Viscosity Effects on Cerebral Blood Flow
Blood viscosity can influence CBF. Hematocrit is the single most
important determinant of blood viscosity.[85]
In
healthy subjects, variation of hematocrit within the normal range (33% to 45%) probably
results in only trivial alteration in CBF. Beyond this range, changes are more substantial.
[85]
[86]
In anemia,
cerebral vascular resistance is reduced and CBF increases. However, this increase
in CBF may result not only from a reduction in viscosity but also
Figure 21-5
Schematic representation of the effect of increasing
concentrations of a typical volatile anesthetic on autoregulation of cerebral blood
flow. Dose-dependent cerebral vasodilation results in attenuation of autoregulatory
capacity. Both the upper and lower thresholds are shifted to the left. MAP, mean
arterial pressure.
in response to reduced oxygen-carrying capacity of blood.[87]
The effect of a reduction in viscosity on CBF is more obvious in the setting of
focal cerebral ischemia, when vasodilation in response to impaired oxygen delivery
is probably already maximal. In this setting, the reduction in viscosity achieved
by hemodilution results in increases in CBF in the ischemic territory.[88]
[89]
[90]
[91]
The best available information suggests that in the setting of focal cerebral ischemia,
a hematocrit of 30% to 34% will result in optimal oxygen delivery.[92]
[93]
However, manipulation of viscosity in patients
with acute ischemic stroke has not been shown to be of benefit in reducing the extent
of cerebral injury.[94]
Hence, viscosity is not
a target of manipulation in patients at risk from cerebral ischemia, with the possible
exception being those with hematocrit values in excess of 55%.
