Hemodilution impairs hypocapnia-induced vasoconstrictor responses in the brain and spinal cord in dogs

Despite the increasing use of plasma expanders in the perioperative period, there have been few studies of cerebrovascular responsiveness during hemodilution. The present study was performed to evaluate the influence of isovolemic hemodilution on vasoconstrictor responses in the brain and spinal cord during hypocapnia. Sixteen mechanically ventilated, halothane-anesthetized dogs were randomly divided into two equal groups: Group 1, control group (hematocrit [Hct], 42% +/- 2%); Group 2, isovolemic hemodilution with 5% dextran 40 (Hct, 19% +/- 2%). Hypocapnia (22 +/- 1 mm Hg) was induced in both groups by removal of dead space tubing without altering mechanical ventilation. Regional blood flow in the brain and spinal cord was measured with 15-microns radioactive microspheres and used to calculate regional vascular resistance (RVR). In Group 1, hypocapnia caused increases in RVR (ranging from 44% +/- 10% in the cerebral cortex to 93% +/- 17% in the thoracic spinal cord). In Group 2, hemodilution itself decreased RVR relatively uniformly throughout the brain and spinal cord. After hemodilution, hypocapnia had no significant effect on RVR in the cerebral cortex, cerebellum, pons, and medulla, and caused less pronounced increases in RVR within the spinal cord. We conclude that hemodilution either attenuated or completely abolished vasoconstrictor responses within the brain and spinal cord during hypocapnia. Furthermore, the present findings suggest that induced hypocapnia may be less effective as a clinical maneuver to reduce increased intracranial pressure during hemodilution.