Effects of repetitive bursts of vagal activity on heart rate.

The effect of the timing of discrete bursts of efferent vagal impulses on heart rate was determined in anesthetized dogs. Two modes of stimulation were employed. In the first mode, one stimulus burst was delivered per cardiac cycle. As the time from the beginning of the P wave to the vagal stimulus (P-St interval) was progressively increased, there was first a progressive lengthening of the cardiac cycle (P-P interval), then a rapid decrease in P-P interval, and finally a gradual augmentation of the P-P interval. The amplitudes of curves of P-P interval as a function of P-St interval increased as the number of stimuli per burst was augmented, with 10 stimuli/burst yielding nearly maximal effects. Vagal stimuli applied at P-St intervals which coincided with the negative-slope (d[P-P]/d[P-St]) region of such curves tended to evoke sinus arrhythmias. With 5 stimuli/burst or more, these arrhythmias were pronounced and consisted of alternate short and long P-P intervals. In the second mode of stimulation, bursts of stimuli were delivered to the vagus nerve at a frequency independent of heart rate. The cardiac pacemaker tended to become synchronized in some fixed ratio of vagal stimuli to P waves, and this tendency became greater the larger the number of stimuli per burst of impulses. Within any range of synchronization, a paradoxical effect was manifest—increasing frequencies of vagal stimulation produced increasing rather than decreasing heart rates.