Brain Natriuretic Peptide

Brain natriuretic peptide (BNP) is named as such, because it was initially identified in brain tissue, however it's primarily released by the myocardium. There are two peptides produced during processing, the amino-terminal BNP and the biologically active BNP. Along with atrial natriuretic peptide (ANP), BNP is involved in salt and water balance in response to myocardial stretch from increased filling pressures. It's influences lead to diuresis, natriuresis and hypotension.

Clinical evaluation of these hormones can be performed using multiple assays. There are tests available for both NT-BNP and BNP, some of which are rapid point of care testing, which can be used in the emergency department. It should be recognized that these assays have different cut-offs for normal values and can be influenced by age, gender and renal failure (higher in elderly and women, low in obesity). There have also been studies in specific ethnic groups which have identified the different reference ranges for certain ethnicities (Choi, 2007). Chronic congestive heart failure may produce persistently elevated levels, making it difficult to interpret spot samples without documented baselines.

In 2002, the Breathing Not Properly study (BNP), evaluated patients presenting to hospital with dyspnea, documenting BNP levels on all patients and comparing level in those eventually felt to have CHF versus alternative diagnoses. Patients with CHF had levels over 400pg/ml with high specificity,  while levels less than 50pg/ml  had a negative predictive value of 96%. Higher levels were also found in patients with atrial fibrillation and renal failure. The authors felt BNP was superior to any one individual clinical factor in predicting CHF exacerbation. A similar trial (PRIDE), examined the ability for NT-BNP to predict CHF in acute dyspnea. They found that levels over 450pg/ml (in patients under 50) and over 950pg/ml (in patients over 50) were sensitive and specific for CHF. NT-BNP was superior to clinical judgement in predicting CHF in this trial. The use of BNP markers has been shown to decrease ICU and hospital duration of stay by ~ 24 hours but have no statistically significant impact on overall mortality (Porapakkham, 2010). 

Despite this information, laboratory testing shouldn't replace clinical judgement, as individual variability contributes to lab interpretation. BNP is however a useful marker in the setting of evaluating acute dyspnea, and there is some additional literature suggesting it may be useful in the outpatient setting, where levels may predict mortality in chronic CHF and be used as a target for treatment.

Cardiac biomarkers NEJM