Fetal breathing and pressures in the trachea and amniotic sac during oligohydramnios in sheep

Oligohydramnios commonly leads to fetal lung hypoplasia, but the mechanisms are not fully understood. Our aim was to determine, in fetal sheep, the effects of prolonged oligohydramnios on the incidence and amplitude of tracheal pressure fluctuations associated with fetal breathing movements (FBM), on tracheal flow rate during periods of FBM (VtrFBM) and periods of apnea (Vtrapnea), on tracheal pressure relative to amniotic sac pressure, and on amniotic sac pressure relative to atmospheric pressure. In five sheep, oligohydramnios was induced by draining amniotic and allantoic fluids from 107 to 135 days of gestation (411.8 +/- 24.4 ml/day), resulting in fetal lung hypoplasia. In five control sheep, amniotic fluid volume was 732.3 +/- 94.4 ml. Oligohydramnios increased the incidence of FBM by 14% at 120 and 125 days and the amplitude of FBM by 30-34% at 120-130 days compared with controls. From 120 days onward, VtrFBM was 35-55% lower in experimental fetuses than in controls. Influx of lung liquid during FBM was 87% lower in experimental fetuses than in controls. Vtrapnea, tracheal pressure, and amniotic sac pressure were not significantly altered by oligohydramnios. Our tracheal flow rate data suggest that transient changes in lung liquid volume during periods of FBM and periods of apnea were diminished by oligohydramnios. We conclude that the primary factor in the etiology of oligohydramnios-induced lung hypoplasia is not an inhibition of FBM (as measured by tracheal pressure fluctuations) or a reduction in amniotic fluid pressure.(ABSTRACT TRUNCATED AT 250 WORDS)