Understanding the Heartbeat: How is it Generated?

Cardiac Conduction System

1. Sinoatrial (SA) Node: Right atrial wall near SVC. Spontaneous phase 4 depolarization (funny current If, Ca²⁺ clock) → 60–100 bpm. Primary pacemaker.
2. Atrial Conduction: Bachmann's bundle → left atrium; internodal tracts → AV node. Atria contract (P wave).
3. Atrioventricular (AV) Node: Koch's triangle. Slow conduction (decremental) → ~120 ms delay (PR interval) → allows atrial kick (20–30% ventricular filling).
4. Bundle of His: Penetrates central fibrous body → bifurcates.
5. Right & Left Bundle Branches: Left splits into anterior & posterior fascicles.
6. Purkinje Fibers: Subendocardial network → rapid conduction (2–4 m/s) → synchronous ventricular activation (QRS complex).
7. Ventricular Contraction: Base-to-apex, endocardium-to-epicardium.
8. Repolarization: T wave; refractory periods prevent tetanus.

Autonomic Modulation

• Sympathetic (β₁): ↑ cAMP → ↑ If, ↑ Ca²⁺ current → ↑ rate, ↑ contractility, ↑ conduction velocity.
• Parasympathetic (M₂, vagus): ↑ K⁺ current (I_KACh) → hyperpolarization → ↓ rate, ↓ AV conduction.

Clinical Relevance

• SA node dysfunction → sick sinus syndrome.
• AV block: 1st° (PR>200ms), 2nd° (Mobitz I/Wenckebach, Mobitz II), 3rd° (complete).
• Bundle branch blocks (LBBB, RBBB) → wide QRS, dyssynchrony.
• Arrhythmias: reentry (AVNRT, AVRT, VT), automaticity, triggered activity.