Calorimetry and respirometry in guinea pigs in hydrox and heliox at 10-60 atm

We used direct calorimetry and respirometry to measure the total rate of he at loss (Q(Sigma)) and of oxygen consumption (VO2) in guinea pigs in 1-atm (0.1 MPa) air and at 10-60 atm in either heliox (98% H-2, 2% O-2) or hydrox (98% H-2, 2% O-2). Our objective was to determine if the physiological res ponses to these two gas mixtures were different and, if so, whether the dif ferences were attributable to the thermal characteristics of the gases alon e or were confounded by additional mechanisms. At 10-40 atm, Q(Sigma) and V O2 were not significantly different in the two gas mixtures, whereas at 60 atm, Q(Sigma) and VO2 were significantly higher in heliox than in hydrox. T he VO2/Q(Sigma) ratio suggested that the animals were not in thermal equili brium in hyperbaria. Based solely on the differing thermal properties of th e gas mixtures, a mathematical model predicted a Q(Sigma) that was higher i n hydrox than in heliox at all pressures. Two plausible explanations are su ggested: one is an adaptive lowering of the surface temperature as a physio logical response of the animal to the thermally more stressful hydrox envir onment, and the other is related to the narcotic suppression of the animal' s activity by hydrox.