Discussion
In each of the six experiments, the system demonstrated stability across the trial. Metabolic parameters were maintained within a physiological range. The hearts demonstrated predominantly an aerobic metabolism throughout the experiments as demonstrated by the neutral lactate metabolism and the stable lactate levels across the perfusion period.
We were able to complete a comprehensive echocardiography assessment with SAM and PAM without interfering with the operators and manipulating the heart. This proved the effectiveness of our custom-made setup for echocardiographic assessment during ESHP.  Looking at the trend in echocardiographic parameters throughout the trials, the expected LV systolic dysfunction consisted of a reduction in EF, FAC, FS and RAD as well as in GLS and GCS throughout the experiment. However, the bigger drop was from baseline to ESHP and was partially due to cold ischemia. During the 5.5 hours of ESHP we detected by echocardiography a slow decrease in LV systolic function which is consistent with the metabolic trends and while excludes heart reconditioning, given the difference of only one hour between PAM and SAM2 towards the end of our experiment we would assume that the heart condition would be similar. When comparing the LV parameters at PAM and SAM2 we noticed a larger difference in load dependent parameters such as LVEF when compared to strain. This is likely a confirmation that the two working modes while both allowing functional heart assessment may not be comparable due to different setup and loading conditions.
Reliable, easy to use and reproducible methods are required to evaluate the myocardial function during ESHP prior to transplantation. Numerous approaches allow the assessment of organ viability during ESHP, including biomarkers of tissue injury (i.e. lactate and troponin I), metabolic measurements (i.e. myocardial oxygen consumption), and hemodynamic and contractility parameters (i.e. pressure-volume loops and echocardiography).26 In clinical practice, the heart is preserved in a unloaded mode which does not allow for evaluation of contractile function. The assessment of the allograft with the OCS utilizes therefore only lactate levels and veno-arterial lactate extraction as markers of heart viability and suitability for transplantation.21 This is sustained by the work of Hamed and colleagues, who found that a serum lactate level above 4.96 mmol/L was a strong predictor of graft dysfunction at 30 days,27 and has been used to identify suitable DCD hearts for clinical transplantation.5 However, lactate concentration only demonstrated weak to moderate correlations and correlated with fewer outcomes compared with hemodynamic parameters. Dornbierer26 and White10 have reported similar findings of the limited applicability of metabolic measurements. Biomarkers of myocardial damage like troponin I and creatine kinase-MB have proven to be of limited value in predicting organ viability due to their natural elevation with the warm ischemia and preservation insult.
The evaluation of heart structure and function prior to cardiac transplantation is therefore auspicable to better identify suitable organs. Previous studies have suggested the advantage of contractility measurements over metabolic parameters during ESHP, but these studies were focused on the use of conductance catheters, which provide a broad range of functional parameters and is the traditional gold-standard when researching myocardial performance.12,14,18 However, these are invasive and difficult to utilize, thereby decreasing result reproducibility. Ideally, methods to evaluate suitable donor hearts should be non-invasive, easy to use and quick to perform. Two-dimensional echocardiography has also been randomly used to obtain qualitative assessments of myocardial contraction,22,28 however, a standardized approach to echocardiography in the ex situ perfused heart has not been developed.
Our study demonstrates the feasibility of a complete non-invasive quantitative echocardiographic assessment of LV systolic function during ESHP by transitioning from LM to two different working modes. It is an important step in facilitating a standardized non-invasive functional assessment of the heart during ESHP to predict the suitability for transplantation.  It also provides some quantitative measurements that may establish some references to set a benchmark for future research.