Comparison of cerebral blood flow measured by laser-Doppler flowmetry and hydrogen clearance in cats after cerebral insult and hypervolemic hemodilution.

Laser-Doppler flowmetry provides a continuous measurement of blood flow without violating the natural state of circulation. The linearity of the laser-Doppler and hydrogen clearance methods of blood flow measurement were compared using a protocol that produced changes in cerebral blood flow that might be experienced in a neurosurgical setting. Cerebral blood flow was measured in both hemispheres of 12 adult cats during the snaring of one common carotid artery, the intracisternal injection of 5 mg of 5-hydroxytryptamine creatinine sulfate, and hypervolemic hemodilution, which produced a 25% reduction in blood hematocrit. The percentage of baseline laser-Doppler flowmetry and hydrogen clearance flows showed an acceptable degree of correlation (R2 = 0.762) over the range of cerebral blood flows measured. More rigorous analysis using Bland and Altman's difference against mean test showed that 10 minutes after hemodilution, the two methods displayed a level of variation outside the limits of agreement (-21.85 to 22.03%). Laser-Doppler flowmetry provided a noninvasive and continuous measure of blood flow, increasing the ability to observe instantaneous changes in cerebral microcirculation. However, laser-Doppler flowmetry did not record absolute blood flow, was affected by cerebral tissue shrinkage, and did not accurately measure flow under conditions of changed blood hematocrit.