Cardiorenal Syndromes: Advances in Determining Diagnosis, Prognosis, and Therapy

The term cardiorenal syndrome (CRS) implies acute or chronic injury to the heart and kidneys that often involves a temporal sequence of disease initiation and progression. The classification of CRS is divided into five subtypes. Types 1 and 2 involve acute and chronic cardiovascular disease (CVD) scenarios leading to acute kidney injury (AKI) or accelerated chronic kidney disease (CKD). Types 3 and 4, describe AKI and CKD, respectively, leading primarily to heart failure, although, it is possible that acute coronary syndromes, stroke, and arrhythmias could be CVD outcomes in these forms of CRS. Finally, CRS type 5 describes a systemic insult to both heart and the kidneys, such as sepsis, where both organs are injured simultaneously in persons with previously normal heart and kidney function at baseline. This manuscript will summarize key issues and future opportunities in challenging patients with CRS. Because most CRS occur in patients with pre-existing myocardial disease or chronic kidney disease, we will emphasize the chronic condition which puts individuals at risk for acute events. In the setting of a hospitalization, acute CRS can occur which have been consistently associated with inpatient complications, longer lengths of intensive care unit and hospital stay, need for renal replacement therapy, rehospitalization and death. While there are several common diagnostic and therapeutic targets for the heart and kidney, there remains considerable opportunity for both in-vitro diagnostics and medicinal therapy to favorably influence the occurrence and natural history of CRS. Citation: McCullough PA, Tumlin JA, Szerlip H, Vijayaraghavan K, Jyothinagaram S, et al. (2015) Cardiorenal Syndromes: Advances in Determining Diagnosis, Prognosis, and Therapy. J Cardiovasc Dis Diagn 3: 221. doi: 10.4172/2329-9517.1000221


Introduction
The term cardiorenal syndrome (CRS) implies acute or chronic injury to the heart and kidneys that often involves a temporal sequence of disease initiation and progression. The classification of CRS is divided into five subtypes each of which has complicated and poorly understood pathogenetic factors, yet holding promise for research and clinical opportunities to improve patient outcomes ( Figure  1). Types 1 and 2 involve acute and chronic cardiovascular disease (CVD) scenarios leading to acute kidney injury (AKI) or accelerated chronic kidney disease (CKD). Types 3 and 4, describe AKI and CKD, respectively, leading primarily to heart failure, although, it is possible that acute coronary syndromes, stroke, and arrhythmias could be CVD outcomes in these forms of CRS. Finally, CRS type 5 describes a systemic insult to both heart and the kidneys, such as sepsis, where both organs are injured simultaneously in persons with previously normal heart and kidney function at baseline. It has been long-recognized that the most significant predictor of cardiovascular outcomes in coronary atherosclerosis and myocardial disease is baseline renal function as reflected by the serum creatinine, blood urea nitrogen, cystatin-C, calculated creatine clearance, and estimated glomerular filtration rate (eGFR) [1][2][3]. While it is believed that a considerable amount of the association between decreased renal function and cardiovascular outcomes is due to confounding by older age, diabetes, hypertension, nephropathy, when we discuss chronic CRS we consider either diabetes or hypertension or both to be nearly universally present.

Impact of Blood Pressure Control on the Heart and Kidneys
For many years, systemic blood pressure has been a clinical target in both heart and kidney disease [7,8]. For the reduction in disease progression in both organs, blood pressure control is a cornerstone of management. The cardiovascular outcomes most responsive to (chronic HF leading to progressive CKD) and Type 4 (CKD and its related hemodynamic and metabolic changes leading to myocardial dysfunction and HF). A notable feature of Type 4 CRS is accelerated myocardial and tubulointerstitial fibrosis. Fibrosis in the myocardium is directly related to mechanical dysfunction and arrhythmogenesis (atrial fibrillation and re-entrant ventricular tachycardia resulting in ventricular fibrillation). The most common conditions that lead to chronic CRS are hypertension and diabetes mellitus. While it is possible that diseases without either of these conditions can be present such as polycystic kidney disease, vesiculo-ureteral reflux, or IgA

Cardiorenal Syndrome (CRS) General Definition:
A pathophysiologic disorder of the heart and kidneys whereby acute or chronic dysfunction in one organ may induce acute or chronic dysfunction in the other organ

CRS Type I (Acute Cardiorenal Syndrome)
Abrupt worsening of cardiac function (i.e. acute heart failure) leading to acute kidney injury •Major mechanisms: neurohormonal, abnormal cell signaling, hemodynamic derangements •Opportunities: identify the ~25% of HF that will develop AKI, test novel drugs/strategies

CRS Type II (Chronic Cardiorenal Syndrome)
Chronic abnormalities in cardiac function (i.e. chronic heart failure) leading to progressive and permanent chronic kidney disease •Major mechanisms: chronic neurohormonal hyperactivation •Opportunities: control risk of hyperkalemia and intensify ACEI or ARB, MRA, BB

CRS Type III (Acute Renocardiac Syndrome)
Abrupt worsening of renal function (i.e. AKI) leading to acute cardiac disorder (e.g. acute heart failure, arrhythmia) blood pressure control are stroke and heart failure ( Figure 2) [9]. Likewise, blood pressure control in CKD is associated with reductions in proteinuria, a slower decline in loss of eGFR, and reduced rates of incident ESRD. Thus a major current therapeutic target for management of chronic CRS is blood pressure with the caveat that for both organs there is a U-shaped relationship with outcomes as illustrated in the Figure 2 [9]. Given the dozens of available agents to treat hypertension, there is little development activity at this time for novel agents that would be of particular or unique benefit in CRS. The National Institutes of Health Systolic Blood Pressure Intervention Trial (SPRINT) will test in 9250 adults over age 50 a strategy of intensive blood pressure control versus standard control on the outcome of first occurrence of a myocardial infarction, acute coronary syndrome, and stroke, heart failure, or CVD death. In this study, there is a prespecified CKD subgroup would serve as an excellent validation for the event curve presented in Figure 2 [10].

Treatment of Diabetes and Newer Antidiabetic Agents
The control of blood glucose over time as reflected in the hemoglobin A1C has been a mainstay of therapy to slow the progression of CKD (termed "microvascular disease" in trials) and reduce the incidence of binary events such as myocardial infarction or stroke. As a general observation, the impact of glycemic control has a larger benefit on the kidneys than the heart particularly with respect to atherosclerosis and heart failure. In fact, there are particular caveats for the cardiorenal system and treatment of diabetes that are twofold: 1) tight glycemic control that results in hypoglycemia may trigger myocardial infarction and cardiovascular death, 2) certain antidiabetic agents have direct adverse myocardial effects that can either mask ischemia or lead to the development of myocardial dysfunction, fluid overload, and heart failure (sulfonylureas, peroxisome proliferatoractivated receptor (PPAR) agonists, and possibly dipeptidyl peptidase 4 (DPP4) inhibitors) [11][12][13]. We recognize there is considerable controversy around the cardiovascular safety of antidiabetic drugs, but we make the point that if a safety risk exists, it is in patients with CKD for the outcome of heart failure which has been overlooked by regulatory guidance documents and in the design of many clinical trials which have focused on myocardial infarction, stroke, and cardiovascular death. Thus, unlike antihypertensive agents, there is considerable interest in novel anti diabetic agents that could overcome these limitations and more favorably affect patients with chronic CRS. The discovery and development of inhibitors of the sodium-glucose transporter-2 (SGLT) have generated considerable interest in the cardiorenal community. These agents inhibit the tubular reabsorption of sodium and glucose in the proximal tubule and thus enhance losses of both sodium and glucose resulting in lower blood pressure (without a concomitant heart rate increase), and reduced serum glucose and hemoglobin A1C. In addition to alpha-glucosidase inhibitors, SGLT inhibitors (canaglifozin, dapaglifozin, empagliflozin) work to limit the availability of glucose to cells and result in a net loss of glucose containing calories to the body [14]. This creates a scenario favorable to weight loss and improved cardiometabolic status; which along with osmotic dieresis and lower blood pressure may theoretically decrease the risk of developing HF (although this has yet to be formally tested). Clinical trials are underway to evaluate SGLT-2 inhibitors for major cardiac and renal events including myocardial infarction, stroke, cardiovascular death, progression of CKD, and ESRD) [15]. It should be noted that the efficacy of SGLT-2 inhibitors depends on renal filtration in order to clear glucose in the urine, and thus these agents would have declining efficacy in very low eGFR patients.

Current Medical Therapy for Chronic Myocardial and Renal Disease
In terms of medicinal therapy, several classes of agents have favorable effects on both the heart and the kidneys primarily in the setting of chronic management. It has been long-recognized that agents that antagonize the renin-angiotensin system in general have beneficial effects on both the heart and kidneys including reductions in left ventricular hypertrophy, reductions in heart failure hospitalization and death, and to a lesser extent reductions in myocardial infarction and stroke; and for the kidneys, lessening of proteinuria, and delayed progression of CKD to ESRD. In general, angiotensin-converting enzyme inhibitors (ACEI) have the most solid base of evidence for benefit in heart disease as well as kidney disease. For those who ACEIintolerant, angiotensin II receptor antagonists (ARB) have a clinical role. However, despite early enthusiasm with dual therapy, the use of ACEI and ARB together has resulted in an unfavorable risk/benefit balance with higher rates of acute kidney injury, hyperkalemia, and hypotension [16]. The additional use of direct renin-inhibitors has also not been shown to have added benefit over ACEI or ARB alone [17]. The chronic use of beta-adrenergic receptor antagonists and mineralocorticoid receptor antagonists (MRA) in general has been associated with improved cardiorenal outcomes provided they are tolerated and free of toxicities including hypotension and hyperkalemia [18]. It is reasonable to infer that the true potential impact of MRA use in patients with significant CKD has not been realized because of dose limiting or prohibiting effects of hyperkalemia [19]. Thus, as indicated below, novel agents to control potassium could indirectly allow a greater use of both RAAS-i and MRAs with potential positive impact on the outcomes of patients with CKD (particularly when combined with HF) in the community.

Therapeutic Targets Beyond Blood Pressure and Glucose
Considerable efforts have been undertaken to understand the range of metabolic perturbations that occur in CKD and evaluate their modification in terms of both renal and cardiac outcomes. Many of these factors (acidosis, iron, vitamin D, calcium, phosphorus, parathyroid hormone, fibroblast growth factors, erythropoietin, anemia, uric acid, oxidative stress, etc.) have their own set of confounders that can range from diet, lifestyle, socioeconomic status, to complex biologic interplays and medication effects [20]. Needless to say, it has been difficult to identify a single causative factor for incident or accelerated cardiovascular disease in CKD patients. Clinical trials have worked through many of these factors in search of fulfilling Koch's hypothesis, that is, change the putative factor and influence the outcome. To date, modifications of a factor such as hemoglobin, parathyroid hormone, or oxidative stress have resulted in no demonstrable benefit, direct toxicity, or off-target deleterious effects leading to cessation of clinical development. Importantly, most of these trials have broadly included patients with CKD with little profiling of populations or strategic use of therapies. Thus, important subgroups for benefit or harm may have been missed. In addition, there have been little or no attempts at mitigating risk for toxicities. For example, in clinical trials of erythrocyte simulating agents (erythropoietin, darbepoetin) there was no effort to exclude patients with resistance to these agents, and as a result patients were subjected to supraphysiologic doses of these agents which had cardiovascular toxicity (e.g. hypertension, stroke, heart failure) [21]. In the case of studies on anemia, the practicing community has been left with considerable confusion over the cause of the adverse outcome: was it secondary to raising hemoglobin or supraphysiologic dosing of the study drugs. Another notable development was the attempt to modify detoxification enzymes and reduce oxidative stress with bardoxolone, which improved eGFR but in a large clinical trial precipitated acutely decompensated heart failure and related mortality [22]. In this example, there were insufficient efforts to mitigate the potential risks of fluid overload [23]. As a result, many CKD trials exclude patients with a prior history of HF or elevations of BNP in order to mitigate against potential cardiotoxicity [24].
Despite these prior setbacks, the future remains bright in terms of exploration of treatments for CKD that would reduce the rate of progression and at the same time lessen the burden of cardiovascular disease. Among hopeful strategies are the use of novel oral and intravenous iron preparations, inhibitors of hypoxia-inducible factor prolyl hydroxylase prolyase, endothelin receptor antagonists, xanthine oxidase inhibitors, treatment of acidosis, designer natriuretic peptides and anti-fibrotic agents including partial inhibitors of galectin-3. Two particularly advanced areas of therapies include newer antidiabetic agents and potassium binders.

Current in vitro Diagnostic Tests in Chronic Management of Cardiorenal Disease
In terms of in vitro diagnostic tests, it has been appreciated that blood B-type natriuretic peptide (BNP) and NT-terminal proBNP (NT-proBNP) reflect a heart-kidney hormonal system that signals increased wall tension in the cardiac chambers with a message to the kidneys to increase natriuresis, diuresis, and lower blood pressure [25]. In asymptomatic patients with CKD, elevations in BNP are strongly predictive for the future development of HF, and thus are used clinically in risk stratification and in clinical trials of novel CKD treatments for risk mitigation for the development of HF as a serious adverse event [26]. There are now a host of acute and chronic disease markers in cardiorenal disease which are in either clinical use or development that will play a major role in the screening, detection, prognosis, and management of disease [27]. A list of selected chronic cardiorenal biomarkers is shown given in Table 1. Cardiac blood biomarkers in the chronic setting that reflect accelerated apoptosis and cell turnover include troponin I and T. With the use of super-sensitive assays for these proteins, most patients with CKD have detectable levels and the plasma concentration at steady state is related to increased risk of heart failure hospitalization and death in patients with CKD. Suppressor of tumorigenicity 2 (ST2) is a decoy protein produced by the endothelial lining of the left ventricle and aortic outflow tract that blocks the interleukin-33 (IL-33) receptor on cardiomyocytes and satellite cells and impairs the favorable IL-33 signal to these cells, and thus turns on cellular processes that promote myocyte dysfunction and fibrosis [28]. ST2 appears to be unaffected by reductions in eGFR, although there is evidence that with increase in renal fibrosis, there may be elevations in ST2 levels. Galectin-3 is an animal lectin which is secreted by macrophages in solid organs (heart, kidney, liver) and stimulates fibroblasts to proliferate and secrete procollagen I which is crosslinked to mature collagen fibers in the extracellular matrix. Galectin-3 has been found to be reflective of both cardiac and renal fibrosis and is the first directly pathogenic measurable blood factor in cardiorenal disease [29][30][31]. All four of these markers (BNP/NT-proBNP, troponin I/T, ST2, and galectin-3) are recommended for use by the most recent American College of Cardiology/American Heart Association Guideline for the diagnosis and management of heart failure [32].
In terms of biomarker guidance in the management of CKD, the reliance on serum creatinine and urine albumin or protein as markers of disease progression has been both a guiding principal but also a retardant for breakthrough treatments. A doubling of serum creatinine which is a common endpoint in CKD trials requires a 57% reduction in eGFR [33]. Thus to witness an "important clinical signal" is to observe greater than half of organ function to be lost which cannot recover. This paradigm has led to stagnation in clinical development of agents to reduce the progression of CKD. A recent proposal that

Glomerulus-specific
Albumin † An emerging marker of monitoring progression of CKD.
Low concentration of protein in urine in terms of its large size and selectivity of glomerular filtration barrier and tubular reabsorption.

No No
A prognostic marker for monitoring reduced GER, progression of CKD, mortality risk. [66] Cystatin-C † An emerging marker of kidney filtration capacity.
A protein of cysterine proteinase inhibitory family, which passes glomerular basement membrane and then it is reabsorbed. TFF3, a small peptide, is expressed in tubular epithelial cells. Injury to tubular cells leads to increase of TFF3 in sera and urine. However, glomerular injury is not associated with urinary TFF3 excretion. TFF3 may play roles in regeneration and restitution, and in kidney damage repair, as well as in EMT process.
A lipocalin protein produced by kidney. Injury to proximal tubule leads to urinary level increase. NGAL mRNA is transcribed and overexpressed in loop of Henle and collecting duct, and thus urinary levels increases significantly.

Yes Urine
A prognostic marker for more accurate estimating GFR and WRS in CHF pts. A weak marker of risk for renal function decline, but might be a predictor of cardiac event in HF pts with and without renal disease. [52,66] N-acetyl-β-Dglucosaminidase An emerging marker of renal dysfunction and worse outcome in CHF.
A protein produced in proximal tubular cells, tubular injury leads to its excretion into urine.

No Urine No No
A prognostic marker for more accurate estimating GFR and WRS in pts with CHF. [66] Interleukin-18 An emerging marker of tubular injury CHF.
A cytokine secreted by proximal tubular cells.

No Yes No No
A prognostic marker for WRS in pts with CHF. Not high specific; [66] L-type fatty acid binding protein An emerging marker of tubular injury CHF.
A protein binding free fatty acids. Tubular injury leads to excretion of L-FABP. A useful marker of early podocyte injury in ESRD pts. [76] *For biomarkers from preclinical biomarkers translated into clinical setting, several processes are needed, including bridging preclinical biomarkers to clinical biomarkers, validation of the biomarker, transplantation application of preclinical biomarkers into clinical setting, and clinical qualification. The table includes FDA approved 10 biomarker qualification (cTnI, cTnT, Alb, Β2M, Clu, CysC, TUP, TFF3, Kim-1, RPA-1) for nonclinical use. The FDA approved biomarkers are also accepted for clinical observation under certain circumstances. Qualification is a conclusion that these biomarkers are sensitive and specific biomarkers for myocardial or kidney damage. FDA also approved 3 biomarkers (Gal3, TIMP-2/IGFBP7) for clinical setting, by using Galectin-3 Assay TM (BG Medicine) and NephroCheck ® test (Astute Medical Inc., USA). § FDA regulates in vitro diagnostic devices (IVD), and the biomarkers used in IVD typically include genomic, proteomic, and metabolomic biomarkers. † Cardiac troponins were reviewed by FDA as qualified biomarkers for nonclinical use on February 23, 2012. Qualification is a conclusion that cTnI and cTnT are sensitive and specific biomarkers for myocardial damage. † 7 biomarkers were selected from 23 markers for kidney toxicity by FDA and EMA as qualified biomarkers for nonclinical use. Qualification is a conclusion that these biomarkers are sensitive and specific biomarkers for kidney damage.  a 30% or 40% loss of function over one to two years would be a valid surrogate for the progression of CKD offers some modest hope that clinical trials could be undertaken that would be feasible using this endpoint [24]. As for the albumin: creatinine ratio or urine: protein ratio in the urine, there is considerable controversy over wether or not this endpoint is a valid surrogate. In general, small reductions in urine albumin in the range of 300 mg/g down to 30 mg/g are probably not indicative of major changes in clinical cardiorenal outcomes. However, in patients with diabetic CKD with heavy proteinuria (> 1 g urine protein per day), a large reduction (30-50%) may be a large enough signal that CKD is stabilizing, therefore influencing the future risk of cardiovascular events such as heart failure and the need for renal replacement therapy or all-cause mortality [34]. Of note, the Study Of Diabetic Nephropathy With Atrasentan (SONAR) trial testing low dose atrasentan (endothelin receptor A antagonist) has a unique design that will also test the validity of urine protein as a surrogate in patients with diabetic nephropathy (eGFR 25 to 75 mL/min/1.73 m 2 and a urine albumin creatinine ratio (UACR) 300-5,000 mg/g [35]. In this trial, all subjects (N=4148) are given atrasentan 0.75 mg p.o. qd initially and then are characterized as "responders" who experience a 30% or more reduction in urine protein. Then both the "responders" and "nonresponders" are randomized to atrasentan or placebo for several years with a composite renal endpoint: doubling of serum creatinine (confirmed by a 30-day serum creatinine) or the onset of end stage renal disease (needing chronic dialysis or renal transplantation or renal death). If only the responders benefit from the study drug for the clinical outcomes, then proteinuria will be validated as a surrogate. However if both the responders and nonresponders either benefit or both fail to benefit over placebo, then the uncertainty around the surrogacy of proteinuria as a treatment target will continue.

Agents to Prevent and Control Hyperkalemia
Patiromer calcium and sodium zirconium cyclosilicate are two new oral potassium binders expected to be approved and in clinical use shortly (Figure 3) [36]. Patiromer calcium is a novel exchange resin formulated as beads that contain sorbitol as an adjunctive cathartic which accounts for 50% of weight (2 g sorbitol for every 4.2 g of patiromer) as well as calcium (1.6 g calcium for every 4.2 g of patiromer). Patiromer is insoluble in typical solvents and passes through the GI tract without degradation and has its principal site of potassium in the colon about 7 hours after ingestion. The Polymeric Potassium Binder, in a Double-blind, Placebo-controlled Study in Patients with Chronic Heart Failure (PEARL-HF) demonstrated that in HF patients with hyperkalemia randomized to patiromer calcium were more successfully treated with spironolactone 50 mg/day (91% vs 74% placebo; P = .019) [37,38]. Sodium zirconium cyclosilicate (ZS-9, ZS Pharma, Inc) is another novel agent under development as a treatment for acute and long-term chronic hyperkalemia. Sodium zirconium cyclosilicate is an inorganic cation exchanger engineered to have a highly selective, high-capacity crystalline lattice structure to preferentially entrap monovalent cations (specifically excess potassium ions) over divalent cations (e.g., calcium and magnesium). Sodium zirconium cyclosilicate also appears to bind ammonium resulting in net acid loss and systemic elevation in plasma bicarbonate. In a double-blind, placebo-controlled clinical trial in patients with hyperkalemia (~66% on at least one ACEI, ARB, or MRA), a total of 753 patients with potassium levels 5.0-6.5 mEq/L, which included patients with CKD, heart failure, diabetes, and those on ACEIs, ARBs, or MRAs, were randomized to receive 1 of 4 doses of sodium zirconium cyclosilicate versus placebo [39]. At 48 hours, there were absolute mean reductions of 0.73 mEq/L in the 10-g group (P<0.0001), as compared with a mean reduction of 0.25 mEq/L in the placebo group. The mean reduction from baseline to 1 hour after the first 10-g dose of sodium zirconium cyclosilicate was 0.11 mEq/L (P = .009) suggesting a potassium binding effect in the upper gastrointestinal tract at the level of the stomach and proximal small intestine. A total of 543/753 patients (72.1%) achieved a normal serum potassium of 3.5-4.9 mEq/L during the initial 48 hour phase, and proceeded to the randomized extended use phase which demonstrated significantly lower potassium levels with zirconium cyclosilicate 5 g and 10 g daily versus the placebo for up to 12 days.
In a second multicenter trial, 258 stable outpatients with a potassium concentration ≥ 5.1 mEq/L (range 5.1 -7.1 meq/L) at baseline (~67% on at least one ACEI, ARB, or MRA), received 10 g of zirconium cyclosilicate PO t.i.d. during the initial 48-hour open-label phase. Patients achieving normokalemia (3.5-5.0 mEq/L) were randomized to receive sodium zirconium cyclosilicate, 5 g (n = 45 patients), 10 g (n = 51), or 15 g (n = 56), or placebo (n = 85) daily for 28 days [40]. The rates of achieving normokalemia at 24 and 48 hours were 84 and 98%, respectively. The primary end point was the mean serum potassium levels between placebo and each treatment group (highest to lowest) during days 8 through 29 of the randomized phase which was achieved for all doses of zirconium cyclosilicate versus placebo. The proportion of patients with normokalemia during the randomized phase of the study was significantly higher in all zirconium cyclosilicate groups vs. placebo (80%, 90%, and 94% for the 5 g, 10 g, and 15 g groups, vs. 46% with placebo; P < .001 for each dose vs. placebo). Thus it appears that there will be two new products available in the near future to aide with the management of hyperkalemia in patients with acute and chronic CRS where the use of ACE, ARB, and MRA could be facilitated with better control of serum potassium.

Acute Cardiorenal Syndromes Requiring Hospitalization
Unlike the ambulatory setting, the acute hospitalization of a patient with chronic cardiorenal disease presents considerable risks for acute kidney injury, decompensation of heart failure, need for the intensive care unit, prolonged hospital stay, ESRD, and death. In the schema of five subtypes of CRS, the acute syndromes are Type 1 (cardiac decompensation leading to AKI), Type 3 (AKI leading to HF), and Type 5 (simultaneous cardiac and renal failure in sepsis, etc). In almost all circumstances, the acute CRS involve a complex and multifaceted pathophysiology of acute on chronic heart and kidney disease as shown in Figure 4. Unfortunately, there are no approved therapies that have been shown to reduce or attenuate the degree of acute/injury to both the heart and kidneys. Many acute therapies have been tested in clinical trials, but all have failed to improve outcomes in a convincing fashion,   [41]. As a result, clinical practice in the hospital setting has not changed much over the past decade with respect to cardiorenal patients. Therefore, we believe that there is a considerable opportunity to develop new vistas in this field to advance the understanding and care for this population. Most of this opportunity will be dependent on phenotypic characterization of the CRS with respect to predominant mechanism (cell signaling, neurohormonal, hemodynamic, electrolyte imbalance) and then a specific therapy applied to the major mechanism. It is unlikely that a "one size fits all" approach to any novel agent in acute development will be broadly successful, including agents such as serelaxin (analogue to the pregnancy hormone relaxin) and omecamtiv mecarbil (myosin activator inotropic agent) [42,43]. A shortcoming of the trials programs for both of these agents is a 48 hour infusion which is simply not long enough to have a meaningful impact on decompensated cardiac or renal function with respect to any of the major mechanisms of acute organ failure. A more realistic approach would be a prolonged infusion (5 or more days) and then conversion to subcutaneous or oral therapy in analogy to the use of antibiotics, corticosteroids, or anticoagulation [44].

Acute Kidney Injury
The definition of acute kidney injury (AKI ) was unified by the Kidney Disease Global Outcomes Initiative (KDIGO) in a set of guidelines that incorporated prior AKI definitions by the Acute Kidney Injury Network (AKIN) and the Risk, Injury, Failure, Loss, and End-Stage Kidney Disease (RIFLE) classifications. The KDIGO criteria for AKI (based on the rise in serum creatinine and urine output shown in Figure 5) were helpful to the practice and research community in setting a standard for recognition of acute dysfunction and a baseline definition from which future modifications can be made as have been done for acute myocardial infarction. A list of selected biomarkers for AKI is shown in Table 2. There have been many studies that have shown that independently and together, a rise in serum creatinine (threshold ≥ 0.3 mg/dl) or a reduction in urine output (threshold < 0.5 cc/kg/hr for six hours) is prognostic for the development of more serious renal events such as permanent loss of renal filtration function, ESRD, rehospitalization, and death. However, many of these subtle changes in renal status can be due to transient hemodynamic changes and not damage or death of functional renal parenchymal cells. Thus, the advent of markers of acute tubular injury has represented a major advance in the early detection and confirmation of AKI ( Figure 6). Because serum creatinine can take 48 hours to significantly rise, the recognition of AKI based on creatinine is always delayed. While urine output can abruptly decline in AKI, it is commonly manipulated by intravenous fluid administration and use of diuretics. Hence, urine output cannot be a reliable indicator of AKI alone and can be misleading in many cases. However, in the setting of oliguria, it has been shown that a "renal stress test" with a one-time administration of high-dose furosemide can be prognostic for the short-term outcome of ESRD or death [45].
Novel markers that are indicators of tubular cell-cycle arrest (tissue inhibitor of metalloproteinase-2 [TIMP-2], insulin-like growth factor binding protein-7 [IGFBP-7]), protectors against catalyticiron induced oxidative stress (siderocalin-2 or neutrophil gelatinase associated lipocalin), or promotors of tubular cell recovery (kidney injury molecule-1) have all been shown to be assistive in the diagnosis and prognosis of AKI. In addition, cytosolic enzymes and housekeeping proteins involved in normal cell biologic functions such as alpha and pie-glutathione S-transferase, L-type fatty acid binding protein, and F-isoprostanes can indicate tubular cell injury or death. Increased urinary levels of proteins normally reabsorbed by the proximal tubules can also indicate tubular dysfunction including urinary albumin,  Renal-derived CNP suppresses fibroblast proliferation and collage production; Hypoxia, cytokines, and fibrotic growth factors stimulate its production; Urinary CNP reflects renal structural remodeling prior to kidney disease, and detects in pts with acute compensated HF.

No Yes
Increased CNP mRNA expression in diabetic rat kidney. CNP holds promise for diagnostic and therapeutic targets.
Pts with HF and renal disease states CNP service as a double role's marker for heart failure and renal remodeling (involving tubulointerstitial fibrosis).

Glomerulus-specific
Albumin † FDA approved qualification for nonclinical use.
A most abundant protein in blood plasma that functions as a regulator of colloidal osmotic pressure of blood. Dysfunction of glomerular filtration leads to increased levels of Alb measured in blood and urine

No Yes No No
In pts with cisplatin treatment, urinary excretion of Alb >20mg/L. [89] Total urine protein † FDA approved TUP qualification for preclinical use.
Increased total urinary proteins are due to dysfunction of glomerular filtration and tubular reabsorption.

No Urine No No
Correlation of increased amounts with diabetes or drug-induced nephrotoxicity. [89]

α1-microglobulin
An emerging marker of AKI.
A protein that is synthesized in liver with half of circulated bound to IgA complex. Its free form is filtrated by glomerulus and reabsorbed by proximal tubular cells.

No Urine (ELISA) No
Pts with postcardiac surgery and ATN.
A sensitive marker for proximal tubular dysfunction in pts with cardiac surgery.
A prognostic marker of outcome in pts with nonoliguric acute tubular necrosis.
A useful marker of tubular dysfunction in infants at high risk of AKI. [90] β-2 microglobulin † FDA approved qualification for nonclinical use.
A protein present on all nucleated cells. Increased levels of urine Β2M indicate dysfunction of glomerular filtration and reabsorption of proximal tubules.

Yes Yes
Gene specific for rat (qPCR). No Increased levels of Β2M in pts with idiopathic membrane nephropathy, in pts suffering from with druginduced kidney damage, and in transplant pts with acute tubulointerstitial rejection. [89] Cystatin C † FDA and EMA approved its qualification for preclinical use of drug-induced AKI.
A low molecular weight protein, which is freely filtrated through glomerulus and completely by tubular cells. CysC strongly related to GER.

Yes Yes
Gene specific for rat (qPCR).
Pts with acute HF or cardiac surgery. A diagnostic marker of tubular injury in pts with AKI. [92] Cysterin-rich protein-61 An emerging of marker of AKI.
A cysterine-rich, heparin-binding protein and a novel ligand for integrins. It is rapidly induced in proximal straight tubules by ischemia.

No West blot. No No
A potential marker of AKI (secretion of urinary Cyr16 within 3-6 hrs after ischemia in rat).
No clinical study demonstrating its use. [92] Netrin-1 An emerging of marker of AKI.
A laminin-like protein that was secreted in urine within 1 hr following functional injury and rises 30-40 folds in 3 hrs and reaches its peak in 6 hrs after insult. Increase of netrin-1in urine may be due toinduction of protein synthesis and release of presynthesis protein. A diagnostic marker of AKI pts, but in normal subjects or pts with CKD.
In ischemic (I/R) and nephrotoxic (cisplatin) AKI rat, excretion of urinary exosomal AFT3 was time dependent. [76] * For biomarkers from preclinical biomarkers translated into clinical setting, several processes are needed, including bridging preclinical biomarkers to clinical biomarkers, validation of the biomarker, transplantation application of preclinical biomarkers into clinical setting, and clinical qualification. The table includes FDA approved 10 biomarker qualification (cTnI, cTnT, Alb, Β2M, Clu, CysC, TUP, TFF3, Kim-1, RPA-1) for nonclinical use. The FDA approved biomarkers are also accepted for clinical observation under certain circumstances. Qualification is a conclusion that these biomarkers are sensitive and specific biomarkers for myocardial or kidney damage. FDA also approved 3 biomarkers (Gal3, TIMP-2*IGFBP7) for clinical setting, by using Galectin-3 Assay TM (BG Medicine) and NephroCheck ® test (Astute Medical Inc., USA). § FDA regulates in vitro diagnostic devices (IVD), and the biomarkers used in IVD typically include genomic, proteomic, and metabolomic biomarkers. † FDA approved as qualified biomarkers for nonclinical use. ¥ FDA and EMA qualified KIM-1 as a urinary biomarker in clinical studies on case-by case basis. ‖ AUC-ROC of 0.5 represents no better than expected by random change, AUC-ROC of 0.75, a good biomarker, and AUC-ROC of 0.9, an excellent biomarker. Biomarker abbreviations Alb = albumin; AP = alkaline phosphatase; ATF3= activating transcription factor3; BNP = Brain natriuretic peptide; CK-MB = creatine kinase-MB; Clu = clusterin; CNP = C-type natriuretic peptide; cTnI = cardiac troponin I; cTnT = cardiac troponin T; Cyr61 = cysterin-rich protein; CysC = cystatin C; h-FABP = heart-type fatty acid binding protein; HGF = hepatocyte growth factor; IGFBP7 = insulin-like growth factor-binding protein 7; IL-6 = interleukin 6; IL-18 = interleukin 18; Gal-3 = galectin-3; Kim-1 = kidney injury molecule-1 (in preclinical study); KIM-1 = kidney injury molecule-1 (in human study); L-FABP = the liver type fatty acid binding protein; NAG = N-acetyl-β-dglucosaminidase; NGAL = neutrophil gelatinase-associated lipocalin; NAG = N-acetyl-beta-D-glucosaminidase; NHE3 = sodium/hydrogen exchanger isoform;NT-proBNP = NT-pro-B type natriuretic peptide; RBP = retinol binding protein; RPA-1= renal papillary antigen-1;TFF3 = trefoil factor 3; TIMP-2 = tissue inhibitor of metalloproteinase 2; TUP = total urinary proteins; uAGT = urinary angiotensinogen; α-GST = alpha-glutathione S-transferase; b2M = β2-Microglubin; γGT = gamma glutamyl transpeptidase; π-GST = π-glutathione S-transferase. Other abbreviations AKI = acute kidney injury; ADHF = acute decompensated heart failure; AH = athlete's heart; AHF = acute heart failure; AKI = acute kidney injury; ATL= controls; AUC = area under a ROC curve; CHF = chronic heart failure; CRS = cardiorenal syndrome; ELISA = enzyme-linked immunosorbent assay; EMA = European Medicines Agency; GI = gastrointestinal; GFR = glomerular filtration rate; HF = heart failure; HR = hazard ratio; LVEF = left ventricular ejection fraction; pts = Patients; RAS = rennin-angiotensin system; ROC = receiver-operating characteristic; WRF = worsening renal function; yr = year. cystatin-C, and urinary N-acetyl-beta-D-glucosaminidase (NAG). These markers are ideally measured in the urine where they are heavily concentrated. Proteins that are constitutively upregulated in response to chronic kidney disease are less helpful (NGAL, KIM-1) clinically as the baseline value has to be considered in the interpretation of the acute value, and thus one must have multiple measurements at time points [46]. The development of commercial grade assays with the precision and reliability has been a challenge which has been recently overcome with the September, 2014 approval by the U.S. Food and Drug Administration of the NephroCheck ® test (Astute Medical, San Diego, CA). This test is a mathematical product of TIMP2 and IGFBP7 expressed in a unit less number with a range of 0.00 to 10.0 and a cutpoint of 0.3 as listed in the package insert. In patients admitted to the intensive care unit, a NephroCheck of < 0.3 had a >90% negative predictive value for the development of moderate or greater AKI [47]. The grey zone for this test appears to be 0.3 to 1.2 where there is uncertainty and likely repeat test will be needed later in time.
NephroCheck ® values of >2.0 had a high positive predictive value for moderate or severe AKI and may be a future trigger for clinical actions including switching away from nephrotoxic agents (aminoglycosides, vancomycin, non-steroidal anti-inflammatory agents, etc) as well as reducing intravenous fluid administration to avoid impending volume overload that will come with reductions in urine output [48]. A novel biomarker for chronic diabetic nephropathy is urine angiotensinogen, which appears to be 5-6 fold elevated in this condition compared to controls, and appears to risk stratify for the progession of CKD in this population [49].
Thus, novel diagnostic markers will rapidly take a support position in addition to serum creatinine and urine output to: 1) rule out AKI in the setting of pre-renal azotemia, 2) confirm AKI when serum creatinine and urine output are subjective, and 3) serve as a harbinger for moderate or severe AKI to develop over the following 12 to 48 hours in the intensive care unit [50]. Importantly, biomarkers for Type 5 CRS, while probably not helpful in recognizing the clinical condition, may be a critical step forward in the identification of cell, tissue, or organ-specific protective strategies against overwhelming injury. Table  3 gives select biomarkers under investigation for Type 5 CRS. A future hopeful vista for cardiorenal disease is the idea that early detection can lead to strategies that prevent or attenuate renal injury/dysfunction or enhance renal recovery and lead to improved outcomes (Figure 7).

Future Trials and the Need for Continuous Measures of Disease Progression
Acute and chronic CRS do not have specific groups of symptoms that can be used to evaluate benefit of therapies; hence, we have been left to conduct clinical trials with binary endpoints such as the doubling of serum creatinine, ESRD, hospitalization, and death as outcomes. These trials are large, have to be broadly inclusive, and in general have failed to allow the clinical development of products in the hospital and clinic setting. They have been fraught with problems including competing risks (e.g. death before the opportunity to demonstrate and organ effect) and lack of control of concurrent therapies (Figure 8). In addition, the renal functional reserve, or the ability of the kidney to adapt to a decreased mass of functioning nephrons and still carry out filtration sufficiently to keep the plasma pool of creatinine in a reasonable range, makes the assessment of CKD difficult. Renal functional reserve often dictates weather AKI will become clinically recognized in the setting of an acute renal stress such as AHF or sepsis. Hence there has been little opportunity for benefit and considerable opportunity for off-target harmful effects (Figure 8). There has been approximately 10 years with no new disease modifying agents to offer patients with CRS. Thus, despite the concern over the legitimacy of surrogate outcomes, reliable continuous clinical measures are needed that precede hard clinical outcomes in order for the field to progress. Other specialities have witnessed progress with such surrogates as human immunodeficiciency viral load, hepatitis viral load, hemoglobin A1 C, low-density lipoprotein cholesterol, and stages of disease progression in malignancies. So the pursuit of trusted biomarkers and other integrated measures of disease progression are crucial to the future of cardiorenal medicine. In addition, subgroup analyses, as well as more sophisticated approaches to identify heterogeneity of treatment benefit (and harm) should be pursued, and tested in confirmatory trials. These efforts should reduce the time course of development and implementation of beneficial therapies. Continued efforts on safety observations and registries can aide in assuring freedom from unexpected events over prolonged use as well as in the calculus in risk benefit tradeoffs.  [105,106] † This table is not intended to be exhaustive. *For biomarkers from preclinical biomarkers translated into clinical setting, several processes are needed, including bridging preclinical biomarkers to clinical biomarkers, validation of the biomarker, transplantation application of preclinical biomarkers into clinical setting, and clinical qualification. Biomarker abbreviations Alb = albumin; BNP = B-type natriuretic peptide; FF = functional fibrinogen; IL-6= interleukin 6; IL-8 = interleukin 8; sCD14-ST= soluble CD14 subtype (presepsin); TNF= tumor necrosis factor. Other abbreviations AUC = area under a ROC curve; CAD = coronary artery disease; CRS = cardiorenal syndrome; ELISA = enzyme-linked immunosorbent assay; eGFR = estimated glomerular filtration rate; CRS = cardiorenal syndrome; LBP = lipopolysacchride binding protein; LPS = lipopolysaccharide; pts = Patients; ROC = receiver-operating characteristic; UACR = urine albumin/creatinine ratio. A panel diagnostic marker for coagulation and fibriolysis in pts with severe sepsis. Plasma levels of Syndecan-1, thrombomodulin, protein C) were independently associated whole blood hypocoagulability, but fibrinolysis (FF) was inversely associated with syndecan-1 quarties. [107]

Summary
In the past 20 years, we have witnessed progress in common understanding and agreement in the concepts and definitions of acute myocardial infarction, heart failure, CKD, and CRS. A unform lexicon has aided patients, clinicians, and researchers as they deal with the issues of acute and chronic heart and kidney disease. The future is bright in terms of diagnostic and therapeutic strategies that will lead to prolonged quantity and quality of life for patients with these common conditions.