1. Veterans Affairs Health Care System, Salem, VA 24153, USA
2. Department of Basic Science Education, Virginia Tech Carilion School of Medicine, Roanoke, VA 24016, USA
3. 1st Department of Critical Care Medicine & Pulmonary Services, GP Livanos and M Simou Laboratories, Evangelismos Hospital, National and Kapodistrian University of Athens, 10675 Athens, Greece
4. Kansas City VA Medical Center, Kansas City, MO 64128, USA