Regional conductance changes during hemorrhage in pregnant and nonpregnant conscious rabbits

Late pregnant (P) conscious rabbits are less able to maintain arterial pressure during hemorrhage than nonpregnant (NP) animals. This study tested the hypothesis that the difference is due in part to less reflex vasoconstriction when the rabbits are P. Rabbits (n = 14) were instrumented with arterial and venous catheters as well as ultrasonic flow probes around the superior mesenteric, renal, and/or terminal aortic arteries. Pregnancy increased (P < 0.05) blood volume [235 ± 5 (P) vs. 171 ± 3 (NP) ml], terminal aortic conductance [1.88 ± 0.11 (P) vs. 0.98 ± 0.06 (NP) ml · min^-1 · mmHg^-1], mesenteric conductance [1.20 ± 0.19 (P) vs. 0.80 ± 0.05 (NP) ml · min^-1 · mmHg^-1], and heart rate [191 ± 4 (P) vs. 162 ± 3 (NP) beats/min] and decreased arterial pressure [59 ± 1 (P) vs. 67 ± 2 (NP) mmHg; P < 0.05]. Renal conductance was unaltered. The rabbits were bled in both the NP and P states at 2% of the initial blood volume per minute until arterial pressure fell below 45 mmHg. Arterial pressure fell with less blood loss in P rabbits [28 ± 2% (P) vs. 39 ± 2% (NP) of initial blood volume; P < 0.001]. Terminal aortic conductance decreased (P < 0.001) before the pressure fall in both groups, but the response was reduced in P rabbits. Mesenteric and renal conductances did not change in either group before the blood pressure fall. During the pressure fall, terminal aortic conductance increased (P < 0.05) only in NP rabbits. Mesenteric conductance increased in both groups. In summary, rabbits in late gestation are less able to maintain arterial pressure during hemorrhage, at least in part because of reduced vasoconstriction in tissues perfused by the terminal aorta.