Heart's Pacemaker: SA Node vs. Purkinje Fibers – Conduction System Explained

The sinoatrial (SA) node is the heart's primary pacemaker. The Purkinje fibers are specialized conduction pathways, not the pacemaker.

Cardiac Conduction Sequence

1. SA Node (Right Atrium): Spontaneous depolarization (60–100 bpm) → "sinus rhythm."
2. Atrial Conduction: Bachmann's bundle → left atrium; internodal tracts → AV node.
3. AV Node (Interatrial Septum): Delay (~120 ms) → allows atrial contraction before ventricular.
4. Bundle of His → Bundle Branches: Right (→ Purkinje → RV), Left (anterior/posterior fascicles → Purkinje → LV).
5. Purkinje Fibers: Large-diameter, fast-conducting (2–4 m/s), glycogen-rich fibers that rapidly distribute impulse to ventricular myocardium → synchronized contraction.

Backup Pacemakers (If SA Node Fails)

Site	Intrinsic Rate	Clinical Context
AV Node / Junctional	40–60 bpm	AV block, beta-blocker/CCB toxicity, inferior MI
Bundle Branches / Purkinje	20–40 bpm	High-grade AV block, diffuse conduction disease
Ventricular Myocardium	<20 bpm	Complete heart block; agonal rhythm

Why Purkinje Fibers Are Not the Primary Pacemaker

• Higher threshold for automaticity (require stronger depolarization).
• Overdrive suppression: SA node's faster rate continuously depolarizes them before they reach threshold.
• Function = rapid, synchronous ventricular activation, not rhythm initiation.

Clinical Correlates

• SA node dysfunction: Sinus brady, sinus arrest, sick sinus syndrome → pacemaker indication.
• AV block: Delay/block at AV node → junctional or ventricular escape rhythm.
• Bundle branch block: Widened QRS (>120 ms); LBBB/RBBB patterns.
• WPW / Accessory pathways: Bypass AV node → pre-excitation (delta wave).

Understanding the hierarchy guides ECG interpretation, pharmacologic choices (e.g., avoid AV-nodal blockers in WPW), and device therapy.