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 investigat ed 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 viscosi ty at native hematocrit (p<0.01) explained by a gradual increase in both he matocrit (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.s a s well as the h/eta ratio (188.4 +/- 2.7 mPa(-1).s(-1)). The RBC aggregatio n index 'M' remained almost equal to zero (mean value: 0.04 +/- 0.01) befor e 32 wk gestation and then increased (p<0.05) until delivery. The upper phy siological limit for this parameter before 32 wk (mean +/- 2 SD) is 0.18. T he RBC aggregation index 'M1' remained constant during pregnancy at 2.98 +/ - 0.26, i.e., the upper physiological limit for this parameter during the i ntrauterine 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/fib rinogen ratio remained stable. We then studied with the laser retrodiffusio n 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: m others (M), maternal RBCs resuspended on newborn plasma (MF), newborn RBCs resuspended on maternal plasma (FM), and newborns (F). Aggregability is hig her 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 th e partial disaggregation threshold (M>MF=FM>F). Thus RBC disaggregability i s higher in newborns, and suspensions on maternal and newborn plasma sugges t that half of this difference in aggregability (and disaggregability) betw een fetal and adult blood results from plasma factors and another half from erythrocytes.