RED-BLOOD-CELL AGGREGATION AND BLOOD-VISCOSITY IN AN ISOLATED HEART PREPARATION

We studied the effects of moderate changes in red blood cell RBC aggre gation on blood flow in the vasodilated vascular bed of an isolated ra t heart. We compared a non-aggregating RBC suspension (in Krebs-albumi n medium) with RBC suspensions in 1% and 2% Dextran 70 (MW 70000), exh ibiting two different degrees of moderate aggregation. Degrees of aggr egation were precisely estimated by in vitro laser aggregometry. Each heart was perfused by the nonaggregating RBC suspension and by one agg regating RBC suspensions. Blood flow was measured in a range of perfus ion pressure from 40 to 80 mm Hg. For the three RBC suspensions, linea r pressure/flow relationships were found. From the comparison between the pressure/flow relationships obtained with Krebs albumin medium and either 1% or 2% Dextran, it was possible to compare in vivo the contr ibution of RBC to the viscosity (i.e., the relative apparent viscosity ) in the 2 aggregating RBC suspensions with that of the non-aggregatin g RBC suspension. The contribution of RBC to the viscosity was found t o be 20% to 25% lower in the 1% RBC suspension than in the non-aggrega ting RBC suspension. With 2% Dextran which induced a higher degree of aggregation no differences were found between the relative apparent vi scosities of the aggregating and the non-aggregating suspension. From the comparison between RBC in 1% Dextran and Krebs-albumin, we conclud ed that in vivo a moderate RBC aggregation reduces viscous resistance due to the presence of blood in a vascular network. Since no more effe ct of RBC aggregation per se was found when the degree of aggregation was higher (with RBC in 2% Dextran), this suggests that, in this case, aggregation induces opposite effects along the myocardial vascular ne twork which cancel each other out, thus inducing a nil net balance.