What are the physiological effects of hemodilution?

Physiological 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

Mechanisms and Effects of Hemodilution

Blood Viscosity and Flow

• Hemodilution decreases whole-blood viscosity by reducing red cell aggregation and improving red cell deformability 1
• The reduced viscosity leads to increased blood flow velocity and improved microcirculation 2
• Cardiac output increases markedly to compensate for the decreased oxygen-carrying capacity 3
• Total peripheral resistance decreases progressively with hemodilution 4

Oxygen Delivery and Consumption

• As hematocrit is reduced, oxygen delivery increases up to 28-30%, but begins to decline at lower hematocrit levels 1
• The body maintains oxygen consumption through two compensatory mechanisms:
  • Increased cardiac output (primary mechanism) 3
  • Increased oxygen extraction from blood (secondary mechanism) 5
• A hematocrit level greater than 18% is needed to maintain systemic oxygen delivery and consumption during normothermic conditions 4

Cerebral Blood Flow and Oxygenation

• 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 1
• Maximal oxygen delivery to the brain occurs at hematocrit levels of 40-45%, challenging earlier beliefs that 30-33% was optimal 1
• PET scan studies in normal volunteers showed that despite increased cerebral blood flow with hemodilution, oxygen delivery to cerebral tissue still decreased 1

Organ-Specific Effects

• Hemodilution increases blood flow to all organs, improving tissue perfusion 6
• Surface tissue oxygenation on skeletal muscle and liver improves during hemodilution despite raised plasma viscosity 6
• The critical hematocrit value for the whole body may be higher than that required specifically for the brain 4

Clinical Implications and Limitations

Efficacy in Ischemic Conditions

• Hemodilution is particularly effective in increasing oxygenation in ischemic tissue (up to 66% increase in oxygen delivery) compared to normal conditions (only 5-15% increase) 2
• 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

Clinical Outcomes

• Hemodilution does not significantly reduce deaths within the first 4 weeks in stroke patients but may influence deaths within 3-6 months 1
• It has no significant influence on death, dependency, or institutionalization/long-term care 1
• Some trials showed a tendency toward reduction in deep venous thrombosis and pulmonary embolism at 3-6 months 1
• The risk of significant cardiac events does not increase with hemodilution 1

Limitations and Contraindications

• Intentional hemodilution, with or without venesection, is not recommended for treatment of patients with acute ischemic stroke (Class III, Level of Evidence A) 1
• The only possible exception for the use of hemodilution is in stroke patients with severe polycythemia 1
• Moderate hemodilution to hematocrit values of approximately 25% appears to be well tolerated even in single vessel coronary artery disease 3

Practical Considerations

• 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
• Of the rheological factors influencing oxygen delivery, hematocrit plays the predominant role while plasma viscosity is of minor importance 6