 - experimental setting : this experiment will be performed in a randomized, double masked, three way cross-over design. Three inhalation periods of different mixtures of oxygen and carbon dioxide are scheduled. Three LDF measurements of 30 seconds are performed before inhalation, 5 and 10 minutes during the inhalation, and then after resting period of 10 minutes. The following gas mixtures are administered for 10 minutes each: 100 O2, Bactal 92O2 8CO2, and air 21,8O2 78,2N2. All gases are delivered through a partially expanded reservoir bag at atmospheric pressure using a two valves system to prevent re-breathing. Transcutaneous carbon dioxide tension TINA PtcCO2 monitoring system, Radiometer, Copenhagen, Denmark, MAP and IOP are monitored at 5, 10 and at the end of the experiment. 