CEREBRAL OXYGEN-TRANSPORT AND METABOLISM DURING GRADED ISOVOLEMIC HEMODILUTION IN EXPERIMENTAL GLOBAL-ISCHEMIA

To verify the optimal hematocrit (Hct) level in the treatment of cereb ral ischemia, cerebral oxygen transport (CTO2) and cerebral oxygen met abolism (CMRO2) in graded isovolemic hemodilution were evaluated durin g cerebral ischemia. Isovolemic hemodilution with low molecular weight dextran to stepwise lower Hct from 43% to 36%, 31%, and 26% was carri ed out in 13 splenectomized dogs, 6 h after global cerebral ischemia. Global ischemia of the animals was produced by multiple intra- and ext racranial ligations of cerebral arteries. Cerebral blood flow (CBF) wa s measured with radioisotope labeled microspheres. CTO2, CMRO2, and ox ygen extraction fraction (OEF) were calculated from CBF, arterial oxyg en content (CaO2), and venous oxygen content (CvO(2)). In dogs with gl obal cerebral ischemia, CBF increased with graded isovolemic hemodilut ion (r=-0.73, P<0.05). CTO2 reached its highest value at a Hct level o f 31.3%. CTO2 at Hct of 36.1% and 31.3% was statistically different fr om the value measured at a Hct of 43.3%, and there was a decrease when Hct was lowered to 25.9%. CMRO2 was the highest when Hct was at 31.3% and differed significantly from the value measured at a Hct of 43.3%. There was a 10% increase of OEF when Hct was at 25.9%; however this c hange was not statistically significant compared with the OEF at Hct o f 36.1% and 31.3%: respectively. These findings indicate that CTO2 and CMRO2 were the highest when Hct was reduced to 31% in hemodilution. H ct at 31% is the optimum for cerebral metabolism in ischemic status. U ncoupling of CTO2, CMRO2 with CaO2 was also observed in this study. Th is phenomenon suggests that hemodilution to augment cerebral circulati on may be at least partially attributed to the beneficial effects of h emorheologic improvement in the microcirculation of the ischemic brain . (C) 1997 Elsevier Science B.V.