How should patients supported with Impella 5.5 (percutaneous left ventricular assist device) be prioritized for heart transplantation under the 2018 UNOS (United Network for Organ Sharing) heart allocation system?

Prioritization of Impella 5.5-Supported Patients Under the 2018 UNOS Heart Allocation System

Patients supported with Impella 5.5 should be listed at Status 1 or Status 2 under the 2018 UNOS allocation system, with Status 1 designation reserved for those requiring intensive care unit-level support, while Status 2 applies to those with temporary mechanical circulatory support who are more stable—recognizing that Status 2 patients face substantially longer wait times (median 36 days vs 13.5 days for Status 1) and should be proactively monitored for status extensions and alternative donor strategies including DCD organs. 1

Understanding the 2018 Allocation Framework

The 2018 UNOS policy fundamentally restructured heart transplant prioritization by creating a more granular status system that specifically prioritized patients requiring temporary mechanical circulatory support (tMCS), including Impella devices. 1 This represented a major departure from the older two-tier system where mechanical support devices like ventricular assist systems qualified patients for the highest urgency Status I designation. 2

Key Changes Affecting Impella 5.5 Patients

• The revised system introduced expanded status tiers with time-limited high-urgency listing periods for candidates supported with temporary MCS, requiring formal extension designations (Status 1e or 2e) beyond the initial period. 1
• The policy was designed to reduce disparities in wait times for patients with unique hemodynamic profiles who require temporary support devices. 1
• Geographic allocation principles now allow high-priority patients to access nationwide or macro-area allocation, making broader sharing necessary for Impella-supported patients. 1

Status Assignment Algorithm for Impella 5.5 Patients

Status 1 Criteria (Highest Priority)

Assign Status 1 when the patient meets BOTH criteria:

• Patient is in an intensive care unit setting 2
• Patient requires the Impella 5.5 device to maintain adequate cardiac output 2

Expected outcomes with Status 1 listing:

• Median wait time of approximately 13.5 days to transplantation 1
• Highest probability of rapid transplantation under current allocation framework 1

Status 2 Criteria (High Priority)

Assign Status 2 when:

• Patient requires Impella 5.5 support but does not meet both ICU-level criteria for Status 1 2
• Patient has temporary MCS but is sufficiently stable to be managed outside intensive care 1

Critical caveat for Status 2 patients:

• Status 2 recipients comprise the majority of Impella-supported transplant candidates (72% in contemporary cohorts) 1
• Median wait time is substantially longer at 36 days, with significant variability 1
• National registry data demonstrate progressive congestion within Status 2, with increasing waitlist durations and declining probability of transplantation within 90 days 1
• Longer waitlist duration is independently associated with increased 30-day post-transplant mortality (OR 1.01 per day; 95% CI 1.00-1.01; p=0.02), meaning each additional day of waiting incrementally increases early post-transplant death risk 1

Management of Time-Limited Status Designations

Status Extensions (1e and 2e)

Approximately 15% of Impella 5.5-supported recipients require status extensions beyond the initial time-limited period. 1

Status 1e patients:

• Median wait time of 17 days 1
• Extension serves to preserve listing priority for patients with ongoing clinical urgency 1

Status 2e patients:

• Median wait time of 29 days 1
• Total wait time not substantially longer than Status 2 without extension, suggesting extensions preserve priority rather than reflecting prolonged delay 1

Key consideration: Status extensions remain common despite timeline restrictions, with marked regional variability in utilization, underscoring ongoing debate regarding whether fixed time windows optimally reflect contemporary clinical practice. 1

Device Duration and Bridging Considerations

FDA-Approved vs. Real-World Use

• Impella 5.5 is FDA-approved for up to 14 days of support 3
• In clinical practice, the device is being used for substantially longer durations, especially as a bridge to heart replacement therapies 3
• Patients supported beyond 14 days demonstrate high rates of favorable outcomes (75%), with no linear increments in serious adverse events with prolonged duration 3

Outcomes Data for Extended Support

For patients supported >14 days (median 23.9 days):

• Unadjusted survival of 80% 3
• 58% successfully bridged to heart replacement therapy 3
• Only 19.1% experienced serious adverse events 3
• These patients were younger (mean age 57 vs 61 years) and more likely to have heart failure-related cardiogenic shock (78% vs 61%) 3

Exceptional cases demonstrate feasibility:

• Successful bridging documented up to 123 days of Impella 5.5 support with excellent post-transplant outcomes 4
• Mean support duration of 70 days (maximum 83 days) reported in case series with successful transplantation 5

Post-Transplant Outcomes

Waitlist and transplant outcomes are excellent:

• One-year post-transplant survival of 91-95% for Impella 5.5-supported patients 6
• Comparable outcomes between Impella 5.0 and 5.5 devices 6
• Similar 2-year survival compared to IABP-bridged patients (88-90%) despite Impella patients being significantly sicker at baseline 7

Donor Allocation Patterns

DCD Organ Utilization

A higher proportion of donation-after-circulatory-death (DCD) donors are utilized among Status 2/2e Impella-supported recipients compared with Status 1/1e recipients. 1 This pattern likely reflects:

• Broader allocation pressures within the post-2018 system rather than a defined status-based strategy 1
• Substantial expansion of the Status 2 cohort with increasing waitlist durations 1
• Transplant programs adapting to prolonged waitlist exposure by accepting alternative donor profiles 1
• Status 1/1e recipients continuing to undergo transplantation within shorter listing intervals, allowing more selective donor acceptance 1

Critical Pitfalls and Management Strategies

Avoiding Status 2 Congestion

The primary challenge is Status 2 tier congestion:

• National registry analyses demonstrate progressive congestion characterized by increasing numbers of candidates, longer waitlist durations, and declining probability of transplantation over time 1
• Most Impella-supported patients are transplanted at Status 2 yet experience the longest and most variable wait times 1

Proactive management approach:

• Early identification of patients likely to remain in the congested Status 2 tier 1
• Judicious use of status extensions to preserve timely access 1
• Consideration of evolving donor strategies including DCD organs 1
• Close monitoring for clinical deterioration that would warrant Status 1 escalation 1

Pre-Transplant Assessment Priorities

Key parameters to monitor during Impella 5.5 support:

• Hemodynamic profile and end-organ function, given the risk of post-transplant acute kidney injury requiring dialysis 1
• Device positioning with continuous monitoring 4
• Ambulation and rehabilitation participation to reduce physical deconditioning 5
• Adequate left ventricular unloading and maintenance of end-organ function 5

Operational Challenges

Be aware of policy-related complications:

• Increased graft ischemic times and distances between donor and recipient hospitals resulting from broader geographic sharing for high-acuity patients 1
• Regional variability in status extension utilization 1
• Need for institutional protocols addressing appropriate candidate selection 1

Comparison to Historical Allocation System

The older UNOS system (pre-2018) used a simplified two-tier approach where patients requiring ventricular assist systems, intra-aortic balloon pumps, or ventilators qualified for Status I (highest priority), while all other patients were Status II. 2 The 2018 revision created more granular tiers specifically to address the unique needs of temporary MCS patients and reduce waitlist mortality. 1

Historical concerns that informed the 2018 revision:

• Widespread inequity in identifying patients who "urgently" required transplantation 2
• Programs with enlarged ICU facilities could board patients on inotropic support to allow earlier transplantation 2
• Critically ill patients had higher operative mortality (14% vs 6% for stable patients), though computer models demonstrated that prioritizing sicker patients maximized overall survival unless post-transplant mortality approached 50% 2