In the electrophysiology lab, testing for re-entrant tachycardias is a frequently performed procedure. It is necessary to find the mechanism or the substrate that supports the tachycardia. Atrio-ventricular nodal re-entry tachycardia (AVNRT) is the most easily described tachycardia. There are several substrates that invoke re-entry. For example, bypass tracts as in Wolfe Parkinson White (WPW) syndrome, AVRT (atrio-ventricular reciprocating tachycardia) and sinoatrial (SA) node tachycardia all have conduction properties that differ within the circuit. One limb of the circuit exhibits slower conductive properties, another limb conducts faster. A re-entrant circuit requires two limbs of tissue that have different conduction velocities around a non-conductive barrier that connects with common tissue.

When trying to induce a re-entrant tachycardia, progressively premature extra stimuli can be used to cause unidirectional conduction block to allow re-entry to occur. The extra stimuli will conduct down both the fast and the slow pathway. The fast pathway conducts faster, so that it reaches the common excitable tissue first. The extra stimulus brought in progressively earlier will reach a point that the fast pathway tissue is refractory. The extra stimulus conducts through the slow pathway (which takes longer), eventually reaching the common excitable tissue, and depolarizes it. The fast tissue recovers and accepts the stimulus that comes through the slow tissue, then reenters up the fast and down the slow. This initiates the tachycardia (Figure 1).
AVNRT is a common re-entrant tachycardia. To initiate the arrhythmia, we bring in the extra stimulus earlier and earlier. In the electrograms below (Figures 2-5), look at the last beat in the train on the HIS signal. The A-V node is normally decremental in character. This protects our ventricles from responding too fast. The HIS channel shows the atrial deflection and the HIS deflection getting farther apart. The time between the atrial deflection and the HIS deflection suddenly elongates, causing a jump. In a extra stimulus protocol, a jump by definition is at least 50 ms longer from the atrial event to the HIS event. This is representative of the fast pathway in refractory and the slow pathway taking over. In the next electrogram, the extra stimulus goes down the slow pathway and then up the fast pathway, because it is no longer blocked and tachycardia is initiated.

Characteristics of a re-entrant AVNRT tachycardia on an EKG are as follows:

The initial P wave is upright and the next
P wave is retrograde (inverted).

There is a prolongation of the first P-R interval.

There is no acceleration in the beginning
of the tachycardia.

A premature beat may cause the tachycardia
to terminate.

Terminating re-entrant tachycardia is possible by causing a simultaneous block in both limbs of the circuit. In re-entrant tachycardia, the tissue that is trailing the advancing wavefront is refractory. There is excitable tissue in front of the advancing wavefront and refractory tissue trailing the wavefront (Figure 6). The tissue between the advancing wavefront and the refractory tissue is the excitable gap. It is during this excitable gap that an extra stimulus must penetrate and propagate towards both the advancing wavefront and chase the refractory tail until it blocks in both directions. This will terminate the re-entrant arrhythmia.

There are factors that affect termination: