Mathematical, clinical, and laboratory study of hemodynamic changes in the placental circulation.

Various rheological properties of blood were investigated in pregnant and non-pregnant women of similar age. The results from 27 women with abnormal pregnancies were compared to those obtained from 17 non-pregnant women. In abnormal pregnancies we found a reduction of the erythrocyte filtration and a pathological use in red cell aggregation and in the shear resistance of the aggregates. Fractional light transmission (T7/T0) was higher than 1.0, suggesting that pressure gradients in the capillary circulation are not sufficient to disperse red cell aggregates. Thus, rheological factors may be partly responsible for the clinical consequences of pre-eclampsia. Pathological red blood cell aggregation and stagnation are consequences of the changed flow properties in the microcirculation. Even a small decrease of the local pressure gradients in the microcirculation of patients with pre-eclampsia would impair the dispersal of red blood cells and lead to their increased aggregation. Complete occlusion of the placental circulation by these aggregates would impair oxygen transfer at a speed that has been calculated from a diffusion equation based on the concept of the placenta as a hollow cylinder.