It's not common for a new diagnostic test to have an immediate impact on clinical practice, but BNP (B-type natriuretic peptide) is just such a test. Best of all, it's a simple, safe blood test that can help doctors evaluate complex cardiac functions.
What is BNP?
BNP belongs to a family of protein hormones called natriuretic peptides. Each member of the group is produced by a different part of the circulatory system. These natriuretic peptides have an important role in regulating the circulation. Scientists have learned the most about ANP and BNP. Both act on blood vessels, causing them to dilate, or widen. They also work on the kidneys, causing them to excrete more salt and water.
The net effect of natriuretic peptides is to promote urine excretion, relax blood vessels, lower blood pressure, and reduce the heart's workload. They are part of the body's natural defense mechanisms designed to protect the heart from stress. And they surge into action when they are needed most, when the heart itself is under siege.
BNP in CHF
It sounds like alphabet soup, but it's actually modern cardiology at its best. BNP helps the body compensate for congestive heart failure (CHF); measurements of BNP help doctors diagnose and treat this serious condition.
CHF results when the heart muscle is weakened. The most common causes are coronary artery disease and hypertension.
The heart's job is to collect blood from the veins, then pump it through the arteries to all the body's tissues. In CHF, the weakened pump is not up to the task; the tissues don't get all the oxygen-rich blood they need, and blood backs up in the lungs and then the veins.
About five million Americans have CHF, and more than a half-million more join the ranks each year. It's a very serious condition, yet treatments have produced major advances. But to treat CHF, you first have to diagnose it — and that's where BNP fits in.
Doctors can usually diagnose advanced CHF on clinical grounds, confirmed by simple studies like chest x-rays, EKGs, and routine blood tests. But milder CHF can be tricky to recognize, and various lung diseases, liver diseases, and kidney diseases can mimic CHF. So when doctors suspect CHF, they usually order an echocardiogram to confirm the diagnosis and assess its severity. Echocardiograms are safe, but they are relatively expensive and may be hard to come by in some communities. While the BNP test will never replace echocardiography, it has already helped doctors reserve the test for patients who really need it.
In CHF, the heart chambers are dilated, or enlarged. Heart muscle cells are stretched as the chambers swell with extra blood that can't be pumped out efficiently. The stretched muscle cells produce extra BNP, which pours into the bloodstream. And doctors now have simple, accurate, inexpensive tests to measure BNP in blood samples. A bedside test can even yield diagnostic information right in the Emergency Department.
BNP is very helpful in diagnosing CHF. A normal BNP level is about 98% accurate in ruling out the diagnosis, freeing doctors to hunt for other conditions that may be causing shortness of breath or fluid retention.
BNP is very helpful in determining the outlook for patients with CHF. In general, the higher the level, the worse it is. Finally, BNP is very helpful in guiding the treatment of CHF. Effective therapy reduces the backup of blood in the heart. The heart chambers get smaller, and as the muscle cells recover from being stretched, they produce less BNP. When doctors see falling BNP levels, they breathe a sigh of relief; more important, their patients breathe better.
BNP in CAD
The cardiologists are at it again, this time exploring the role of BNP in coronary artery disease (CAD). Patients who have CAD without CHF don't have stretched heart muscle cells, but if their coronary artery blockages are extensive or if the vascular inflammation in the plaques is active, the muscle cells will be ischemic — that is, they won't be getting enough oxygen to keep them happy. Like stretched muscle cells, ischemic cells make and release extra BNP, though in smaller amounts than with CHF. Research suggests that this simple test may soon help doctors tell which patients need fancy tests and which don't. And the test has already improved the diagnosis and treatment of CHF. That's a big gain for a small protein.February 2007 update