Protective effect of hypothermia in cerebral oxygen deficiency caused by arterial hypoxia

To study the cerebral protective effects of hypothermia in arterial hypoxia, anesthetized (70% N2O), mechanically ventilated rats were cooled to a body temperature of 27 C. Hypoxia was induced by decreasing the oxygen content in the inspired gas mixture either to 6-7 per cent or to 2.5-3 per cent. This reduced mean PaO2 to about 25 and 11-12 torr, respectively. At PaO2 torr, there was no change in cerebral blood flow (CBF), cerebrla oxygen consumption (CMRO2), or labile tissue metabolites. The absence of signs of cerebral hypoxia could be attributed to an effect of temperature and pH on the hemoglobin-oxygen dissociation curve. Thus, at 27 C with a PaO2 of 25 torr the total oxygen content (TO2) of arterial blood remained greater than 15 ml (100 ml)-1, about three times the value obtained at this PO2 in normothermic rats. At PaO2 11-12 torr, arterial TO2 was reduced to about 5 ml (100 ml) (-1). The hypoxia induced no change in CMRO2, a threefold increase in CBF, a moderate lactacidosis in the tissue, and a small decrease in phosphocreatine content, but no change in ATP, ADP, or AMP. These changes are less marked than those occurring at the same arterial TO2 in normothermic rats. It is concluded that hypothermia exerts a pronounced protective effect on the brain in hypoxic hypoxia, and that two mechanisms are involved. First, since hypothermia shifts the oxyhemoglobin-dissociation curve towards the left, and prevents or minimizes a rightward shift due to acidosis, it maintains a high TO2 in arterial blood at a given PaO2. Second, by reducing CMRO2, and thereby presumably also cellular energy requirements, hypothermia exerts a protective effect at the cellular level.