Cardiorenal syndrome (CRS) is the spectrum of disorders in which acute or chronic dysfunction of the heart or kidneys leads to acute or chronic dysfunction of the other.
The condition is classified into five subtypes based on the primary organ dysfunction and whether the disease process is acute or chronic. The heart and the kidneys maintain hemodynamic stability and organ perfusion through an intricate network. CRS results from a complex interplay of hemodynamic alterations, neurohormonal activation, inflammatory mediators, and endothelial dysfunction, all contributing to progressive organ injury. Cardiorenal syndrome is commonly associated with conditions such as heart failure, chronic kidney disease (CKD), acute kidney injury (AKI), and systemic hypertension.
Management of CRS primarily focuses on addressing the underlying cause while mitigating the complications associated with the syndrome. Since volume overload is a predominant feature in most patients, treatment typically involves fluid removal, primarily through loop diuretics, with thiazides as adjuncts for diuretic resistant cases. Ultrafiltration is reserved for refractory cases. Depending on the case, additional therapies such as ACE inhibitors, angiotensin II receptor blockers, mineralocorticoid receptor antagonists, and inotropes may be utilized. Despite available treatments, CRS remains associated with high morbidity and mortality.
Signs and symptoms Cardiorenal syndrome (CRS) encompasses a spectrum of disorders in which acute or chronic dysfunction in the heart or kidneys leads to dysfunction in the other organ. Therefore, the clinical signs and symptoms are consistent with congestive heart failure and chronic kidney disease. The clinical presentation of most patients typically involves fluid overload, reduced cardiac output, and worsening renal function. Patients with acute cardiorenal syndrome often present with clinical features of pulmonary or systemic congestion and acute kidney injury.
Symptoms of peripheral edema and shortness of breath are common both in patients with CHF and CKD or a combination thereof. Patients will frequently exhibit signs of acute decompensated heart failure, such as volume overload characterized by peripheral edema, pulmonary congestion, jugular venous distension, and shortness of breath. Prolonged effects of heart failure, such as fatigue and exercise intolerance, may also be present.
Symptoms of acute cardiorenal syndrome also often present with classic indicators of renal dysfunction. Increased serum levels of creatinine and BUN, as well as reduced urine production may indicate worsening renal function.
Risk factors The primary risk factors for the development of cardiorenal syndrome are pre-existing cardiac or renal disease. The following risk factors have been associated with increased incidence of CRS.
Clinical: Older age Comorbid conditions (diabetes mellitus, uncontrolled hypertension, anemia) Drugs (anti-inflammatory agents, diuretics, ACE inhibitors, ARBs) Heart: History of heart failure with impaired left ventricular ejection fraction Prior myocardial infarction Elevated New York Heart Association (NYHA) functional class Elevated cardiac troponins Kidney: Chronic kidney disease (reduced eGFR, elevated BUN, creatinine, or cystatin)
Pathophysiology Cardiorenal syndrome (CRS) pathophysiology involves a complex bidirectional interaction between the heart and kidneys. The underlying mechanisms are broadly categorized into hemodynamic and non-hemodynamic factors. Hemodynamic factors primarily include changes in blood flow, such as reduced cardiac output and elevated central venous or intra-abdominal pressures. Non-hemodynamic factors include neurohormonal activation, oxidative stress, and systemic inflammation. These mechanisms often act synergistically, contributing to the progressive dysfunction of both organs.
Hemodynamic factors Reduced cardiac output, commonly due to heart failure or other cardiovascular conditions, leads to decreased renal perfusion. Historically, this reduction in perfusion was considered the primary driver of kidney dysfunction in heart failure. However, recent studies suggest that venous congestion may play an even more critical role. Heart failure increases central venous and intra-abdominal pressures, which are important regulators of renal blood flow. Elevated venous pressures reduce the net glomerular filtration pressure, promoting renal injury. These changes contribute to worsening volume overload and further deterioration of cardiac and renal function.
The renin-angiotensin-aldosterone system (RAAS) is activated in response to reduced renal perfusion. Although RAAS normally helps maintain blood pressure and organ perfusion, chronic over-activation leads to inappropriate sodium and water retention. This exacerbates volume overload and perpetuates a cycle of worsening heart and kidney function.
Non-hemodynamic factors In addition to hemodynamic changes, several non-hemodynamic mechanisms contribute to the progression of CRS. These include neurohormonal activation, oxidative stress, and systemic inflammation, all of which are associated with structural and functional deterioration in both the heart and kidneys.
Neurohormonal systems, primarily the RAAS and sympathetic nervous system (SNS), are activated in response to reduced renal perfusion. In heart failure, these systems become over-activated, causing peripheral vasoconstriction and extracellular fluid retention. Beyond hemodynamic effects, RAAS and SNS activation stimulate oxidative and inflammatory pathways and contribute to cardiac remodeling and progressive dysfunction.
Oxidative stress and inflammation also play critical roles. Elevated levels of reactive oxygen species (ROS), endothelin, and arginine vasopressin contribute to endothelial dysfunction, myocardial hypertrophy, and fibrosis, as well as to renal tubular injury and glomerular dysfunction. An imbalance between nitric oxide and ROS exacerbates endothelial dysfunction and impairs organ perfusion. There is a close interaction within these cardio-renal connectors as well as between these factors and the hemodynamic factors which makes the study of CRS pathophysiology complicated.
Diagnosis
Diagnosing cardiorenal syndrome (CRS) is challenging due to the complex and interconnected nature of cardiac and renal dysfunction. It is critical to diagnose CRS at an early stage in order to achieve optimal therapeutic efficacy. There is no specific test to diagnose cardiorenal syndrome. Instead, diagnosis relies on clinical evaluation, laboratory data, and imaging, often in the context of known heart failure, kidney disease, or both. The diagnosis of heart failure requires the presence of clinical signs and symptoms supported by evidence of a structural or functional cardiac abnormality. This diagnostic requirement for cardiorenal syndrome includes similar evidence for both the heart and kidneys.
Cardiac Biomarkers Cardiac biomarkers can help to identify cardiac dysfunction in the evaluation for Cardiorenal syndrome. Brain-naturetic peptide (BNP) is a peptide that is elevated in the presence of cardiac stress and volume overload. Cardiac troponin is a biomarker that can be useful to indicate ongoing myocardial damage and stress. The use of BNP and troponin can be confounded by decreased renal clearance in patients with chronic kidney function. Despite the limitations, these biomarkers can help to identify cardiac dysfunction crucial to the diagnosis of CRS. Though less frequently used in clinical practice, Galectin-3 and ST2 (suppressor of tumorigenicity 2), markers of fibrosis and myocardial stress, can add prognostic value.
Renal Biomarkers Unlike markers of heart damage or stress such as troponin, creatine kinase, natriuretic peptides, reliable markers for acute kidney injury are lacking. Recently, research has found several biomarkers that can be used for early detection of acute kidney injury before serious loss of organ function may occur. Several of these biomarkers include neutrophil gelatinase-associated lipocalin (NGAL), N-acetyl-B-D-glucosaminidase (NAG), Cystatin C, and kidney injury molecule-1 (KIM-1) which have been shown to be involved in tubular damage. Other biomarkers that have been shown to be useful include BNP, IL-18, and fatty acid binding protein (FABP). However, there is great variability in the measurement of these biomarkers and their use in diagnosing CRS must be assessed.
Non-Invasive Imaging The diagnosis of Cardiorenal syndrome utilizes imaging to provide key insights into structural and functional changes in the heart and kidneys. Echocardiography is the primary tool to assess heart function and can provide data on heart chamber performance, valvular abnormalities, and estimates of filling pressures. Lung ultrasound has recently emerged as a commonly used bedside tool to identify pulmonary edema. Imaging of the kidneys is commonly performed using ultrasound to assess size, structural changes, and perfusion.
Classification Ronco et al. first proposed a five-part classification system for CRS in 2008 which was also accepted at ADQI consensus conference in 2010. These include:
Type Inciting event Secondary disturbance Example Type 1 (acute CRS) Abrupt worsening of heart function kidney injury acute cardiogenic shock or acute decompensation of chronic heart failure Type 2 (chronic CRS) Chronic abnormalities in heart function progressive chronic kidney disease chronic heart failure Type 3 (acute renocardiac syndrome) Abrupt worsening of kidney function acute cardiac disorder (e.g. heart failure, abnormal heart rhythm, or pulmonary edema) acute kidney failure or glomerulonephritis Type 4 (chronic renocardiac syndrome) Chronic kidney disease decreased cardiac function, cardiac hypertrophy and/or increased risk of adverse cardiovascular events chronic glomerular disease Type 5 (secondary CRS) Systemic condition both heart and kidney dysfunction diabetes mellitus, sepsis, lupus
Type 1: Acute cardio-renal syndrome Acute cardiorenal syndrome occurs in patients who experience an abrupt decrease in cardiac function, causing an acute kidney injury and/or dysfunction. The sudden reduction in kidney function is frequently caused by acute decompensated heart failure, acute coronary syndrome, cardiogenic shock, and/or low flow syndrome following cardiac surgery. The extent of renal injury can vary, often causing acute kidney injury (AKI), but it can also result in acute renal failure. Type 1 CRS carries the risk of advancing to more severe stages of chronic kidney disease and end-stage renal disease (ESRD).
Type 2: Chronic cardio-renal syndrome Type 2 CRS refers to the specific situation in which chronic heart dysfunction results in progressive kidney dysfunction. Cardiac conditions such as heart failure with reduced or preserved ejection fraction, atrial fibrillation, ischemic cardiomyopathy, and congenital heart disease can result in adaptive changes in renal perfusion and neurohormonal activation over time. These conditions cause a time-dependent and progressive decline in renal function.
The distinction between CRS type 2 and CRS type 4 is based on the assumption that, also in advanced and chronic disease, two different pathophysiological mechanisms can be distinguished, whereas both CKD and HF often develop due to a common pathophysiological background, most notably hypertension and diabetes mellitus. Furthermore, the feasibility of the distinction between CRS type 2 and 4 in terms of diagnosis can be questioned.
Type 3: Acute reno-cardiac syndrome