Dynamic interaction of vagal activity and heart rate on atrioventricular conduction

The dynamic effects of 1) brief bursts of vagal stimuli and 2) changing heart periods (A-A interval) on the atrioventricular conduction time (AVCT) were studied in open-chest, anesthetized (pentobarbital, 30 mg/kg) dogs (n = 8) under beta-adrenergic blockade (propranolol, 1 mg/kg). Four different atrial and/or vagal stimulation paradigms were used to quantitate the interactions between these two separate effects. We measured the AVCT responses to identical vagal stimuli both when the heart was paced and when it was not paced. When the heart was not paced, we collected and stored in computer memory the sequences of dynamically changing heart periods that followed each vagal stimulus burst. Later, without vagal stimuli, we played these sequences of changing intervals back into the atrial pacing electrodes and measured the subsequent AVCT responses to those changes of heart period alone in the absence of vagal activity. We also measured the AVCT responses when we played these intervals back into the atrial pacing electrodes but when identical vagal stimuli were simultaneously applied, so as to absolutely fix the site of atrial activation. We found that these two separate effects of vagal stimuli and changing A-A intervals on AVCT were not linearly additive. For example, the response of the atrioventricular (AV) node to the two combined effects was less than the linear sum of the AV response to the two separate perturbations. Our results further suggest strongly that shifts in the pacemaker site may not be an important mechanism for the paradoxical response of the AV node to vagal stimulation under the present experimental conditions.