Impact of the 2018 UNOS Heart Allocation Policy on Impella 5.5-Supported Patients
The 2018 UNOS heart allocation policy revision successfully reduced waitlist times and improved transplantation rates for high-acuity patients supported with Impella 5.5 devices, with 87% achieving transplantation at a median waitlist time of 19 days, without compromising post-transplant survival or increasing early complications. 1
Key Policy Changes and Their Impact
The 2018 allocation policy fundamentally restructured heart transplant prioritization to address waitlist mortality by creating a more granular status system that prioritized patients requiring temporary mechanical circulatory support (tMCS), including Impella devices 2, 3. This policy change specifically addressed previous disparities where patients with smaller ventricular cavities and unique hemodynamic profiles—such as those supported with Impella devices—faced prolonged wait times and limited options once listed 2.
Waitlist Outcomes by UNOS Status
For patients bridged with Impella 5.5 under the revised allocation system:
Status 1-3 (High-Acuity): These patients experienced the most dramatic benefit, with 81% directly bridged to transplant while on the device and only 7% waitlist mortality 1. The policy change resulted in a targeted reduction in waitlist death and deterioration for high-priority patients without compromising post-transplant survival 4.
Status 2 Classification: Impella 5.0/5.5 devices are now prioritized as Status 2 under current allocation policy, providing appropriate urgency classification for these critically ill patients 5.
Median Waitlist Duration: The revised policy achieved a median waitlist time of only 19 days for Impella 5.5-supported patients, representing a substantial reduction compared to pre-policy era 1.
Device-Specific Performance Metrics
The Impella 5.5 demonstrated robust clinical performance as a bridge to transplant:
Device complications and failure occurred in less than 5% of cases, indicating excellent mechanical reliability during the bridging period 1.
Clinical deterioration requiring waitlist removal occurred in only 5% of patients, substantially lower than historical rates for critically ill candidates 1.
Comparable outcomes between Impella 5.0 and 5.5: Both devices achieved similar 1-year post-transplant survival (91.3% vs 94.6%, p=0.661), with no significant differences in transplantation likelihood or waitlist mortality 5.
Post-Transplant Outcomes
Early post-transplant complications remained acceptable despite higher acuity recipients:
One-year post-transplant survival reached 89.5% for Impella 5.5-supported patients, comparable to other bridging strategies 1.
Acute kidney injury requiring dialysis was the most common complication at 16%, but rates of treated rejection, infection-related hospitalization, and all-cause hospitalization were not increased compared to pre-policy era 3, 1.
The policy change improved access to transplantation for higher acuity patients without increasing early posttransplant rates of treated rejection or hospitalization, factors that typically predict long-term mortality 3.
Allocation System Adaptations
The revised policy drove significant changes in mechanical support utilization patterns:
Increased use of temporary support devices: Post-policy implementation saw significant increases in patients transplanted while supported with IABP, tMCS (including Impella), and ECMO (all p<0.01) 6.
Decreased durable LVAD utilization: Fewer patients were transplanted while on durable LVADs post-policy (p<0.01), as the new system deprioritized stable LVAD patients 6.
Device progression patterns: Among patients initially listed on inotropes, 57.4% progressed to another form of support post-policy (compared to 27.4% pre-policy), reflecting the system's emphasis on escalating support for deteriorating patients 6.
Impact on Lower-Priority Patients
Critically, the benefits to high-acuity patients did not come at the expense of lower-priority candidates:
Low-priority patients (Old Status 1B/2, New Status 4-6) showed a trend toward benefit with reduced waitlist death/deterioration (adjusted sdHR: 0.86, p=0.07) and increased transplantation rates (adjusted sdHR: 1.08, p=0.007) 4.
No difference in 1-year post-transplant survival was observed when stratifying by policy era and priority status (log-rank p=0.22) 4.
Statistical interaction analysis confirmed policy benefits were concentrated in high-priority patients (interaction sdHR: 0.59, p<0.001) without harming lower-priority candidates 4.
Donor and Logistical Considerations
The policy change introduced new operational challenges:
Increased graft ischemic times and distances between donor and recipient hospitals resulted from broader geographic sharing for high-acuity patients 2.
Multiorgan transplantation remained uncommon at 7% of all heart transplants in 2018, reserved for highly selected candidates 2.
Geographic allocation principles: High-priority patients already access nationwide or macro-area allocation, making the broader sharing necessary for Impella-supported patients 7.
Pre-Transplant Assessment Considerations
For patients being evaluated with Impella 5.5 support, key assessment parameters include:
Hemodynamic profile: Severity of congestion and adequacy of perfusion must be continuously assessed, as these patients often present with cardiogenic shock requiring more than IV inotropes alone 2.
End-organ function monitoring: Given the 16% rate of post-transplant acute kidney injury requiring dialysis, careful attention to renal function during the bridging period is essential 1.
Device duration tolerance: The median 19-day waitlist time suggests Impella 5.5 provides adequate support for the typical bridging period, though individual tolerance varies 1.
Common Pitfalls and Caveats
Several important considerations for clinical practice:
Do not delay transplant evaluation: The policy change has significantly increased transplantation rates (78.9% pre-policy vs 89.8% post-policy for inotrope-supported patients), making early referral critical 6.
Recognize device limitations: While Impella 5.5 shows excellent bridging outcomes, patients with smaller LV cavities and differing hemodynamic profiles may still face challenges, though the 2018 policy specifically addressed these disparities with separate listing criteria 2.
Monitor for clinical deterioration: Despite low rates (5%), deterioration remains a risk, and the temporary nature of Impella support requires vigilant assessment for potential escalation to durable support or urgent transplantation 1.
Post-transplant immunosuppression: The confluence of transplantation trends and marked increase in critically ill patients may warrant reconsideration of tailored immunosuppression strategies, though specific guidelines remain under development 2.