Donor-Recipient Size Mismatch in Pediatric Heart Transplantation.

Objective To assess whether the association between donor-recipient total cardiac volume (TCV) mismatch and post-transplant outcomes is modified by etiology of underlying heart failure etiology in pediatric heart transplantation. Methods We identified isolated pediatric heart transplant recipients aged 1–17 years in the United Network for Organ Sharing database from 1987 to 2025. The association between TCV mismatch and post-transplant outcomes was evaluated using entropy-balanced, covariate-adjusted Cox proportional hazards models with restricted cubic splines for 1-,5-, and 10-year mortality conditional on 90-day survival and in the full cohort. Adjusted interaction analyses assessed heterogeneity by recipient heart failure etiology. Results Among 7,971 patients, TCV mismatch was associated with all-cause mortality at 1 year (P=0.022) and 5 years (P=0.014) among 90-day survivors, with significant nonlinearity (P=0.007 and P=0.004, respectively). TCV mismatch was not significantly associated with 10-year mortality (P=0.171) However, significant heterogeneity across etiologies of heart failure emerged at 10 years (P=0.021) and in full-cohort analysesDilated cardiomyopathy demonstrated a monotonic pattern characterized by relatively lower hazard with increasing mismatch, whereas congenital heart disease showed a distinct nonlinear pattern. No statistically significant dose-response relationships were identified at 10-years in less common etiologies. Conclusions In pediatric heart transplantation, the association between donor-recipient TCV mismatch and mortality varied by recipient etiology, suggesting that a single universal size-matching strategy may be overly simplistic. These findings challenge a unified size matching framework and suggest that the clinical significance of TCV mismatch may be etiology-dependent rather than uniform across heart failure etiologies.