In conclusion, our study demonstrates that PP induces a decrease in PaCO 2 and VD alv . This is related to an improvement in respiratory mechanics, with a decrease in Pplat and an increase in compliance. Testing the response to PP appeared to be physiologically more relevant using PaCO 2 changes than PaO 2 /FiO 2 changes. How this may affect management at the bedside remains to be studied. Estimated VD physiol /V T ratios systematically underestimated measured VD physiol /V T ratios.