Control of Hemodynamic Adjustments during Acute Volume Expansion in the Rabbit

Abstract

The influence of reflexes mediated by the carotid sinus, aortic, and vagus nerves on control of blood pressure were investigated in the pentobarbital-anesthetized rabbit during an acute intravascular volume expansion. Blood, kept at 37°C, was gradually infused at 2.5-min intervals until the blood volume of each animal was expanded to 10, 20, 30, and 40% above normal. Responses in sinoaortic-vagally denervated rabbits were compared to intact rabbits. Both intact and denervated animals showed a significant increase in central venous pressure with each 10% addition of blood. Heart rate did fall significantly in the intact group but not in the denervated group. No significant changes were identified in mean arterial pressure (MAP) in either group. In both groups changes in cardiac output were significantly greater than control for 20, 30, and 40% expansion and calculated total peripheral resistance fell in both groups at all levels of expansion when compared to control. The absence of significant changes in MAP within the intact and denervated groups suggests that sinoaortic-vagal reflexes are not affecting control of MAP in response to slow, acute volume expansion in the intact rabbit. On the other hand, since total peripheral resistance fell in both groups, a non-sinoaortic-vagal mechanism appears to be functional. This mechanism may assist in increasing vascular capacitance in order to prevent significant increases in blood pressure.