We gratefully acknowledge funding to SNT from the US National Institute of Health (K99/R00 HL1431149; R01HL157803; R01DK134590; R24OD034189), the National Science Foundation under Grant No. EEC 1941543, the Claflin Distinguished Scholar Award on behalf of the MGH Executive Committee on Research, and the Polsky Family Award for Leaders in Surgery.&#160;We acknowledge research funding to AAO from the Hassenfeld Family Foundation, the MGH Executive Committee&#160;on Research, and the MGH Center for Diversity and Inclusion.&#160;We acknowledge research funding to GO from the Sarnoff Cardiovascular Research Foundation.

This study was conducted in accordance with the Institutional Animal Care and Use Committee (IACUC), Massachusetts General Hospital, and Jove&#39;s animal guidelines. Hearts (170 - 250 g) were harvested from Yorkshire pigs (30 - 35 kg, age 3-4 months, mixed sex) using a model of donation after brain death and perfused retrogradely (Langendorff) for 6 h prior to loading. All grafts were exposed to a cold ischemia time of approximately 1h during instrumentation.
1. System design</...

Enhanced Cardiac Assessment in Ex Vivo Perfusion with Left Ventricular Loading

Normothermic machine perfusion is a powerful modality for organ preservation and assessment that has greatly impacted the field of cardiac transplantation by expanding the donor pool of adult hearts36. This expansion is the result of the ability to currently utilize a small pool of hearts previously considered unsuitable for transplantation. Normothermic machine perfusion preserves cardiac grafts in a beating state, offering the opportunity for both functional and metabolic assessment. However, de...

Orthotopic heart transplantation is the current gold standard of care for end-stage heart failure1. Unfortunately, the field is significantly limited by a severe donor shortage crisis, resulting in only 2,000 heart transplants being performed each year when over 20,000 people would benefit from the lifesaving procedure2. This organ shortage is expected to worsen as the prevalence of heart failure in the United States alone is projected to surpass 8 million individuals by 20303. Steady increases in waitlist survival times - as a result of improved medical management, advances in mechanical circulatory support, and amendments to the UNOS allocation policy - have resulted in a further increase in the number of patients in need of transplantation at any given moment4,5.
Ex vivo machine perfusion or normothermic machine perfusion (NMP) is a preservation modality that has facilitated the expansion of the supply pool by allowing the use of organs donated after circulatory death (DCD) while achieving some extension of preservation times5,6,7,8. Unlike static cold storage, the current gold standard for preservation, NMP maintains organs in a metabolically active state, which creates the opportunity for real-time monitoring and graft assessment, becoming the standard preservation method for DCD grafts8,9. However, NMP devices currently used clinically are restricted to the Langendorff perfusion mode, which lacks quantitative metrics to predict transplantation outcomes and is unable to capture functional parameters6. For instance, lactate accumulation during Langendorff perfusion has been denoted as the best metabolic predictor of post-transplantation outcomes and is currently used in the clinical setting as a proxy for cardiac graft health10. However, even as the best assessment biomarker, it fails to reliably anticipate the need for mechanical circulatory support post-transplantation11,12. Similarly, the predictive capabilities of commonly utilized hemodynamic parameters (i.e., aortic pressure and coronary blood flow) are largely limited by the retrograde nature of current clinically used configurations for heart machine perfusion9.
The development of assessment protocols for accurate and precise determination of cardiac graft health during NMP would have an immense impact in the field beyond improving post-transplantation outcomes. Objective predictive tools would enable the reliable evaluation and likely utilization of marginal or extended criteria organs (i.e., prolonged warm (&#62; 30 min) and cold ischemia times (&#62; 6 h), increased donor age (&#62; 55), other comorbidities, etc.) from both DCD and brain death donors (DBD) that are currently rejected for transplantation due to the stringent selection criteria13. By enabling the use of marginal hearts, NMP could facilitate an increase in the organ supply as it is estimated that successful transplantation of half the hearts that currently go unused would be sufficient to eliminate the heart waitlist within 2-3 years14. Hemodynamic measurements obtained from left ventricular loading during NMP have garnered significant attention within the field due to their potential as objective assessment parameters. Previous studies have demonstrated that these parameters, such as left ventricular pulse pressure, contractility, and relaxation, are more indicative of cardiac graft function than metabolic trends15,16,17.
In effect, efforts have been dedicated to the development and identification of optimal loading methods to maximize assessment accuracy. Through these efforts, other groups have identified the most relevant mode of aortic perfusion during loading, whereby a stronger correlation between hemodynamic parameters and post-transplantation function was seen when implementing a passive afterload (i.e., no retrograde perfusion to the aorta during loading) when compared with pump-supported afterload (i.e., retrograde perfusion to the aorta during loading)18. This indicates that assisted coronary perfusion likely masks functional deficiencies. Previous studies have successfully incorporated passive afterloads into perfusion setups by implementing systems that mimic the Windkessel effect18,19,20. The Windkessel effect aids in dampening the fluctuation in blood pressure, maintaining continuous blood flow to the tissues and improving coronary perfusion. This protocol achieves the Windkessel-based passive afterload using a modified intravenous (IV) bag enclosed in two spring-loaded plates, where coronary perfusion is exclusively dependent on heart ejection (patent pending).
The use of passive left atrium (LA) pressurization (i.e., gravity-dependent pressurization) during loading, although common practice in small animal heart perfusions, is rarely utilized in the loading of large hearts21,22,23. Instead, the large majority of methods reported in the literature rely on secondary pumps for LA pressurization18,24,25,26,27,28. The pressurization of the LA through a gravity-dependent reservoir, rather than by pump, significantly simplifies the implementation of loading protocols. The use of gravity provides a fixed and constant pressure source, which greatly decreases the need for complicated control systems to achieve and maintain adequate LA pressurization. Moreover, through this pressurization approach, the requirement for a secondary pump is eliminated, facilitating the incorporation of loading capabilities into current Langerdoff setups, as only an extra reservoir is needed. The integration of loading capabilities into clinically utilized machine perfusion systems would amplify the application of cardiac NMP devices by facilitating detailed assessment of cardiac grafts during the preservation period. In effect, maximizing the utility of a system that poses significant financial commitment for patient care due to transportation and device utilization29.
This protocol demonstrates the feasibility of employing both passive afterload and passive LA pressurization during left ventricular loading. Through the validation of passive afterload/LA pressurization as a loading method, this protocol also provides an easy and effective manner of incorporating loading capabilities into established Langendorff perfusion systems. Importantly, it highlights the capability of functional assessment to uncover differences in viable versus failing hearts after extended periods of preservation (&#707;6 h).

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cardiac loading using passive left atrial pressurization and passive afterload for graft assessment

Ex vivo machine perfusion or normothermic machine perfusion is a preservation method that has gained great importance in the transplantation field. Despite the immense opportunity for assessment due to the beating state of the heart, current clinical practice depends on limited metabolic trends for graft evaluation. Hemodynamic measurements obtained from left ventricular loading have garnered significant attention within the field due to their potential as objective assessment parameters. In effect, this protocol provides an easy and effective manner of incorporating loading capabilities to established Langendorff perfusion systems through the simple addition of an extra reservoir. Furthermore, it demonstrates the feasibility of employing passive left atrial pressurization for loading, an approach that, to our knowledge, has not been previously demonstrated. This approach is complemented by a passive Windkessel base afterload, which acts as a compliance chamber to maximize myocardial perfusion during diastole. Lastly, it highlights the capability of capturing functional metrics during cardiac loading, including left ventricular pulse pressure, contractility, and relaxation, to uncover deficiencies in cardiac graft function after extended periods of preservation times (&#707;6 h).

Author Spotlight: Enhancing Graft Viability Assessment Through Quantitative Metrics and Innovative Reservoir Systems

Cardiac Loading using Passive Left Atrial Pressurization and Passive Afterload for Graft Assessment

The scope of this research covers two important areas of cardiac ex vivo machine perfusion. First, it highlights the shortcomings of current clinically-used assessment metrics in determining graft viability and transplantability. Second, it provides an alternative assessment technique capable of discerning between viable and unviable grafts, even when conventional metrics fail to do so.Through this work, we developed a gravity-dependent loading reservoir that significantly simplifies and standardize the incorporation of loading capabilities into existing ex vivo machine perfusion systems. This, in combination with passive afterload, seems to significantly increase the predictive capabilities of the technique. The field has been in dire need of quantitative assessment metrics that eliminate the subjectivity of current practices.This work offers a method to obtain quantitative objective metrics for assessing graft variability. This assessment method is expected to significantly improve the ability to draw conclusions from both experimental and clinical setups.

Hearts from 4 Yorkshire pigs (30 - 35 kg) were harvested and preserved via Langendorff NMP for 6 h prior to 4 h of continuous loading. This experimental condition was chosen since 6 h is the average clinical preservation duration (5.1 &#177; 0.7 h)34. Through the addition of 4 extra hours of continuous loading (total of 10 h ex vivo time), some degree of heart failure was expected as a clear correlation between perfusion time and myocardial function decline has been previously reported<su...

The protocol describes a porcine ex vivo heart perfusion system in which direct loading of the left ventricle may serve as an assessment technique for graft health while simultaneously providing a holistic evaluation of graft function. A discussion of the system design and possible assessment metrics is also provided.

Ex vivo machine perfusion or normothermic machine perfusion is a preservation method that has gained great importance in the transplantation field. ...

cardiac-loading-using-passive-left-atrial-pressurization-passive