Regional conductance changes during hemorrhage in pregnant and nonpregnant conscious rabbits

Abstract

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.

Pregnancy alters hemodynamic responses to hemorrhage in conscious rabbits

“…However, whether pregnancy modifies the changes in either cardiac output or total peripheral resistance (TPR) evoked during hemorrhage has not been studied in conscious animals. Nevertheless, changes in renal and mesenteric conductances were similar in pregnant and nonpregnant conscious rabbits, and decreases in conductance of the terminal aortic vascular bed during hemorrhage were only slightly attenuated in pregnant animals (4). Thus pregnancy appears to have only modest effects on the regulation of regional resistance during hemorrhage, indirectly suggesting that regulation of cardiac output may also be altered during pregnancy.…”

“…5). In addition, in pregnant conscious rabbits, the decrease in pressure during the hypotensive phase is less profound (4,6).…”

Pregnancy and the endocrine regulation of the baroreceptor reflex

“…Moreover, in many species, pregnancy interferes with the normal ability to maintain arterial pressure during hemorrhage (26,36,99,152,186). Hemodynamic studies have revealed that in pregnant animals, arterial pressure falls with less blood loss due to both a failure to maintain cardiac output and also inadequate peripheral vasoconstriction (22,36). Importantly, 11-22% of maternal mortality is attributed to peripartum hemorrhage (156,166), a common event during vaginal delivery and C-section and after delivery.…”

Pregnancy increases baroreflex-independent GABAergic inhibition of the RVLM in rats

“…sympathetic nerve activity; brain stem; cardiovascular regulation; angiotensin II PREGNANCY IS CHARACTERIZED by increased blood volume and cardiac output, mild tachycardia, and decreased arterial blood pressure due to a significant decrease in total peripheral resistance (21, 47). Pregnant women and animals are more susceptible to orthostatic (3) and hemorrhagic hypotension (6,8). Although decreased vascular sensitivity to endogenous vasoconstrictors likely contributes to decreased compensatory responses (21), there is evidence that alterations in central nervous system (CNS) autonomic control mechanisms play a major role in regulation of sympathetic outflow and blood pressure during pregnancy.…”

“…Pregnant women and animals are more susceptible to orthostatic (3) and hemorrhagic hypotension (6,8). Although decreased vascular sensitivity to endogenous vasoconstrictors likely contributes to decreased compensatory responses (21), there is evidence that alterations in central nervous system (CNS) autonomic control mechanisms play a major role in regulation of sympathetic outflow and blood pressure during pregnancy.…”