The heart has specialized cells collected into nodes and tracts. The sinoatrial (SA) node, near the junction of the superior vena cava and right atrium, has a rich vagal and sympathetic nerve supply and normally controls heart rate. Conduction of impulses from the SA node to the atrioventricular (AV) node occurs without a specialized conducting system. Slow cell-to-cell conduction through atrial myocardium explains the relatively long duration of the P wave.
The AV node is in the interatrial septum just anterior and superior to the mouth of the coronary sinus; it is also innervated by vagal and sympathetic fibers and consists of a mesh of very thin fibers that conduct impulses very slowly. As a result, the AV node delays conduction, giving time for ventricular filling; in atrial fibrillation, the AV node limits the number of impulses reaching the ventricles. The bundle of His passes from the AV node through the central fibrous body into the ventricular septum. The bundle of His has large, rapidly conducting fibers and has vagal nerves only proximally; more distally, there is only sympathetic innervation of conduction tissues. The bundle of His gives off the compact right bundle branch, which has wide fibers, and then the left branch bundle, which is a diffuse fan of thinner fibers. The bundle is insulated from surrounding myocardium and normally does not activate the ventricular myocardium until it branches and ramifies into the Purkinje fibers. These peripheral conducting fibers ramify just beneath the endocardium so that the ventricular walls are depolarized from subendocardium to subepicardium. Rapid conduction down the His-Purkinje system allows the entire ventricular myocardium to contract nearly simultaneously, explaining the narrow QRS complex in normal humans.
Some people have accessory pathways connecting atrial and ventricular myocardium. In patients with Wolff-Parkinson-White syndrome, the bundle of Kent is a muscular bridge spanning the AV groove. With some bundles, conduction is possible in both anterograde and retrograde directions; in others, it is exclusively retrograde. Anterograde conduction causes early depolarization of the ventricles (pre-excitation); retrograde conduction causes rapid reentry between the atria and the ventricles, causing sustained tachyarrhythmias. A second type of accessory pathway, the Mahaim fiber, is thought to be made up of specialized conducting fibers and to connect directly to the specialized conducting system. The most common type, called the atriofascicular connection, connects atrial myocardium with elements of the right bundle branch. In general, with Mahaim pathways, only anterograde conduction is thought to be possible, and tachycardia occurs as a result of anterograde conduction from the atrium to the distal conducting system and then retrograde conduction to the bundle of His, AV node, and back to the atrium, resulting in a maximally pre-excited, or wide complex, tachycardia.
Finally, some people have dual AV node pathways. These pathways are functionally and anatomically distinct, with the atrial approaches to the slow pathway being posterior ...