Relationship between inspired and expired gas temperatures in a hyperbaric environment

Area of Science:

• Hyperbaric physiology
• Respiratory thermoregulation
• Gas physiology

Background:

• Expired gas temperature (TE) typically increases with inspired gas temperature (TI) at sea level.
• Previous hyperbaric studies assumed a consistent TE vs. TI relationship across different breathing gas mixtures.

Purpose of the Study:

• To investigate if diluent gases like helium (He) or hydrogen (H2) affect the TE vs. TI relationship under hyperbaric conditions.
• To test the hypothesis that gas properties (density, specific heat) influence respiratory heat exchange.

Main Methods:

• Studied 3 professional divers during the COMEX Hydra IX experiment.
• Tested three hyperbaric conditions (He-H2-O2, H2-O2, He-O2) with varying inspired temperatures.
• Measured minute ventilation, inspired temperature (TI), and expired temperature (TE) simultaneously.

Main Results:

• A linear relationship between TE and TI was observed in all conditions.
• Regression line slopes were significantly lower in mixtures containing hydrogen (H2) compared to helium-oxygen (He-O2).
• Convective respiratory heat loss (Cr) was 1.6 times higher in H2-O2 mixtures than He-O2 at +10°C inspired temperature.

Conclusions:

• The use of hydrogen as a diluent gas significantly alters respiratory thermoregulation under hyperbaric conditions.
• A single equation cannot accurately predict the TE vs. TI relationship across all hyperbaric breathing gas mixtures.
• Findings highlight the importance of gas properties in hyperbaric respiratory heat exchange.