PROne POsition and Volumetric CAPnography

Beneficial effect of prone position in ARDS on mortality is not linked to increase in PF ratio. It is probably due to improvement in ventilation-perfusion ratio (V/Q). Volumetric capnography allows assessment of dead space and homogeneity of V/Q. All prone positionned patients will be included in the study with usual monitoring (i.e. respiratory system basic mechanics, volumetric capnography, blood gas samples). Epidemiologic and monitoring data will be collected during all prone position sessions.