Heart transplantation remains the standard treatment for patients with advanced heart failure. However, access to a donor organ depends on complex allocation systems designed to balance clinical severity, urgency, and fairness in organ distribution.

In the United States, the 2018 reform of the United Network for Organ Sharing (UNOS) allocation system introduced a more granular patient classification and gave higher priority to patients requiring temporary mechanical circulatory support, such as the intra-aortic balloon pump (IABP), microaxial flow devices such as Impella, or extracorporeal support with ECMO.

This change rapidly altered bridge-to-transplant strategies. Traditionally, many patients had been stabilized with durable ventricular assist devices (dVADs). However, the new system encouraged the use of temporary devices, which allow access to higher-priority waiting list categories.

This study examines the evolution of mechanical circulatory support use after implementation of the new allocation system. Using the national UNOS database, the authors identified 5325 adult patients who underwent heart transplantation during 2 comparative periods: 2019 and 2024.

The type of circulatory support at the time of waitlist registration and at the time of transplantation was assessed, together with patient clinical characteristics and 90-day survival. The analysis showed a marked increase in Impella use both at listing and at the time of transplantation. In contrast, the use of durable ventricular assist devices and the intra-aortic balloon pump declined significantly, whereas ECMO use showed a modest increase.

Despite these changes in support strategies, 90-day post-transplant survival was similar across both study periods, with no relevant differences according to the type of support used.

The authors conclude that, following introduction of the new allocation system, there has been a marked increase in the use of Impella as a temporary circulatory support strategy, with no apparent effect on early post-transplant survival.

COMMENTARY:

The 2018 reform of the heart allocation system in the United States represents one of the most influential changes in heart transplant organization in recent decades. This study shows how modifications in allocation rules can rapidly reshape clinical practice and circulatory support strategies in patients with advanced heart failure.

One of the most striking findings is the exponential growth in Impella use as temporary support before transplantation. Whereas its use was anecdotal in 2019, by 2024 it had become one of the predominant strategies. This shift illustrates how incentives embedded within a prioritization system can influence the choice of mechanical support. Under the new UNOS framework, patients receiving temporary circulatory support are granted higher waiting list priority than those with stable durable devices.

In parallel, the new heart transplant prioritization criteria adopted in Spain in 2023 also explicitly incorporated advanced temporary mechanical circulatory support within the highest urgency levels, including devices such as CentriMag or Impella 5.5. This change reflects an international trend toward prioritizing patients receiving temporary circulatory support in the absence of established multiorgan failure.

A particularly relevant point is that this growth appears to be driven almost exclusively by the Impella 5.5. Whereas earlier practice was dominated by devices such as the Impella 5.0 or Impella CP, by 2024 almost all Impella devices present at the time of transplantation corresponded to the 5.5 model. This device can deliver flows of up to 5.5 L/min, offers greater stability for prolonged support, and can be implanted through axillary access, thereby facilitating patient mobilization.

It should be noted that this evolution cannot be analyzed with the same level of detail in other national registries. For example, the Spanish heart transplant registry groups all continuous-flow support systems within a single category, including both temporary centrifugal pumps such as Levitronix/Centrimag and microaxial devices such as Impella. Therefore, although a growing predominance of this type of support is apparent, it is not possible to determine what proportion corresponds specifically to Impella use.

From a pathophysiological standpoint, devices such as Impella offer several advantages: they provide effective hemodynamic support, allow left ventricular unloading, and can be implanted relatively quickly using percutaneous or surgical techniques. In particular, the Impella 5.5 requires surgical implantation, usually through axillary access in arteries with an adequate caliber (>7 mm), and can deliver flows of up to 5.5 L/min, providing sufficient hemodynamic support in severe shock, with a lower complication burden and greater support durability.

In current clinical practice for cardiogenic shock, many centers use stepwise strategies in which patients initially stabilized with VA-ECMO in profound shock states (INTERMACS 1) are subsequently transitioned to Impella 5.5 support. This transition allows maintenance of effective hemodynamic support, reduces some of the complications associated with prolonged ECMO, and facilitates progression to transplantation under better clinical conditions.

Along these same lines, early experiences from Spanish centers using the Impella 5.5 in patients with advanced cardiogenic shock have recently been reported. These studies have shown favorable results both when the device is used as primary support and when it is implanted after transition from VA-ECMO, suggesting that it may help stabilize critically ill patients and support progression toward definitive strategies such as heart transplantation.

However, this trend also raises important questions. Support with temporary devices is not free from complications, including hemolysis, bleeding, or thromboembolic events. In addition, prolonged use of systems originally designed for short-term support may generate new forms of morbidity that have not yet been fully characterized.

Despite changes in support strategies, early post-transplant survival has remained stable. This finding suggests that the transition toward temporary support strategies has not compromised immediate outcomes.

Nevertheless, this stability should not necessarily be interpreted as equivalence among the different bridging strategies. Patients selected for each type of support have distinct clinical profiles, and long-term outcomes were not explored in depth.

Lastly, the study illustrates how allocation systems can decisively influence the therapeutic strategies adopted by transplant centers.

Ultimately, beyond changes in technology or allocation policy, the goal remains the same: to maintain the patient in the best possible clinical condition until transplantation and to ensure the best possible outcomes after the procedure.

REFERENCES:

Potel KN, Nguyen S, McLaughlin N, Schaffer E, Huddleston SJ, Kelly RF, et al. The changing landscape of temporary mechanical circulatory support devices in the new heart allocation system – a United States nationwide analysis. JTCVS Open. 2026. doi:10.1016/j.xjon.2026.101692.

González-Vílchez F, Almenar-Bonet L, Gómez-Bueno M, Crespo-Leiro MG, Cobo-Belaustegui M, Crespín-Crespín M, et al. Spanish heart transplant registry. 36th official report of the Heart Failure Association of the Spanish Society of Cardiology. Rev Esp Cardiol (Engl Ed). 2025 Oct;78(10):906-915. English, Spanish. doi: 10.1016/j.rec.2025.04.011. Epub 2025 Jun 23. PMID: 40562161.