PHYSIOLOGICAL EVIDENCE FOR ARTERIOVENOUS ANASTOMOSES IN THE UTERINE CIRCULATION OF LATE-PREGNANT EWES

1. The objective of the present stud was to determine whether arteriov enous anastomoses (AVA) are present in the uterine circulation of cons cious, late-pregnant ewes. 2. Twenty late-pregnant ewes were assigned to two groups. In group 1, 15 mu m coloured microspheres were injected into the uterine artery of the pregnant horn and the relative proport ion of microspheres trapped in the uterus and lungs was determined. Th e percentage shunting of blood measured by this method was 17 +/- 3%, representing a blood flow of 164 +/- 39 mL/min (n = 12). Any contribut ion of ovarian, cervical, rectal and other vascular beds was specifica lly excluded in this group. 3. In group 2, total uterine capillary flo w was measured using the reference blood flow method, by injecting 15 mu m microspheres into the heart. Calibrated flow probes, placed on bo th main uterine arteries, concurrently measured total uterine blood fl ow. Total blood flow to the uterus (flow probe) was significantly grea ter (t = 3.415; P = 0.027) than uterine capillary flow (microspheres), indicating AVA shunting in the uterine circulation. Mean total blood flow to the uterus was 1749 +/- 160 mL/min, of which an average 25 +/- 5% (n = 5 sheep) was shunted. 4. The percentage AVA shunting for grou ps 1 and 2 was not significantly different (t = 1.219; P = 0.24) and, when combined, the mean percentage of uterine blood flow passing throu gh AVA in the late-pregnant sheep was determined to be 19 +/- 3%. 5. T he present study shows that AVA are present and patent in the uterine circulation of late-pregnant ewes and that they account for the shunti ng of approximately 20% of the uterine blood flow into the maternal ve nous circulation.