1. Pulmonary, Allergy, and Critical Care Division, Department of Medicine
2. Center for Clinical Epidemiology and Biostatistics, and
3. Department of Physical Medicine and Rehabilitation, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, Pennsylvania
4. Philadelphia Veterans Affairs Medical Center, Philadelphia, Pennsylvania
5. Pulmonary and Critical Care Unit, Department of Medicine, Massachusetts General Hospital, Boston, Massachusetts
6. Department of Medicine, Pulmonary and Critical Care Medicine Division, Intermountain Medical Center, Murray, Utah
7. Psychology Department and Neuroscience Center, Brigham Young University, Provo, Utah; and