Hemodilution and blood substitutes.

The primary consequence of the substitution or replacement of blood with a surrogate is the dilution of the original constituents. This hemodilution produces systemic and microvascular phenomena that underlie all forms of blood replacement and provides a physiological reference for comparison for blood substitutes. The basic features of hemodilution become evident when the procedure is carried out in isovolemic and isoocotic conditions where blood viscosity and oxygen carrying capacity are changed. Blood viscosity is decreased, which redistributes macro and microcirculatory blood pressure increasing the arterio/venular pressure difference and central venous pressure, which improves cardiac filling and cardiac output and therefore blood flow velocity. These effects coupled to the oxygen carried by the diluted blood maintains the rate of oxygen delivery to the microcirculation up to hematocrit reductions of one third. The increased flow velocity counteracts the diffusive losses of oxygen from the microvessels. The increased flow velocity increases shear stress at the vessel wall lowering the tendency of activated leukocytes to adhere. Hemodilution with dextran 70 also maintains functional capillary density to hematocrit decreases of one half. Hemodilution with alpha alpha-hemoglobin presents all the features found with non-oxygen carrying colloids, however the increased oxygen carrying capacity is not fully exploited because systemic and microvascular effects due to colloids do not develop to the same extent.