Introduction

In the United States, the system to allocate solid organs for transplantation is regulated and maintained by the United Network for Organ Sharing (UNOS) under contract to the Organ Procurement and Transplantation Network (OPTN). The UNOS/OPTN recently charged its Thoracic Organ Transplantation Committee to propose changes to the 2006 allocation system to further reduce waitlist mortality and improve post-transplant survival.¹ Table 1 represents the current allocation system.

Table 1: Current US Heart Allocation System

Status	Current Criteria
1A	Mechanical Circulatory Support      - 30 day elective ventricular assist device (VAD) time      - Total artificial heart      - Intra-aortic balloon pump      - Extracorporeal membrane oxygenation VAD with complications Mechanical ventilation Pulmonary artery catheter with 1 high dose or multiple lower dose inotropes Exception granted by regional review board
1B	Mechanical circulatory support beyond 30 day interval Continuous intravenous inotropic support Exception granted by regional review board
2	Those who do not meet criteria for status 1A or 1B

Remaining Issues

Despite the positive effects of the 2006 revisions, where groups intended to benefit from the 2006 changes experienced the most pronounced improvement in outcomes, the UNOS/OPTN Thoracic Committee identified four major deficits with the current allocation system:

1. Too many status 1A patients
   While the number of patients in need of a transplant has doubled in the last 10 years, the number of Status 1A candidates has increased five-fold.1 Furthermore, status 1A patients are three times more likely to die on the waitlist than candidates in any other status. Lastly, waitlist mortality is not uniform for status 1A patients and varies considerably between each of the 1A subpopulations.²
2. Certain candidates are not well served by the current system
   Too many exception requests are needed. A total of 605 status 1A exceptions were submitted between the years of 2009 to 2011. More than 90% of those were granted and a majority of these patients went on to receive heart transplants.³ The majority of exceptions were requested for patients with ventricular arrhythmias, the inability to administer or tolerate inotropes, and congenital heart disease.
3. Allocation rules for mechanical circulatory support device (MCS) patients are not aligned with prognosis
   Since 2006, the use of MCS as a bridge therapy has increased significantly albeit with some geographic variability.⁴ From 2007 to 2014, the percentage of transplant recipients supported with MCS doubled from 16.2% to 35.8%.¹ In many regards, VAD use has changed the landscape of heart allocation. For example, stable VAD patients have a lower mortality risk than many other status 1A patients,^2,5 but they have preferential status for at least 30 days. The new allocation system should aim to improve MCS patient stratification based upon device type and patient condition.
4. Unintended geographical disparities
   Regional differences in waiting times and status disparities remain despite the 2006 implementation.⁶ The current allocation system favors less critical candidates in the donor service area (DSA) over sicker ones in Zone A. Sometimes critically ill patients in Zone A may be as close as a few miles from the donor hospital, but receive lower allocation priority if they reside outside of the DSA. Consequently, a goal for the new guidelines was to broaden organ sharing to align with the Final Rule and eliminate disparities.

New Allocation System

The Thoracic Organ Transplantation Committee acknowledged that the most precise way to allocate organs would be to create a heart allocation score. Notably, UNOS does not currently collect sufficiently detailed waitlist and post-transplant data to develop an evidence-based score that would prioritize allocating organs to the most critically ill patients without jeopardizing post-transplant outcomes. Instead, a decision was made to increase the number of statuses and address the aforementioned problems in the existing allocation system. Data analysis included a review of waitlist mortality and post-transplant outcomes as well as committee members' experience with these patients. A multi-tier ranking system was created that grouped patients according to their acuity. The proposed system was subsequently tested with a simulator (thoracic simulation allocation model).¹ The first revision of the proposed allocation system was presented for public comment in January 2016. The suggestions and voiced concerns led the Committee to develop a second version of the proposal and release for pubic comment in August 2016 (Table 2).

Table 2: Proposed New Allocation System

Status	Proposed Criteria
1	ECMO (up to 14 days of support) Non-dischargeable BiVAD or RVAD Mechanical circulatory support with life-threatening ventricular arrhythmia
2	Intra-aortic balloon pump (up to 14 days of support) Acute percutaneous endovascular circulatory support device (up to 14 days of support) Ventricular tachycardia/ventricular fibrillation, mechanical support not required Mechanical circulatory support with device malfunction/mechanical failure Total artificial heart Dischargeable BiVAD or RVAD
3	LVAD for up to 30 days Multiple inotropes or single high-dose inotropes with continuous hemodynamic monitoring (up to 14 days of support) Mechanical circulatory support with device-related complications such as infection, hemolysis or thromboembolism, Mechanical circulatory support with device infection Mechanical circulatory support with thromboembolism
4	Diagnosis of congenital heart disease (CHD) Diagnosis of ischemic heart disease with intractable angina Diagnosis of hypertrophic cardiomyopathy Diagnosis of restrictive cardiomyopathy Stable LVAD candidates after 30 days Inotropes without hemodynamic monitoring Diagnosis of amyloidosis Re-transplant
5	Combined organ transplants

Several major changes of the new allocation guideline proposal need to be outlined:

1. The new allocation system consists of five statuses (not including status 6: "all remaining active candidates" and status 7: "inactive"). Proposed statuses 1-3 are generally defined by current status 1A criteria, proposed status 4 is generally defined by current status 1B, and proposed status 5 and 6 are defined by current status 2 criteria. The major novelty is the stratification of patients within the current 1A status into three groups of decreasing acuity. Status 1 will include patients with ECMO, non-dischargeable right ventricular and bi-ventricular assist devices and MCS with life-threatening ventricular arrhythmias. Patients supported by other MCS devices will be designated for a lower status.
2. The new guideline proposal more clearly defines MCS complications leaving less open to interpretation and reducing the need for review boards to consider exception requests.
3. Another significant change includes the incorporation of broader sharing for the highest tier patients. Now, the two sickest patient groups will draw organs from a 500-mile radius (DSA + Zone A) in the first round of organ allocation as opposed to a stepped approach. Modeled waitlist mortality and transplant rates were tested and found to be improved with the new system.¹

The transplant community has repeatedly voiced a number of concerns regarding the new heart allocation proposal:

1. The prioritization of patients on ECMO into the highest status remains a major concern. The worry is that preferentially allocating hearts to ECMO-supported patients may reduce waitlist mortality at the expense of post-transplant survival.
2. Many fear that certain types of MCS might be used to "game" the system. The proposed stratification could incentivize transplant programs to insert or keep patients on MCS such as IABP, percutaneous endovascular devices and BiVADs to increase their status. Special attention is given to IABP, which now supports Status 2 listing but some consider the urgency of candidates on IABP not to be equal to other devices in the same Status.
3. Some question whether the stratification of MCS with device-related complications are justified. While the old system classified all device-related complications under Status 1A the new guidelines spread these patients across Status 2 and 3 based on complication type.
4. Concerns remain regarding costs and organ damage (due to longer ischemic time) with the proposed broader sharing.

The second version of the new allocation guidelines was recently approved by the UNOS/OPTN Board and goes into effect pending implementation and notification to members.

Conclusion

Heart allocation is about to change in a significant way. The intent to provide the sickest patients with an organ in the shortest possible time is admirable and hopefully will lead to improved outcomes. Nevertheless, we must also focus efforts on increasing the utilization of organs. As shown by Khush et al.,⁷ there has been a steady decline of organ acceptance perhaps driven by the alternative of MCS therapy. Until the day when an MCS device can deliver survival comparable to a transplant, the long-term outcomes favor transplantation, even with its attendant risks, for eligible patients.

References

1. https://optn.transplant.hrsa.gov/governance/public-comment/modify-adult-heart-allocation-2016-2nd-round/. Last accessed 10/09/2016.
2. Dardas T, Mokadam NA, Pagani F, Aaronson K, Levy WC. Transplant registrants with implanted left ventricular assist devices have insufficient risk to justify elective organ procurement and transplantation network status 1A time. J Am Coll Cardiol 2012;60:36-43.
3. Meyer DM, Rogers JG, Edwards LB, et al. The future direction of the adult heart allocation system in the United States. Am J Transplant 2015;15:44-54.
4. Lund LH, Edwards LB, Kucheryavaya AY, et al. The registry of the International Society for Heart and Lung Transplantation: thirty-first official adult heart transplant report--2014; focus theme: retransplantation. J Heart Lung Transplant 2014;33:996-1008.
5. Pinney SP. Timing isn't everything: donor heart allocation in the present LVAD era. J Am Coll Cardiol 2012;60:52-3.
6. Schulze PC, Kitada S, Clerkin K, Jin Z, Mancini DM. Regional differences in recipient waitlist time and pre- and post-transplant mortality after the 2006 United Network for Organ Sharing policy changes in the donor heart allocation algorithm. JACC Heart Fail 2014;2:166-77.
7. Khush KK, Zaroff JG, Nguyen J, Menza R, Goldstein BA. National decline in donor heart utilization with regional variability: 1995-2010. Am J Transplant 2015;15:642-9.

Keywords: Amyloidosis, Cardiomyopathy, Hypertrophic, Cardiomyopathy, Restrictive, Coronary Artery Disease, Critical Illness, Extracorporeal Membrane Oxygenation, Heart, Heart Transplantation, Heart-Assist Devices, Hemodynamics, Hemolysis, Intra-Aortic Balloon Pumping, Pulmonary Artery, Respiration, Artificial, Tachycardia, Ventricular, Thromboembolism, Transplantation, Tissue and Organ Procurement, Ventricular Fibrillation

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