MATHEMATIC MODELING OF HUMAN AMNIOTIC-FLUID DYNAMICS

OBJECTIVE: We sought to develop a model quantifying the relative contr ibutions of fetal swallowing and intramembranous flow to amniotic flui d dynamics during human gestation. We then used the model to simulate the impact of absent swallowing on amniotic fluid volume. STUDY DESIGN : The model was developed with published data for normal human amnioti c fluid volume and composition, human fetal urine flow rate and compos ition (11 to 42 weeks), and extrapolated data from ovine lung fluid pr oduction. Fetal swallowing and intramembranous flow were calculated wi th assumptions that (1) swallowed fluid is isotonic to amniotic fluid, (2) intramembranous flow is free water diffusion, and (3) 50% of lung fluid is swallowed. The model was then applied to simulate absent fet al swallowing and variable (0%, 50%) proportions of swallowed lung flu id were used as a representation of esophageal atresia-tracheal fistul a variations. RESULTS: Fetal swallowed volume and intramembranous flow linearly increase until 28 to 30 weeks. Daily swallowed volume then e xponentially increases to a maximum of 1006 ml/day at term, whereas in tramembranous flow continues on a linear trend to reach 393 ml/day at term. With absent swallowing and variable amounts of lung fluid swallo wed (0%, 50%), predicted amniotic fluid volume is similar to normal va lues through 20 weeks, exceeds the 95% confidence interval for normal amniotic fluid volume at 29 to 30 weeks' gestation (similar to 2000 mi ), and then exponentially increases. Predicted amniotic fluid osmolali ty (280 to 257 mOsm/kg) is slightly lower than actual values although within the clinically normal range. CONCLUSIONS: This model indicates that the normal reduction in amniotic fluid volume beginning at 34 wee ks results from the marked increase in swallowed volume during the thi rd trimester. Additionally, this model correlates well with the timing of the initial clinical presentation of polyhydramnios observed in so me fetuses with conditions that result in absent or reduced swallowing or gastrointestinal atresia. Modeling of amniotic fluid dynamics can predict normal changes in fetal fluid exchange and may aid in understa nding of amniotic fluid imbalances.