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

Abstract

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.

Control of hepatic and intestinal blood flow: effect of isovolaemic haemodilution on blood flow and oxygen uptake in the intact liver and intestines.

“…Messmer (1975) stated that increased tissue blood flow serves as the primary means of compensation for reduced oxygen content, while oxygen extraction remains unaltered. Von Restorff, Hofling, Holtz & Bassenge (1975), on the other hand, supported earlier work which demonstrated an increased oxygen extraction as well as an elevated cardiac output. The observation that venous oxygen tensions are different in blood draining different regions (Murray, 1964) suggests that not all organs respond to haemodilution in an identical manner.…”

“…Von Restorff et al (1975) came to a similar conclusion. The reduced haematocrit would however permit a greater degree of vasodilation than the maximum obtained at normal viscosity.…”

Reassessing the risk of hemodilutional anemia: Some new pieces to an old puzzle

“…41 However, at more severe levels of hemodilution (hematocrit * 10%), animal studies demonstrate an overall reduction in the metabolic consumption of oxygen during acute hemodilution. 74,75 This reduction in metabolic rate is associated with heterogeneous and organ-specific changes in oxygen consumption in which the cerebral metabolic rate remains unchanged 76 while oxygen consumption increases in the heart 26,77 and decreases in the kidney. 78 The overall reduction in the metabolic requirement for oxygen at very low hematocrit may help to balance oxygen demand with its reduced supply during acute hemodilution.…”

Do We Have Enough Evidence to Know when to Transfuse Erythrocytes?

“…In doing so, in several separate studies, Cain [4,5] repeatedly established that the critical oxygen delivery in anesthetized dogs is approximately 10 ml O2/kg/min, which occurred in those dogs at an hematocrit of 10-15%. Von Restorff et al [6] were unable to find systemic evidence of inadequate oxygen delivery in awake dogs at similar hematocrits, likely because they did not study more severe anemia. Furthermore, the value of critical oxygen delivery varies among species [7], making extrapolation of these data to humans not possible.…”