Effect of mild hypothermia on cerebral oxygen uptake during gradual cerebral perfusion pressure decrease in piglets

Objective: To study the effect of mild hypothermia on cerebral oxygen metabolism and brain function in piglets during reduced cerebral blood flow because of gradual reduction of the effective cerebral perfusion pressure (CPP). Design: Comparison of two randomized treatment groups: normothermic group (NT; n = 7) and hypothermic group (HT; n = 7). Setting: Work was conducted in the research laboratory of the Institute for Pathophysiology, Friedrich Schiller University, Jena, Germany. Subjects: Fourteen piglets (14 days old) of mixed German domestic breed. Intervention: Animals were anesthetized and mechanically ventilated. An epidural balloon was gradually inflated to increase intracranial pressure to 25 mm Hg, 35 mm Hg, and 45 mm Hg every 30 mins at adjusted mean arterial blood pressures. After determination of baseline CPP (NT, 79 +/- 14 mm Hg; HT, 84 +/- 9 mm Hg), CPP was reduced to similar to 70%, 50%, and 30% of baseline (NT, 38.1 +/- 0.5 degrees C; HT, 31.7 +/- 0.5 degrees C). Measurements and Main Results: Every 25 mins after the gradual CPP reductions. Mild hypothermia induced a reduction of the cerebral metabolic rate of oxygen (CMRO2) to 50% +/- 15% of baseline values (baseline values, 352 +/- 99 mu mol.100 g(-1).min(-1)) (p < .05). Moreover, the electrocorticogram was altered to a pattern of reduced delta activity (p < .05) but unchanged higher frequency activity. The cerebral oxygen balance in HT animals remained improved until CPP reduction to 50%, indicated by a reduced cerebral arteriovenous difference of oxygen but elevated brain tissue PO2 (p < .05). Further CPP reduction gave rise to a strong CMRO2 reduction (NT, 19 +/- 21%; HT, 15 +/- 15%; p < .05). However, the high-frequency band of electrocorticogram was less reduced in hypothermic animals (p < .05). Conclusions: Mild whole body hypothermia improves cerebral oxygen balance by reduction of brain energy demand in juvenile piglets. The improvement of brain oxygen availability continues during a mild to moderate CPP decrease. A loss of the difference in CMRO2 between the hypothermic and normothermic piglets together with the fact that brain electrical activity was less suppressed under hypothermia during severe cerebral blood flow reduction indicates that hypothermic protection may involve some other mechanisms than reduction of brain oxidative metabolism.