A study of haemorheological behaviour in 150 pregnant subjects

Rheological behaviour in 150 normotensive pregnant subjects has been studied. All patients had uncomplicated pregnancies and consisted of 66 primagravidae and 84 multigravidae subjects. Whole blood and plasma viscosities were measured on a Carri-Med controlled Stress Rheometer and, with the exception of plasma fibrinogen quantitation, all other parameters were calculated from the results of these two basic techniques. These included red cell rigidity and “oxygen delivery” quotient. The latter is a relative calculation based on the fact that oxygen delivery to the tissues is directly proportional to the haemoglobin concentration and blood fluidity. Given that viscosity is inversely proportional to fluidity then the haemoglobin/whole blood viscosity quotient indicates the oxygen delivery relative to the control group. Whole blood viscosity produces a decline (not statistically significant by the least squares method which was used throughout this study) reaching a minimum at 26 to 28 weeks of gestation then an increase to its original value by delivery (r=0.74, p=<0.0l). Plasma viscosity shows an increase from 32 weeks (r=0.74, P=<0.05). The mean haematocrit produces similar results to those of whole blood viscosity while the red cell rigidity peaks at 26 weeks then peaks again at a value outside the reference range at delivery. Similarly relative oxygen delivery to the tissues steadily decreases throughout pregnancy and reaches a minimum at 28 weeks (r=0.64, p=<0.001). An immediate increase to a value at 32 weeks then occurs followed by a further decrease to delivery at which point only some 55% relative oxygen delivery occurs. Clearly, further haematological investigation is indicated at the 26th to 28th week of gestation in view of these physiological changes.