**5.2. Cardio‐renal syndrome**

Cardio‐renal syndrome (CRS) is a clinical syndrome broadly in which dysfunctional hearts and dysfunctional kidneys can "initiate and perpetuate disease in the other organ though common hemodynamic, neurohormonal, and immunological/biochemical feedback path‐ ways" [134]. General definition of CRS is a pathophysiologic disorder of the heart and kidneys, whereby acute or chronic dysfunction in one organ may induce acute or chronic dys‐ function in the other organ. According to human medical literature [134], the CRS is largely divided into five types: (1) Type I (acute CRS) is acute kidney injury induced by acute heart failure (e.g., acute cardiogenic shock or acutely decompensated CHF), (2) Type II (chronic CRS) is permanent and progressive chronic kidney disease induced by chronic heart failure (e.g., chronic abnormalities in cardiac function), (3) Type III (acute reno‐cardiac syndrome, RCS) is acute heart failure induced by acute kidney diseases (e.g., acute kidney ischemia or glomerulonephritis), (4) Type IV (chronic RCS) is chronic heart failure induced by chronic kidney disease (e.g., chronic glomerular or interstitial disease), and (5) Type V (secondary CRS) is heart and renal failure induced by systemic diseases (e.g., diabetes mellitus, sepsis). Major mechanisms of CRS include renal hypoperfusion directly resulting from a decreased cardiac output and neurohormone‐mediated renal damage as hypertensive nephropathy via activation of the renin‐angiotensin‐aldosterone system (RAAS) among others.

In veterinary study, the prevalence of azotemia is high in dogs with CMVI and increases with the severity of the heart failure and azotemia is associated with a decrease in GFR [135]. Azotemia and renal impairment increase with the severity of CHF and are frequent findings in dogs with CMVI [129]. One retrospective study of 33 dogs with CMVI demonstrated that the prevalence of azotemia (defined as abnormally elevated serum levels of Cys‐C, SDMA, and creatinine) was increased in dogs with CMVI [28]. Azotemia and renal impairment increase with the severity of HF and are frequent findings in dogs with CMVI [129]. Keys for success‐ ful management of CRS are: (1) try to decrease the dosage of furosemide if azotemia was worsen during the CHF treatment, (2) increase water intake, (3) consider IV fluid, if patients have clinical sings of azotemia (e.g., 2.5% dex + 0.45% saline, 5% dextrose; 30–40 ml/kg/day), and (4) monitor patient's condition regularly to maintain proper dose of furosemide/ACEI.
