U.S. doctors suggest the depression sometimes linked to a stay in the intensive care unit may be rooted in care issues.

The study, published online in Critical Care Medicine, tracked 160 intensive care patients that survived six months after coming to one of 13 Baltimore hospitals for acute lung injury -- a respiratory distress syndrome that despite greatly reduced mortality rates still kills about 40 percent of those affected.

The researchers found 26 percent scored above the threshold for possible depression. The depressed patients were found to be more likely to have suffered greater severity of organ failure and to have received 75 mg or more of a benzodiazepine sedative daily.

Study leader Dr. O. Joseph Bienvenu of The Johns Hopkins University School of Medicine says severe organ failure often leads to a longer physical recovery period and thus perhaps depression, but he is not sure how to explain the association between depression and benzodiazepine dose that was not exhibited by other intensive care sedatives.

This is clearly a question that needs further study, Bienvenu said in a statement. Historically, the only goal for critical care physicians, understandably, was to keep people alive, but now there is interest in longer-term outcomes, such as patients' mental health and well-being."