Frequency limitation in the human baroreceptor reflex

In 10 unsedated normotensive subjects, low-level bilateral electrical stimulation of the carotid sinus nerves (0.35 ms, 1-2 V) was applied with frequencies of 20-200 Hz for therapeutic purposes. Both peak and steady-state reflex effects increased with the stimulus rate up to 80 Hz. After 1 min of stimulation, the largest fall of arterial pressure (21.0 +/- 5.8%, mean +/- S.D.) was produced by stimulus frequencies of 80-120 Hz. Right atrial pacing did not modify the results. After 1 min of stimulation at 200 Hz, the arterial pressure fall was reduced to 13.6 +/- 5.2% (P less than 0.001), whereas peak effects, reached after 15-40 s, were near maximal. The reduction of the depressor response emerged after about 10 s of stimulation and was complete after about 1 min. Peak RR-interval prolongation occurred within 10 s and showed no adaptation. Comparison of continuous stimulation and R-wave-triggered intermittent stimulation in 3 subjects suggested that the reflex effect was determined by the mean stimulus frequency. We conclude that frequency limitation in the baroreceptor reflex begins when the stimulus rate exceeds about 80 Hz. It leads to a frequency-dependent, gradual adaptation of the reflex effects when the stimulus rate exceeds 120 Hz. This conclusion is limited to the part of the reflex that is subserved by myelinated afferent fibers.