To include the vast array of interrelated derangements and to stress the 
bidirectional nature of the heart-kidney interactions, the classification of the 
cardiorenal syndrome today includes 5 subtypes whose terminology reflects their 
primary and secondary pathology, time frame, and the presence of concomitant 
cardiac and renal dysfunction. Cardiorenal syndromes (CRSs) are pathophysiologic 
disorders of the heart and kidneys whereby acute or chronic dysfunction of one 
organ may induce acute or chronic dysfunction of the other. Type 1 CRS reflects 
an abrupt worsening of cardiac function leading to acute kidney injury. Type 2 
CRS describes chronic abnormalities in cardiac function causing progressive 
chronic kidney disease. Type 3 CRS consists in an abrupt worsening of renal 
function causing acute cardiac disorder. Type 4 CRS describes a state of chronic 
kidney disease contributing to decreased cardiac function, cardiac hypertrophy, 
and/or increased risk of adverse cardiovascular events. Type 5 CRS reflects a 
systemic condition (eg, sepsis) simultaneously causing both cardiac and renal 
dysfunction. Biomarkers can help characterize the subtypes of CRS as well as 
suggest the timing of treatment initiation and its likely effectiveness. The 
identification of patients and the pathophysiologic mechanisms underlying each 
syndrome subtype, including fluid overload or, in general, altered conditions of 
fluid status, can help physicians understand clinical derangements, provide the 
rationale for management strategies, and allow the design of future clinical 
trials with more accurate selection and stratification of the population under 
investigation.