Hemodilution & blood

A) VISCOSITY

1. Blood pressure = cardiac output x total peripheral resistance

2. Flow (cardiac output) µ perfusion pressure

total peripheral resistance

3. Flow µ perfusion pressure

vascular resistance x viscosity

4. viscosity = shear stress

shear rate

As the blood vessel diameter decreases —> shear rate decreases —> increase viscosity —> increase peripheral resistance (ie most of resistance is in smaller vessels, not in aorta & larger vessels)

However during CPB:

1) reduced flows

—> increased viscosity

—> increased peripheral resistance

—> decreased tissue perfusion

2) hypothermia ‑>

i)	—> increased viscosity	ii)	—> vasoconstriction
	—> increased peripheral resistance		—> increased peripheral resistance
	—> decreased tissue perfusion		—> decreased tissue perfusion

Haemodilution

significantly reduces blood viscosity

—> marked reduction in total resistance

—> increase in tissue perfusion

note: during hemodilution on CPB, perfusion pressure does not indicate adequacy of flow as pressure is a function of flow and viscosity

B) HEMOGLOBIN

· the diminished oxygen carrying capacity of the red cell depleted blood is offset by the increased capillary flow rates achieved as a result of lowered viscosity

C) COAGULATION FACTORS

· hemodilution reduces the concentrations of all coagulation factors & platelets

—> coagulation factor deficiencies

D) PLASMA PROTEINS

· hemodilution reduces the concentrations of allplasma proteins

—>reduced oncotic pressures

—>increased tissue oedema