BNP: The blood test that detects heart failure

Today I am going to talk about a really interesting blood test that detects weakness of the heart muscle (heart failure). This test is called BNP.  BNP stands for brain natriuretic peptide and this test is already being used commonly in medical practice.

Before I talk about this BNP, I wanted to start off by talking about heart failure.  In heart failure, the fundamental problem is that the heart is unable to pump blood to match the body’s requirements. The common symptoms therefore include breathlessness, fatigue, ankle swelling and exercise intolerance. Patients with heart failure can’t push themselves as much as normal people and therefore have a much more limited exercise capacity. It is also worth understanding that in the early stages, the heart failure may not cause any symptoms at all and therefore can be very difficult to detect.

There are several problems with heart failure:

  1. People with a weak heart don’t, in general, live as long as people with a strong heart (but with treatment, we can definitely improve prognosis)
  1. People with a weak heart,in general, don’t have as good a quality of life as people with a strong heart (but with effective treatment we can definitely improve quality of life)
  1. We recognise that without treatment, the weakness of the heart can continue to worsen and therefore early detection and treatment is essential
  1. The symptoms can be quite non specific and don’t necessarily immediately point to the diagnosis (Breathlessness/ tiredness/leg swelling etc). They also don’t correlate with severity of heart failure.
  1. The gold standard test to make the diagnosis is an echocardiogram which is a specialist investigation, needs trained personnel and therefore not as easily available.

So in essence, Heart failure is an important, disabling and dangerous condition which can be treated quite effectively (if treated early) but the symptoms are non-specific and the gold standard test is not as easily or quickly available; and therefore the idea of a simple blood test to make the diagnosis is really very attractive. This is where the BNP blood test comes in.

In heart failure, there is difficulty in emptying of the heart and therefore the pressure within the heart is increased. In response to this increase in pressure, the atria and the ventricles secrete these chemicals called Natriuretic peptides. The atria secrete atrial natriuretic peptides and the ventricles secrete BNP and the role of these natriuretic peptides is to cause the kidneys to get rid of more salt and water and also open up all the blood vessels – the idea being that if the heart is congested, then opening up all the blood vessels and getting rid of salt and water from the circulatory system will reduce the congestion. Researchers started thinking that if they could measure these natriuretic peptides then this would allow them to work out which patients with these non-specific symptoms of breathlessness were actually more likely to have heart failure which would then allow the more appropriate patient to be referred for echocardiography. They, therefore, developed a blood test for BNP.

The BNP test is a simple blood test and costs about 20-30 pounds. The normal value in a healthy population is about 10 pmol/L. Patients with a BNP level of less than 100pg/mL are very unlikely to have heart failure. Patients with a BNP levels of > 400 pg/mL are significantly more likely to have heart failure and therefore should have an echocardiogram. People with BNP levels of between 100-400 may or may not have heart failure but the BNP could be explained by other causes some of which I will discuss later.

Since the blood test was developed we have made several interesting observations.

  1. People with heart failure in general have much higher BNP levels than patient who have normally functioning hearts. This is generally true even if the patient has minimal symptoms. A normal BNP makes it very unlikely (although not impossible) that the patient has heart failure. A raised BNP however does not always mean that the patient has heart failure but does mean that that patient should undergo further evaluation
  1. The level of BNP elevation can also point to prognosis – people who have very very high BNP levels and heart failure tend to have a poorer prognosis without treatment compared to patients with lower levels and heart failure
  1. The level of BNP can also be used to measure response to treatment and as the patient gets better and the heart gets stronger or stabilises the BNP levels can come down to reflect that.
  1. Perhaps the most interesting and exciting development  about BNP is that scientists have now realised that these natriuretic peptides may be the body’s own way of trying to heal the heart and therefore if there was some way of either manufacturing these chemicals or stopping your intrinsic BNP from breaking down then perhaps one could improve the prognosis from heart failure and actually this hypothesis has been proved because we now have a new medication called Entresto which works in part by stopping the breakdown of these natriuretic peptides and has been shown to be very effective at reducing mortality in heart failure.

It is also worth knowing a few other facts:

There are lots of other situations in which BNP can be elevated and therefore having an elevated BNP does not automatically mean you have heart failure. Some of the other conditions include:

Kidney disease
Lung disease
Liver disease
Heart rhythm disorders (like Afib)
Infections
Burns
Chemotherapy
Hypertension

I guess the reason for me doing this video is to make people aware of this blood test which is commonly being done in general practice and to make them understand the strength of test as well as the weaknesses of the test. A normal test is extremely reassuring but an abnormal test simply requires further investigation ideally in the form of a cardiology review and an echocardiogram.

I think the measurement and propagation of the natriuretic peptides may hold the key to the diagnosis and treatment of many other cardiac conditions and this will become increasingly apparent as more research and learning accrues.

I hope you found this post informative and I would love to hear your thoughts and comments.

Here is the link to a youtube video I have done on this subject.

 

About the Author:

Dr Sanjay Gupta
I'm Dr Sanjay Gupta, a Consultant Cardiologist with specialist interest in Cardiac Imaging at York Teaching Hospital in York, UK. I believe that high quality reliable jargon-free information about health should be available at no cost to everyone in the world.

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