Derivation of rate of arterio-arterial anastomotic transfusion between monochorionic twin fetuses by Doppler waveform analysis.

ABSTRACT

We aimed to determine rates of interfetal transfusion along arterio-arterial (AA) anastomoses in monochorionic twins in vivo from analysis of Doppler waveform patterns. Twenty-one monochorionic twin pregnancies in which an AA anastomosis was identified antenatally underwent serial Doppler velocimetry. Unidirectional AA anastomotic flow rates increased with increasing gestational age (log y = 8 x 10(-9)x - 5 x 10(-8); p = 0.0002). The mean net rate of flow through an AA anastomosis at 28 weeks gestation was 7.6 x 10(-8) l/s (SD = 4.9 x 10(-8) l/s). This flow was significantly related to the distribution of arterio-venous (AV) anastomoses (p = 0.009) and birthweight discordancy (p = 0.006). We derived estimates of flow along individual AV anastomoses by assuming that net AA countertransfusion is shared equally among uncompensated AV anastomoses, and speculate that the median AV flow rate at 28 weeks is in the order of 6 x 10(-8) l/s. In conclusion, this study demonstrates that flow rates along AA anastomoses can be quantified antenatally. These are the first estimates of flow rates in vivo along placental anastomoses. Although AA net flows are modest, chronic unbalanced counterflow of this magnitude in the absence of compensatory superficial anastomoses could lead to significant haemodynamic compromise.