The influence of oligohydramnios on thoracic dimensions of fetal sheep.

Oligohydramnios frequently leads to lung hypoplasia in the fetus, but the underlying mechanisms are incompletely understood. Our aim was to determine the effects of oligohydramnios on the dimensions of the fetal thorax. Using pairs of implanted ultrasound transducers in 6 fetal sheep, we measured 4 thoracic dimensions (transverse, anterior-posterior, manubrium to left and right hand sides of the diaphragmatic dome) for 2 control days, 3 days of amniotic and allantoic fluid drainage (oligohydramnios), and 2 days after the return of drained fluids. The effect of oligohydramnios, which began at 121-2 days of gestation (term being c.145 days), on each dimension was quantified daily as the difference between the measured value and the value predicted from the growth of that dimension over the study period. Oligohydramnios led, within 48 hours, to significant reductions in the transverse dimension (5.9-6.1%) and in the distance between the manubrium and the dome of the diaphragm (1.7-2.2%). There was no change in the anterior-posterior dimension. We conclude that oligohydramnios causes alterations, within 48 hours, in the dimensions of the fetal thorax which can be reversed, at least partially, by re-expansion of the fluid sacs. These changes, which are expected to produce reductions in thoracic volume, may, if prolonged, lead to lung hypoplasia.