HEMODILUTION IMPAIRS HYPOCAPNIA-INDUCED VASOCONSTRICTOR RESPONSES IN THE BRAIN AND SPINAL-CORD IN DOGS

Despite the increasing use of plasma expanders in the perioperative pe riod, there have been few studies of cerebrovascular responsiveness du ring hemodilution. The present study was performed to evaluate the inf luence of isovolemic hemodilution on vasoconstrictor responses in the brain and spinal cord during hypocapnia. Sixteen mechanically ventilat ed, halothane-anesthetized dogs were randomly divided into two equal g roups: Group 1, control group (hematocrit [Hct], 42% +/- 2%); Group 2, isovolemic hemodilution with 5% dextran 40 (Hct, 19% +/- 2%). Hypocap nia (22 +/- 2 mm Hg) was induced in both groups by removal of dead spa ce tubing without altering mechanical ventilation. Regional blood flow in the brain and spinal cord was measured with 15-mu m radioactive mi crospheres and used to calculate regional vascular resistance (RVR). I n 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 thro ughout the brain and spinal cord. After hemodilution, hypocapnia had n o significant effect on RVR in the cerebral cortex, cerebellum, pens, and medulla, and caused less pronounced increases in RVR within the sp inal cord. We conclude that hemodilution either attenuated or complete ly abolished vasoconstrictor responses within the brain and spinal cor d during hypocapnia. Furthermore, the present findings suggest that in duced hypocapnia may be less effective as a clinical maneuver to reduc e increased intracranial pressure during hemodilution.