Cardiorenal syndrome: refining the definition of a complex symbiosis gone wrong

Background: The term "cardiorenal syndrome" has generally been reserved for declining renal function in the setting of advanced congestive heart failure. Considering the complex and bi-directional relationship between the heart and the kidneys, we postulate refining the definition to recognize the symbiotic nature of these organs. Discussion: We divide the cardiorenal syndrome into five subtypes: type I, acute cardiorenal syndrome; type II, chronic cardiorenal syndrome; type III, acute renocardiac syndrome; type IV, chronic renocardiac syndrome; and type V, secondary cardiorenal syndrome. Conclusions: As early recognition of dysfunction in one organ may prove important in mitigating the spiral of co-dysfunction in both, the need for early and treatment-guiding biomarkers, along with their characteristics, are also discussed.