1. Department of Anesthesiology, University of Massachusetts Medical Center, Worcester, Massachusetts 01655 and Department of Surgery, Children's Hospital, Boston, Massachusetts 02115.
2. Department of Surgery, Children's Hospital and Harvard Medical School, Boston, Massachusetts 02115.
3. Department of Surgery, Beth Israel Hospital and Harvard Medical School, Boston, Massachusetts 02115.
4. Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139.