Hemodilution in acute stroke.

IT SEEMS APPROPRIATE to attempt to improve ce-rebral blood flow (CBF) in stroke. By so doing, oxygen delivery to the brain will increase and removal of the waste products of metabolism will be more efficient. Better flow should also increase the fragmentation and dispersal of existing thrombi and inhibit extension and spread of intravascular thrombosis. One approach to increase CBF is to improve the flow properties of blood by lowering viscosity. The two main determinants of blood viscosity in cerebral vessels are flow and hematocrit. As flow falls, viscosity rises. So any manoeuvre which raises CBF will lower viscosity. Lowering hematocrit by hemodilution reduces vicosity dramatically. Hemodilution increases CBF impressively. 1 This improvement in flow may be due either to the reduction in viscosity or to the reduction in the oxygen carrying capacity of the blood, which would necessitate an increase in flow to maintain oxygen delivery. Initially, it was suggested 1 that viscosity was likely to be more important, but subsequent work to clarify the problem from the same laboratory 2 3 and from elsewhere" indicates that oxygen delivery is the dominant factor. Therefore, since the improvement in flow after hemodilution is largely due to a reduction in oxygen carrying capacity, should it be used in acute stroke? There is little debate about the need to reduce elevated hematocrit in polycythemic patients in the immediate aftermath of stroke. Their blood flow is often extremely low and reducing hematocrit increases CBF and often improves conscious level and alertness. Furthermore , hemodilution actually achieves a small increase in oxygen delivery. 5 But should patients without polycythemia who have a hematocrit in the accepted "normal range" have the hematocrit reduced in order to improve cerebral blood flow? Wood and Kee, in this issue, argue that they should. Several studies have shown that reducing hematocrit in this group produces an increase in flow 1 and Wood et al, 1984, 6 have shown that it also produces an improvement in EEG in acute stroke. Harrison et al, 1981, 7 have shown that infarct size after carotid occlusion increases with rising hematocrit. It was proposed that efficiency of flow through collateral vessels was reduced by high hema-tocrit and viscosity and that infarct size increased ac-8 have published some very encouraging results of a pilot study of hemodilu-tion in acute stroke which has indicated that the size of handicap following stroke is reduced by hemodilution. In …