The prone position (PP) may also be part of the ventilatory strategy. This method was proposed more than 30 years ago, initially in pathophysiological studies [6, 7] . Recently, Sud et al. [8] suggested, on the basis of pooled data from randomized, controlled trials, that PP may improve survival in the subgroup of patients with the most severe ARDS, that is, those with a ratio of partial pressure of arterial O 2 to fraction of inspired O 2 (PaO 2 / FiO 2 ) < 100 mmHg. Many questions remain unresolved. In particular, response to PP is usually defined according to changes in PaO 2 , with responders being those in whom the PaO 2 /FiO 2 ratio increases > 20 mmHg after one to six hours in the PP [9] [10] [11] . However, we have previously reported that PP allows recruitment of a slow compartment previously excluded from ventilation [12] . This was associated with a decrease in partial pressure of arterial CO 2 (PaCO 2 ), an indirect reflection of the reduction of the alveolar dead space (VD alv ) [12] . Gattinoni et al. [10] also reported that the prognosis is improved in patients in whom PaCO 2 declines after an initial PP session. Finally, VD alv appears to be an independent risk factor for mortality in patients with ARDS [13] . In a recent study, Siddiki et al. [14] proposed evaluating the physiological dead space fraction (VD physiol / V T ) by using a rearranged alveolar gas equation for PaCO 2 without any expired CO 2 measurement.