Relationship between local cerebral blood flow and metabolism during mild and moderate hypothermia in rats

Background: Hypothermia may interfere with the relationship between cerebra l blood now (CBF) and metabolism. Because this conclusion was based on the analysis of global values. the question remains whether hypothermic CBF/met abolism uncoupling exists on a local cerebral level. This study investigate d the effects of hypothermic anesthesia on local cerebral blood now (LCBF) and local cerebral glucose utilization (LCGU). Methods: Thirty-six rats were anesthetized with isoflurane (1 minimum alveo lar concentration) and artificially ventilated to maintain normal arterial carbon dioxide partial pressure (pH-stat). Pericranial temperature was main tained as normothermic (37.5 degrees C, n = 12) or was reduced to 35 degree s C (n = 12) or 32 degrees C (n = 12), Pericranial temperature was maintain ed constant for 60 min until LCBF or LCGU were measured by autoradiography. Twelve conscious rats served as normothermic controls. Results: Compared with conscious animals, mean CBI: remained unchanged duri ng normothermic anesthesia. Mean CBI: significantly increased during mild h ypothermia but was unchanged during moderate hypothermia. During normotherm ic anesthesia, mean CGU was 45% lower than in conscious controls (P < 0.05) , No further CGU reduction was found during mild hypothermia, whereas CGU f urther decreased during moderate hypothermia (48%; P < 0.05), Local analysi s showed a Linear LCBF/LCGU relationship in conscious (r = 0.94) and anesth etized (r = 0.94) normothermic animals, as well as in both hypothermic grou ps (35 degrees C: r = 0.92; 32 degrees C: r = 0.95; P < 0.05), The LCBF-to- LCGU ratio increased from 1.4 (conscious controls) to 2.4 (normothermic iso flurane) and 3.6 ml/mu mol (mild and moderate hypothermia, P < 0.05), Conclusions: Decrease of mean CGU at unchanged or increased mean CBF during hypothermic anesthesia may not indicate uncoupling. Local analysis shows a maintained linear relationship that is reset to a higher CHF/CGU ratio.