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

Abstract

SUMMARYThe 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 (/» < 0.05) than the AV responses to changes in heart period (AAA) alone without vagal stimulation (using the identical AAA 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.IT HAS BEEN SHOWN by us previously that a single stimulus applied to the vagosympathetic trunk of an unpaced dog heart preparation can lead to changes of atrioventricular (AV) conduction time that are extremely complex, with several different mechanisms contributing to the resultant response.1 Thus, a generalized increase in vagal activity at the sinoatrial (SA) and AV nodes results in two oppositely directed effects on AV conduction time (the AV interval): (1) the acetylcholine (ACh) released will prolong the AV interval via a direct effect on the AV node, and (2) it will indirectly decrease the AV interval by increasing heart period (AA interval), primarily an AV nodal effect.2 I wished here to define quantitatively the interaction between these two responses. The specific question asked was: Do the two independent effects on AV conduction combine as a simple sum when applied simultaneously, or is there a more complex, nonlinear interaction that produces the resultant response? The second alternative was found to prevail in almost all animals.The basic protocol used an open-chest, anesthetized dog and consisted of noting the AV conduction responses to the following sequence of experiments: (1) single vagal stimulus bursts delivered at different times throughout the cardiac cycle in the spontaneously beating heart prepara-

Baroreceptor control of atrioventricular conduction in man.

“…In our study the pre-His conduction time was markedly increased or decreased by the baroreflexes only during atrial pacing, and only small effects were observed when heart rate was allowed to decrease and increase as a result of the baroreceptor manipulation. This finding reproduces in man a phenomenon obtained in experimental animals, in which the direct effects of vagal stimulation on the atrioventricular node were largely masked by simultaneous changes in cardiac cycle (Martin, 1977;Beer et al, 1977), and raises an obvious question: is the direct baroreceptor influence on this structure an unimportant component of the reflex in ordinary life, or does it represent a physiologically meaningful effect? If we consider that baroreflexes markedly alter the cardiac cycle (Robinson et al, 1966;Gribbin et al, 1971;Pickering et al, 1972b;Korner et al, 1974;Mancia et al, 1977), and that this alteration profoundly affects atrioventricular nodal conduction (Damato et al, 1969), then the influence of the baroreceptors on the nodal tissue appears as a homeostatic mechanism that counteracts the effects of alterations in cardiac cycle length to maintain atrioventricular conduction values close to normal despite large changes in heart rate.…”

The basis of vagal efferent control of heart rate in a neotropical fish,the pacu,Piaractus mesopotamicus

“…Levy and colleagues (Levy et al, 1972) and Spear and colleagues (Spear et al, 1979) confirmed that a pulse of stimulation could be effective in causing two inhibitory phases separated by a brief phase of cardio-acceleration, depending on the phase of the cardiac cycle in which it was delivered. Martin (Martin, 1977) noted that atrio-ventricular conduction time in the heart of the dog could be shorter in the presence of vagal stimulation. Similar complex relationships were reviewed by Levy and colleagues (Levy et al, 1981).…”

Morphology and electrophysiology of the mammalian atrioventricular node

“…Arrival of an atrial impulse at the time N cells were more depolarized than normally led to a shorter conduction time (presumably because the difference between actual membrane potential and threshold potential for activation of the slow inward current was smaller so that less axial current was necessary to activate the N cells). This could explain the "paradoxical" shortening of A V conduction time after a single vagus nerve volley in the in situ canine heart (104).…”