Oxygen pulse in guinea pigs in hyperbaric helium and hydrogen

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 (VO2) was measured by gas chromatographic analysis. Core temperature and heart rate were measured by using surgically implanted radiotelemeters. The VO2 was modulated over a fourfold range by varying chamber temperature from 25 to 36 degrees C. There was a direct correlation between VO2 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.