Cervical efferent vagal stimulation related or unrelated to cardiac cycles: comparison of negative atrial inotropic effects.

Efferent cervical vagal stimulations were performed in 4 anesthetized dogs, either with trains of stimuli of constant frequency (CONT), or with brief trains, triggered by the atrial electrical activity (TRIG). Heart rate was increased by cardiac pacing during vagal stimulations. Negative inotropic effects of vagal stimulations, evaluated as changes in atrial contractility, were partially inhibited by cardiac acceleration during CONT, but were unaffected during TRIG. It is concluded that the number of stimuli per cycle, not the number of stimuli per second, is the critical factor for parasympathetic heart regulation.

Heart rate variability at the onset of efferent vagal stimulations in the rabbit.

Vagal stimulations were performed in 6 anesthetized rabbits for single cardiac cycles. The distribution of effects over 6 consecutive cardiac cycles was described as a function of varying intervals between P waves and stimuli (P-St): the response on the first couple of cycles is affected by P-St whilst the effects decline regularly over successive cycles. Stimulation of 6 consecutive cycles showed rapid rise and subsequent slow increment of the response. Results interpreted as evidence for intracellular accumulation of a restraining factor for heart rate due to vagal stimulation.

Effects of combined stimulation of right and left vagus nerves on heart rate in rabbits.

The negative chronotropic effects of combined stimulations of the right and left vagus nerves were compared with the effects of single nerve stimulations in 10 urethan anesthetized rabbits. The combined stimulations gave smaller effects than single nerve stimulations at double frequency, over a wide range of frequencies: this was more evident for the right vagus compared with right plus left, rather than vice versa. It is concluded that the effects of combined stimulations are partially occluded and that the left vagus has smaller effects than the right vagus, although such difference becomes apparent only with combined stimulations. Possible mechanisms of occlusion are discussed.