DEFINITION AND CLASSIFICATION

There are a number of important interactions between heart disease and kidney disease. The interaction is bidirectional as acute or chronic dysfunction of the heart or kidneys can induce acute or chronic dysfunction in the other organ. The clinical importance of such relationships is illustrated by the following observations:

• Mortality is increased in patients with heart failure (HF) who have a reduced glomerular filtration rate (GFR). (See "Cardiorenal syndrome: Prognosis and treatment", section on 'Reduced GFR and prognosis'.)
• Patients with chronic kidney disease have an increased risk of both atherosclerotic cardiovascular disease and heart failure, and cardiovascular disease is responsible for up to 50 percent of deaths in patients with renal failure [1,2]. (See "Chronic kidney disease and coronary heart disease", section on 'Introduction'.)
• Acute or chronic systemic disorders can cause both cardiac and renal dysfunction.

The term cardiorenal syndrome (CRS) has been applied to these interactions, but the definition and classification have not been clear. A 2004 report from the National Heart, Lung, and Blood Institute defined CRS as a condition in which therapy to relieve congestive symptoms of HF is limited by a decline in renal function as manifested by a reduction in GFR [3]. The reduction in GFR was initially thought to result from a reduction in renal blood flow. However, various studies have demonstrated that cardiorenal interactions occur in both directions and in a variety of clinical settings [4]. (See 'Pathophysiology' below.)

The different interactions that can occur led to the following classification of CRS that was proposed by Ronco and colleagues [5]:

• Type 1 (acute) — Acute HF results in acute kidney injury (AKI, previously called acute renal failure).
• Type 2 — Chronic cardiac dysfunction (eg, chronic HF) causes progressive chronic kidney disease (CKD, previously called chronic renal failure).
• Type 3 — Abrupt and primary worsening of kidney function due, for example, to renal ischemia or glomerulonephritis causes acute cardiac dysfunction, which may be manifested by HF.
• Type 4 — Primary CKD contributes to cardiac dysfunction, which may be manifested by coronary disease, HF, or arrhythmia.
• Type 5 (secondary) — Acute or chronic systemic disorders (eg, sepsis or diabetes mellitus) that cause both cardiac and renal dysfunction.