Background Because of the critical shortage of adult donor hearts, many recipients die awaiting transplantation of an organ of appropriate size.Undersized hearts (donor/recipient weight ratio <0.7) have been used for heterotopic heart transplantation. We report on 6 moribund adult heart transplant candidates who were rescued with orthotopic heart transplantation of undersized pediatric hearts.Methods and Results Recipients were hypotensive (mean blood pressure, 62.3+-13.4 mm Hg), had high pulmonary artery pressures (mean pulmonary artery pressure, 42.4+-6.3 mm Hg), and had mean cardiac indexes of 1.7+-0.6 L min-1m-2. Four had pretransplant intra-aortic balloon pumps, and one was on a Thoratec left ventricular assist device complicated by fungemia. Since conventionally sized donors were unavailable (+-30% recipient weight), the patients were listed in a wider weight range (+-60%). Donor characteristics were age, 8.7+-1.5 years; weight, 32.8+-7.0 kg; and donor/recipient weight ratio, 0.44+-0.2, with average ischemic time of 236.0+-59.3 minutes. Technical considerations during transplantation included (1) opening the donor right atrium from the inferior vena cava to superior vena cava to facilitate size matching, (2) performing size-mismatched pulmonary artery and aortic anastomoses end to end, (3) infusing prostaglandin E112 ng kg-1min-1to decrease pulmonary and systemic vascular resistance, (4) pacing donor and recipient atria synchronously to improve ventricular filling, (5) maintaining high heart rates up to 140 beats per minute (initially with isoproterenol or pacing, chronically with theophylline), (6) hyperventilating with sedation and paralysis as necessary, (7) reperfusing with triiodothyronine, and (8) minimizing afterload. All patients were discharged from the hospital. At 1 week, hemodynamics were normal and echocardiograms demonstrated left ventricular growth.Conclusions Hence, undersized pediatric hearts can be used successfully to salvage moribund patients and expand the potential donor pool for adult orthotopic heart transplantation.(Circulation. 1994;90(part II):II-74-II-77.)