Respiratory effects of cold-gas breathing in humans under hyperbaric environment

Changes in total lung resistance (RL) during inhalation of cold gas mixtures were measured in 4 human volunteers during an experimental dive at 46 ATA. The subjects breathed helium-nitrogen-oxygen mixtures during the decompression schedule, and measurements were performed at 46, 36, 21, 12.5, 6 and 2 ATA (1 ATA = 100 kPa). RL was measured during eupneic ventilation when individuals inhaled either ambient gas at +30 to +33 degrees C (control condition), or cooled gas at +7 to +18 degrees C. RL values measured in control conditions increased with gas density. Thus, the changes in RL induced by cold gas breathing were expressed in percent of the corresponding control values. No cold-induced bronchospasm occurred at low ambient pressure, even at the lowest inspired temperature, +7 degrees C. However, the airway response was present at pressure up to 21 ATA and then occurred at higher level of inspired gas temperature. The convective respiratory heat loss (Cr), calculated at each pressure level and experimental condition, was linearly related to cold-induced changes in RL; the value of Cr inducing 20% increase in RL was around 1.4 kcal.min-1. The bronchomotor response was related to the increase in respiratory heat loss induced by the high thermal capacity of the gas mixture used in hyperbaric environment. The present observations confirm previous data obtained under hyperbaric conditions (25 ATA) as well some experiments performed at sea level in normal individuals breathing very cold air.