In summary, none of the present studies on prone positioning, including the one by Gattinoni and colleagues,2
should be considered definitive. All should, however, be looked at carefully to help design future studies. Clearly, more studies are needed before drawing any final conclusion regarding the effect (eg, mortality) of prone positioning on outcome. We need particularly to determine the target population that might benefit the most from prone positioning, and the optimal timing for its use (ie, at what stage of the natural course of ARDS, and how often and for how long should we turn patients to the prone position and keep patients prone). It is also likely that the best prone ventilatory strategy differs from the best supine ventilatory strategy and that the intensity of abdominal compression that is allowed for the patient in the prone position also may be important. These issues have not been comprehensively addressed so far. As improved gas exchange per se is not the ultimate goal, we also need to determine how to best manage responders (ie, shall we aim at reducing airway pressure or the fraction of inspired oxygen?), and we should pay closer attention to how prone positioning affects the lungs rather than to how much it impacts Pao2 in future studies. Although clinical outcome studies are very important, the premature or suboptimally designed study carries the risk of inappropriately changing the way we look at reality. Given the established safety, the frequent gas exchange improvement, and the potential beneficial effects on ventilator-induced lung injury that are associated with prone positioning, I do not see any compelling reason not to turn my next patient with severe ARDS to the prone position. I still believe the glass is half-full, not half-empty.