Carotid baroreflex heart rate control during the active and the assisted breathing cycle in man.

The interaction between the phase of the breathing cycle and the carotid baroreflex heart rate control was studied in fifteen healthy subjects with special reference to respiratory sinus arrhythmia (RSA). Carotid baroreceptors were stimulated by neck suction. With a standardized breathing pattern, two types of experiment were performed. (a) Neck suction applied in counter phase with the respiratory arterial pressure variations; this reduced the amplitude and shifted the phase of the transmural carotid arterial pressure oscillations but did not influence the RSA. (B) During active as well as ventilator assisted breathing, brief periods of neck suction were applied during either inspiration (insufflation) or expiration. Neck suction during active inspiration did not change the amplitude of the RSA but when applied during expiration, increased it by 6.8 beats X min-1 (P less than 0.001). In the ventilator experiments, neck suction during insufflation and expiration augmented the RSA equally, by 5.3 beats X min-1 (P less than 0.05). The results indicate a decreased sensitivity of the carotid arterial baroreflex during active inspiration, with no equivalent during assisted breathing, and suggest that the change in baroreflex sensibility is a prerequisite for the rise of the RSA.