Rheological properties of fetal red cells with special reference to aggregability and disaggregability analyzed by light transmission and laser backscattering techniques.

Blood viscosity factors and fetal erythrocyte aggregability were investigated with light transmission (Myrenne device) during a cross-sectional study of blood drawn in utero by cord venepunctures in 119 normal fetuses between 18 and 39 weeks gestation. There was a progressive increased blood viscosity at native hematocrit (p < 0.01) explained by a gradual increase in both hematocrit (from 33% to 40%, p < 0.05) and Dintenfass' 'Tk' RBC rigidity index (p < 0.05), while plasma viscosity remained constant at 1.18 +/- 0.01 mPa x s as well as the h/eta ratio (188.4 +/- 2.7 mPa(-1) x s(-1)). The RBC aggregation index 'M' remained almost equal to zero (mean value: 0.04 +/- 0.01) before 32 wk gestation and then increased (p < 0.05) until delivery. The upper physiological limit for this parameter before 32 wk (mean +/- 2 SD) is 0.18. The RBC aggregation index 'M1' remained constant during pregnancy at 2.98 +/- 0.26, i.e., the upper physiological limit for this parameter during the intrauterine life (mean +/- 2 SD) is 7.85. Both fibrinogen (r = 0.479, p < 0.05) and albumin (r = 0.494, p < 0.01) correlated with time so that the albumin/fibrinogen ratio remained stable. We then studied with the laser retrodiffusion technique the venous blood of 20 women (18-43 yr, 37-40 wk gestation) and the cord blood of their newborns at birth, comparing RBC aggregation of: mothers (M), maternal RBCs resuspended on newborn plasma (MF), newborn RBCs resuspended on maternal plasma (FM), and newborns (F). Aggregability is higher in M (RBC aggregation time M < MF < FM < F; p < 0.01); RBC aggregation index at 10 s M > MF > FM > F; p < 0.01), with in turn the symmetric inverse picture for the partial disaggregation threshold (M > MF = FM > F). Thus RBC disaggregability is higher in newborns, and suspensions on maternal and newborn plasma suggest that half of this difference in aggregability (and disaggregability) between fetal and adult blood results from plasma factors and another half from erythrocytes.