Effect of increased blood fluidity through hemodilution on general circulation at rest and during exercise in dogs

During progressive normovolemic hemodilution with dextran-60, circulatory functions (cardiac output, oxygen delivery to tissues, arterial pressure and mixed venous oxygen saturation) and total body oxygen consumption were studied in conscious dogs at rest and during two levels of submaximal treadmill exercise. At rest, cardiac output rose continuously with progressive hemodilution. This increase, however, was not sufficient to compensate for the reduced arterial oxygen content. Consequently oxygen delivery fell significantly from 23.3 +/- 1.8 ml/min with kg at hematocrit 47.5% to 15.7 +/- 0.9 ml/min with kg at hematocrit 12.5%. The constant oxygen consumption was maintained by a simultaneous increase in oxygen extraction from blood. During the superimposed stress of exercise, a constant oxygen consumption was maintained between hematocrit ranges of 50 to 15 or 25%, respectively. Again, the increase of cardiac output due to hemodilution did not compensate for the reduced arterial oxygen content and consequently oxygen extraction rate was increased. These data demonstrate that at rest (and even more during submaximal treadmill exercise) the reduced whole blood viscosity or improved fluidity during hemodilution does not initiate an increase in cardiac output that is sufficient to maintain a constant oxygen delivery to the tissues.