The role of arterial oxygen tension in the respiratory response to localized heating of the hypothalamus and to hyperthermia.

1. Rectal temperatures, respiratory rates, arterial blood gas tensions, arterial pH and the percentage of red cells in arterial blood have been measured in the unanaesthetized ox in a cool environment (15/12 degrees C, dry bulb/wet bulb [DB/WB]), in a hot, dry environment (40/21 degrees C, DB/WB), during hyperthermia, during infra-red irradiation, and during localized heating of the anterior hypothalamus. In some experiments the gas tensions and pH of mixed venous blood, and the percentage saturation of the arterial blood with oxygen, were also measured.2. In the cool environment at a mean rectal temperature (T(r)) of 38.8 degrees C and a respiratory rate (f) of 28/min the mean values obtained from six animals were: arterial oxygen tension (P(a, O) (2)), 93 mm Hg; arterial carbon dioxide tension (P(a, CO) (2)) 42 mm Hg; arterial pH 7.49; arterial oxygen saturation (S(a, O) (2)) 94%; arterial oxygen capacity (Cap(a, O) (2)) 13.6 vol.%; arterial packed cell volume (P.C.V.) 29%.3. Exposure to the hot, dry environment resulted in a small increase in the rectal temperature and thermal polypnoea, but there were no statistically significant changes in the blood gas tensions.4. During hyperthermia statistically significant increases occurred in rectal temperature, respiratory rate, P(a, O) (2), pH and arterial haematocrit, while the P(a, CO) (2) decreased. The venous oxygen tension (P(v, O) (2)) decreased also, and the tentative conclusion was made that although the oxygenation of arterial blood remained unimpaired during hyperthermia, tissue hypoxia may supervene. At very high levels of deep body temperature, some evidence for a secondary decrease in P(a, O) (2) was obtained.5. Localized heating of the anterior hypothalamus caused an increase in respiratory rate and in P(a, O) (2). The P(v, O) (2) increased also. These changes were considered to be due to increased cardiac output and diversion of blood to the skin.6. During infra-red irradiation of three animals at an environmental temperature of 40/21 degrees C, the respiratory rate increased, but the P(a, O) (2) decreased.