Objective: This study aimed to evaluate aerobic capacity in children and adolescents following a diagnosis of Multisystem Inflammatory Syndrome in Children (MIS-C) compared to healthy matched controls.

Design: Prospective cohort study.

Setting: Quaternary Pediatric intensive care unit and Pediatric cardiology unit.

Participants: 14 children diagnosed with MIS-C.

Interventions: None.

Measurements: Cardiopulmonary fitness parameters at the time of post-Pediatric intensive care unit follow-up (mean 3.6 months) such as maximum oxygen uptake (VO_2max ) and the first ventilatory anaerobic threshold (VAT), as a marker of muscular deconditioning.

Main results: A total of 14 patients (12 boys) with a confirmed diagnosis of MIS-C and 70 healthy children were included. The median age was 13.2 years (range 10.7-13.6 years). All participants had a normal echocardiogram and normal lung function at the time of cardiopulmonary exercise test. As measured by VO_2max Z-score, exercise capacity was significantly lower in the MIS-C group compared to healthy controls (median-0.91 vs 0.13, p < 0.01), and a significantly higher proportion of children in the MIS-C group had impaired aerobic capacity (VO_2max Z-score<-1.64) compared to controls (29% vs. 3%, p < 0.01). The VAT was also significantly lower in the MIS-C group with a higher proportion of children with an impaired VAT (VAT Z-score<-1.64) compared to controls (43% vs. 3%, p = 0.03). Impaired aerobic capacity in the MIS-C group was associated with higher BMI, higher PELOD 2 score and lower platelet count at the PICU admission, and lower hemoglobin level at the cardiopulmonary exercise test time.

Conclusions: This study suggests that children with MIS-C experience significant reductions in aerobic capacity compared to healthy controls, primarily due to muscular deconditioning. These findings highlight the importance of considering post-ICU consultations and implementing strategies to address physical deconditioning in this population.