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Anatomy and Physiology

The heart has specialized cells collected into nodes and tracts. The sinoatrial node, near the junction of the superior vena cava and right atrium, has a rich vagal and sympathetic nerve supply and controls heart rate. Conduction of impulses from the sinoatrial node to the atrioventricular node occurs without a specialized conducting system. However, there are preferential pathways that have been termed internodal tracts. In addition, preferential conduction from the sinus node to the roof of the left atrium occurs over the Bachmann bundle, which is also not part of a specialized conduction system. The Bachmann bundle is important, however, in timing of left atrial contraction in relation to mitral valve opening. The slow cell-to-cell conduction through atrial myocardium explains the relatively long duration of the P wave.

The atrioventricular (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. Also, in the presence of atrial fibrillation, the AV node limits the number of impulses reaching the ventricles. From the AV node, the bundle of His passes through the central fibrous body into the ventricular septum just behind and below its membranous portion.1 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. Near the summit of the muscular ventricular septum, the bundle of His gives off the compact right bundle branch with wide fibers and then the left branch bundle with 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 hearts.

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 atrioventricular groove.2 With some Kent bundles, conduction is possible in both anterograde and retrograde directions; in others, it is exclusively retrograde. Anterograde conduction causes early depolarization of the ventricles (preexcitation); 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.3 The most common type, called the atriofascicular connection, connects atrial myocardium with elements of the right bundle branch. In general, only anterograde conduction is thought to be possible ...

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