Rehabilitation for Cardiorenal Multimorbidity: Epidemiology, Functional Phenotypes, and Effects on Physical Function,...

Background/Objectives: Cardiac and renal dysfunction frequently coexist and interact bidirectionally, constituting cardiorenal syndrome (CRS). In aging societies, this overlap is increasingly conceptualized within cardiovascular-kidney-metabolic (CKM) syndrome, in which metabolic risk factors, chronic kidney disease (CKD), and cardiovascular disease cluster and worsen prognosis. Patients with cardiorenal multimorbidity exhibit reduced exercise tolerance, physical activity, and skeletal muscle function, leading to frailty, disability, recurrent hospitalization, and reduced tolerance of disease-modifying therapies. Although exercise-based rehabilitation is central to cardiovascular care and increasingly recognized in nephrology, its role in combined cardiac and renal dysfunction remains insufficiently integrated. Methods: This narrative review synthesizes cardiology and nephrology evidence using a functional framework. We address (i) the epidemiology and clinical significance of cardiorenal overlap across CRS/CKM, (ii) functional phenotypes defined by inactivity, low exercise capacity, sarcopenia/frailty, and disability, (iii) rehabilitation effects on physical function and renal trajectories, including renal endpoint validity (creatinine vs. cystatin C), and (iv) prognostic implications and evidence gaps. Results: Evidence from heart failure trials demonstrates that exercise-based cardiac rehabilitation improves health-related quality of life and reduces hospital admissions. In CKD, systematic reviews support exercise benefits for physical function and cardiometabolic risk. Conclusions: Although evidence remains limited, data support rehabilitation as a biologically plausible, function-centered therapeutic strategy.