From the Department of Surgery and the Department of Comparative Medicine, The Milton S. Hershey Medical Center, The Pennsylvania State University, Hershey, Pennsylvania and the Biomedical and Enginneering and Instrumentation Branch, National Institutes of Health, Bethesda, Maryland, U.S.A. Supported by PHS Grant NO. 5 R01 HE 13426 and Pennsylvania Heart Associaton.