Lung water in pregnancy and the early puerperium
Background. Today, Mendelson's syndrome (pneumonitis following the aspiration of gastric contents) is a major cause of the small, but apparently irreducible number of maternal deaths associated with anaesthesia. Previous research has been directed at the relationship between the physical characteristics of the aspirate and the severity of the ensuing pneumonitis. Prophylaxis, logically based on this knowledge has been directed at reducing the volume and increasing the pH of the stomach contents of women in labour, and at specific measures to reduce the risk of aspiration at the induction of anaesthesia. Despite this, the maternal mortality attributed to Mendelson's syndrome remains essentially unchanged. The operation of an additional, but previously unconsidered variable might explain both the apparent increase in severity of Mendelson's syndrome and the failure of prophylaxis. This thesis reports attempts to elucidate the place of lung water content in this context. Review of the literature and theoretical discussion. This falls into two distinct but inter-related categories: (a) Circumstantial evidence, already existing in the literature for lung water handling being altered in pregnancy and the puerperium, for example the reports of B2 sympathomimetics inducing pulmonary oedema when used for tocolysis. (b) A detailed review of the present understanding of the pathogenesis of pulmonary oedema, together with the alterations in maternal physiology which accompany normal pregnancy. The two have not previously been considered in conjunction. From this base it is possible to proceed to a theoretical prediction of the consequences of such common interventions as the use of intravenous fluids, oxcytocics and endotracheal intubation under light general anaesthesia. Clinical Study - Normal Primagravidae. Twenty normal primagravidae were investigated for evidence of increased lung water. Detectable increases were found in seven. There was a positive correlation with plasma volume and water turnover. Clinical Study - Pre Eclampsia. Seventeen primagravidae having a pregnancy complicated by pre eclampsia were investigated for evidence of increased lung water. Detectable increases were found in six. There was a positive correlation with fluid balance. Mouse Model. The lungs of pregnant and non pregnant MF1 mice were examined by electron microscopy to confirm and if possible elucidate the expansion of the interstitial tissue detected by the clinical study. A stereological method of point counting allowed volumetric quantification of the tissue components and measurement of surface areas and thicknesses. The results showed that in pregnant mice: (1) Total lung volume was increased but not significantly; (2) The volume and surface area of alveolar lining cells was increased; (3) Pulmonary blood volume was unchanged; (4) Capillary surface area was unchanged; (5) The interstitial compartment was expanded. Within this compartment: The cellular component was unchanged The volume of collagen was unchanged The amorphous component was enlarged Basal lamina volume was increased Appendix A. A magnetic resonance imaging method of lung water measurement was developed with the intention of using it in pregnancy but was not sufficiently sensitive for this purpose.