SYSTEMATIC CHARACTERIZATION OF THE REENTRANT CIRCUIT DURING ATRIOVENTRICULAR NODAL REENTRANT TACHYCARDIA

This study was conducted to systematically characterize the excitable gap and conduction properties of the reentrant circuit during atrioventricular nodal reentrant tachycardia (AVNRT). Previous studies have attempted to analyze these properties by introducing single ventricular extrastimuli during tachycardia. These studies have been limited, however, by the inability of single extrastimuli to engage the circuit in the majority of patients studied. Thus, in most cases, the nature of the excitable gap and the conduction properties of the anterograde and retrograde limbs of the circuit during tachycardia remain undefined. In this series, 11 patients with typical AVNRT were studied. During tachycardia, bath single and double ventricular extrastimuli (the first extrastimulus acting as a conditioning stimulus) were used to scan diastole. The resetting response of the reentrant circuit, as well as the conduction properties of the retrograde fast and anterograde slow pathways, was recorded and analyzed. Whereas strial pre-excitation and resetting of the reentrant circuit could he demonstrated in only 1 patient with single ventricular extrastimuli, resetting was achieved in all 11 patients with closely coupled double ventricular extrastimuli. Over the full range of coupling intervals used, no retrograde delay in fast pathway conduction could be demonstrated before tachycardia termination or ventricular refractoriness. Penetration of the reentrant circuit resulted in a progressive increasing delay in the anterograde of portion of the subsequent return cycle and an increasing resetting response pattern in all cases. Thus, the reentrant circuit during AVNRT demonstrates heterogeneous excitability. While the fast pathway remains fully excitable during tachycardia, the slow pathway uniformly demonstrates decremental conduction, resulting in an increasing resetting response pattern. An increasing resetting response pattern does not preclude the presence of fully excitable tissue within a reentrant circuit.