Pregnancy alters hemodynamic responses to hemorrhage in conscious rabbits

By: Kathy A. Clow, George D. Giraud, Bryan E. Ogden and Virginia L. Brooks (AJP - Heart 284:1110-1118, 2003. First published Dec 5, 2002; doi:10.1152/ajpheart.00626.2002)

The onset of a hemorrhage will result in two different phases. The first phase is the "nonhypotensive" phase, where arterial pressure (AP) is sustained. Once blood loss becomes excessive, the onset of the second "hypotensive" phase will occur, and that is when AP rapidly decreases. Both of these phases are altered in pregnancy. Baroreflex control of heart rate and SNA is decreased in pregnancy, which may be suggested by inadequate reflex responses. The hypothesis is during hemorrhage, pregnant animals exhibit a lesser ability to maintain cardiac output as well as reduced reflex vasoconstrcition as measured by increases in total peripheral resistance (TPR). A second purpose of the study was to determine whether the response to hemorrhage of the femoral vascular bed, which perfuses the hindquarters, was smaller in pregnant rabbits.

10 female rats were used in this study. Catheters were inserted to record AP. Flow probes were also implanted. Rabbits were first hemorrhaged in the nonpregnant state. They were then bred with male breeders, and another hemorrhage was induced 28-30 days of pregnancy. The animals were bled as a function of their initial volume using body weight, since blood volume increases during pregnancy.

At basal levels, pregnant rabbits had lower AP and TPR, as well as higher HR, cardiac output, and total peripheral conductance. During the first phase of hemorrhage, AP was significantly less with less blood loss during pregnancy. Cardiac output decreased much quicker in pregnant rabbits. Decrease after 12.5 min was also greater during pregnancy. TPR increased, while total peripheral conductance decreased. AP fell below 45 mmHg after 25% blood loss during pregnancy, where before pregnancy, AP fell blow 45 mmHg after 39.4% blood loss. During the second phase of hemorrhage, the decrease in pressure in pregnancy was less. CO, TPR, and SV also decreased less in pregnancy.

This study showed that pregnancy alters how the body responds to a hemorrhage. AP fell much more rapidly in pregnant rabbits than non pregnant. Cardiac output decreased during pregnancy, but TPR increased. In theory, TPR would go up if cardiac output went up. We see that during the hemorrhage TPR seems to go up as a reflex of the CO going down so much. It was the body's way of compensating for the severe loss of blood.

This study relates to what we are doing in the lab, by comparing how the baroreflex controls the blood loss. We are studying females now, in addition to males, so they behave much differently. This study focuses how pregnancy affects this entire response system, and it was much different than the basal values. This is important info as we start studying females more.

-Tsetse Fly