Mild Hyperbaric Oxygen Exposure Is Associated with Calculation-Method-Dependent Changes in VE/VCO₂ During Seated Rest

Mild hyperbaric oxygen (HBO) exposure might influence ventilatory and gas-exchange responses. We investigated the effects of mild HBO exposure at 1.3 atmospheres absolute (ATA) on VE/VCO₂ and related variables at rest in healthy young men. Using a single-group repeated-measures design, pre, steady (20-50 min at 1.3 ATA), and post-phases were compared in 16 participants. The primary outcome was VE/VCO₂, calculated as minute ventilation (VE) divided by carbon dioxide output (VCO₂); VE, VCO₂, and partial pressure of end-tidal CO₂ (PETCO₂) were secondary outcomes. Linear mixed models and Bonferroni-adjusted pairwise comparisons were used. Using the primary bin-wise calculation, VE/VCO₂ increased significantly from 39.9 ± 1.0 in the pre-phase to 43.8 ± 1.0 in the steady phase and decreased to 38.8 ± 1.0 in the post-phase. VE and PETCO₂ showed no phase effects; VCO₂ decreased significantly during the post-phase. However, an alternative calculation based on phase-averaged VE divided by phase-averaged VCO₂ produced the opposite phase-dependent pattern. Among eight participants completing both HBO and normobaric normoxic conditions, the exploratory Condition × Phase interaction was significant for both calculations, although phase patterns differed between methods. Thus, the direction of the VE/VCO₂ change depended on the calculation method, and its physiological significance remains uncertain.