Carotid baroreceptor reflex in man, its modulation over the respiratory cycle.

Carotid baroreceptors were stimulated in 40 healthy volunteers by a rapid increase of the carotid transmural pressure due to subatmospheric pressure applied to a tight neck chamber (neck suction). Heart rate, arterial blood pressure and respiratory response were recorded. Reflex vagal inhibition of the sinus node preceded a blood pressure fall. Cardiac response adapted in a few seconds during prolonged baroreceptor stimulation, whereas reflex hypotension was maintained over the whole stimulation period applied up to 30 s. Augmented inspiratory movements were observed during neck suction. Short-lasting baroreceptor stimuli produced much less cardiac inhibition if applied during the early inspiratory than during the expiratory phase. On the peak and on the plateau of the voluntarily sustained deep inspirations, a spontaneous sinus bradycardia and a facilitation of the baroreceptor-cardiac reflex appeared. Voluntarily delayed expiration following deep and sustained inspiration coincided with cardioacceleration and baroreflex inbibition. Thus voluntarily Imposed, breathing patterns may reverse both respiratory sinus arrhythmia and cyclical modulation of the baroreflex responsiveness over the respiratory cycle.