In 1981, de Bold et al.1 provided the definitive demonstration of the endocrine function of the heart by the description of atrial natriuretic peptide (ANP). This was followed by the identification of brain natriuretic peptide, or B-type natriuretic peptide (BNP), in 1988.2,3 This natriuretic peptide (NP) was originally discovered in the porcine brain, but it is synthesised, stored and released mainly in the myocardium.4 These discoveries were a breakthrough and proved the long-predicted endocrine link between the heart and the kidneys. It is now apparent that the heart is not only a mechanical pump; rather, it is also an endocrine gland that releases NPs from haemodynamically stressed myocardium by increased atrial or ventricular myocardial stretch or strain. Soon after its discovery the first report of increased ANP plasma concentrations in heart failure (HF) patients was published.5 The concept of a plasma marker in HF was thereby introduced and has since been intensely pursued, with a strong focus on clinical applications. In subsequent comparative clinical studies, BNP and N-terminal proBNP (NT-proBNP) emerged as the superior diagnostic markers compared with ANP and other proANP-derived peptides.
Physiology and Pathophysiology
ANP and BNP promote natriuresis and diuresis, act as vasodilators, and antagonise the vasoconstrictor effects of the renin–angiotensin– aldosterone system (RAAS).6 The NP system has emerged as one of the most important hormonal systems in the control of cardiovascular homeostasis and function via co-ordinated central and peripheral actions. In the brain stem, NPs decrease the sympathetic tone; in the hypothalamus the secretion of arginine vasopressin and corticotropin; and in areas adjacent to the third ventricle they inhibit salt appetite and water drinking.7 More recently, evidence has been found that this family of cardiovascular NPs plays an autocrine and paracrine role in the control of myocardial and vascular structure and function as well. The known physiological effects of NPs are summarised in Table 1. NPs are characterised by a common 17-amino-acid ring structure with a di-sulphide bond between two cysteine residues. This ring structure shows high homology between different NPs and is essential for receptor binding and the biological activity of NPs.8 There are three known types of membrane-bound NP receptors (NPR), including NPR-A and NPR-B, the guanylyl cyclase-coupled receptors responsible for most if not all biologic effects; and NPR-C, the short cytoplasmic domain receptor responsible for peptide clearance and possibly regulation of cell proliferation.
Thus, the second messenger of NPs is cyclic guanosine monophosphate (cGMP), which regulates ion channels, protein kinases and phosphodiesterases. BNP exerts most if not all of its biological activities via binding to NPR-A. BNP is produced primarily in the cardiac atria under normal conditions in men.9 In left ventricular dysfunction (LVD), ventricular hypertrophy and other cardiac pathologies with chronic haemodynamic pressure or volume overload, ventricular myocytes undergo phenotypic modifications and re-express several foetal genes including ANP and BNP. Ventricles become an important source of NPs, particularly BNP,8 and, given the greater ventricular mass, most circulating cardiac BNP derives from the ventricles under pathophysiological conditions. However, NPs are also increased in all oedematous disorders10 with salt and fluid overload and increased atrial or ventricular wall tension (e.g. in heart and renal failure), and increases are not specific for HF (see Table 2).
Current understanding of the biochemistry of BNP and its circulating forms is far from complete.11 The human gene for BNP encodes a 134- amino-acid pre-proBNP precursor, which, after removal of a 26-amino-acid signal peptide, gives rise to a 108-amino-acid proBNP polypeptide.12 The regulation of BNP secretion occurs mainly at the level of gene transcription with only minor stores within cardiomyocytes. proBNP is believed to be split into BNP 1–32 and NT-proBNP 1–76, mainly upon secretion although limited amounts of processed BNP have also been described in the secretory granules of atrial cardiomyocytes. The processing site in proBNP occurs immediately downstream from the Arg73-X-X-Arg76 sequence, at a cleavage site similar to that recognised by the ubiquitous endoprotease furin.13 However, other endoproteases such as corin14 or prohormone convertases15 may be involved in the post-translational maturation of proBNP. BNP exhibits the biological activity, whereas no defined biological function has been found to be associated with NT-proBNP.16 proBNP can be also detected in the circulation.17 It circulates as a monomer and has only weak biological effects compared with BNP.16 There is no evidence for relevant processing of proBNP into BNP 1–32 and NT-proBNP in the circulation. Only small amounts of the intact hormone BNP 1–32 appear to circulate in plasma, and there are no known BNP-binding proteins.
The major circulating BNP forms appear to be split products of BNP 1–32, but are still not sufficiently characterised. Dipeptidyl-peptidase IV degrades BNP 1–32 to BNP 3–32 at its N-terminal end. NTproBNP appears to be rapidly truncated at both ends as well.18 proBNP and NT-proBNP are both glycosylated to a variable degree.19 BNP and NT-proBNP are extracted by the kidneys to a comparable extent of only about 15–20%.20 The calculated biological half-lives of BNP range from 13 to 20 minutes and of NT-proBNP from 25 to 70 minutes. 11 BNP and NT-proBNP are cleared differentially: BNP is actively removed from the bloodstream (binding to clearance receptors and to a much lesser extent by enzymatic degradation by neutral endopeptidase) and also has passive clearance mechanisms, including renal clearance; NT-proBNP is cleared more passively by organs with high rates of blood flow (e.g. muscle, liver, kidneys). There is evidence that the relative ratio of circulating proBNP to BNP varies from patient to patient and is disease-dependent, and the substantially less hormonally active proBNP may be the major immunoreactive form of BNP in patients with severe HF.21
Pre-analytical and Analytical Considerations
For a proper interpretation of BNP and NT-proBNP test results in clinical practice, clinicians must also be aware of the requested blood sampling conditions and pre-analytical and analytical features of current routine assays. A summary checklist is provided in Table 3.
Blood Sampling Conditions
Blood sampling conditions are not critical for patients presenting with acute symptoms, because in acute HF and other acute diseases (e.g. renal failure) BNP and NT-proBNP concentration increases are marked. However, as with other hormones regulating fluid homeostasis, blood sampling conditions in stable patients may be important and have to be tested and compared carefully. There are no significant circadian BNP and NT-proBNP variations or significant NP plasma concentration variations throughout the menstrual cycle.22–24 It has been reported that BNP and NT-proBNP do not significantly increase after daily physical activities in controls and HF patients.22,25 Body posture during blood sampling also does not significantly affect BNP and NT-proBNP concentrations. It can be concluded that there is no marked influence of blood drawing conditions for BNP measurements,26 and the requirements for blood sampling for NT-proBNP are even less critical than for BNP.22
It has to be emphasised that BNP and NT-proBNP are different analytes, and NT-proBNP concentrations are usually much higher than BNP values measured in the same blood samples. However, BNP and NT-proBNP measuring results also depend on the assay used in the laboratory, because BNP and NT-proBNP assays are not yet standardised. However, BNP assays are harmonised well to achieve good agreement in classification of patients as having values > or <100ng/l (the originally proposed cut-off value for the BNP TRIAGE® assay, which was the first BNP assay to be approved for routine use). Similarly, NT-proBNP assays show good harmonisation at a concentration around 125ng/l (originally proposed cut-off value for the NT-proBNP Roche® assay, which was the first NT-proBNP assay approved for routine use). However, it is important to be aware that other BNP or NT-proBNP cut-off values are method-dependent and cannot be used with another BNP or NT-proBNP assay without verification. Both BNP and NT-proBNP show a considerable biological variation and, therefore, only marked changes from baseline values (NT-proBNP >50% and BNP >60%) correlate well with clinical changes,27 and consequently frequent measurements for disease monitoring are not necessary.
In general, as with cardiac troponin I and cardiac troponin T, there are no relevant differences in the clinical utilities of BNP and NT-proBNP except for some very specific clinical settings (e.g. montoring of acute HF treated with BNP). BNP and NT-proBNP results always have to be interpreted in conjunction with the available clinical information and different thresholds are needed in different clinical settings. Using a single decision limit for all clinical scenarios is an oversimplification and unfortunately inappropriate, which makes the interpretation of BNP and NT-proBNP concentrations complex. Performance characteristics of both markers are similar in many clinical scenarios, and either can be used in patient care. In general, BNP and NT-proBNP concentrations correlate very well, but due to the longer half-life NT-proBNP concentrations are usually higher and show slower fluctuations compared with BNP. Clinicians must understand their differences (see Table 4), and absolute levels of BNP and NT-proBNP are not interchangeable and additionally depend on the assay used for measurement. The choice of which NP to use is an institutional decision and often comes down to which analysers are available in the local hospital laboratory.
Heart Failure Diagnosis
Measurements of BNP and NT-proBNP are now discussed in all international guidelines for the diagnosis and management of HF.28 A major limitation is that the substantial data that are now available have been obtained with different assays, which makes it frequently very difficult to report generally applicable marker decision limits. The best established role of BNP and NT-proBNP is in distinguishing HF from other causes in symptomatic patients presenting to the emergency department or to primary care with new onset dyspnoea. In this setting, rule-in and rule-out threshold values have been established for both BNP and NT-proBNP (see Figures 1 and 2). Values of <100ng/l for BNP and <300ng/l for NT-proBNP carry high negative predictive values (>90%) for the exclusion of acute HF for all commercially available assays.29,30 These tests significantly reduce diagnostic uncertainty in the emergency department and their use has been shown to be cost-effective by accelerating patient management and care in the acutely dyspnoeic population.31,32 The higher the values the more likely is HF, but for both markers there is a grey zone (BNP: 100–500ng/l; NT-proBNP: 300–1,800ng/l; see Figures 1 and 2), and their decision limits need to be adjusted for renal function as well.33,34 In case of an estimated glomerular filtration rate (eGFR) <60ml/minute/m2, a BNP rule-out limit of 200 (TRIAGE BNP) and NT-proBNP rule out limit of 1,200ng/l (Roche NT-proBNP assay) must be used. However, as all large multicentre trials on the diagnostic performance of BNP and NT-proBNP excluded patients with severe or end-stage renal failure, there are insufficient data on appropriate cut-off values in this patient population presenting with acute dyspnoea. Additionally, at least for BNP there appears to be a relevant inverse relationship of BNP with body mass index (BMI) as well, and in cases of severe obesity (BMI >35) a lower BNP rule-out cut-off value for HF (55ng/l, TRIAGE BNP) has been suggested.35
Most of our knowledge regarding the value of BNP and NT-proBNP testing is derived from studies in patients presenting with dyspnoea to the emergency department. The majority of HF patients with novel or increasing dyspnoea, however, are seen primarily by general practitioners or internists in private practice. Fewer data in this patient group are available,36 but in patients below 50 years of age with mild symptoms, lower cut-off values should be used for the exclusion of HF because of the significant influence of age on BNP and NT-proBNP reference values.
Age- (by decade) and sex- (females have significantly higher values than males) adjusted assay-specific upper reference limits are appropriate decision limits for the exclusion of HF in patients with mild symptoms; for patients presenting with severe symptoms the diagnostic algorithms of the emergency department studies appear to be appropriate.
Heart Failure Risk Stratification and Disease Monitoring
In the context of acute HF, measurements of NP on admission and pre-discharge allow improved risk stratification. Failure of marked NT-proBNP decreases (at least 30% of baseline values) during HF treatment and persistently high discharge values are associated with markedly increased 30-day and six-month risk of death and/or readmission.37 A pre-discharge BNP >700ng/l (TRIAGE BNP) was associated with a much higher risk of death or readmission within six months than those patients with BNP <350ng/l.38 In chronic HF patients, BNP and NT-proBNP provide comparable powerful prognostic information about survival and are predictors of functional status deterioration as well.39
A promising application of serial BNP and NT-proBNP measurements is the monitoring and titration of therapy in chronic HF. According to recent studies, the mortality and HF readmission rates are significantly reduced in the group receiving BNP- or NT-proBNP-guided adjustment of pharmacotherapy compared with clinical or usual care groups in the majority of studies. A BNP-guided treatment targeting a BNP <100ng/l (TRIAGE BNP) significantly reduced HF deaths and hospital stays for HF.40 Thus, for patients regularly followed in an outpatient clinic, an increase in BNP or NT-proBNP >50% from an individual’s steady state concentrations should trigger an evaluation for confirmatory signs and symptoms followed by adjustment of treatment.
Patients with Pulmonary Embolism
Elevated troponins and NPs (TRIAGE BNP >90ng/l and Roche NT-proBNP >500ng/l) have been identified as risk markers in patients with acute pulmonary embolism.41,42 In these patients, echocardiography should be immediately performed and in case of significant right ventricular dysfunction, thrombolysis or embolectomy should be considered.
Acute Coronary Syndromes and Acute Myocardial Infarction
In acute coronary syndromes (ACS) and acute myocardial infarction (AMI), BNP and NT-proBNP are powerful prognostic markers; the direct therapeutic consequences, however, remain to be determined, although high values should trigger more careful surveillance and aggressive application of appropriate treatment. In ACS, BNP >80ng/l (TRIAGE) and NT-proBNP >240ng/l (Roche assay) identify high-risk patients.43,44 In AMI, BNP and NT-proBNP concentrations during the subacute phase are strong independent predictors of prognosis and correlate with infarct size and left ventricular dysfunction.43,45
Valvular Heart Diseases
In the setting of cardiac valvular disorders, NPs are becoming a useful tool for monitoring patients. In general, NP concentrations rise with increasing severity of the valvular abnormality. The most convincing data are available for patients with aortic stenosis, e.g. NT-proBNP concentrations <680ng/l (Roche assay) and BNP values <130ng/l (TRIAGE BNP) have been found to predict a benign course within the subsequent six months in patients with severe aortic stenosis.46
BNP and NT-proBNP for Risk Stratification in Other Diseases
BNP and NT-proBNP improve risk stratification in most cardiovascular diseases, e.g. stable coronary artery disease (CAD), hypertension, diabetes, primary pulmonary hypertension, renal failure, and even in the general population. The common finding in all studies on risk stratification for the development of future cardiovascular events is that the lower the BNP or NT-proBNP, the lower is the risk for the individual patient. The major limitation is the current lack of evidence that lowering NPs results in improved outcome in clinical settings other than HF.
BNP and NT-proBNP are predominantly secreted from the heart in response to haemodynamic stress mediated by volume and pressure overload. They have a comparable clinical utility, and both have an established role in excluding the diagnosis of HF in the newly dyspnoeic patient in the hospital and primary care settings. The higher the values, the more likely HF will occur, but for both markers there is a grey zone. Current evidence also supports the use of NPs prior to discharge in patients hospitalised with acute HF for risk stratification and the long-term management of HF patients. An increasing body of evidence suggests that adjustment of treatment in response to serial NP testing in addition to clinical assessment improves outcomes in the management of chronic HF. There is an increasing body of evidence for the use of NP testing in the management of patients with heart valve disease. In almost all clinical studies NPs improved cardiovascular risk stratification.