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 atrioven tricular nodal reentrant tachycardia (AVNRT). Previous studies have at tempted to analyze these properties by introducing single ventricular extrastimuli during tachycardia. These studies have been limited, howe ver, by the inability of single extrastimuli to engage the circuit in the majority of patients studied. Thus, in most cases, the nature of t he excitable gap and the conduction properties of the anterograde and retrograde limbs of the circuit during tachycardia remain undefined. I n this series, 11 patients with typical AVNRT were studied. During tac hycardia, bath single and double ventricular extrastimuli (the first e xtrastimulus acting as a conditioning stimulus) were used to scan dias tole. The resetting response of the reentrant circuit, as well as the conduction properties of the retrograde fast and anterograde slow path ways, was recorded and analyzed. Whereas strial pre-excitation and res etting of the reentrant circuit could he demonstrated in only 1 patien t 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 fas t pathway conduction could be demonstrated before tachycardia terminat ion or ventricular refractoriness. Penetration of the reentrant circui t resulted in a progressive increasing delay in the anterograde of por tion of the subsequent return cycle and an increasing resetting respon se pattern in all cases. Thus, the reentrant circuit during AVNRT demo nstrates heterogeneous excitability. While the fast pathway remains fu lly excitable during tachycardia, the slow pathway uniformly demonstra tes decremental conduction, resulting in an increasing resetting respo nse pattern. An increasing resetting response pattern does not preclud e the presence of fully excitable tissue within a reentrant circuit.