Cue Column | Notes Column

SAN — where, rate? | Located in upper right atrium near superior vena cava opening. Intrinsic rate: 70-75 bpm. PRIMARY pacemaker. Has highest automaticity in heart. Impulse spreads through both atria via gap junctions (atrial syncytium).

AVN — where, role? | Located at atrioventricular junction (base of right atrium near interatrial septum). Receives atrial impulse. Slows conduction to 0.05 m/s — the SLOWEST conducting tissue. Introduces 0.1 s delay: allows atria to finish systole, maximizing ventricular filling.

Bundle of His? | Also called atrioventricular bundle. Runs through interventricular septum. Splits into LEFT and RIGHT bundle branches. Only electrical connection between atria and ventricles in normal heart. Purkinje fibres are fastest conducting cardiac tissue (2-4 m/s).

Why apex upward? | Purkinje fibres deliver impulse to apex first → ventricular muscle contracts from apex upward → blood pushed toward outflow tracts (pulmonary artery, aorta). Efficient ejection design.

ECG correlations? | P wave = atrial depolarization (SAN fires → atria activate). PR interval = AVN conduction time (0.12-0.20 s; prolonged in 1st-degree heart block). QRS = ventricular depolarization (Bundle of His + Purkinje). QT interval = ventricular depolarization + repolarization. T wave = ventricular repolarization.

What if SAN fails? | AVN becomes pacemaker at 40-60 bpm (junctional rhythm). If AVN also fails → Purkinje/ventricular escape at 20-40 bpm. Hierarchical backup ensures heart continues beating. Clinical: complete heart block requires artificial pacemaker.

Summary

The conducting system has a strict hierarchy: SAN (fastest intrinsic rate, primary pacemaker) → AVN (delay station) → Bundle of His (only AV conduction pathway) → Purkinje fibres (fastest velocity, apex-upward activation). Each ECG wave maps to a specific conducting event. The AVN's 0.1-second delay is functionally critical — not a defect.