HEMODYNAMIC AND HORMONAL RESPONSES TO HEMORRHAGE IN CONSCIOUS RABBITSAT MID-GESTATION AND LATE-GESTATION

This study tests the hypothesis that conscious rabbits late in pregnan cy (P), but not at midgestation (MP), are less able to maintain arteri al pressure during hemorrhage. Blood volume (BV) was elevated (P < 0.0 5) 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 pres sures of 70 +/- 2 mmHg until 38 +/- 2% of the initial BV was removed a nd then rapidly fell to reach a nadir at 35 +/- 2 mmHg. In contrast, i n P rabbits, basal arterial pressure was lower (61 +/- 2 mmHg; P < 0.0 5) and gradually decreased to below control after <25% of the initial BV was removed, Moreover, the rapid hypotensive phase was triggered wi th 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 i n 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 ra bbits 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.