Patrick A Stokes1, Patrick L Purdon2,3. 1. Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Boston, MA 02114. 2. Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Boston, MA 02114; patrickp@nmr.mgh.harvard.edu. 3. Department of Anaesthesia, Harvard Medical School, Boston, MA 02115.