Normothermic Ex Vivo Heart Preservation: Assessment of Graft Viability Prior to Transplantation

A critical component of organ preservation is the ability to evaluate organ functionality before transplantation. Current preservation methods for cardiac transplantation predominantly use cold static storage (CSS) at 5 ºC, which is typically restricted to 6 h. Although CSS is effective for brain-dead donor hearts, it is not commonly used for hearts from donors after circulatory determination of death (DCD). Furthermore, CSS is constrained by time limitations, restricted transportation options, and a lack of comprehensive assessments of organ function before transplantation. Prolonged CSS times are associated with graft dysfunction caused by ischemia/reperfusion injury, which occurs during the preservation and transport time of the organs.1 Normothermic ex vivo heart preservation aims to minimize ischemic damage during storage, enable longer preservation periods, and provide opportunities for detailed evaluations of marginal donor hearts before transplantation.2 The existing Food and Drug Administration–approved organ care system has shown comparable effectiveness to CSS, yet it primarily measures lactate trends during ex vivo perfusion as a surrogate for organ viability, offering limited insights into actual heart function. In contrast, many organs have established methods for assessing their functions in ex vivo systems, such as (1) specific hemodynamic parameters for perfused organs, (2) urine production in kidneys, (3) toxin filtration and bile production in livers, and (4) digestive enzyme and insulin production in pancreases. Additionally, obtaining biopsies from these organs to assess tissue health in real time is relatively straightforward. In stark contrast, the fundamental role of the heart in pumping blood through the body presents challenges; current preservation systems fail to reliably replicate the pumping function of the heart, and taking biopsies of cardiac tissue is both difficult and risky. The study by Clavier et al3 investigated the effect of ischemic duration on cardiac recovery using an isolated rat heart model, specifically focusing on DCD situations. The study involved 30 rats, using an isolated heart perfusion system at a constant pressure of 60 mm Hg with modified acellular Krebs-Henseleit bicarbonate buffer. Results indicated that hearts exposed to >27 min of ischemia exhibited significantly reduced left ventricular work, cardiac output, and left ventricular power. Early reperfusion after 21 min of ischemia also showed increased coronary flow, with notable elevations in troponin T and lactate dehydrogenase levels after 60 min of reperfusion for longer ischemic periods. Lactate release during reperfusion was significantly higher among the ischemic groups, and principal component analysis effectively distinguished between ischemic and nonischemic hearts and among varying ischemic durations. Receiver operating characteristic analysis identified glycogen phosphorylase and pyruvate kinase as promising novel biomarkers for evaluating cardiac graft quality. The authors concluded that proteins released during early reperfusion hold potential as biomarkers for assessing graft quality during normothermic perfusion. Key proteins, including HSPA4 (heat shock protein 70), ITIH1 (inter-alpha-trypsin inhibitor heavy chain 1), LDHA (lactate dehydrogenase A), PFKM (phosphofructokinase, muscle), and VCP (valosin-containing protein), were identified via least absolute shrinkage and selection operator regression as strong indicators of cardiac recovery, with glycogen phosphorylase, muscle associated emerging as a particularly promising novel biomarker. Limitations of the study included (1) the murine model of isolated heart perfusion lacking cellular perfusate components present in standard care during normothermic regional perfusion and organ care system and (2) the absence of histological evaluations of cardiac tissue. Nonetheless, the findings suggest that proteins released during early reperfusion could be effective biomarkers for evaluating DCD cardiac graft quality, thereby aiding transplantation decisions and enhancing clinical practices. One of the most pressing objectives in heart transplantation is to refine preservation methods so that nearly all accepted hearts can be successfully transplanted. Emerging graft preservation techniques should focus on extended (>12 h) preservation coupled with objective functional assessments of the organ before transplantation. Normothermic ex vivo heart perfusion (NEHP) is one such technique. Research from the extracorporeal life support laboratory at the University of Michigan has reported4,5 on an intermittent working-heart model transitioning from NEHP in a nonworking state to a working-heart configuration after initial resuscitation, allowing for hemodynamic assessments. Preliminary findings in porcine models using this intermittent perfusion approach demonstrated that hemodynamic function could be maintained throughout a 24-h perfusion period, highlighting the feasibility of this method. These results underscore the importance of direct hemodynamic assessments for determining transplant suitability and suggest the potential for extending heart preservation times beyond current limitations, ultimately improving heart transplantation outcomes. NEHP has significant potential as a platform for interventions on donor hearts, with opportunities to enhance donor–recipient matching, facilitate broader organ sharing across the nation, and apply conditioning regimens to foster immune tolerance in transplant recipients—even in cases of DCD. This advancement could substantially reduce the toxicity and side effects associated with maintenance immunosuppression, lower the incidence of acute and chronic graft rejection, and ultimately enhance allograft survival, function, and patient quality of life. Overall, the study by Clavier et al represented a critical initial step in identifying early biomarkers as surrogates for cardiac injury and viability during NEHP. Future developments could further improve the successful application of biomarker analyses—such as the creation of multiplexed protein panels and identification of molecular pathways associated with ischemia tolerance—in conjunction with objective cardiac function assessments (including echocardiography and pressure–volume curves under varying preload and afterload conditions). This integrated approach aims to enhance outcomes in heart transplantation and expand the pool of viable organs available for recipients.