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The feline cardiac physical exam can be a tricky entity. While the intensity of a murmur in a dog often correlates with the severity of heart disease, cats don’t always play by the same rules. We commonly see benign murmurs in cats, and on the flip side, some cats with very severe cardiac disease may have a normal auscultation. In these scenarios it is helpful to have a screening tool that can help us catch some of the patients with significant heart disease that might otherwise fly under the radar based on a physical exam alone. While there is certainly no perfect screening test, a good place to start is the NT-proBNP.

In simplistic terms NT-proBNP is a peptide that is released into the bloodstream in response to stimuli such as stretch of the heart muscle. This allows us to use its concentration as a biomarker for cardiac disease. Currently, we have two options when evaluating the NT-proBNP, a quantitative test available for both dogs and cats, and a point of care “snap” test which is available only for use in cats.  A quantitative NT-proBNP will always give us the most information, and in most situations, this is the test I recommend. If added to annual bloodwork this value can be trended over time for an individual patient, and a significant change in the value would signal a need for that patient to be evaluated further. This can aid in earlier detection of heart disease in our feline patient population.

The Idexx point of care “snap” NT-proBNP is also very useful in the correct setting. A positive result on this test indicates the NT-proBNP concentration is > 150pmol/L. While this does not replace the value of a quantitative measurement of NT-proBNP, it can give immediate information in an emergency setting. I find the most useful scenario for this test is in trying to differentiate the cardiac vs non-cardiac causes of respiratory distress in the cat. In this situation, a negative result on the NT-proBNP snap test would indicate that congestive heart failure is unlikely to be the cause for respiratory distress. However, it is important to note that the opposite is not always true. For example, a cat with respiratory distress due to asthma may also have enough underlying cardiac disease to result in an abnormal snap result.  Therefore, when a positive result is obtained, further investigation is still needed. All in all, while a positive result may not be quite as helpful on its own, a negative result can be extremely helpful information in the right situation. In patients with a positive NT-proBNP snap test, a follow up quantitative test is always a good next step.

Lastly, we know that there are certain non-cardiac disease processes that may affect the NT-proBNP levels. The one we encounter most commonly is that significant renal disease may cause the NT-proBNP level to rise. This occurs as patients with a significant reduction in their GFR will have decreased clearance of the NT-proBNP, causing the levels in the blood to rise. Therefore, mild NT-proBNP elevations in patients with significant renal disease should be interpreted with this in mind. Significant systemic hypertension, pulmonary hypertension and likely sepsis may also affect the NT-proBNP results.

In summary, while often overlooked, the NT-proBNP can be your friend in clinical practice, especially in our tricky feline patients!

Katie Lopez DVM, DACVIM (Cardiology)