Low Lung Allocation Score Predicts Poor Waitlist Outcomes for Patients on Extracorporeal Membrane Oxygenation as a Bridge to Lung Transplantation

Abstract

Extracorporeal membrane oxygenation (ECMO) is increasingly used as a bridge to lung transplantation in the United States. However, ECMO is not separately distinguished from assisted ventilation in the lung allocation score (LAS), and there is a wide range in LAS for candidates on ECMO. In this study, we examined the relationship between LAS and waitlist outcomes in patients on ECMO. This is a retrospective cohort study of lung transplant candidates on ECMO from 5/4/05 until 12/31/18 using data from the United Network for Organ Sharing. We modeled LAS as a continuous variable and as high LAS (top 50% of scores) versus low LAS (bottom 50% of scores). The LAS used was the first one calculated within two days of ECMO initiation. The primary outcomes were transplantation and a composite of death or removal from the waitlist for clinical deterioration. The secondary outcome was post-transplant survival. We used an adjusted competing risk regression model to compare waitlist outcomes and an adjusted Cox regression model to examine post-transplant survival. In total, 954 patients were bridged on ECMO with 594 (62.2%) receiving transplants and 329 (34.4%) dying or being removed from the waitlist. The mean LAS in the low and high LAS cohorts were 78.2 (SD ± 15.6) and 90.7 (SD ± 1.36), respectively. The low LAS cohort had decreased chance of transplant (SHR 0.80, 95% CI 0.67-0.96, p=0.02) and increased risk of death or waitlist removal (SHR 1.34, 95% CI 1.05-1.70, p=0.02) (Figure 1). For every 10 point increase in LAS, the risk of transplant increased by 14% (p<0.01) and the risk of death decreased by 11% (p=0.02). There was no difference in post-transplant survival time between the low and high LAS cohorts (HR 0.82, 95% CI 0.60-1.12, p=0.21). Among patients on ECMO as bridge to transplant, a lower LAS was associated with decreased risk of transplant and increased risk of death or clinical worsening without any difference in post-transplant survival.