CAROTID BAROREFLEX CONTROL DURING HEMORRHAGE IN CONSCIOUS AND ANESTHETIZED DOGS

The hypothesis was tested that carotid baroreflex gain is increased after 20% hemorrhage. The baroreceptor reflex responses to changes in carotid sinus pressure (CSP) were measured in control, 20% hemorrhage, and reinfusion conditions in three experimental groups: conscious intact (n = 7), anesthetized intact (n = 8), and anesthetized vagotomized (n = 8) dogs. Mean arterial pressure (MAP), heart rate (HR), cardiac output (CO), stroke volume (SV), and calculated total peripheral resistance (TPR) responses to changes in CSP were measured. At any given CSP, MAP, CO, and SV all decreased significantly with the 20% hemorrhage, as reflected by a downward shift in the reflex characteristic curve with no change in overall reflex range or gain. In contrast, TPR and HR responses to CSP were not significantly altered by 20% hemorrhage; reflex curves and gains were comparable to control conditions. In the conscious intact dogs, the maximal reflex gain, G(max), for the MAP response was -1.365 +/- 0.25, -1.298 +/- 0.33, and -1.324 +/- 0.25 in control, 20% hemorrhage, and reinfusion conditions, respectively, and was not significantly altered by hemorrhage. In the same group, the G(max) for the HR response was -1.792 +/- 0.65, -1.709 +/- 0.33, and -1.986 +/- 0.67 in control, 20% hemorrhage, and reinfusion conditions, respectively; baroreflex gain on HR was not increased with hemorrhage. Plasma arginine vasopressin (AVP), an increase in which has been proposed to augment baroreflex gain, increased from a control level of 0.98 +/- 0.27 to 9.66 +/- 2.67 pg/ml during 20% hemorrhage in the conscious intact dogs; despite the increase in plasma AVP during hemorrhage, augmentation of baroreflex gain was not observed. These results suggest that baroreflex control of multiple efferent mechanisms is not augmented after a 20% decrease in blood volume.