Hyperviscosity is less frequent in myeloma than in Waldenström macroglobulinemia, where higher-molecular-weight IgM molecules frequently cause an increase in viscosity. Munshi, Sundar Jagannath, in Hematology (Seventh Edition), 2018 Hyperviscosity Therefore, viscosity is not a target of manipulation in patients at risk as a result of cerebral ischemia, with the possible exception of those with hematocrit values higher than 55%. However, manipulation of viscosity in patients with acute ischemic stroke is not of benefit in reducing the extent of cerebral injury. In patients with focal cerebral ischemia, a hematocrit of 30% to 34% will result in optimal delivery of oxygen. In this situation, reducing viscosity by hemodilution increases CBF in the ischemic territory. 11 The effect of a reduction in viscosity on CBF is more important with focal cerebral ischemia, a condition in which vasodilation in response to impaired oxygen delivery is probably already maximal. 15 Although arterial oxygen content can be reduced by both hypoxia and by hemodilution, the increase in CBF that accompanies hypoxia is of a greater magnitude than that by hemodilution induced reduction in oxygen delivery. However, this may result not only from a reduction in viscosity but also as a compensatory response to reduced oxygen delivery. In anemia, cerebral vascular resistance is reduced and CBF increases. Beyond this range, changes are more substantial. 14 In healthy humans, variation of the hematocrit within the normal range (33%-45%) probably results in only modest alterations in CBF. Hematocrit is the single most important determinant of blood viscosity. Gropper MD, PhD, in Miller's Anesthesia, 2020 Effects of Blood Viscosity on Cerebral Blood Flowīlood viscosity can influence CBF.
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