798-2 Differential Sensitivity of AV and VA Conduction to Adenosine in Humans

Abstract

Studies in rat isolated hearts suggest that adenosine is more potent for slowing antegrade atrioventricular (AV) than retrograde ventriculo-atrial (VA) conduction. The potency (EC 50 ) and maximal effect (E max ) of adenosine to slow AV and VA conductions were determined in 16 patients (mean age 48 years). These patients were selected from a group that had a common form of AV nodal reentrant tachycardia who had prior to the study been subjected to selective radiofrequency ablation of the slow pathway conduction, but with intact antegrade and retrograde fast pathway conduction. During high right atrial and ventricular pacing at comparable cycle lengths (400–500 msec). adenosine was injected as an intravenous bolus at an initial dose of 0.5 mg followed by stepwise increase of 1 mg given every 5 minutes until AV or VA block occurred. The dose-response curves for adenosine-induced prolongation of AV and VA conduction time were determined. The 1:1 AV and VA conduction cycle lengths before injection of adenosine were 340 ± 50 and 326 ± 75 msec, respectively. Adenosine induced dose-dependent prolongation of AV nodal conduction time and caused typical Wenckebach AV block in all 16 patients with EC 50 and E max values of 1.4 ± 0.8 mg and 54 ± 2%, respectively. Adenosine also induced dose-dependent prolongation of VA conduction time and VA block, however, with significantly (P < 0.051 higher EC 50 and shorter E max , i.e., 6.1 ± 1 mg and 20 ± 2%, respectively. The mean dose of adenosine to induce AV block is significantly lower than that to induce VA block (3.3 ± 1.6vs. 7.7 ± 3.3 mg, P < 005). Notably, adenosine induced Wenckebach VA block in 10 of 16 patients and Mobitz type II block in 6 of 16 patients. In conclusion, adenosine is more potent in slowing AV than VA conduction, suggesting that the electrical wavefront entering the N zone of AV node is less affected by adenosine during VA than during AV conduction. In addition, retrograde fast pathway may at least in part involve conduction through the N zone of AV node.