[1] Stanford Univ, Sch Med, Dept Anesthesia, Stanford, CA 94305 USA
[2] Stanford Univ, Sch Med, Dept Pediat, Stanford, CA 94305 USA
[3] Johns Hopkins Med Inst, Dept Anesthesiol, Pediat Pain Serv, Baltimore, MD 21205 USA
[4] Johns Hopkins Med Inst, Dept Crit Care Med, Baltimore, MD 21205 USA
[5] Johns Hopkins Med Inst, Dept Pediat, Baltimore, MD 21205 USA