Hemodilution & gas exchange

· Hemodilution produces an absolute reduction in the number of red cells and increased likelihood of exposure to oxygen in the oxygenator

· Arterial PO₂ is higher on CPB

· Effect of blood flow on metabolic use of oxygen

- total amount of O₂ available each minute for use in any given tissue is determined by:

a) quantity of O₂ transported in each 100 ml blood

b) rate of blood flow

· Hemodilution —> reduced hematocrit —> marked decrease in total resistance —> increase in tissue perfusion [blood flow]

· Reduction of Hb levels to below normal will still permit adequate oxygen transfer

· Hemodilution produces an absence of metabolic acidosis, interpreted as an index of good tissue perfusion with concommitant absence of peripheral vasoconstriction or cyanosis, stable electrolyte pattern.

· When not on CPB, normovolaemic haemodilution increases cardiac output to a greater magnitude than the corresponding reduction in blood oxygen carrying capacity thereby yielding a net increase in oxygen availability to the tissues

1) Cardiac input increases without changes in HR or BP 2^o

i) increased VR

ii) reduced afterload

2) However, O₂ availability falls sharply below a Hct of 20%

· During CPB, oxygen transfer from oxygenator to tissues is a product of:

1) flow rate

2) O₂ saturation

3) Hb concentration