Temporal and spatial summation caused by aortic nerve stimulation in rabbits. Effects of stimulation frequencies and...

Frequency-modulated stimulus patterns, controlled by heart rate or blood pressure feed-back, are proposed for therapeutic use of electrical stimulation of the carotid sinus nerves (CSNS). Continuous CSNs with equidistant pulses is applied in hypertensive or angina pectoris patients who are refractory to therapy. Frequency modulation simulates transmission properties of single baroreceptors, while modulation of stimulus amplitudes simulates recruitment of baroreceptors by pressure pulses in the carotid sinus. Using aortic nerves in the anesthetized rabbit as a model, it was investigated if amplitude modulation could substitute frequency modulation in electrical stimulation. First, 'step stimulation' was applied (protocol 1): starting from a basal low frequency with the basal amplitude set at a level of 10% between threshold and the maximum, either the frequency or the amplitude of pulses was increased in a single step for 20 s (frequency step, FS; amplitude step, AS). The blood pressure response and its course of recovery was analyzed. Fifteen different basal stimulus frequencies (1, 2, ...15 Hz) were investigated. For FS, each given frequency was increased 10-fold (10, 20, ...150 Hz) while the basal amplitude level was kept constant. For AS, the basal amplitude level was increased to the maximum, while each given frequency was kept constant. Depressor responses to AS were equivalent to FS at a 10-fold higher frequency. After step stimulation it was either stopped immediately (type A), or was resumed for 90 s with a basal low frequency and the basal low amplitude level (type B). After type A and B stimulations, the recovery time of blood pressure response was found to be longer after cessation of AS, compared to FS. In further experiments (protocol 2), an 'electronic baroreceptor analog' generated stimulus patterns which were feed-back controlled by the blood pressure signal. Either frequencies (FM) or amplitudes (AM) of stimuli were modulated by the pressure pulse signal. Since in feed-back controlled stimulation the mean electrical current flow is reduced by depressor responses, stimulus current flow in relation to the blood pressure response served to compare the efficacy of different stimulus patterns. Equidistant stimulation (EDS) was also applied for comparison. Both feed-back controlled stimulus patterns (FM and AM) were more effective than EDS. It is suggested that AS predominantly activated C-fibers, whereas low level stimulation predominantly excited A-fibers.(ABSTRACT TRUNCATED AT 400 WORDS)