Hemodynamic and hormonal responses to hemorrhage in conscious rabbits at mid- and late gestation

Abstract

This study tests the hypothesis that conscious rabbits late in pregnancy (P), but not at midgestation (MP), are less able to maintain arterial pressure during hemorrhage. Blood volume (BV) was elevated ( P < 0.05) by an average of 13 ± 4 (MP) and 35 ± 3% (P). Rabbits were bled in both the nonpregnant (NP) and P state at 2% of the initial BV per minute. The hemorrhage was stopped after arterial pressure decreased. In NP rabbits, arterial pressure was well maintained near control pressures of 70 ± 2 mmHg until 38 ± 2% of the initial BV was removed and then rapidly fell to reach a nadir at 35 ± 2 mmHg. In contrast, in P rabbits, basal arterial pressure was lower (61 ± 2 mmHg; P < 0.05) and gradually decreased to below control after <25% of the initial BV was removed. Moreover, the rapid hypotensive phase was triggered with a lower percent BV removal (33 ± 2%; P < 0.05). Basal heart rate was higher during P (149 ± 5 vs. 189 ± 9 beats/min; P < 0.05), and reflex increases were delayed. The slope of the relationship between arterial pressure and vasopressin was not modified during P, although the line was shifted to a lower pressure ( P < 0.05). Larger increases in plasma renin activity and ANG II concentration were produced during hemorrhage in P rabbits. In contrast, no differences in the changes in arterial pressure, heart rate, and vasopressin were found between NP and MP rabbits during hemorrhage, although increases in renin and ANG II were greater at MP ( P < 0.05). In summary, although P conscious rabbits are less able to maintain blood pressure during hemorrhage, this change is not evident at MP. These data suggest that the factors that mediate the P-induced alterations in arterial pressure regulation are not operative until late in gestation.

Pregnancy alters hemodynamic responses to hemorrhage in conscious rabbits

“…Therefore, to produce equivalent hemorrhages in the rabbits when they were pregnant and nonpregnant, the animals were bled as a function of their initial blood volume, which was estimated before each hemorrhage protocol. A blood volume of 49 ml/kg was assumed and calculated for each rabbit in the nonpregnant state (6,17). However, because blood volume is more variable as a function of body weight during pregnancy (6), blood volume was estimated in the pregnant animals the day before the experiment by measuring the volume of distribution of technetium-labeled red blood cells (1).…”

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

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.…”

Visceral pain decreases tolerance to blood loss in conscious female but not male rabbits

“…In contrast, term pregnancy, a physiological state associated with decreased arterial pressure, decreases tolerance to blood loss in conscious rabbits (8,12). For these reasons, it was surprising to find that colorectal distension, a painful stimulus associated with increased arterial pressure, did not increase blood loss required to produce hypotension.…”