Outline the physiologic response to AIHD.
To understand how AIHD may be safely accomplished, it is necessary to understand the physiologic response to its use.
As previously discussed,DO2 is the product of CO and CaO2. As isovolemic hemodilution occurs and red cells are removed from the circulation, CaO2 of blood decreases.
It might be expected that DO2 would therefore decrease.
Surprisingly, as the Hct decreases to 30%, DO2 actually increases over baseline. This is because hemodilution alters the rheologic properties of blood. Blood viscosity is decreased by hemodilution, which effectively lowers systemic vascular resistance (SVR). An increase in venous return results in an increased stroke volume. CO increases proportionally more than CaO2 decreases, and DO2 rises.
Furthermore, the rise in CO is accomplished without an increase in heart rate, if intravascular volume is main-tained. Hemodilution to a Hct of 30% causes a 30–50% increase in CO. DO2 does not fall to control values until a Hct of approximately 20% is reached. Additionally, during isovolemic hemodilution, local tissue oxygenation is preserved and even enhanced by a more homogeneous distribution of capillary blood flow. Studies with tissue electrodes have shown that hypoxic microareas do not occur during isovolemic hemodilution.