Ice, ice, maybe? Is it time to ditch the igloo cooler? Benefits of machine perfusion preservation of donor hearts

Geographic Variation in Heart Transplant Extended Criteria Donors in the United States

BACKGROUND

Recent research has explored the use of higher risk extended criteria donors (ECDs) as a means of expanding the donor pool for heart transplantation. Here we sought to explore the current geographic distribution and survival outcomes of ECD utilization in various regions across the United States.

METHODS

The United Network for Organ Sharing database was retrospectively queried for adult primary heart transplantation from 2000 to 2019. The EXPAND trial definition of ECD was used: ischemic time ≥ 4 hours, ejection fraction < 50%, age > 55 years, and history of coronary artery disease. Geographic data and 2019 population estimates were obtained from the US Census Bureau.

RESULTS

Of the 42 642 transplants included in our analysis, 11 750 (27.6%) used ECDs. Region utilization of ECDs ranged from 18.4% to 46.5% of transplants. Region 6 had the highest utilization rate at 46.5%; this was primarily driven by the number of transplants with ischemic time ≥ 4 hours. Region 6 encompasses the largest total area (1.08 million square miles) and smallest population density (15.6 people per square mile). Region 8 had the lowest marginal donor utilization rate at 18.4%. Regions with high utilization of low ejection fractions, older donors, and donors with coronary artery disease (ie, regions 1 and 2) were able to maintain an average utilization rate of ECDs by maintaining short ischemic times.

CONCLUSIONS

Large discrepancies in the use of ECDs exist across the different United Network for Organ Sharing regions. This is primarily driven by longer ischemic times, likely guided by variance in population densities between different regions.

Machine perfusion of donor organs for transplantation

The ever-widening gap between organ supply and demand has resulted in an organ shortage crisis that affects patients all over the world. For decades, static cold storage (SCS) was the gold standard preservation strategy because of its simplicity and cost-effectiveness, but the rising unmet demand for donor organ transplants has prompted investigation into preservation strategies that can expand the available donor pool. Through ex vivo functional assessment of the organ prior to transplant, newer methods to preserve organs such as perfusion-based therapy can potentially expand the available organ pool. This review will explain the physiologic rationale for SCS before exploring the advantages and disadvantages associated with the two broad classes of preservation strategies that have emerged to address the crisis: hypothermic and normothermic machine perfusion. A detailed analysis of how each preservation strategy works will be provided before investigating the current status of clinical data for each preservation strategy for the kidney, liver, pancreas, heart, and lung. For some organs there is robust data to support the use of machine perfusion technologies over SCS, and in others the data are less clear. Nonetheless, machine perfusion technologies represent an exciting frontier in organ preservation research and will remain a crucial component of closing the gap between organ supply and recipient demand.

Myocardial Substrate Oxidation and Tricarboxylic Acid Cycle Intermediates During Hypothermic Machine Perfusion

BACKGROUND

The optimal substrate for hypothermic machine perfusion preservation of donor hearts is unknown. Fatty acids, acetate, and ketones are preferred substrates of the heart during normothermic perfusion, but cannot replete the tricarboxylic acid (TCA) cycle directly. Propionate, an anaplerotic substrate, can replenish TCA cycle intermediates and may affect cardiac metabolism. The purpose of this study was to determine myocardial substrate preferences during hypothermic machine perfusion and to assess if an anaplerotic substrate was required to maintain the TCA cycle intermediate pool in perfused hearts.

METHODS

Groups of rat hearts were perfused with carbon-13 (¹³C)-labeled substrates (acetate, β-hydroxybutyrate, octanoate, with and without propionate) at low and high concentrations. TCA cycle intermediate concentrations, substrate selection, and TCA cycle flux were determined by gas chromatography/mass spectroscopy and ¹³C magnetic resonance spectroscopy.

RESULTS

Acetate and octanoate were preferentially oxidized, whereas β-hydroxybutyrate was a minor substrate. TCA cycle intermediate concentrations except fumarate were higher in substrate-containing perfusion groups compared with either the no-substrate perfusion group or the no-ischemia control group.

CONCLUSIONS

The presence of an exogenous, oxidizable substrate is required to support metabolism in the cold perfused heart. An anaplerotic substrate is not essential to maintain the TCA cycle intermediate pool and support oxidative metabolism under these conditions.