Methods Astral 150, Elisee 150, Trilogy 200 and Monnal T50 equipped with non-vented double limb circuit or equivalent were set in volume controlled mode (tidal volume 500 ml, breathing frequency 15 cycles/min, inspiratory time 0.8 sec, constant flow inflation shape, positive end expiratory pressure 5 cmH2O) and connected to ASL 5000 lung model set in a condition mimicking COPD patient (compliance 75 ml/cmH2O, inspiratory and expiratory airways resistance 15 and 25 cmH2O/L/s, respectively). Oxygen was supplemented by connecting the low pressure oxygen port at the rear of ventilator to the wall oxygen supply via a flow-meter. Three oxygen flow rates were delivered: 0, 3 and 15 L/min. FI02 was measured with Citrex H4TM (imtmedical, Switzerland) and continuously monitored at the screen of the device. Once FI02 reached a plateau oxygen administration was maintained for an additional one minute and FI02 was measured over the last 10 breaths of the recording at the steady state defined as changes in FI02 less than 10% from the previous sampling window. The sampling rate was 200 Hz. Citrex H4 TM device was calibrated before each experiment. The values of FI02 were expressed as mean ± SD. Comparisons were made by using two-factor ANOVA and multiple comparisons between ventilators by using Tukey test.