Hypothermia impairs but hyperthermia does not promote inert gas elimination in the rat

The volume of nitrogen eliminated (VN2) and washout rate constant (k) during a 2-h period of O2 breathing in the rat was altered in response to changes in body temperature. VN2 and cardiac output (Q) were determined simultaneously on 3 groups of unanesthetized but restrained male rats at 3 different body temperatures: normothermic (36 degrees C), hyperthermic (39 degrees C), and hypothermic (33 degrees C). Under normothermic conditions, the rats eliminated 12.8 ml N2/kg in 2 h of O2 breathing while Q, determined by a thermodilution technique, was 430 ml X min-1 X kg-1. A significant reduction in VN2 (33%) and k (44%) was seen during hypothermic exposures in which rectal temperature dropped 3 degrees C and Q decreased to 395 ml X min-1 X kg-1. During hyperthermic exposures, VN2, 11.7 ml N2/kg, and k decreased while Q increased to 468 ml X min-1 X kg-1, however, these changes were not significant when compared to the normothermic conditions. The overall relationship between VN2 and Q is described by: VN2 = 4.44 + 0.019 Q (r = 0.536, P less than 0.01). Under conditions of altered body temperature, Q accounts for only 29% of the VN2 response. Characteristic changes in regional blood flow, such as a decreased adipose tissue blood flow, may account for the observed reduction in VN2 and k during hypo- and hyperthermic conditions. The results of this study indicate that hypothermia prolongs the duration required for desaturation of nitrogen.