[1] Hosp Univ Penn, Div Cardiac Surg, 3400 Spruce St,Silverstein 6, Philadelphia, PA 19104 USA
[2] Hosp Univ Penn, Div Cardiovasc Med, Philadelphia, PA 19104 USA
[3] Univ Penn, Leonard Davis Inst Hlth Econ, Philadelphia, PA 19104 USA
[4] Michael J Crescenz VA Med Ctr, Philadelphia, PA USA