• Laetitia Bodet-Contentin, Denis Frasca, Elsa Tavernier, Fanny Feuillet, Yohann Foucher, and Bruno Giraudeau.
• Université de Tours, Université de Nantes, INSERM, SPHERE U1246, Tours, France.
• Crit. Care Med. 2018 Mar 1; 46 (3): 425-429.

IntroductionAcute respiratory distress syndrome often requires invasive mechanical ventilation, with both mortality and mechanical ventilation duration as outcomes of interest. The concept of ventilator-free days has been proposed as an outcome combining these two outcomes. Here we analyzed the construction of the ventilator-free day outcome and provided a hypothetical scenario to alert physicians that such an outcome can lead to misleading interpretations.MethodsWe proposed the isoventilator-free day curve concept and, using an analytical development, illustrated how a median ventilator-free day value can actually result from very different combinations of death rates and mechanical ventilation durations. We also used a hypothetical example to compare the Student t test, Wilcoxon rank-sum test, and Gray test (which accounts for death as a competing event with extubation) in comparing exposition to mechanical ventilation.ResultsA median ventilator-free day value of 10 days may mean that 10% of the patients died while survivors were ventilated during a median of 14 days or that 40% died while survivors were ventilated during a median of 5 days. Changing the time horizon affected the Student t test but not the Wilcoxon rank-sum result. The Gray test was more relevant than both the Student t test and Wilcoxon rank-sum test in identifying differences in groups showing highly different mechanical ventilation duration, despite equal median ventilator-free days. This approach was also illustrated using real data.ConclusionsUse of ventilator-free days as an outcome appears to have many drawbacks. Suitable methods of analyzing time to extubation should be preferred.

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