Single esophageal catheter (Type Cooper, French caliber #5) and disposable pneumotachs were used. Lidocain spray and K-Y gel were applied during the insertion of the catheter. Its placement was done at 37.6±5.7 cm across the subject and a Mueller test was performed for each subject. In regard of spontaneous breathing, an Hudson mask was used while a C-4 Gas Mask with a canister was employed (Manufacturer: Airboss Defence, Bromont, Canada). Hypoxemia mixture was home-design with usual nitrous and medical gas and maintaining a FiO2 target at 14%. Prototyped Free O2 System was employed for the correction of the hypoxemia.