Abstract Body (Do not enter title and authors here): Background: Cardiac magnetic resonance (CMR) is increasingly used for rejection surveillance in pediatric heart transplant recipients (PHTR). It is unclear if CMR-derived tissue characterization and myocardial perfusion correlate with functional status, i.e. VO_2peak as derived by cardiopulmonary exercise testing (CPET). Lower VO_2peak has been associated with poor cardiometabolic health and development of coronary artery vasculopathy in PHTR.

Aim: Our aim was to evaluate the correlation between CMR derived variables and CPET outcomes.

Methods: PHTR who underwent CMR within 1 year of CPET were retrospectively reviewed. Those with submaximal effort on CPET (RER<1.1) or clinical events (rejection, new or worsening CAV, or cardiac hospitalizations) between CMR and CPET were excluded. CMR variables were biventricular volumes, ejection fraction, myocardial T2, T1/extracellular volume fraction (ECV) which are biomarkers for edema and fibrosis respectively, and myocardial perfusion reserve index (MPRI). CPET variables were VO_2peak, O₂ pulse, age-predicted maximum heart rate (APMHR), HR reserve and exercise duration. Relationships between variables were studied using Pearson’s correlations and regression.

Results: Fifty PHTR met inclusion criteria (age 16.5 ± 2.9 yrs, female=25). Time from transplant and time between CPET and CMR were 5.9 ± 0.5 yrs and 0.5 ± 0.3 yrs, respectively. CPET mean baseline HR was 89 ± 11 bpm and peak HR was 167 ± 21 bpm; 8 patients were on beta blockers. Mean LVEF was 60 ± 4.5%. Higher T2 correlated with reduced VO_2peak (R= -0.346, p=0.029) and higher global MPRI correlated to increased exercise duration (R=0.323, p=0.048). Associations were also noted between CMR variables and HR-dependent CPET variables including O₂ pulse, HR reserve and %APMHR (table 1). However, given known abnormal HR response to exercise related to denervation and variable timing of reinnervation in PHTR, the significance of these associations is unclear.

Conclusion: In PHTR with normal LV function, CMR cardiac structure and tissue characteristics have correlations with exercise capacity and duration. Larger studies are needed to understand the role of multiparametric CMR in the functional surveillance of PHTR.