[1] Univ N Carolina, UNC Hosp, Dept Emergency Med, Chapel Hill, NC 27599 USA
[2] Univ Pittsburgh, Sch Med, Dept Anesthesiol, Pittsburgh, PA 15260 USA
[3] Univ Pittsburgh, Sch Med, Dept Crit Care Med, Pittsburgh, PA 15260 USA
[4] Univ Pittsburgh, Sch Med, Safar Ctr Resuscitat Res, Pittsburgh, PA 15260 USA
[5] Univ Pittsburgh, Sch Med, Natl Inst Environm Hlth Serv, Pittsburgh, PA 15260 USA