Cardiovascular Physiology · Cardiology · High Yield

Cardiac Conduction System

The cardiac conduction system generates and propagates the electrical impulse that coordinates myocardial contraction. Understanding its anatomy explains every arrhythmia and its ECG correlate.

✦ The Mnemonic

"Systematic Atrial Beats Launch Rhythmic Pulses"

SA node → Atria → AV node → Bundle of His → Bundle Branches → Purkinje fibres

SA Sinoatrial Node Natural pacemaker; rate 60–100 bpm; crista terminalis of RA; supplied by RCA (60%) or LCx (40%)

A Atrial conduction Impulse spreads through atrial myocardium → produces P wave on ECG

AV Atrioventricular Node Delays impulse 0.1 s (PR interval); Koch's triangle; supplied by RCA (90%)

H Bundle of His Penetrates fibrous skeleton; divides into left and right bundle branches

BB Bundle Branches Left BB: anterior + posterior fascicles; Right BB: single; LBBB = left axis deviation

P Purkinje Fibres Rapid conduction (4 m/s); subendocardial; depolarise ventricular myocardium from apex upward

📚 Clinical Breakdown

The SA node sits at the junction of the SVC and the right atrium, within the crista terminalis. Its intrinsic rate (60–100 bpm) is the fastest, so it normally controls the heart rate. If the SA node fails, the AV node takes over at 40–60 bpm; if that fails, the ventricles generate an escape rhythm at 20–40 bpm.

AV node delay is essential — it allows atrial systole to complete before ventricular systole begins, optimising stroke volume. The AV node is supplied by the RCA in 90% — explaining why inferior MI (RCA) is commonly associated with heart block. The PR interval on ECG represents AV nodal conduction time (normal: 120–200 ms).

Bundle branch blocks: LBBB (left bundle branch block) causes a wide QRS (>120 ms) with a characteristic M-shaped complex in V5–V6 and W-shape in V1. New LBBB with chest pain = STEMI equivalent — treat as such. RBBB causes an RSR' pattern in V1 ('M' in V1, 'W' in V6). RBBB is more commonly an incidental finding than LBBB.

SA node rate 60–100 bpm — fastest, so dominates

AV node escape rate 40–60 bpm

Ventricular escape rate 20–40 bpm

PR interval AV nodal conduction — normal 120–200 ms

⭐ Clinical Pearl

Wolff-Parkinson-White (WPW): an accessory pathway (bundle of Kent) bypasses the AV node, pre-exciting part of the ventricle. ECG shows short PR interval, delta wave, and wide QRS. The dangerous arrhythmia in WPW is pre-excited AF — the accessory pathway can conduct at very high rates (up to 300 bpm) → ventricular fibrillation. Do NOT give AV-nodal blocking drugs (adenosine, verapamil, digoxin) in pre-excited AF — they preferentially block the AV node, forcing all conduction down the accessory pathway.

← Heart Chambers