Kayar, Susan R., and Erich C. Parker. Oxygen pulse in guinea pigs in hyperbaric helium and hydrogen. J. Appl. Physiol. 82(3): 988-997, 1997.We analyzed O₂ pulse, the total volume of O₂ consumed per heart beat, in guinea pigs at pressures from 10 to 60 atmospheres. Animals were placed in a hyperbaric chamber and breathed 2% O₂ in either helium (heliox) or hydrogen (hydrox). Oxygen consumption rate (O₂) was measured by gas chromatographic analysis. Core temperature and heart rate were measured by using surgically implanted radiotelemeters. The O₂ was modulated over a fourfold range by varying chamber temperature from 25 to 36°C. There was a direct correlation between O₂ and heart rate, which was significantly different for animals in heliox vs. hydrox (P = 0.003). By using multivariate regression analysis, we identified variables that were significant to O₂ pulse: body surface area, chamber temperature, core temperature, and pressure. After normalizing for all nonpressure variables, the residual O₂ pulse was found to decrease significantly (P = 0.02) with pressure for animals in heliox but did not decrease significantly (P = 0.38) with pressure for animals in hydrox over the range of pressures studied. This amounted to a roughly 25% lower O₂ pulse for normothermic animals in 60 atmospheres heliox vs. hydrox. These results suggest that reduction of cardiovascular efficiency in a hyperbaric environment can be mitigated by the choice of breathing gas.