Impact of the United Network for Organ Sharing allocation criteria changes on temporary mechanical circulatory support use as a bridge to transplant

Abstract

Temporary mechanical circulatory support (tMCS) devices such as intra-aortic balloon pumps, veno-arterial extracorporeal membrane oxygenation, and percutaneous ventricular assist devices, play a major role in supporting patients with end-stage heart failure and bridging them to transplant. In 2018, the United Network for Organ Sharing heart allocation criteria was modified by increasing the number of statuses in the heart transplant waitlist to differentiate and favor the sickest patients awaiting transplantation. Within this new system, patients with tMCS devices receive the highest priority statuses. While the 2018 allocation system has reduced waitlist times and mortality for the highest-priority patients, some studies have shown a concomitant rise in the utilization of tMCS devices as bridge to transplant after its enaction. In this narrative review, we describe these changes in tMCS utilization and provide insights on how the upcoming creation of a continuous distribution allocation system may further impact these trends.

Key Words: Mechanical circulatory support; Heart transplantation; Ventricular assist device; Allocation criteria; Continuous distribution

Core Tip: With the introduction of the new allocation system for heart transplants in 2018, there has been a change in the usage of temporary and durable mechanical circulatory devices. Reports from multiple registries have shown that the biggest change was noticed with the use of intra-aortic balloon pumps, which grant the second highest status within the current transplantation system. While this represents deficiencies in determining medical urgency within the current system, the proposed continuous allocation model and candidate risk score aim to provide equitable distribution of donor hearts.

INTRODUCTION

Heart Failure affects about 1 million people above the age of 55 per year and has a prevalence of 8% in adults above 50 years of age in the United States[1,2]. Advanced heart failure is the cause of mortality of over 85000 people per year in the United States[2]. Even with current advancements in guideline-directed medical therapy, a significant proportion of patients develop refractory HF and require heart replacement therapies to achieve considerable improvement in quality of life and survival[3]. Orthotopic heart transplantation (OHT) has evolved to be the standard of care for these patients.

The number of yearly heart transplants in the United States has increased from about 2000 in 2010 to more than 4000 in 2022[4]. Since the development of intra-aortic balloon pumps (IABPs), temporary mechanical circulatory support (tMCS) devices have played a prominent role in the stabilization of patients with end-stage heart failure as they await OHT.

In the first allocation system developed in 1988, patients on temporary or durable mechanical circulatory support (MCS) were offered the highest tier status[5]. As durable left ventricular assist devices (dLVAD) became more popular, the system was expanded to three tiers in 1999[6]. Patients on tMCS devices were still allocated the highest tier status 1A and those with uncomplicated dLVADs at tier 1B[5]. However, the prolonged waitlist duration for patients at 1A status was unacceptable and the allocation system was revised in 2018 to differentiate between tMCS device types[7]. This system used the increasing invasiveness and level of support of different tMCS devices as a surrogate for disease severity and aimed to prioritize those with the more advanced tMCS devices to the highest status (Table 1). As a result, the type of MCS device used as a bridge to transplant (BTT) started playing a large role in determining the transplant status of the patient and, indirectly, the amount of time spent on the waitlist[7]. Hence, there was significant concern that the allocation change could bring with it a shift in management driven by a desire to list patients at a high status, even if it was discrepant with their disease severity[7]. Considering this possibility, the framers of the current OPTN criteria added the cardiogenic shock criteria, where hemodynamic evidence (both invasive and non-invasive) justifying the need for tMCS is explicitly required to request certain statuses[8].

Table 1 Mechanical circulatory support and transplant status over the years.

1989-1998, status	1999-2018, status	2018-present, status	Criteria
1	1A	1	VA-ECMO1
			Non-dischargeable surgically implanted BiVAD
			MCSD with life threatening ventricular arrythmias
		2	Non-dischargeable surgically implanted LVAD
			TAH, BiVAD, RVAD
			Malfunctioning MCSD
			Percutaneously inserted MCSD1
			IABP1
			VT/VF
		3	Dischargeable LVAD (during 30-day discretionary period)
			Multiple inotropes or single high dose inotrope with hemodynamic monitoring1
			MCSD with either hemolysis, thrombosis, right heart failure, mucosal bleeding or aortic insufficiency
			VA-ECMO after 7 days
			Non-dischargeable surgically implanted LVAD after 14 days
			Percutaneously inserted MCSD after 14 days
			IABP after 14 days
	1B	4	Dischargeable LVAD (without 30-day discretionary period)
			Inotropes (without hemodynamic monitoring)
			Congenital heart disease
			Ischemic heart disease with intractable angina
			Amyloidosis/Hypertrophic/Restrictive cardiomyopathy
			Heart re-transplant
2	2	5	Dual organ transplant candidates on waitlist
		6	All other candidates

¹Indicates that patients will also need to meet invasive and non-invasive hemodynamic criteria for cardiogenic shock to be listed under this status. VA-ECMO: Veno-arterial extracorporeal membrane oxygenation; BiVAD: Bi-ventricular assist device; MCSD: Mechanical circulatory support device; LVAD: Left ventricular assist device; RVAD: Right ventricular assist device; TAH: Total artificial heart; IABP: Intra-aortic balloon pump; VT: Ventricular tachycardia; VF: Ventricular fibrillation.

Data from OPTN reports show that before 2018, around 25% of the waiting list comprised patients at the highest status (1A) which included tMCS devices[9]. After the revision of the allocation policy, 41% of patients continue to be added as status 1A equivalent (status 1, 2, or 3 in current allocation criteria) implying an increase in tMCS use[9]. The overall ventricular assist devices (VAD) status at listing had increased from 22.4% to 35% between 2012 and 2022, with a disproportionate increase in tMCS compared to durable VAD[9]. Additional independent studies have also shown an increase in the use of ECMO and IABP use following the policy update[10,11]. Concomitantly, the latest Interagency Registry for Mechanically Assisted Circulatory Support (INTERMACS) reports have shown a shift in dLVAD use towards long-term support and away from BTT[12,13]. Therefore, the policy changes appear to have created a clinical paradigm where tMCS devices play an important role in acute bridging to OHT, while dLVADs are reserved for chronic support.

Considering the anticipated change to a continuous distribution allocation model from OPTN by 2025, our review aims to provide a brief overview of the tMCS devices, the impact of the 2018 criteria change on the utilization of these devices, and an outlook on future directions of the allocation criteria[14]. Our review is primarily aimed at general cardiologists, cardiology fellows, residents, and other providers who take care of patients who are candidates for cardiac transplants.

CHANGES IN UTILIZATION OF TMCS DEVICES

Temporary MCS devices are designed to provide hemodynamic support and ensure adequate organ perfusion in cases of cardiogenic shock refractory to medical therapy, as a bridge to recovery or definitive therapies such as durable left ventricular assist devices (LVAD) or heart transplant. Currently, available tMCS devices include the IABP, percutaneous ventricular assist devices, veno-arterial extracorporeal membrane oxygenation (VA-ECMO), and surgically implanted temporary ventricular assist devices (VADs)[15]. Specific details about each tMCS device are summarized in Figure 1 and Table 2.

Figure 1 Central Illustration: Changes in temporary mechanical circulatory support and durable left ventricular assist devices use as a percentage of total mechanical circulatory support use at time of transplant listing in the 5 years before allocation criteria change (Pre) and 5 years after allocation criteria change (Post). Data for temporary mechanical circulatory support and left ventricular assist devices use is derived from the United Network for Organ Sharing/Scientific Registry of Transplant Recipients 5-year monitoring report. IABPs: Intra-aortic balloon pumps; VA-ECMO: Veno-arterial extracorporeal membrane oxygenation; MCS: Mechanical circulatory support; LVAD: Left ventricular assist devices; VAD: Ventricular assist devices; BiVAD: Bi-ventricular assist device.

Table 2 Summary of important temporary mechanical circulatory support devices.

Device	Insertion method	Ventricular support	LV unloading	Cardiac output increase (L/min)	Oxygenation	Pump mechanism	Insertion location	Location of device	Cardiac synchronization	Mandatory anticoagulation	Durability	Ambulation possibility (If yes, not commonly performed)	FDA approval for BTT	Major contraindications	Major adverse events
IABP	Percutaneous	Left	Indirect	0.5	No	Pneumatic	Femoral or axillary artery	Proximal descending aorta	Yes	No, but recommended	Days	Yes if subclavian approach	No	Significant AI, aortic dissection, PVD	Limb ischemia, bleeding, vessel injury, aortic rupture, thrombocytopenia
Impella CP	Percutaneous	Left	Yes	3-4	No	Axial	Femoral or axillary artery	Across aortic valve	No	No with BBPS1, but recommended	Approved for ≤ 4 days	Possible with axillary approach	No	Significant PVD, AI, VSD, metallic aortic valve	Limb ischemia, valve injury, hemolysis, bleeding, ventricular arrhythmia
Impella 5.5	Surgical cutdown	Left	Yes	6	No	Axial	Axillary artery or directly into aorta	Across aortic valve	No	No with BBPS1, but recommended	Approved for upto 14 days; Often used for days to few weeks	Possible with axillary approach	No	Significant PVD, AI, VSD, metallic aortic valve	Limb ischemia, valve injury, hemolysis, bleeding, ventricular arrhythmia
Impella RP	Percutaneous	Right	Yes	4	No	Axial	Femoral vein	Pulmonary artery	No	Yes	Approved for upto 14 days. Often used for days to few weeks	Possible with axillary approach	No	Disorders of pulmonary artery wall, mechanical valves, PVD, mural thrombus of right atrium	Bleeding, vascular complication, hemolysis, thrombus, valve injury, arrhythmia
VA-ECMO	Percutaneous	Left, right, or both	No	10	Yes	Centrifugal	Femoral artery, femoral vein, internal jugular vein, or central cannulation	Extracorporeal	No	Yes	Approved for 6hrs	Yes, with portable ECMO devices	No	AI, aortic dissection, LV thrombus, severe PVD	Vascular injury, limb ischemia, stroke, intracranial hemorrhage
Centri-Mag VAD	Surgical	Left, right, or both	Yes	10	Yes- with oxygenator	Centrifugal	Left ventricle/Right superior pulmonary vein, Aorta	Extracorporeal	No	Yes	Weeks to months	Yes	No	AI, aortic dissection, LV thrombus, severe coagulopathy	Bleeding, thrombus, infection, stroke
Tandem Heart pVAD	Percutaneous	Left	Yes	5	No	Centrifugal	Femoral artery and vein, requires transseptal puncture	Femoral artery	No	Yes	Approved for upto 6 hours	No	No	AI, PVD, known atrial thrombus	Vascular injury, limb ischemia, stroke, intracranial hemorrhage
Tandem Heart Protek Duo	Percutaneous	Right	No	4.5	Yes- with addition of oxygenator	Centrifugal	Internal jugular vein	Right atrium to pulmonary artery	No	Yes	Approved for 6 days, used weeks to months	Yes, if internal jugular access	No	Severe PVD, severe pulmonary hypertension	Bleeding, vascular complications, thrombus, hemolysis
Syncardia TAH	Surgical	Both (total heart replacement)	Not applicable, device replaces both ventricles	7-9	No	Pneumatic	Device directly attached to atria	Fully implantable	No	Yes	Months to years	Yes	Yes	Severe systemic illness, coagulopathy, inadequate body size	Stroke, infection, thromboembolism, device malfunction

¹Heparin-free bicarbonate-based purge solution. VA-ECMO: Veno-arterial extracorporeal membrane oxygenation, VAD: Ventricular assist device, TAH: Total artificial heart, IABP: Intra-aortic balloon pump; AI: Aortic insufficiency; PVD: Peripheral vascular disease; VSD: Ventricular septal defect; LV: Left ventricle; pVAD: Percutaneous ventricular assist device.

Intra-aortic balloon pump

The development of the IABP began in the 1960s with early prototypes showing benefits in increasing coronary blood flow and reducing afterload. It uses the concept of counterpulsation, where the intra-aortic balloon inflates during diastole to augment coronary perfusion and deflates during systole to reduce left ventricular afterload[16]. By the 1970s, IABP had become widely used for managing cardiogenic shock and in the perioperative setting, with advancements like percutaneous catheter insertion driving its expanded clinical application[16]. It is commonly inserted percutaneously through the femoral artery into the thoracic aorta and increases cardiac output by around approximately 0.5 L/min[16]. The increase in aortic diastolic blood pressure provided by this device improves coronary perfusion and reduces myocardial oxygen consumption[15,16]. While it has not shown consistent benefit in cardiogenic shock associated with acute myocardial infarction, IABPs have been widely used in cardiogenic shock secondary to decompensated or de-novo heart failure in the absence of AMI[17].

The use of IABP in OHT candidates meeting hemodynamic criteria for cardiogenic shock affords them status 2 in the new allocation criteria. An early study by Huckaby et al[10] in 2020 showed increased use of IABPs immediately after the allocation change, with IABP-supported status 2 patients having predictably shorter waitlist time, longer ischemic times, longer distance traveled for donor hearts, and improved waitlist survival. The baseline characteristics of the recipients pre- and post-2018 remained similar in that study[10]. According to OPTN data, while the use of IABP rose from 5.7% to 9.1% between 2013 and 2018, by the end of 2022, the rate increased to nearly 26%[18]. In a study by Parker et al[19] looking at the immediate periods of pre and post-allocation change, the use of IABP increased by 3.7%, and low-dose inotrope use decreased by 18% by April 2019. In the OPTN 5-year monitoring report, compared to a pre-2018 cohort, the use of IABPs more than doubled from 13.2% to 29.5% by October 2023[9]. Therefore, IABP has grown to become the most prominent tMCS device used as a bridge to OHT in patients with cardiogenic shock. In the same OPTN analysis, the most common criteria being met for status 2 was IABP use (38.5%) followed by exception invocation (37.8%). Hanff et al[20] showed that patients listed for OHT as status 2 due to IABP or exceptions had lower waitlist mortality compared to other status 2 Listings, like patients with ventricular tachycardia/ventricular fibrillation, surgical VAD, RVAD, or BiVADs. The waitlist mortality of IABP status 2 Listings was similar to status 3 Listings with high-dose inotropes, inferring that low-risk patients on status 2 due to IABPs may be impeding access to other higher-risk status 2 patients[20]. Additionally, Parker et al[19] argue that the low barrier to status 2 recertification every 14 days after an initial listing with the use of pulmonary capillary wedge pressure > 15 as the sole criteria makes it an attractive option compared to high-dose inotropes.

In conclusion, with the growing number of status 2 Listings, driven primarily by the use of IABPs, objectively prioritizing “sick” and “stable” patients has become increasingly challenging.

Percutaneous left ventricular assist devices and microaxial flow pumps

The predominant form of percutaneous left ventricular assist device currently in use is the Impella (Abiomed, Danvers MA). The development of the Impella was inspired by the Archimedes screw pump used to raise water for irrigation in 200 BC[21]. The Impella is placed across the aortic valve with the proximal end in the left ventricle and the distal end in the aorta, thereby offloading the left ventricle and improving forward blood flow[22]. The device is inserted percutaneously through the femoral artery and advanced till it reaches the left ventricle, however, larger models like Impella 5.5 are inserted surgically via a conduit created in the axillary artery or directly into the aorta in some cases[22]. All models of Impella devices, including the Impella 2.5, Impella cardiac power, and Impella 5.0 are commonly used in the management of cardiogenic shock, supporting high-risk percutaneous coronary intervention, and as a stop-gap treatment for patients waiting for heart transplant[15,23]. The Impella RP has recently been developed for right ventricular support, providing over 4 L/min of flow to the pulmonary artery[24]. Combined with left-sided devices, it offers percutaneous support to patients with biventricular failure awaiting heart transplants[24]. Tandem Heart (Cardiac Assist Inc, Pittsburgh, PA, United States) is another type of percutaneous VAD that employs an external centrifugal pump bypassing the left ventricle using inflow from transeptal left atrial access and outflow to the iliac artery[25]. It can provide around 3.5 to 4 L/min of forward flow[25]. While there is a hemodynamic benefit with using Tandem Heart, studies are yet to demonstrate a mortality benefit over other devices like IABP[26,27]. Tandem Heart is not frequently used as a BTT strategy due to the increased technical difficulties with transseptal puncture[15].

The yearly OPTN reports do not provide granular data on these microaxial flow devices, instead reporting aggregate data for all LVADs, temporary and permanent[18]. However, important information regarding the role of Impella can be collected from the 5-year follow-up report of the allocation change[9]. In this report, we see that the use of percutaneous endovascular MCSD made up about 16.3% of all listed patients in status 2[9]. When comparing pre- and post-allocation change, it was noted that overall LV Impella use at listing (including cardiac power, 2.5, 5.0, 5.5) grew from 1.75% to 9.85% of all LVAD use (temporary or permanent) while the overall LVAD use saw a significant decrease between 2013 and 2023[9]. Pahwa et al[28] analysis of United Network for Organ Sharing (UNOS) data from 2015 to 2019 showed an increase in Impella use during the same period, with the most substantial change in October 2018, rising from 1% to 4% without a change in baseline demographics of the patients. There was a significant decrease in waitlist time and an increase in transplant rates for patients who were listed for OHT after the policy change, without affecting post-transplant survival[28]. Additionally, only 3% of patients on Impella were converted to durable LVAD support, compared to 13% before the allocation change[28]. Following the FDA approval of the Impella 5.5 in 2019, there was an increase in use due to an improved side effect profile compared to VA-ECMO. The risk of stroke with Impella 5.5 was only marginally lower at 4% compared to 5.9% with VA-ECMO[29,30]. However, the risk of major bleeding is less than 3% when compared to 41% with VA-ECMO[30,31].

In all-comer patients who required Impella 5.5 support, Impella removal without escalation of care was more likely in patients who presented in Society for Cardiovascular Angiography & Interventions stage A or B, while dLVAD or OHT was required in most patients who presented in stage C or D[32]. The highest mortality, as expected was in patients who presented in stage E shock[32]. Cevasco et al[29] showed that in patients listed for OHT with an Impella 5.5, 87% ultimately underwent transplant, of which 96% were directly bridged from the Impella with a median waitlist time of 17 days demonstrating excellent short-term durability. However, as seen with IABP use, a very low percentage (1.9%) of patients planned for OHT were bridged to a durable LVAD, likely due to the opportunity for elevated status 2 Listing for patients on Impella[29]. Nordan et al[33] directly compared status 2 patients listed with an Impella to IABP and found that while they had similar post-transplant outcomes, patients supported by Impella had significantly higher waitlist mortality. The use of Tandem Heart as a bridge to transplant has overall been low, comprising only 0.05% of all LVAD use both before and after the change in transplant criteria[9].

In conclusion, percutaneous LVADs, similar to other tMCS devices, are seeing a significant increase in use as BTT. While they are afforded the same status 2 as IABPs, these patients may be sicker at baseline and less likely to tolerate complications[33].

VA-ECMO

ECMO was first utilized between 1971 and 1975 in patients who had potentially fatal respiratory or cardiac failure resistant to conventional therapy[34]. It works by circulating deoxygenated blood through an oxygenator that oxygenates the blood outside the body before returning it to the arterial system using a pump, providing both respiratory and hemodynamic support[34]. There are different configurations of ECMO, including the veno-venous ECMO with both catheters in the venous system used purely for oxygenation in respiratory failure, and the veno-arterial ECMO that provides both oxygenation and hemodynamic support[22]. VA-ECMO can provide around 6 L/min of biventricular support and is of great value in the management of severe cardiogenic shock, extracorporeal cardiopulmonary resuscitation, and post-cardiotomy shock[15]. ECMO is also commonly used in patients with critical cardiogenic shock refractory to traditional tMCS while they wait for a heart transplant[35]. The use of ECMO however carries significant risks, with major bleeding events occurring in 41% of patients, thromboembolism in approximately 10% and stroke in 5.9% of patients[30,36].

It also has a higher mortality with a low 1-year survival at 71% compared to 80%-90% with other forms of temporary mechanical circulatory support[37,38].

VA-ECMO support currently qualifies for status 1 for 7 days after listing, which can be further extended with appropriate justification[8]. Patients on VA ECMO made up 56% of all patients listed at status 1 for OHT post-allocation criteria change[9]. Total ECMO use at listing nearly doubled from 4.6% to 8.4% after the allocation criteria change, while ECMO use at the time of transplant increased from 1.5% to 8.8% in the same period[9]. The largest increase in ECMO use as a BTT strategy was seen from 2018-2019, similar to other tMCS devices[39]. Zalawadiya et al[40] analyzed UNOS registry data on patients who underwent OHT on VA-ECMO from 2000-2015 and found that the overall 1-year survival for patients bridged to OHT with VA-ECMO was 57.8%, compared to all patients undergoing OHT being above 90%. The mortality risk was the highest in the first 30 days following OHT[40]. However, Patel et al[41] found that post-transplant survival at 6 months, 1 year, and 3 years has shown significant improvement compared to the pre-2018 era. Additionally, 3-year survival for patients transplanted on ECMO is similar to patients transplanted without ECMO support in the current allocation system[41]. Stratified analysis of the current system showed that of tMCS devices, VA-ECMO had the lowest median days to transplant at 5 days compared to IABP (11 days) and percutaneous endovascular MCS (15 days)[9]. Compared to the previous allocation system, the median ECMO run time for transplanted patients has significantly decreased from 7 to 4 days, with a notable reduction in waitlist mortality from 19.3% to 6.3%[41]. Similarly, Nordan et al[42]. also demonstrated that there was a significant increase in 180-day post-transplant survival in patients bridged to transplant with ECMO from 69.6% to 90.2% after the change in allocation criteria. There was also a lower waitlist mortality or deterioration under the new allocation scheme[42].

In conclusion, there has been an overall positive impact of the allocation system change on outcomes for patients on ECMO. However, similar to other tMCS devices, there has been an overall increase in use as a bridge to transplant strategy that cannot be readily explained by patient characteristics.

Other non-dischargeable mechanical circulatory support

The other less commonly used temporary mechanical circulatory support devices in the United States are CentriMag, Protek Duo, and total artificial heart (TAH). Specific details on these devices are summarized in Table 2.

Compared to isolated LVAD use, non-dischargeable BiVAD and isolated RVAD use remained low before and after the change in allocation criteria[9]. CentriMag was the most common BiVAD used both before and after the change, with a slight overall drop in use from 48.6% to 44.5%[9]. HeartMate 3 replaced Heartware HVAD as the next common BiVAD used after the change, likely due to the FDA notice finding increased neurological adverse events and mortality with HVADs in June 2021[9,43]. The use of isolated RVADs showed a small increase from 0.28% to 0.52% after the change. Cardiac Assist ProtekDuo overtook CentriMag as the RVAD of choice after the change, likely due to the minimally invasive nature of insertion and the versatility to be used with other tMCS devices[9,44]. Total artificial heart use showed a significant decrease from 1.39% to 0.48% after the change, however, SynCardia CardioWest continued to be the most common TAH system implanted[9]. Itagaki et al[45] showed that before the change in allocation, TAH was a viable strategy as a bridge to transplant, with high-volume centers having lower 6-month mortality rates and post-transplant mortality rates compared to low-volume centers. The relatively high mortality as well as the complexity of implantation may be the reason behind the decreasing utilization compared to other MCS devices like VA-ECMO which also afford status 1[45].

IMPACT ON LVAD UTILIZATION

Examining the use of LVADs provides additional perspective on the impact of MCS choice on transplant status. In the INTERMACS 2024 report by Meyer et al[46], there was a significant decrease in the use of LVADs as either a bridge to transplant or a bridge to candidacy strategy, with a significant increase in use as destination therapy (DT) or long-term use after the 2018 allocation system change. This change occurred despite there being no significant changes in the profiles of patients who had LVAD implantations[13]. By the end of 2023, DT was the device strategy for 81.5% of patients who received a durable LVAD, compared to just 44.6% in 2012[12,13]. While some of this change can be attributed to the FDA approval of HeartMate 3 as DT in 2018, the deprioritization of patients on durable LVADs for OHT in the new allocation criteria likely had a significant effect as well. Overall, cases where LVADs were used as a BTT strategy had better 1 and 5-year survival outcomes compared to cases where a DT strategy was employed[13]. Importantly, between the two allocation criteria eras, tMCS use occurred at similar rates before LVAD implantation[13]. As of 2023, only 3.7% of LVADs were implanted as BTT strategy in the US while it was about 30% in 2014[46]. This is in stark contrast to Europe, where LVAD implantation as BTT has been stable between 62%-68% over the past decade[47,48]. However, the relative lower donor heart availability and increased transplant wait time may also play a role in the increased use of LVADs as BTT in Europe[48]. These findings taken together suggest that at least some of the changes in tMCS and LVAD use in bridging to OHT could be directly related to organ allocation policy changes.

DISCUSSION

The overall increase in tMCS use after the 2018 change in allocation criteria, without an appropriate change in patient profiles has been demonstrated in both OPTN and independent analyses. This trend was not entirely unexpected. Parker et al[49] predicted in 2017 that the new cardiogenic shock criteria for selected tMCS devices would cause a reduction in priority status for at least 19% of listed candidates. However, contrary to expectations, there was no subsequent increase in the use of non-dischargeable surgically implanted LVADs, which do not require the cardiogenic shock criteria[9,49]. Conversely, there has been an increase in the use of conventional percutaneous tMCS devices, with a heavy reliance on the use of exception requests for uplisting patients who do not meet the shock criteria[50,51]. Independent analysis has shown that this increase in tMCS devices after the change in allocation criteria was limited only to United States transplant centers when compared to cardiac intensive care units at non-transplant centers[52]. There was also a concomitant increase in pulmonary artery catheter usage, isolated again to transplant centers, which may have been driven by the need to meet the new cardiogenic shock requirement for tMCS initiation[52]. While the cardiogenic shock criteria were implemented to prioritize the sickest patients, some argue that the hemodynamic criteria are too lenient[53]. The current criteria do not require evidence of end-organ hypoperfusion like the presence of elevated lactate. This makes it difficult to differentiate between compensated advanced heart failure patients who are ambulatory and decompensated patients who truly require immediate tMCS support[53]. Thus, there may not be sufficient protection to prevent the misuse of tMCS devices to obtain early transplant offers in patients without decompensated heart failure. The overuse of tMCS devices, especially IABPs may cause overcrowding of higher statuses, triggering an unfavorable increase in waitlist time for patients on durable LVADs. The decreasing use of LVADs as a BTT strategy suggests that this phenomenon is occurring in the real world[46]. Even in patients who do not meet the current hemodynamic criteria for shock, the use of exceptions to request status upgrades also contributes to this discrepancy[53]. The increasing use of tMCS devices also leads to poor resource utilization and increased hospital costs[54].

While the overall goal of the new OPTN allocation criteria is to increase the number of transplants while ensuring equity in organ distribution, there are concerns that we may be prematurely directing patients toward heart transplantation[55]. The time spent on the waiting list reduced significantly for the highest status patients (status 1A pre-implementation and status 1 post-implementation of new allocation criteria) from a median of 80 days to 5 days[9]. While the decreased waitlist mortality and time to transplantation in the new allocation system is admirable, there has been that significant concern that rapid access to transplantation does not give a subset of patients with potentially reversible causes enough time to recover cardiac function[56]. Topkara et al[56] demonstrated that a significantly lower number of status 1 patients on ECMO and status 2 patients on IABP or percutaneous LVADs were delisted for cardiac recovery in the new allocation system compared to the prior system. The RESTAGE-HF trial showed that even in patients with Stage D heart failure, aggressive up-titration of guideline-directed medical therapy after mechanical unloading with an LVAD led to cardiac recovery resulting in explant in 40% of patients[57]. Combined with data that shows that there has been a significant decrease in the use of LVADs as BTT, we believe that there may exist a subset of patients who would benefit from a trial of LVAD as a bridge to recovery, before listing for OHT[13,48]. Such patients typically are younger, with non-ischemic etiologies of heart failure, and without concomitant kidney dysfunction[58]. Therefore, careful analysis of patients who may have reversible causes of heart failure needs to be done to consider the likelihood of recovery before listing for a heart transplant[48,55].

FUTURE DIRECTIONS

The current issues regarding the classification-based model of the current allocation system prompted UNOS/OPTN to develop a continuous distribution allocation system[59]. Such allocation systems are already in use for lung and kidney transplants[60]. In the current classification, all patients in a particular class (e.g., status 2) are prioritized over all patients in a lower class (e.g., status 3). This results in an inordinate amount of emphasis on a single variable, in this case, the type of tMCS device used[60]. In a continuous allocation system, a composite allocation score (CAS) will be assigned to each patient, which would be comprised of five sub-scores that are aligned with OPTN’s final rule[61]. The five sub-scores that would comprise the CAS are medical urgency, post-transplant survival, candidate biology, patient access, and placement efficiency[61]. Table 3 The weightage of different subscores on the CAS may be different from each other. The CAS thus formed would be utilized to perform a match run for the candidate, with each match run resulting in different total points based on donor and geographical factors[61].

Table 3 Currently identified attributes as well as attributes in discussion in the 2024 OPTN concept paper for continuous allocation system for heart transplant.

	Medical urgency	Post-transplant survival	Candidate biology	Patient access	Placement efficiency
Currently identified attributes	Type of tMCS; Inotrope use; LVAD use; HCM, RCM, CHD; Re-Tx; IHD w angina; Multi-organ Tx		Blood type; Sensitization using CPRA	Pediatric recipients; Prior living organ donors	Geographic proximity; Proximity efficiency
Attributes still in discussion	Waiting time accrued with an LVAD (increased risk of complications with time)	Post-transplant survival metric	Abnormal stature

HCM: Hypertrophic cardiomyopathy; RCM: Restrictive cardiomyopathy; Tx: Transplant; IHD: Ischemic heart disease; CPRA: Calculated panel reactive antibody.

The latest concept paper released by OPTN in 2024 aimed to identify attributes within different sub-scores that could be utilized[62]. Medical urgency would continue to be determined by the tMCS device used, with a certain percentage of priority points allotted based on the evidence of excess waitlist mortality in patients on ECMO and non-dischargeable BiVADs[62]. However, considering that there are no existing models for post-transplant survival, a decision was made to not include it in the first iteration of the continuous distribution system[62].

Concerns about the medical urgency sub-score also exist as it is still driven by the MCS device used rather than an objective assessment of patient illness[63]. Zhang et al[64] developed and validated a multivariate predictive model that outperformed the current allocation system in determining medical urgency. The newly developed United States-Candidate Risk Score (US-CRS) incorporated prior or current short-term MCS (excluding IABP and percutaneous LVAD), log bilirubin, estimated glomerular filtration rate, log brain natriuretic peptide, albumin, sodium, and current LVAD use to estimate 6-week waitlist mortality[64]. This US-CRS model was based on the French CRS, however it did not consider prior ECMO use or any surgical non-dischargeable VAD use. Like the French CRS, IABPs and percutaneous LVADs were not included in the short-term MCS definition as the dynamically different profiles of these patients would dilute the weightage of MCS use on the score[64]. This was based on a prior study that showed poor predictive value of IABP use on waitlist mortality[65]. In the US-CRS model developed by Zhang, when IABP and percutaneous LVADs were considered short-term MCS devices, the effect size of the term fell significantly from 1.02 to 0.50, while the effect size of the overall score fell from 1.02 to 0.35[64]. Therefore, the decision was made not to dilute the score with the addition of these devices.

When a risk-score-based medical urgency criteria is established that takes into consideration factors other than just the type of MCS use, we think that there would be a definite change in the patterns of tMCS use. Currently, OPTN is expected to release a policy proposal regarding a new continuous allocation system in 2025[14].

CONCLUSION

Overall, the change in treatment strategy in response to the allocation policy change adopted in 2018 by uplisting patients to the highest possible status strikes a delicate balance between optimizing outcomes for individual patients and preserving the broader benefits of heart transplantation for the entire advanced heart failure population[50]. Some argue that clinicians caring for a patient with advanced heart failure are caught in the moral dilemma of choosing between the core tenets of the practice of medicine- beneficence and justice. Should they use tMCS devices or exceptions to shorten waitlist times to benefit their patients, unfairly prioritizing them over other patients who might have a more urgent need for transplant[59]? The evolution and implementation of a continuous allocation system for hearts will attempt to balance the principles of beneficence and justice equitably, reducing the moral burden on individual physicians to make life-altering decisions based on limited criteria. The new system will aim to optimize the type of tMCS used and reduce the number of exceptions that need to be requested. Over time, uncaptured clinical variables in exceptions will need to be studied and incorporated in further iterations of the continuous distribution model. The impact of the newly proposed allocation system on tMCS utilization and its potential to alter current practices positively remains to be determined. To fully grasp the impact of the new allocation system, multiple follow-up reports and studies need to be conducted post-implementation. It remains to be seen the impact of the proposed allocation system on tMCS use and to see if policies will indeed change practices once again.