Dynamic interaction between brief vagal stimulation and heart period on atrial contractility.

We have characterized the dynamic interaction of the simultaneous changes of vagal activity and heart period on atrial contractile force. Our hypothesis was that the linear sum of these two separate effects was not equal to the response of the unpaced heart when both effects were elicited simultaneously. We performed 3 experiments: we measured the beat-to-beat inotropic response to brief vagal stimuli when the heart was paced; this represented the pure vagal effect on contractile force. We measured the responses to identical stimuli when the heart was not paced; this represented the effect of the combined influences. Finally, we measured the contractile responses to the identical sequences of changing cardiac intervals that were recorded in part 2 above but in the absence of vagal activity; this represented the pure effect of changes in cardiac cycle length on contractile force. These experiments were done at 3 levels of vagal stimulus intensity. We first subtracted the responses of the paced from those of the unpaced heart. These derived differences represented the linear elimination of the pure vagal effect from the combined responses. We then compared these differences with the responses to changing cardiac intervals that were measured in the absence of vagal activity (part 3, above). These two sets of responses were not significantly different at the low level of stimulus intensity. For the mid level of stimulus intensity, they were significantly different for only the 3rd and 4th beats after the stimulus. For the high level of stimulus intensity, they were significantly different for each of the first through the 4th beats (only) after the stimulus. We conclude that the independent effects of vagal activity and changing heart period combine non-linearly to produce the resultant inotropic response when the intensity of brief vagal stimuli is increased, but we cannot draw this conclusion at the lower levels of stimulus intensity.