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B-type natriuretic peptide is a substance secreted by the ventricles of the heart in response to heart strain. It has an important physiologic role in decreasing blood pressure and the salt content of the body. B-type natriuretic peptide levels are elevated in the blood of patients with heart failure; therefore, these levels are commonly measured to diagnose heart failure, monitor heart failure, and distinguish heart failure from other disease states.
Also known as brain natriuretic peptide (BNP), B-type natriuretic peptide is a molecule that is made by different parts of the human body in response to stress. It is composed of 32 amino acids, and is created from the progenitor species pro-BNP. B-type natriuretic peptide was initially discovered as a product of the brain, but was later identified as a substance produced by the ventricles of the heart. Normal serum BNP levels are less than 100 picograms per milliliter.
Understanding the physiologic function of BNP helps in understanding why measuring BNP levels can be used clinically for diagnostic purposes. The heart ventricles secrete BNP in response to high blood flow, high pressures, and strong contractions of the heart. The secreted BNP promotes low blood pressure, relaxation of blood vessels, increased excretion of salt in the urine, and increased urination. This peptide has also been shown to decrease remodeling of the heart, a process that has been implicated in heart failure.
In patients with heart failure, levels of BNP are high. Heart failure is associated with high ventricular filling pressure, high blood flow, and strong heart contractility; all of these states stimulate the ventricles to release BNP. Patients with either symptomatic and asymptomatic heart failure typically have elevated BNP levels in their blood.
One specific use of the BNP level is in the evaluation of patients who present with shortness of breath. Without a strong clinical history, it can be difficult to distinguish pulmonary causes of shortness of breath, such as chronic obstructive pulmonary disease, from cardiac causes of shortness of breath, such as heart failure. Elevated BNP levels point towards a cardiac cause of shortness of breath. It is important to distinguish between heart failure and pulmonary disease because the treatment for the two types of diseases is very different.
Another specific use of the B-type natriuretic peptide lies in ruling out heart disease in asymptomatic patients. Almost all patients with heart failure, even if they are asymptomatic, have elevations in BNP. Patients with normal levels of BNP, therefore, likely do not have heart failure, so this measurement can be used to rule it out.
B-type natriuretic peptide has also been used by some doctors to monitor responses to heart failure treatment regimens. With the proper heart failure medications, patients' BNP levels should decrease. Some research groups have argued that medication dosages can be effectively titrated — or adjusted — in response to serial BNP levels.