Effects of temperature on cerebral tissue oxygen tension, carbon dioxide tension, and pH during transient global ischemia in rabbits

Background: A decrease in brain temperature (T(brain)) causes a decrease in the cerebral metabolic rate for oxygen (CMRO₂) and provides potent neuroprotection against ischemic damage. In the present study, the effects of mild to moderate hypothermia on cerebral tissue oxygen tension (P(O₂) brain), carbon dioxide tension (P(CO₂) brain), and pH (pH(brain)) were monitored during short episodes of global cerebral ischemia. Methods: After approval by the Animal Care and Use Committee, 10 New Zealand white rabbits were anesthetized (1% halothane in air) and mechanical ventilation was adjusted to maintain the arterial carbon dioxide tension at 35 mmHg (αstar). A sensor to measure P(O₂) brain, P(CO₂), brain, pH(brain), and T(brain) was inserted into the brain through a burr hole in the skull T(brain) was adjusted to 38°C, 34.4°C, and 29.4°C in a random sequence in each animal. P(O₂) brain, P(CO₂) brain, and pH(brain) (all variables are reported at the actual T(brain)) were recorded every 10 s during a 5-min baseline, 3 min of cerebral ischemia induced by inflation of a neck tourniquet, and 10 min of reperfusion at each level of T(brain). Analysis of variance and Dunnett's test were used for statistical analysis. Data are presented as means ± SD. Results: During ischemia, P(O₂) brain decreased from 56 ± 3 to 33 ± 2 mmHg at 38°C, from 58 ± 3 to 32 ± 3 mmHg at 34.4°C, and from 51 ± 2 to 32 ± 2 mrnHg at 29.4°C (p = NS). P(CO₂) brain increased by 6.7 ± 2 mmHg at 38°C, by 5.1 ± 1.4 mixing at 34.4°C, and by 2.3 ± 0.8 mmHg at 29.4°C. pH(brain) inversely followed the trend of P(CO₂) brain. Conclusions: The attenuated increase in P(CO₂) brain during hypothermic ischemia results from the reduced CMLRO₂. The similar decrease in P(O₂) brain at all temperature levels indicates that despite the reduction in CMRO₂, P(O₂) brain is no better preserved during brief episodes of hypothermic ischemia than during normothermic ischemia.