Carotid baroreflex responsiveness to lower body positive pressure-induced increases in central venous pressure.

Lower body positive pressure (LBPP) was applied at 4 and 30 Torr to increase central venous pressure (CVP). CVP increased with LBPP (r = 0.55, P < 0.01), and the increases were significant at 4 and 30 Torr (7.4 and 7.8 mmHg) from the control (6.0 mmHg). During LBPP cardiac output increased, which was significantly related to the increase in CVP (r = 0.63, P < 0.01). The carotid baroreflex was elicited by trains of neck pressure and suction from 40 to -65 Torr. The carotid-cardiac and carotid-vasomotor baroreflex responses were assessed by determining the maximal gains of heart rate (HR) interval and intraradial mean arterial pressure (MAP) to changes in carotid sinus pressure using a logistic model. The carotid-cardiac and carotid-vasomotor baroreflex gains were negatively related to LBPP, and at 30 Torr, both gains (5.6 ms/mmHg and -0.14 mmHg/mmHg) were significantly smaller than the control (12.4 ms/mmHg and -0.24 mmHg/mmHg). The decreases in carotid-cardiac and carotid-vasomotor baroreflex sensitivity during LBPP were primarily associated with the increases in CVP (r = -0.52, P < 0.01, and r = -0.74, P < 0.01, respectively). These data suggest that the increases in CVP and/or central blood volume diminish the sensitivity of the carotid baroreflex control of HR and MAP by enhancing the tonic inhibitory influence from the cardiopulmonary baroreceptors.