Physiological Effects of Hemodilution

Mechanisms and Effects of Hemodilution

Hemodilution improves blood flow but reduces oxygen-carrying capacity, with optimal oxygen delivery occurring at hematocrit levels of 28-30%, below which oxygen delivery begins to decline 1, 2, 3
Hemodilution decreases whole-blood viscosity by reducing red cell aggregation and improving red cell deformability 1
As hematocrit is reduced, oxygen delivery increases up to 28-30%, but begins to decline at lower hematocrit levels 1, 3

In cerebral infarction, cerebral autoregulatory mechanisms are impaired 1
Hemodilution increases cerebral blood flow in both infarcted and contralateral hemispheres 1
While there is an inverse relationship between hematocrit and cerebral blood flow, there is a linear relationship between hematocrit and oxygen delivery to brain tissue 4
Maximal oxygen delivery to the brain occurs at hematocrit levels of 40-45%, challenging earlier beliefs that 30-33% was optimal 4, 5
PET scan studies in normal volunteers showed that despite increased cerebral blood flow with hemodilution, oxygen delivery to cerebral tissue still decreased 4

In stroke patients, hemodilution has been studied to improve cerebral blood flow to potentially viable brain tissue supplied by collaterals 1
Despite theoretical benefits, clinical trials have not shown significant improvements in mortality or functional outcomes in stroke patients 1, 2
Hemodilution does not significantly reduce deaths within the first 4 weeks in stroke patients but may influence deaths within 3-6 months 1
Intentional hemodilution, with or without venesection, is not recommended for treatment of patients with acute ischemic stroke (Class III, Level of Evidence A) 1, 3

Maintenance of a normal circulating blood volume with regulation of metabolic parameters within physiological ranges is desirable 1
Various solutions have been used for hemodilution including dextran 40, hydroxyethyl starch, and albumin 1