Echocardiographic Assessment 
A standard TEE probe (TEV5Ms Transducer) and echocardiography system (Acuson SC2000, Siemens, Mountain View, CA) were used to perform a comprehensive epicardial echocardiographic examination. In order to allow for an optimal interface between the donor heart and the probes, a custom-made 3D-printed spacer was applied at the tip of the transducer and filled with gel. The probe was covered by a disposable sterile plastic cover bag. Figure 1B illustrates the custom-made 3D-printed spacer and figure 1C depicts the experimental timeline. The following standard echocardiography views were obtained: apical 4 chamber (Ap4), apical 2 chamber (Ap2), apical 3 chamber (Ap3), apical 5 chamber (Ap5), LV short axis (LV SAX) and LV long axis (LAX). LV function was quantified according to current guidelines25using biplane ejection fraction (EF), fractional shorting (FS), fractional area change (FAC), global longitudinal strain (GLS), global circumferential strain (GCS) and radial strain (RAD). Strain was measured off line with Echo Insight software (Epsilon Imaging, trademark, Ann Arbor, Michigan, USA). Global strain was computed as the average of the strain values of all LV segments. FS, FAC, GCS and RAD were measured in the LV SAX.