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Invited Commentary

A New Dawn for Transvalvular Pumps for Ventricular Unloading as a Bridge to Heart Transplantation

Kapur, Navin K.*; Hernandez-Montfort, Jaime; Kanwar, Manreet K.

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doi: 10.1097/MAT.0000000000001778
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The exponential rise in the number of patients with end-stage heart disease awaiting heart transplantation (HT) under temporary mechanical circulatory support (tMCS) has enhanced the need for appropriate patient selection, matching patient needs to adequate devices and management strategies while on tMCS.1–3 The use of transvalvular microaxial-flow pump Impella platforms (Abiomed, Danvers, MA) in the advanced heart failure population is similarly growing. The Impella family of devices has evolved from the original percutaneous 2.5 device to the Impella CP (3.5–4.0 L/minute flow capacity) and the surgically implanted devices from the 5.0 to the 5.5 L/minute flow devices. More recently, the first Impella Bridge to Recovery (BTR) device was implanted for longer term purgeless transvalvular support. As a result, Impella use is growing as an alternative to intra-aortic balloon pump (IABP)—especially among patients with dilated cardiomyopathy—and, more commonly, as a first-line support strategy for advanced heart failure patients.

The rationale for selecting a transvalvular axial-flow pump in advanced heart failure is driven, in part, by fundamental physiologic principles. First, transvalvular pumps function more efficiently when the ventricle is severely impaired. Using a standard H-Q curve concept, device flow is directly proportional to device speed and inversely proportional to the pressure gradient across the aortic valve.4 For this reason, patients with decompensated advanced heart failure often have high left ventricular end-diastolic pressure and low aortic diastolic pressure. This narrow pressure gradient means that, for a given speed, setting device flow will be higher the more hypotension and congestion are observed. As a result, higher flow capacity Impellas such as the 5.0 and 5.5 pumps start make more sense as the first-line therapy for patients with persistent congestion, hypotension, and low cardiac output. Second, in patients with dilated cardiomyopathy and left ventricular diameters over 6.0 cm, placement of a lower capacity pump may be insufficient to handle that magnitude of preload. It would be beneficial to use higher flow capacity pumps that can more effectively replace cardiac output and reduce left ventricular preload and subsequent wall stress. Third, by actively reducing left-sided pressures and volume, pulsatile load on the right ventricle is reduced, and what was once considered a marginally functioning right ventricle starts to behave more efficiently.5 In some cases, placement of a high-flow left-sided pump may unmask right heart failure, but by avoiding the need for surgical apical coring as is often performed with an left ventricular assist device (LVAD) the interventricular septum and biventricular interaction are preserved, thereby leading to more effective right heart unloading. Finally, the introduction of ambulation using the Impella 5.0 and 5.5 devices has opened up many new possibilities not only for myocardial recovery but also for total body rehabilitation.6 Nearly 10 years ago, we began exercising patients with advanced dilated and decompensated heart failure shorty after Impella 5.0 implantation and observed that many of these patients became more favorable candidates for LVAD implantation or HT. These approaches may now be accelerated by the introduction of the investigational Impella BTR pump, which does not require a purge line and, if successful, may enable longer duration implantation on the order of months. With recent Food and Drug Administration (FDA) approval of donation after circulatory death (DCD) transplantation, endovascular deployment of the Impella 5.5 and BTR pumps may replace the use of durable LVADs as a strategy to bridge patients to decision or transplantation. Where the future of endovascular transvalvular unloading goes from here remains to be determined, but with more focus on clinical studies and continued innovation focused on limiting complications and optimizing patient selection and management, the pathway appears to be bright.

Recent analyses of large registries including extracorporeal life support organization (ELSO) have demonstrated a potential survival benefit with Impella used as an adjuvant to venoarterial extracorporeal membrane oxygenation (VA-ECMO) for myocardial recovery and stabilization of patients with heart failure–related cardiogenic shock as a bridge to potential HT.7,8 However, the long-term use of Impella platforms can be potentially challenged by device-related complications that might further impact post-HT outcomes.9 In this issue of the Journal, Impella platforms are explored by addressing both the clinical aspects in an ICU setting as an overview (and single-center experience) as well as a contemporary registry analysis from the United Network of Organ Sharing (UNOS), describing characteristics and outcomes associated with its use before HT.10,11

Papolos et al.10 describe the critical care-oriented perspective of Impella-based devices with an emphasis on management, repositioning, and monitoring, describing a single-center experience of 91 patients supported with various Impella platforms. The authors describe the Impella system in terms of hemodynamic effects and general principles, with an emphasis on structural design and capacities across the five different available catheters as well as technicalities placement, repositioning, and monitoring of aortic and left ventricular pressures by an optical pressure sensor. The importance of understanding the “anatomy” of the different devices is helpful to best anticipate and avoid common device-related complications related to positioning and repositioning after noticing that among their cohort 26% of the patients required such intervention. Notably, there are detailed perspectives surrounding catheter depth, orientation, and step-by-step description for the bedside clinician attempting to reposition. Additionally, general recommendations on Impella platform controller alarms, invasive hemodynamic-driven management goals related to right heart predominant support, and transitions when patients are on concomitant Impella and VA-ECMO, known as either ECMella or ECPella, are described succinctly.

The authors propose that most of the complications related to Impella platforms are related to positioning and anticoagulation and support their findings with institutional data across multiple devices and cohorts between 2017 and 2021. Most of the patients received femoral delivery of Impella (61%), with 13% of the patients implanted with an Impella 5.5% and 45% of the patients receiving 5.0. Half of the patients (51%) of the patients undergo tMCS for nonacute myocardial infarction (non-AMICS). Limb ischemia occurred in 16% of the study population. The report describes catheter positioning-related complications including hemolysis (45%), ventricular arrhythmias (18%), and mitral valve dysfunction (3%), and further describes anticoagulation-related events including cerebral vascular accident (4%). The total inhospital mortality was 52%.

The new donor heart allocation system was instituted in 2018 in the USA by the UNOS, allowing patients with tMCS in the form of VA-ECMO, IABP, and Impella platforms achieve highest status designation. The goal behind these changes was to decrease mortality rates for recipients on the waiting list with better stratification of candidates most urgently in need of HT. Although the overall assessment is that the new allocation system has brought important positive changes to the allocation of donor organs and HT outcomes, it has resulted in an increase in the donor allograft ischemic time. More importantly, it has had a significant influence on the field of tMCS and bridge to transplant strategies. The comparative use and outcomes of Impella platforms as bridge to HT before and after policy change are described by Pahwa et al.11 Since October 2018, there was incremental changes in Impella platform utilization, with the most common device being Impella 5.0. There were 378 patients that received Impella support before HT, of which 179 were placed on support after UNOS policy change. There were no significant differences in survival before or after policy change (90 days before 94% and after 92%, 180 days before 93% and after 92%, and 1 year before 89% and 86% after) among patients receiving Impella platforms despite increased device utilization.

The changes in the policy landscape for the US HT donor allocation system have also exposed our capacity to project a transition pathway (beyond a daily goal of alive on the waiting list) by the adoption of novel tMCS platforms. The shift toward increased utilization of temporary mechanical unloading merits attention as perhaps is the need for resolution of persistent end-organ congestion, which favors clinicians to transition this device. Although heart allocation policy has traditionally been based on wait-list mortality rather than posttransplant outcomes, the need to balance the risk of complications on tMCS while awaiting HT with long-term posttransplant outcomes remains a priority. Furthermore, in-depth protocolized hemodynamic device and imaging-related assessments by interdisciplinary teams as described raise the topic of whether the physiologic benefit of temporary mechanical unloading with Impella platforms is blunted by limited best care practices. In that regard, device-related complications have the risk of potentially affecting longitudinal outcomes after HT. Based on the International Society for Heart and Lung Transplantation registry, bridge to transplantation (BTT) with a percutaneous VAD (n = 75) was independently associated with greater risk of posttransplant mortality (hazard ratio, 1.83; 95% CI, 1.09–3.08; p < 0.02) with a survival of 1 year at 79.9%.11 The current study does not allow for detailed longitudinal hemodynamic and metabolic profiling, inotrope/vasoactive index, and specific and sequential device utilization compared with contemporary cohorts, which is a limitation of the registry. The small cohort size (limited to only 1 year postallocation policy change) with a limited follow-up time in this submission does not extend our ability to understand the long-term outcomes post HT. Whether an increasing number of patients who present with cardiogenic shock and supported on tMCS get directed to urgent HT with a short waiting time, thereby limiting their ability to recover (had the waiting time been longer), is an unaddressed concern.

We can all agree that the patients and their severity of sickness has not changed since 2018—but our approach on how to manage these patients has. Understanding the impact of temporary MCS support on long-term HT in reference to advancing technology and its limitations can help create a frame of reference for future directions in the persistent search for the ideal temporary MCS indication as a bridge for transplant.

References

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9. Yin MY, Wever-Pinzon O, Mehra MR, et al.: Post-transplant outcome in patients bridged to transplant with temporary mechanical circulatory support devices. J Heart Lung Transplant. 38: 858–869, 2019.
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