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Anatomy and
Physiology
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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.
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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.
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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
in ...