Functional cerebral hyperemia is unaffected by isovolemic hemodilution

The cerebral hyperemic effect of hemodilution is well known; however, its mechanism and potential modifying effect on the functional hyperemic response to neuronal activation are unclear. The authors investigated the effects of isovolemic hemodilution on vibrissal stimulation-induced changes in cerebrocortical laser Doppler flow and tissue oxygen tension in the rat.

The hyperemic response to whisker stimulation was assessed in the whisker barrel cortex of 12 rats anesthetized with chloralose-urethane before and after hemodilution. Graded, isovolemic hemodilution was performed by three repeated withdrawals of 3 ml blood with replacement of equal volume of 5% serum albumin. Measured systemic hematocrit values were 39.3 +/- 1.3% (control), 29.5 +/- 1.0%, 22.3 +/- 1.5%, and 17.0 +/- 1.6% (after the three hemodilution steps). Arterial blood pressure was maintained at control levels with an infusion of methoxamine. Unilateral whisker stimulation was performed with a mechanical actuator at 8 Hz, and 10 cycles of 10 s on-30 s off periods. In six control animals, shed blood was immediately reinfused, resulting in no change in hematocrit, and whisker stimulation was performed using the same timeline as in the other animals. In six additional experiments, resting cerebral cortical oxygen tension was measured using the phosphorescence quenching technique following the same hemodilution protocol.

Graded hemodilution increased baseline laser Doppler flow by 5.5 +/- 0.9%, 13 +/- 1.6%, and 23.7 +/- 2.2%. Vibrissal stimulation transiently increased laser Doppler flow by 17.0 +/- 2.0%. The hyperemic response was unchanged after hemodilution and was identical to that seen in the control group in all conditions. Tissue oxygen tension increased slightly but significantly with hemodilution at a rate of 1.4 mmHg per 10% hematocrit change (r = 0.83). Mean arterial pressure, arterial oxygen tension, carbon dioxide tension, and pH were within normal limits in each experimental group and were not different from control during hemodilution.

The results suggest that an increase in baseline flow during hemodilution maintains cortical oxygen supply and consequently preserves the normal functional hyperemic response.