A study involving 520 intensive care patients who had been put on ventilators for acute lung injury (ALI), of whom 186 patients of the 275 survivors were followed up over the next two years, found that 35% of them had clinically significant symptoms of PTSD. Nearly two-thirds of these (62%) still had symptoms at two years.
ICU survivors with PTSD are unusual in that they often experience flashbacks to delusions or hallucinations they had in the hospital, rather than events that actually occurred
The main risk factors were: depression before hospitalization; greater length of time in the ICU; development of sepsis while in ICU; high doses of opiates. Lowered risk was associated with being given corticosteroids.
 . Post-traumatic stress disorder symptoms after acute lung injury: a 2-year prospective longitudinal study. Psychological Medicine. 2013 ;FirstView:1 - 15.