1. From the Vanderbilt Autonomic Dysfunction Center (L.E.O., C.S., A.G., A.D., S.R.R., B.K.B., D.R., I.B.), Division of Clinical Pharmacology (L.E.O., C.S., A.G., A.D., S.R.R., B.K.B., D.R., I.B.), and Departments of Medicine (L.E.O., C.S., A.G., A.D., S.R.R., B.K.B., D.R., I.B.), Pharmacology (S.R.R., D.R., I.B.), Neurology (D.R.), Biomedical Engineering (A.D.), and Biostatistics (L.C.), Vanderbilt University School of Medicine, Nashville, TN.