In-utero characterization of the blood flow in the Hyrtl anastomosis

An anastomosis between the two umbilical arteries, located approximately wi thin 3 cm from the placental insertion, has been previously described at de livery and in utero. However, the prenatal investigation of the Hyrtl anast omosis is limited to case reports. We report a series of antenatal function al evaluations of this vessel. Forty-one women underwent a target ultrasono graphy to evaluate the blood flow characteristics of the Hyrtl anastomosis. The resistance index of the anastomosis and the umbilical arteries resista nce indices before and after the anastomosis were obtained. The direction o f the brood flow in the anastomosis was determined by color Doppler evaluat ion. An anastomosis between the two stems of the umbilical arteries was pre sent in 36 cases while a fusion of the two umbilical arteries was found in the remaining five cases. The median (range) gestational age at diagnosis w as 33.1 weeks (25.5-40.1). The median diameter of the anastomosis was 2.3 m m (1.3-7.1). The blood flow in the anastomosis was pulsatile with a median resistance index of 0.62 (0.45-0.85) and unidirectional toward the umbilica l artery with lon er resistance index. The difference between the resistanc e indices of the two umbilical arteries was higher after than before the an astomosis [0.07 (0-0.3) versus 0.04 (0-0.17), P=0.05]. The median diameter of the Hyrtl anastomosis was significantly higher when the anastomosis was oblique (n=8) than when it was transverse (n=28) (4.8 mm [2-7.1] versus 2.3 mm [1.3-5.3], P <0.05). In three out of the five cases with fusion of the two umbilical arteries the placental insertion was marginal or velamentous. We conclude that the Hyrtl anastomosis may act as a pressure-equalizing sy stem between umbilical arteries. This supports the hypothesis that the Hyrt l anastomosis plays an important role when the placental territories suppli ed by the umbilical arteries are different in size. (C) 2001 Harcourt Publi shers Ltd.