A hormone the stretched heart releases
Unlike troponin, which leaks from injured cells, the natriuretic peptides are deliberately made and released by heart-muscle cells under strain. When the left ventricle is stretched — because it is handling too much pressure or volume, as in heart failure — the muscle senses the tension on its walls and secretes B-type natriuretic peptide. Its job is helpful: it tells the kidneys to shed salt and water and it relaxes blood vessels, both of which lower the load on the heart. The peptide is, in effect, the heart calling for relief.
The cell makes a long precursor and snips it into two pieces: the active hormone BNP and an inactive fragment, NT-proBNP. Both rise together when the heart is stretched, so a lab can measure either one. NT-proBNP lingers longer in the blood and gives higher numbers; BNP is the active molecule. The choice between them is mostly about which assay a hospital runs — the interpretation is the same: higher means more wall stress.
Telling cardiac breathlessness apart
The classic use is a patient who arrives short of breath. Is the breathlessness coming from the heart or the lungs? A lung problem like asthma or pneumonia usually leaves natriuretic peptides near normal, while heart failure with fluid overload pushes them high. So the test is powerful in two directions. A low value makes heart failure unlikely and is reassuring; a high value strongly supports it. Values in the middle are a grey zone calling for more information.
What pushes the number up or down
Like any biomarker, natriuretic peptides have honest quirks. Several non-cardiac factors shift the number, and knowing them keeps interpretation grounded.
- Rises with age and in women — older people run higher baselines, so thresholds are adjusted by age.
- Rises with kidney impairment — failing kidneys clear the peptide more slowly, lifting the level independent of the heart.
- Lower in obesity — heavier patients tend to have lower readings for the same degree of strain, so a “normal” value can mislead.
- The drug sacubitril-valsartan raises BNP but not NT-proBNP — a reason NT-proBNP is often preferred for monitoring.