Aortic compression and uterine blood flow during pregnancy.

To understand the effects of uterine compression on the abdominal aorta in pregnant women, the abdominal aorta in pregnant animals was experimentally constricted. This procedure produced symptoms identical to toxemia in humans. A physical model simulating the human aorta with blood flowing under physiologic pressure, and a "term uterus" pressing on it showed that blood flow reduction thus produced easily could be within the range observed in the toxemic human. Experimental constriction or compression of the abdominal aorta in pregnant animals produced a parallel drop in the femoral artery pulse pressure and uterine artery blood flow. This observation is applicable to pregnant women in the supine position. When the pregnant uterus presses on the abdominal aorta, reduced uterine artery blood flow was noted by following femoral or popliteal artery pulse pressure reduction. In 379 pregnant patients studied, 40% showed a drop in popliteal artery pulse pressure during uterine contraction and 11% during uterine relaxation. During relaxation, this was observed in 16% of primiparous patients and in 6% of multiparious patients.