Relationship between local cerebral blood flow and metabolism during mild and moderate hypothermia in rats.
BACKGROUND
Hypothermia may interfere with the relationship between cerebral blood flow (CBF) and metabolism. Because this conclusion was based on the analysis of global values, the question remains whether hypothermic CBF/metabolism uncoupling exists on a local cerebral level. This study investigated the effects of hypothermic anesthesia on local cerebral blood flow (LCBF) and local cerebral glucose utilization (LCGU).


METHODS
Thirty-six rats were anesthetized with isoflurane (1 minimum alveolar concentration) and artificially ventilated to maintain normal arterial carbon dioxide partial pressure (pH-stat). Pericranial temperature was maintained as normothermic (37.5 degrees C, n = 12) or was reduced to 35 degrees C (n = 12) or 32 degrees C (n = 12). Pericranial temperature was maintained 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 CBF remained unchanged during normothermic anesthesia. Mean CBF significantly increased during mild hypothermia but was unchanged during moderate hypothermia. During normothermic anesthesia, mean CGU was 45% lower than in conscious controls (P < 0.05). No further CGU reduction was found during mild hypothermia, whereas CGU further decreased during moderate hypothermia (48%; P < 0.05). Local analysis showed a linear LCBF/LCGU relationship in conscious (r = 0.94) and anesthetized (r = 0.94) normothermic animals, as well as in both hypothermic groups (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 isoflurane) and 3.6 ml/micromol (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 CBF/CGU ratio.
Frietsch
T
T
Department of Anesthesiology and Critical Care Medicine, Faculty of Clinical Medicine Mannheim, Germany.
Krafft
P
P
Piepgras
A
A
Lenz
C
C
Kuschinsky
W
W
Waschke
K F
KF
D000136:Acid-Base Equilibrium; D000769:Anesthesia, Inhalation; D018685:Anesthetics, Inhalation; D000818:Animals; D001345:Autoradiography; D001923:Brain Chemistry / Q000502:physiology*; D002560:Cerebrovascular Circulation / Q000502:physiology*; D005947:Glucose / Q000378:metabolism; D007035:Hypothermia / Q000378:metabolism* / Q000503:physiopathology*; D007530:Isoflurane; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley
D018685:Anesthetics, Inhalation; D007530:Isoflurane; D005947:Glucose