bundle of His

The bundle of His originates from the AV node and travels through the fibrous cardiac skeleton (specifically the membranous interventricular septum) before dividing into right and left bundle branches. It provides the sole electrical connection between the atria and ventricles, ensuring that after the AV node delays conduction (allowing atrial contraction and ventricular filling), the impulse rapidly spreads to the ventricles via the bundle branches and Purkinje fibers, producing coordinated ventricular depolarization seen as the QRS complex on ECG.

Clinically, the bundle of His is important in understanding heart blocks: first-degree, second-degree (Mobitz I and II), and third-degree (complete) AV block can involve conduction delay or failure at or below the level of the His bundle. Infra-Hisian blocks (below the bundle) tend to be more dangerous and often require pacemaker placement, whereas supra-Hisian (AV nodal) blocks are usually more benign. Bundle branch blocks (right or left) result from disease distal to the bundle of His, producing characteristic widened QRS morphology (RBBB: RSR' in V1; LBBB: broad QRS with loss of septal Q waves in lateral leads).

The bundle of His can be affected by ischemic heart disease (especially septal MI), infiltrative diseases (amyloidosis, sarcoidosis), Lyme disease (Lyme carditis causing AV block), post-surgical trauma (e.g., valve replacement), and congenital abnormalities. Electrophysiological studies can localize block to the His bundle level (His bundle electrogram) to guide pacemaker decisions. Understanding its anatomy is essential for interpreting ECG conduction abnormalities and for procedures like catheter ablation in arrhythmias (e.g., AV nodal reentrant tachycardia, ablation near the His bundle carries risk of iatrogenic heart block).