EFFECT OF PHASIC VAGAL-STIMULATION AND ATRIAL-PACING SITE ON ATRIOVENTRICULAR-CONDUCTION TIME

We tested the hypothesis that the effect of phasic vagal stimulation o n atrioventricular (AV) conduction time is affected by the site of atr ial pacing in anesthetized dogs. We paced the right atrium at a consta nt cycle length from the interatrial septum (IAS), superior coronary s inus (SCS), or inferior coronary sinus (ICS) regions, and we evaluated the time-dependent effects of vagal stimulation on AV conduction at e ach pacing site. When we stimulated the vagi at stimulus (St)-A phases greater than 136 +/- 40 ms and less than the phase that blocked AV co nduction (182 +/- 70 ms), IAS pacing prolonged A-V intervals by 8.6 +/ - 8.2 ms more than ICS pacing. A change in pacing site affected the A- V intervals by up to 30 ms when we stimulated the vagus at those times that caused the A-V intervals to prolong maximally. Furthermore, the effect of atrial pacing site on A-V intervals was modulated by AV noda l recovery times during the second or third cycles after the vagal sti mulus.