Atrioventricular nodal reentrant tachycardia, abbreviated AVNRT, is the most common cause of supraventricular tachycardia. It is more common in woman than in men and it usually presents over a wide age range, from teenagers and young adults to the elderly. Patients with AVNRT have an abnormal electrical circuit within their AV node. It is not clear if patients are born with the propensity to have this arrhythmia, or whether it develops later in life for unclear reasons. It does not, however, appear to be inherited. In addition, there may be many individuals who have the abnormal circuit but never develop the tachycardia.

Figure 1.  Simplified idealized diagram of the normal electrical circuit in the heart.  The atrioventricular (AV) node and the His bundle are the linchpin of the electrical circuit because normally they are the only electrical connection between the atria and ventricles.

Figure 1 illustrates the normal atrioventricular conduction system. In this figure, both atria and ventricles are depicted as single oval-shaped objects. The AV node and His bundle are shown as a single one-way pathway connecting the atria to the ventricles. In individuals with a normal AV node there is only a single AV nodal pathway; the electrical impulse conducts from the SA node, through the right atrium, and then traverses the single AV node pathway to the ventricles.

Patients with the AVNRT have an abnormal electrical circuit, involving two interconnected electrical conduction pathways in the AV node, as depicted in Figure 2. Having two pathways can result in the electrical impulse getting “stuck” inside the AV node circuit. In this case, the electrical impulse goes “round-and-round” inside the AV node at a very fast rate. Each time the impulse comes back around to the bottom of the AV node, the impulse conducts down the His bundle to the ventricles. Therefore the ventricles beat very fast — as fast as the electrical impulse spins (conducts) around inside the dual AV nodal pathways.

Figure 2. Simplified idealized diagram of the abnormal electrical circuit in the heart responsible for atrioventricular (AV) nodal reentrant tachycardia (AVNRT). In this situation, there are two electrical pathways in the AV node, unlike the normal situation where there is only a single pathway (see Figure 1). Panel A shows how the normal SA node electrical impulse can conduct through one limb of the dual pathway system en route to the ventricles. Sometimes and electrical impulse from the atria not only conducts to the ventricles through one of the two pathways but, in addition, the impulse can also conduct “backwards” up the AV node using the other pathway (panel B). This can lead to the electrical impulse continuing to go “round-and-round” inside the AV node (panel C). Each time the impulse comes back around to the bottom of the AV node it sends another impulse down to the ventricles. Therefore, the ventricles beat very rapidly, as fast as the electrical impulse spins around inside the AV node.