Maternal cigarette smoking and oxygen diffusion across the placenta

The aim of this study was to test whether or not adaptations in partial, to tal and specific oxygen diffusive conductances occur in the placentae of wo men who smoke cigarettes during pregnancy and help to compensate for intrau terine fetal hypoxic stress. Tissue sections mere randomly sampled from hum an term placentae divided into two groups (non-smokers and smokers) accordi ng to maternal smoking status. In smokers, status was expressed as either d eclared smoking rate or level of plasma cotinine (the major metabolite of n icotine). Sections were analysed stereologically to estimate key structural quantities (vascular volumes, exchange surface areas, tissue diffusion dis tances). These were combined with previously-published physicochemical quan tities (oxygen-haemoglobin reaction rates and tissue oxygen diffusion coeff icients) in order to estimate the partial conductances of six tissue compar tments of the oxygen pathway: maternal erythrocytes and plasma, villous tro phoblast, villous stroma (including fetal capillary wall), fetal plasma and erythrocytes. From partial conductances and birthweights, total and specif ic conductances were calculated for each placenta. Results were assessed st atistically by analyses of variance and t-tests. Despite apparent improveme nts in the partial conductances of the maternal erythrocytes and plasma, to tal and specific conductances did not alter significantly in smoking groups . However, the relative biases affecting these estimates may be different i n smokers and non-smokers. We conclude that total conductance does not incr ease in placentae associated with maternal smoking. However, given that the fetus suffers chronic hypoxic stress as a consequence of smoking (evidence d here by elevated haematocrits), even a constant diffusive conductance imp lies a reduced transplacental partial pressure gradient. This could be a co ntributory factor to the reduced birthweight. (C) 2000 Harcourt Publishers Ltd.