Physiological evidence for arteriovenous anastomoses in the uterine circulation of late-pregnant ewes.

1. The objective of the present study was to determine whether arteriovenous anastomoses (AVA) are present in the uterine circulation of conscious, late-pregnant ewes. 2. Twenty late-pregnant ewes were assigned to two groups. In group 1, 15 microm coloured microspheres were injected into the uterine artery of the pregnant horn and the relative proportion of microspheres trapped in the uterus and lungs was determined. The percentage shunting of blood measured by this method was 17+/-3%, representing a blood flow of 164+/-39 mL/min (n = 12). Any contribution of ovarian, cervical, rectal and other vascular beds was specifically excluded in this group. 3. In group 2, total uterine capillary flow was measured using the reference blood flow method, by injecting 15 microm microspheres into the heart. Calibrated flow probes, placed on both main uterine arteries, concurrently measured total uterine blood flow. Total blood flow to the uterus (flow probe) was significantly greater (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 groups 1 and 2 was not significantly different (t = 1.219; P = 0.24) and, when combined, the mean percentage of uterine blood flow passing through AVA in the late-pregnant sheep was determined to be 19+/-3%. 5. The present study shows that AVA are present and patent in the uterine circulation of late-pregnant ewes and that they account for the shunting of approximately 20% of the uterine blood flow into the maternal venous circulation.