Theoretical effects of amniotic fluid volume changes on surfactant concentration measurements.

A baseline model for consideration of the theoretical effects of amniotic fluid volume changes on surfactant concentration measurements has been constructed. The baseline data include: a polynomial equation that best fits experimental data comparing amniotic fluid lecithin concentration measurements to gestational age, inflow of 0.450 L of surfactant-poor fluid into the amniotic fluid per day, outflow of 0.450 L of surfactant-rich fluid per day, and volume of 1.0 L of amniotic fluid between 30 to 31 and 38 to 39 weeks' gestation. With this model the incremental amounts of lecithin necessary to achieve the concentration levels derived from the polynomial equation have been calculated. The quantity of lecithin needed was calculated at intervals of 0.1 day for 56 days. Polyhydramnios and oligohydramnios were then allowed to develop hypothetically by modifying the inflow or outflow of fluid while keeping the quantity of lecithin added exactly the same as was determined from the baseline data. Chronic polyhydramnios or oligohydramnios had minimal effects on the concentration measurements. The effects of acute polyhydramnios (or oligohydramnios) were greater when the change in volume was due to an increased (or decreased) volume of inflow as compared to a decreased (or increased) volume of outflow.