Early and Stage-Dependent Cardiopulmonary Fitness Impairment in Pediatric CKD.

key pointsChildren with CKD exhibit markedly lower cardiopulmonary fitness with an absolute maximal oxygen uptake deficit of 10 ml/kg per minute compared with reference values. More than half of the patients show impaired fitness as early as CKD stage 3, highlighting early functional vulnerability. Maximal oxygen uptake is independently associated with modifiable parameters commonly targeted by rehabilitation, supporting tailored intervention strategies.

backgroundChildren with CKD are at high cardiovascular risk, yet cardiopulmonary fitness is rarely assessed in routine care. Maximal oxygen uptake (VO 2 max) is a powerful prognostic marker, and Z -score-based evaluations may improve early risk stratification.

methodsIn this prospective multicenter study, children aged 6-17 years with CKD stages 2-5 (Kidney Disease Improving Global Outcome) underwent standardized cardiopulmonary exercise testing (CPET). VO 2 max was expressed as Z -scores using contemporary pediatric reference values. Impaired fitness was defined as VO 2 max Z -score <-1.64. Multivariable linear regression was used to identify independent determinants of VO 2 max.

resultsAmong 88 enrolled patients (mean age 12±4 years, 67% male), 77 completed a valid CPET. Mean VO 2 max Z -score was -1.8±2.1, corresponding to 33.5±9.2 ml/kg per minute, that is, an absolute lowering of approximately 10 ml/kg per minute compared with age-matched and sex-matched reference values. Overall, 57% had impaired fitness. VO 2 max was progressively lower across higher CKD stages ( P < 0.001 ). In multivariable analysis, higher VO 2 max was independently associated with higher hemoglobin concentration, better pulmonary function (forced expiratory volume in 1 s), better ventilatory efficiency (VE/VCO 2 slope), and higher ventilatory anaerobic threshold, explaining 77% of VO 2 max variability.

conclusionsCardiopulmonary fitness is markedly lower in children with CKD, with a clear stage-dependent decline that is already evident at early disease stages. The magnitude of VO 2 max impairment is substantial and related to potentially modifiable clinical and functional factors, supporting early CPET-based screening and the development of targeted, multidisciplinary rehabilitation in pediatric CKD. CLINICAL TRIAL REGISTRY NAME AND REGISTRATION NUMBER: Clinicaltrials.gov identifier NCT04897672 and Montpellier University Hospital Institutional Review Board (IRB 202100852).