Bundle Branches: The Heart's Electrical Conduction Highways

The bundle branches are specialized conduction fibers that distribute the cardiac impulse from the atrioventricular (AV) node to the ventricular myocardium, ensuring synchronized contraction.

Anatomy and Pathway

1. SA node (right atrium) → spontaneous depolarization (60–100 bpm).
2. Atrial conduction → AV node (delay ~120 ms — allows atrial kick).
3. Bundle of His (penetrates fibrous skeleton) → bifurcates into:
4. Right Bundle Branch (RBB): Single fascicle → right ventricular septum and free wall → Purkinje network.
5. Left Bundle Branch (LBB): Splits into two fascicles:
   • Left Anterior Fascicle (LAF): Superior/anterior LV.
   • Left Posterior Fascicle (LPF): Inferior/posterior LV.
6. Purkinje fibers: Terminal network penetrating subendocardium → rapid, simultaneous ventricular activation (apex-to-base).

Conduction Velocity

• SA/AV node: 0.05 m/s (slow — delay).
• Bundle branches/Purkinje: 2–4 m/s (fast — synchronization).
• Ventricular muscle: 0.3–0.5 m/s.

Clinical Significance: Bundle Branch Blocks (BBB)

Type	ECG Criteria (V1, V6)	Common Causes
RBBB	V1: RSR' (M-shaped); V6: wide S wave; QRS ≥120 ms	Normal variant, ASD, pulmonary HTN, MI, cardiomyopathy
LBBB	V1: wide QS/rS; V6: broad R, no Q; QRS ≥120 ms	HTN, ischemic CM, aortic stenosis, LAD disease
LAFB	Left axis deviation (-45 to -90°); qR in I/aVL; rS in II/III/aVF	HTN, aging, LAD disease, anterior MI
LPFB	Right axis deviation (>+90°); rS in I/aVL; qR in II/III/aVF	Rare isolated; inferior MI, WPW, lung disease

Clinical Implications

• New BBB in acute chest pain → STEMI equivalent (Sgarbossa criteria for LBBB).
• Chronic BBB: assess for structural heart disease.
• Bifascicular block (RBBB + LAFB) → higher risk complete heart block; consider EP study if syncope.
• CRT (cardiac resynchronization therapy) targets LBBB dyssynchrony.

The bundle branches are the final common pathway for ventricular activation—precision engineering for a synchronized heartbeat.