Determinants of flow along arterio‐arterial anastomoses in monochorionic placentae by dynamic computer modelling of chorionic plate vasculature

Objectives To investigate anatomical and physiological determinants of inter‐fetal transfusion along arterio‐arterial (AA) anastomoses in monochorionic placentae. Methods A computer model of chorionic arterial vasculature was constructed in QuickBASIC using data collected from experimentation and the published literature. After validating the model, the influence of various physiological and anatomical variables on anastomotic flow rates was examined. Results AA anastomotic flow rates were significantly related to changes in fetal mean arterial pressure ( p < 0.0001) and heart rate ( p < 0.0005). AA flow rates were also related to the imbalance in number of arterio‐venous (AV) anastomoses, to placental territory share, and to the branch number of the AA anastomosis (AAAs) from the chorionic arterial tree. Conclusions Net blood flow and direction along AA anastomoses are influenced by fetal cardiac output, by the presence of compensatory AV anastomoses, and by the branch number of the chorionic arteries connected by the anastomosis. This study provides insight into the determinants of chronic transfusional imbalance as well as acute inter‐fetal transfusion. Copyright © 2006 John Wiley & Sons, Ltd.