization |
| J point | Junction between end of QRS and start of ST segment |
| ST segment | Early ventricular repolarization (isoelectric at baseline) |
| T wave | Ventricular repolarization |
| U wave | Small wave after T wave (origin debated; may represent repolarization of Purkinje fibers or papillary muscles) |
| QT interval | Total duration of ventricular depolarization + repolarization (QRS onset to T wave offset) |
The P wave and QRS are depolarization waves. The T wave is a repolarization wave. Because ventricular mass far exceeds atrial mass, the low-amplitude atrial repolarization wave is buried within the QRS and not visible on surface ECG.
Goldman-Cecil Medicine, p. 421; Guyton & Hall Medical Physiology, p. 143
Normal ECG Intervals (Table 42-1)
| Parameter | Normal Value |
|---|---|
| Heart rate | 50–100 bpm |
| P wave duration | < 0.12 sec (120 ms) |
| PR interval | 0.09–0.20 sec (90–200 ms) |
| QRS duration | 0.075–0.11 sec (75–110 ms) |
| QTc (males) | 390–450 ms |
| QTc (females) | 390–460 ms |
| QRS axis | −30° to +90° |
Goldman-Cecil Medicine, Table 42-1
ECG Paper & Speed
- Standard recording speed: 25 mm/sec
- Small box (1 mm) = 0.04 sec (40 ms) horizontally; 0.1 mV vertically
- Large box (5 mm) = 0.20 sec (200 ms) horizontally; 0.5 mV vertically
- Standard recording duration: 10 seconds (groups of leads I–III, aVR/aVL/aVF, V1–V3, V4–V6, each for 2.5 sec + rhythm strip)
Heart Rate Calculation
- Formula: HR = 60,000 ÷ RR interval (in ms)
- Quick method (regular rhythm): Count large boxes between consecutive R waves:
- 1 box = 300 bpm | 2 = 150 | 3 = 100 | 4 = 75 | 5 = 60 | 6 = 50
- For irregular rhythm (e.g., AF): Count QRS complexes in 10-second strip × 6
The 12 Leads
A standard ECG has 12 leads — 6 limb leads + 6 precordial leads:
| Limb Leads | View |
|---|---|
| I, II, III | Bipolar limb leads (Einthoven's triangle) |
| aVR, aVL, aVF | Augmented unipolar limb leads |
| Precordial Leads | Position |
|---|---|
| V1 | 4th intercostal space, right sternal border |
| V2 | 4th intercostal space, left sternal border |
| V3 | Between V2 and V4 |
| V4 | 5th intercostal space, midclavicular line |
| V5 | Anterior axillary line (same level as V4) |
| V6 | Midaxillary line (same level as V4/V5) |
Normal R-wave progression: V1 has a small R and deep S → progressively taller R wave → by V5/V6, predominantly positive. The transition point (where R amplitude first exceeds S) is normally at V3 or V4.
Normal 12-Lead ECG
Electrical Axis
- Normal frontal plane axis: −30° to +90°
- Left axis deviation (LAD): < −30° (seen in LBBB, left anterior fascicular block, inferior MI)
- Right axis deviation (RAD): > +90° (seen in RVH, RBBB, left posterior fascicular block, PE)
- Quick rule: If QRS is mainly positive in both leads I and II → axis is normal
Systematic Approach to ECG Interpretation
Use a stepwise approach to avoid missing findings:
- Rate — Calculate from RR interval
- Rhythm — Regular or irregular? P before every QRS?
- P waves — Present, morphology, upright in I and II?
- PR interval — Short (<120 ms → pre-excitation), prolonged (>200 ms → 1st degree AV block)?
- QRS duration — Narrow (<110 ms) or wide (≥120 ms → BBB, paced, WPW, hyperkalemia)?
- Axis — Normal, LAD, or RAD?
- ST segments — Elevation (injury/STEMI), depression (ischemia/NSTEMI)?
- T waves — Upright, inverted, peaked (hyperkalemia)?
- QT interval — Prolonged (drugs, electrolytes, congenital LQTS)?
- Compare to prior ECG if available
Goldman-Cecil Medicine, p. 424
Key Pathological Patterns at a Glance
| Finding | Classic Cause |
|---|---|
| ST elevation | STEMI, Prinzmetal angina, pericarditis, LBBB |
| ST depression | NSTEMI/UA, LVH strain, digoxin effect |
| Pathological Q wave | Prior MI (width ≥ 40 ms, depth ≥ 25% of R) |
| Wide QRS (≥120 ms) | LBBB, RBBB, WPW, paced rhythm, hyperkalemia |
| Prolonged QTc | Drugs (amiodarone, antipsychotics, antibiotics), hypokalemia, hypomagnesemia, congenital LQTS |
| Delta wave + short PR | Wolff-Parkinson-White syndrome |
| J (Osborn) wave | Hypothermia, hypercalcemia |
| Peaked T waves | Hyperkalemia |
| Diffuse saddle-shaped ST elevation | Pericarditis |
Want to go deeper on any specific topic — arrhythmias, bundle branch blocks, STEMI localisation, axis deviation, or a specific clinical scenario?
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