Fisk, Nicholas Maxwell; (1992) Amniotic pressure in disorders of amniotic fluid volume. Doctoral thesis (Ph.D.), UCL (University College London).

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Abstract

Amniotic pressure (AP) has been assumed to be raised in both oligohydramnios and polyhydramnios, but has not previously been measured. The aims of this thesis were (i) to characterize AP in human pregnancies with normal amniotic fluid volume (ii) to compare with this AP in pregnancies with abnormal amniotic fluid volume and (iii) to investigate the relationship of abnormal AP with the complications of disorders of amniotic fluid volume. AP was measured during invasive procedures by a fluid manometry system attached to a needle positioned within the amniotic cavity. In pregnancies with normal amniotic fluid volume, AP increased with advancing gestation, but was not related to amniotic fluid volume. In comparison, AP was significantly higher in pregnancies with polyhydramnios and lower in those with oligohydramnios, the degree of abnormality in AP correlating with the severity of derangement in amniotic fluid volume; furthermore, AP returned towards normal with drainage and infusion of fluid respectively. In polyhydramnios, amniotic pressure was negatively correlated with fetal pO2 and pH, suggesting that raised AP may impair uteroplacental perfusion. In pregnant sheep however, fetal acid/base status was unaltered when AP was acutely elevated by amnioinfusion. In human pregnancies complicated by severe oligohydramnios, restoration of amniotic fluid volume did not alter the incidence of fetal breathing movements or Doppler indices of umbilical artery downstream resistance; together with the finding of low AP, these results challenge the concept of fetal compression in oligohydramnios that has become widely accepted in the literature. In order to determine whether lung hypoplasia in oligohydramnios is caused by low amniotic pressure disturbing the tracheal-amniotic pressure gradient, fetal sheep were subjected to chronic pharyngeal drainage at subamniotic pressures, but this had no effect on lung development. That tracheal drainage affects lung development was confirmed, but no evidence for this effect being via a reduction in fetal breathing could be obtained. It is concluded that amniotic pressure is elevated in polyhydramnios and reduced in oligohydramnios.

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