Opposite effects of red blood cell aggregation on resistance to blood flow

We summarized the results of two series of independent experiments recently published by our group which studied the consequences of changes in red blood cell (RBC) aggregation on resistance to blood flow in vivo. The first series compared in vivo apparent viscosity in isolated hearts perfused by either normo-aggregating or non-aggregating RBC suspension. We showed that a moderate degree of RBC aggregation induces a significant decrease in apparent relative viscosity of a suspension of RBC when compared to a non-aggregating RBC suspension. The second series studied the effects of normo vs. hyperaggregation in muscle microvascular network. We showed that hyperaggregation of RBC is responsible for a decrease in arteriolar blood flow velocity and for a decrease in density of perfused capillaries. This latter induces an additional increase in the resistance to blood flow and may also have important deleterious consequences for tissue oxygenation. From these two series of experiments, we concluded that RBC aggregation can have two opposite effects on in vivo viscosity and tissue perfusion. A moderate level of RBC aggregation reduces in vivo viscosity whereas a high level of aggregation increases this viscosity and the resistance to blood flow. We believe that the normo-aggregating state is approximately an equilibrium point between these two tendencies. Consequently, RBC hyperaggregation induced either by acute pathological states, or by use of hyperaggregating plasma substitutes or by biological disturbances which increase RBC aggregation, such as high fibrinogen may have important deleterious consequences for tissue perfusion and tissue oxygenation.