Relationship between inspired and expired gas temperatures in a hyperbaric environment.

When breathing room air at sea level the expired gas temperature (Te) increases in proportion to the inspired one (Ti). Previous studies conducted under hyperbaric conditions have assumed that the Te vs Ti relationship was the same when humans breathed room air at atmospheric pressure or helium-oxygen mixture under hyperbaric conditions. We hypothesized that the use of dilutant gases, as helium (He) or hydrogen (H2), having low density but high specific heat compared to nitrogen, could change the Te vs Ti regression. The present study was conducted on 3 professional divers participating in the COMEX Hydra IX experiment. Three conditions were studied: A, (23.5 ATA, HeH2O2 mixture); B, (21 ATA, H2O2 mixture); and C, (21 ATA, HeO2 mixture). In each condition six different inspired temperatures were tested, while minute ventilation, Ti and Te values were measured simultaneously. In all cases a linear relationship was found between Te and Ti, but the slopes of the regression lines obtained in conditions A and B (gas mixture containing H2) were significantly lower than in condition C (HeO2 mixture). Computation of the convective respiratory heat loss (Ċr) revealed that, when the subjects breathed the coldest gas mixtures (+ 10°C), Ċr value was 1.6 times higher in condition B than in C. These data are consistent with theoretical considerations and they demonstrate that a single equation cannot be used to predict the Te vs Ti relationship in all environmental circumstances.