724-6 Resetting Responses During Atrioventricular Nodal Reentrant Tachycardia Suggest an Anisotropic Mechanism

Abstract

We hypothesized that an anatomically determined circuit should have an identical excitable gap regardless of the site of stimulation, while a functional anisotropic circuit could show differences depending on stimulation site. In order to explore the nature of the reentrant circuit, and to characterize the excitable gap during atrioventricular nodal reentrant tachycardia (AVNRT), we determined the resetting response patterns in eight patients with typical AVNRT (meantachycardia cycle length (TCl) 346 ± 18 msec). Premature atrial stimuli were delivered during tachycardia from the area of the slow pathway (SPI in 7 patients, the fast pathway (FP) in 6 patients, and the ventricle (V) in 7 patients and the subsequent return cycles (As-Ar) were analyzed. In all patients, two or more sites were stimulated. Ventricular extrastimulation employed double extrastimuli such that the first extrastimulus did not interact with the circuit and acted as a conditioning stimulus. The excitable gap was defined as the range of coupling intervals from first reset to tachycardia termination. Duration of Excitable Gap (msec) by Stimulation Site Patient 1 2 3 4 5 6 7 8 Mean %TCL FP 10 * 65 20 30 20 10 * 26 ± 21 7 SP 35 80 90 40 * 20 20 20 43 ± 30 12 V 35 75 45 * 50 50 10 20 42 ± 19 12 AVNRT could be reset from the atrium and from the ventricle at all sites studied. In all cases, when the coupling intervals of premature extrastimuli were plotted against the return cycle (As-Ar), an increasing resetting response pattern was obtained. There was no decremental conduction over the retrograde FP with V stimulation. 1) The reentrant circuit of AVNRT can be reset from multiple sites. 2) The circuit as a whole demonstrates partial excitability with heterogenicityof conduction over individual pathways. 3) A difference in the measured excitable gap of > 20 msec dependent on stimulation site was seen in 5/8 patients and suggests a functionally determined circuit and anisotropic reentry.