Hemodilution for cerebral ischemia.

If irreversible cerebral infarction from stroke occurred immediately at the time of arterial occlusion, there would be little hope of intervening therapeutically. However, even though there may be a central "core" of severe ischemia where brain tissue dies after a period of several minutes, in most situations there is a surrounding "ischemic penumbra" where blood flow is reduced enough to result in physiological loss of function but not so much as to cause irreversible loss of cellular integrity. This "penumbra" may persist for days and offers a window of time during which therapeutic intervention to improve the delivery of oxygen and metabolites and the removal of carbon dioxide and products of metabolism may be effective in preventing irreversible damage to this area. Such damage may occur if marginal levels of perfusion persist for hours or days since the development of infarction is related not only to the severity of ischemia but also to its duration. Even a slight improvement in perfusion may be capable of preventing irreversible damage to the "penumbra" until reperfusion occurs, either by clot dissolution in cases of embolic stroke, by increased collateralization in cases of more proximal thrombosis, by resolution of vasospasm in patients with subarachnoid hemorrhage, or by surgical revascularization in some carefully selected cases. There is considerable evidence that hemodilution is an effective way to increase perfusion of ischemic brain. The rationale for using hemodilution in the treatment of cerebral ischemia is based on the wellknown Hagen-Poiseuille equation, which states that, other factors remaining constant, blood flow is inversely proportional to blood viscosity. Hematocrit (Hct) is a major factor influencing blood viscosity, and it is a particularly important determinant of flow at the low velocity gradients (shear rates) present in the microcirculation. With ischemia these