Paradoxical dynamic interaction of heart period and vagal activity on atrioventricular conduction in the dog.

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Abstract
The dynamic interaction of simultaneously changing heart period and single vagal stimuli on atrioventricular conduction (AV interval) was quantified by subtracting the vagally induced responses of the paced heart preparation from those of the unpaced heart preparation. This difference was significantly greater (P less than 0.05) than the AV responses to changes in heart period (delta A) alone without vagal stimulation (using the identical delta AA recorded from the unpaced heart in the same preparation, but with a crushed sinoatrial node). That is, for a given increase in AA interval, the AV conduction time was considerably less when the change in AA was associated with increased vagal activity than in the absence of any vagal activity. Data from some dogs in which a complete AV block was produced for both paced and unpaced hearts suggested that one mechanism of the paradoxical response was located in AV nodal tissue. Data from other dogs, in which two surface atrial recording sites were used, indicated that shifts of pacemaker site and atrial activation patterns also are an important mechanism of the paradoxical response. The relative contribution of these two mechanisms is not fixed, but can vary considerably from animal to animal.