Mending the Model for End-Stage Liver Disease: An in-depth review of the past, present, and future portopulmonary hypertension Model for End-Stage Liver Disease exception

Patients with portopulmonary hypertension (POPH) have an increased cardiovascular and overall mortality risk when undergoing liver transplantation (LT). However, such risk is not captured in their Model for End-Stage Liver Disease (MELD) laboratory score. POPH MELD exception criteria were established in 2006 with the aim of prioritizing these patients for LT prior to pulmonary hypertension (PH) progression and eventual right heart failure. The original criteria emphasized a posttreatment, pre-LT mean pulmonary arterial pressure (mPAP) of <35 mm Hg and pulmonary vascular resistance (PVR) <400 dynes-s-cm⁻⁵ or <5 Wood units (WU). Since 2006, there have been important advances in the treatment of POPH with pulmonary arterial hypertension (PAH)–targeted therapies and newer evidence regarding LT outcomes and risk factors for perioperative mortality. Specifically, PVR rather than mPAP has been shown to be more strongly associated with outcomes, including mortality. In addition, among treated patients with POPH, mPAP may be persistently elevated related to an elevated cardiac output or other factors that do not necessarily reflect POPH disease severity. Thus, in February 2021, the Organ Procurement and Transplantation Network approved proposed modifications to POPH MELD exception criteria, now allowing either of the following posttreatment, pre-LT hemodynamic profiles: mPAP less than 35 mm Hg and posttreatment PVR less than 400 dynes-s-cm⁻⁵ (or less than 5 WU) or mPAP greater than or equal to 35 mm Hg and less than 45 mm Hg and posttreatment PVR less than 240 dynes-s-cm⁻⁵ (or less than 3 WU). This article reviews the history of the POPH MELD exception criteria, describes the recent modifications to the exception criteria and the evidence supporting them, and highlights unanswered questions and areas for future research.