A new method for measuring unidirectional transplacental flux.

A new method for measuring unidirectional transplacental flux has been developed. Unlike indicator-dilution techniques, it does not depend on the use of a second reference tracer to study the substance of interest, nor on assumptions that the placenta can be modelled as a series of long, parallel, tubular capillaries. The only assumptions required are that the placenta behaves as a linear, time-invariant system and does not distinguish between tracer and tracee. The method was validated by measuring unidirectional flux rates of Evans Blue dye in an hydraulic model in which the true flux rates were controlled by peristaltic pumps. There was excellent agreement between the calculated and true flux rates. A preliminary experiment was carried out in vitro using a dually perfused human placenta to measure the unidirectional flux rates of L-lactate. At a steady state lactate concentration of 25.5 mM, maternal to foetal flux rate was 30.1 mumol min-1 and foetal to maternal flux rate was 34.0 mumol min-1, in agreement with measurements reported by other workers.