Maternal cigarette smoking and oxygen diffusion across the placenta

The aim of this study was to test whether or not adaptations in partial, total and specific oxygen diffusive conductances occur in the placentae of women who smoke cigarettes during pregnancy and help to compensate for intrauterine fetal hypoxic stress. Tissue sections were randomly sampled from human term placentae divided into two groups (non-smokers and smokers) according to maternal smoking status. In smokers, status was expressed as either declared smoking rate or level of plasma cotinine (the major metabolite of nicotine). Sections were analysed stereologically to estimate key structural quantities (vascular volumes, exchange surface areas, tissue diffusion distances). These were combined with previously-published physicochemical quantities (oxygen-haemoglobin reaction rates and tissue oxygen diffusion coefficients) in order to estimate the partial conductances of six tissue compartments of the oxygen pathway: maternal erythrocytes and plasma, villous trophoblast, villous stroma (including fetal capillary wall), fetal plasma and erythrocytes. From partial conductances and birthweights, total and specific conductances were calculated for each placenta. Results were assessed statistically by analyses of variance and t -tests. Despite apparent improvements in the partial conductances of the maternal erythrocytes and plasma, total and specific conductances did not alter significantly in smoking groups. However, the relative biases affecting these estimates may be different in smokers and non-smokers. We conclude that total conductance does not increase in placentae associated with maternal smoking. However, given that the fetus suffers chronic hypoxic stress as a consequence of smoking (evidenced here by elevated haematocrits), even a constant diffusive conductance implies a reduced transplacental partial pressure gradient. This could be a contributory factor to the reduced birthweight.