Cardiorenal syndrome (CRS) is defined as progressive, combined cardiac and renal dysfunction. In this mini review, a historical note on CRS is presented, the pathomechanisms and clinical hallmarks of both chronic heart failure and chronic kidney disease are discussed, and an updated classification of CRS is proposed. The current consensus classification relies on the assumed etiology and the course of the disease, i.e., acute or chronic CRS. Five types are described: type-I CRS presenting as acute cardiac failure leading to acute renal failure; type-II CRS presenting as chronic cardiac failure leading to chronic renal failure; type-III CRS presenting as acute kidney injury aggravating heart failure; type-IV CRS presenting as chronic kidney failure aggravating heart failure; and type-V CRS presenting as concurrent, chronic cardiac and renal failure. For an updated classification, information on the presence or absence of valvular heart disease and on the presence of hyper- or hypovolemia is added. Thus, CRS is specified as "acute" (type-I, type-III or type-V CRS) or "chronic" (type-II, type-IV or type-V) CRS, as "valvular" or "nonvalvular" CRS, and as "hyper-" or "hypovolemia-associated" CRS if euvolemia is absent. To enable the use of this updated classification, validation studies are mandated.
Keywords: acute heart failure; acute kidney injury; cardiorenal syndrome; chronic heart failure; chronic kidney disease.