In this context, we conducted a prospective physiological study to evaluate the impact of PP on ventilatory mechanics, gas exchange and VD alv . Our main objective was to validate our hypothesis that changes in PaCO 2 and VD alv might be more relevant than changes in PaO 2 in defining the respiratory response to PP. Our second objective was to validate the method of evaluation of the VD physiol /V T proposed by Siddiki et al. [14] .