Hemodilution and fluid management in neurosurgery.

Early in his career, the senior author (RCH) became interested in the experimental study of cerebral vasospasm, which continues to be one of the most important causes of morbidity from subarachnoid hemorrhage (SAH).21–23,45 The initial approach was to find a single pharmacological agent, the so-called “silver bullet,” to prevent and reverse vasospasm. Arterial smooth muscle relaxants were the first class of tempting agents that were studied. These agents turned out to be “fool’s gold” and were generally ineffective in treating vasospasm.19,20,55,58 While treatments were being developed in experimental models, clinical observations were reported that hypertension may improve neurological deficits resulting from cerebrovascular insufficiency.9 This led to the demonstration that vasospasm-induced cerebral ischemia could be treated successfully with iatrogenic hypertension.13,34 The benefits of intravascular volume expansion combined with hypertension were reported soon afterwards.27 These foundations, along with the additional strides made in the understanding of rheology, cerebral oxygen transport, and cerebral blood flow (CBF) augmentation, led to the formalization of the concept of hyperdynamic therapy. Gaining broad acceptance by the late 1980s, the combination of hypervolemia, hypertension, and hemodilution, colloquially known as “triple-H therapy,” is now considered essential in the treatment for cerebral vasospasm. We have observed that the hypervolemic and hypertensive arms of the hyperdynamic therapy receive the most attention from clinicians. It seems that the hemodilution aspect of the treatment is often overlooked or taken for granted because some degree of hemodilution occurs with an increase in intravascular volume. However, the benefits of hemodilution are well grounded in the laboratory, beginning with the pioneering work of Wood et al.59–61 The senior author concentrated on the effects of hemodilution on cerebral ischemia for longer than 15 years in his laboratory. In this report, we shall discuss the evolution of this work and its translation into clinical practice. We shall also review the current status of hemodilution in clinical practice and describe the implications of this work for fluid therapy in general for neurosurgical patients. RATIONALE FOR HEMODILUTION IN CEREBRAL ISCHEMIA The Hagen-Poiseuille equation indicates that flow is inversely proportional to viscosity (Fig. 26.1). Blood viscosity is a complex variable determined by several factors, including erythrocyte aggregation and flexibility, platelet aggregation, plasma viscosity, and hematocrit. Of these factors, hematocrit is by far the most important determinant of blood viscosity.15,52 In ischemic brain, the regional blood vessels are dilated maximally, and blood viscosity becomes a major determinant of blood flow. The low blood flow inherent to the ischemic region results in a dramatic rise in viscosity, favoring microaggregation and thrombus formation. Thus, the hematocrit becomes an even more important factor in these low-flow states.15 It follows that hemodilution is an effective way to increase perfusion to the ischemic brain, and a large amount of evidence supports this hypothesis. Although hemodilution increases perfusion in the ischemic brain, it also reduces the oxygen-carrying capacity of blood. In ischemic brain, the autoregulation of blood flow is lost, and rheological factors, such as viscosity, hence, hematocrit, become a most important determinant of regional blood flow.