Evaluation of isovolemic hemodilution

Abstract

The procedures of hypervolemic hemodilution (by infusion of colloidal solutions) and of isovolemic hemodilution (by exchange of colloidal solution against whole blood) have been shown to improve cerebral perfusion. In this review, the literature and the personal experience of the author in this field is evaluated. The concept of rheological improvement of cerebral perfusion was originally developped from the epidemiological observations which clearly identified high hemoglobin levels (or high hematocrit) as an important risk factor for cerebrovascular disease. These findings are in keeping with the extensive clinical experience that high hemoglobin concentration is associated with a reduction not only in global cerebral perfusion, but also in cerebral red cell flux (flow multiplied by hematocrit). Conversely, anemia is associated with increased cerebral perfusion; there is no evidence that anemia is associated with neurological deficits. Various procedures to reduce hematocrit resulted in improved perfusion of cerebral and peripheral vascular beds in patients. These global results do not allow to draw firm conclusions about the mode of action: arguments for the action of whole blood and plasma viscosity, of the oxygen transport capacity or of vasodilator responses are reviewed. In closing, the clinical results of retrospective and of prospective studies of the principle of hemodilution, presently available, are summarized, as are adequate procedures for its application to patients.