1 Department of Internal Medicine & Pediatrics and School of Medicine, University of Alabama, Birmingham, AL 35233, USA. Electronic address: swrussell@uabmc.edu.
2 Department of Pulmonary and Critical Care Medicine, The Johns Hopkins Hospital, Baltimore, MD, USA.
3 Medical School, University of Edinburgh, Edinburgh, Scotland.
4 Department of Theory and Practice of Medicine, Stanford University, Stanford, CA, USA.