Worldwide medical and scientific journal for animal health professionals
Veterinary Focus

Issue number 32.2 Cardiology

Cardiac biomarkers in the cat

Published 20/10/2022

Written by Clémence Peyron and Fanny Bernardin

Also available in Français , Deutsch , Italiano and Español

Recent advances have identified biomarkers that can aid in the diagnosis of feline heart disease; this paper discusses the uses – and limitations – of these tests. 

The TnI assay is useful in helping determine if a dyspneic cat has a primary cardiac lesion or not

Key points

NT-proBNP is a marker of cardiac muscle stretch and volume overload; as an indicator of myocardial function, it can assist in the diagnosis of cardiomyopathies.


Troponin I (TnI) is a marker of myocardial injury, particularly ischemic injury, and its assay can help refine the prognosis in cardiac cases, as the serum concentration level correlates strongly with the risk of death.


A qualitative NT-proBNP rapid test provides cage-side results when dealing with animals in respiratory distress, and can quickly identify or rule out a cardiac cause of dyspnea. 


The determination of these two biomarkers gives the clinician useful information, but does not replace imaging and other additional tests.


Introduction

NT-proBNP and troponin I are frequently cited in feline medicine as part of the panel of complementary examinations used by clinicians to investigate cardiac function or to search for possible myocardial lesions. With assay of these two biomarkers now widely available, this raises various questions, including:

  • When should they be measured?
  • Which one(s) should be chosen and why?
  • How do they relate to other examinations such as thoracic imaging?
  • How are the assay results interpreted?

 

Cardiac biomarkers

A biomarker is a substance which is assayed as part of the diagnostic procedure for specific conditions – in this case cardiac conditions. The ideal biomarker contributes to the screening, diagnosis, prognosis, and follow-up of different cardiomyopathies 1, and the assay should permit excellent sensitivity (i.e., minimal false negatives) to allow early detection of disease, as well as good specificity (i.e., minimal false positives) in order to maximize its reliability 2.

Troponin I

Troponins are proteins involved in the contraction and relaxation of cardiac and skeletal muscle. They are located in the myofibrils, along with actin and myosin, and exist in different isotypes (I, T, C) depending on the muscle in question (Figure 1). Troponin I (TnI) is only present in heart muscle, which gives it 100% specificity, and is regarded as a marker of ischemic myocardial injury. Plasma concentrations of TnI increase in the hours following a cardiac lesion, and peak on the first day, although values can remain high for a week afterwards 3,4.

Conformation of the different proteins

Figure 1. Conformation of the different proteins (actin, myosin, tropomyosin and troponins I, T and C), at the origin of the sliding movement of the filaments within the sarcomere in the cardiomyocyte. Credit: Clémence Peyron/Redrawn by Sandrine Fontègne

NT-proBNP

Natriuretic Peptides (NPs) are a family of hormones that regulate intravascular volume and control blood pressure. They are secreted by cardiomyocytes in response to abnormal cardiomyocyte stretch and volume and/or pressure overload, and they are also produced by other tissues. Their main effect is to counteract the action of the renin-angiotensin-aldosterone system, which is hyperactive in heart disease and responsible for cardiac remodeling. As the name indicates, NPs cause natriuresis, and they also influence diuresis, vasodilation, diastole, and vascular permeability, and inhibit multiplication of smooth muscle cells.

Brain Natriuretic Peptide, or BNP, was first isolated from porcine brain, but it is also secreted by the heart ventricles. In pathological conditions its serum levels increase, and it is therefore considered a marker of cardiac function. BNP is initially secreted as a precursor molecule, which is cleaved by serum proteases to form C-BNP, the active form, and NT-proBNP, the inactive form. C-BNP has a very short half-life (90 seconds), hence the use of NT-proBNP, which has greater stability during sampling and storage 2, and a half-life of around 120 minutes.

Biomarkers and feline cardiomyopathies

The term cardiomyopathy refers to a myocardial disorder characterized by one or more structural and/or functional abnormalities of the heart muscle, the most frequently described condition in cats being hypertrophic cardiomyopathy (HCM). This is the condition most often addressed in the literature and in studies that consider the role of biomarkers in cardiac disease; to date, their role in other forms of cardiomyopathies, including restrictive, dilated and non-specific cardiomyopathies, have been studied in far less detail 5.

Investigation of cardiomyopathies requires various tests, some of which are specialist in nature (e.g., echocardiography and electrocardiography). Tests that can be performed in general practice include NT-proBNP, troponin I, chest radiographs and systemic blood pressure measurement. Managing the patient and prioritizing the various complementary examinations will depend on the individual situation (e.g., a cat that simply requires screening for possible heart disease or the patient in respiratory distress). It is essential to recognize that NT-proBNP and TnI assays sometimes have overlapping applications, and it is important to know what each one assesses in order to best target the diagnostic approach (Table 1).

Table 1. A summary of the different uses of NT-proBNP and troponin I in the feline species.

Quantitative NT-proBNP
Qualitative NT-proBNP
(rapid SNAP® test)
Troponin I
  • Discrimination between cardiac vs. non-cardiac origin in dyspneic cats +/- pleural effusion – using plasma or pleural effusion.
  • Discrimination between subclinical heart disease vs. healthy cat: to be considered if echocardiography not feasible. Additional examinations justified if result is abnormal. Does not exclude cardiac disease if the result is within the normal reference interval.
  • Discrimination between occult HCM vs. healthy cat. 
  • Discrimination between obstructive vs. non obstructive HCM.
  • Discrimination between HCM cats with CKD and hypertension vs. CKD cats without hypertension.
  • Discrimination between cat with advanced CKD vs. early/intermediate CKD.
  • Discrimination between cardiac vs. non-cardiac origin in dyspneic cats +/- pleural effusion – using plasma or pleural effusion, preferably diluted.
  • Discrimination between advanced subclinical heart disease vs. healthy cat or cat with intermediate subclinical heart disease – sensitivity is better if a heart murmur is present. Additional examinations justified if result is abnormal. Does not exclude cardiac disease if the result is within the normal reference interval.
  • Discrimination between cardiac vs. non-cardiac origin in dyspneic cats +/- pleural effusion.
  • Prognostic value – association with risk of cardiac death.
  • Discrimination between HCM vs. healthy cat (screening).
  • Aids assessment of the clinical patient – levels increase with the severity of HCM (healthy cat < cat with occult HCM < cat with advanced HCM < cat with HCM and thromboembolism).
  • Discrimination between compensated vs. decompensated HCM.

 

Biomarkers for cats in respiratory distress

It is useful to quickly determine whether the cause of a cat’s respiratory distress is of cardiac origin or not, and can avoid disturbing an often-unstable patient unnecessarily. The question of feasibility and the benefit/risk ratio must be considered (Figure 2) before taking blood samples. However, if radiography is inadvisable because the patient is unstable, a rapid (cage-side) NT-proBNP test like the SNAP® Feline proBNP test (Figure 3) may be considered, coupled (if possible) with a thoracic ultrasound scan. A negative result will suggest, with reasonable reliability, that the dyspnea is most probably non-cardiac in origin 5.

It may be essential to stabilize the feline cardiac patient before considering additional examinations

Figure 2. It may be essential to stabilize the feline cardiac patient before considering additional examinations, especially where there is respiratory distress; this can involve aspects such as minimal handling and use of an oxygen cage. 
Credit: Fanny Bernardin

Studies in cats presented with respiratory distress and pleural effusion showed that the SNAP® Feline proBNP had a sensitivity of 93.9-100% and a specificity of 72.2-87.5% for diagnosing congestive heart failure as the cause of dyspnea using a blood sample 6,7,8. The test has also been performed using pleural effusion samples with very good sensitivity (i.e., few false negatives) but variable specificity due to too many false positives 6. Another study showed better results when the pleural sample was diluted by half with 0.9% NaCl, giving a final sensitivity of 100% and a much better specificity of 85.7% 8. It should be noted, however, that the use of the cage-side SNAP® Feline ProBNP test is only officially validated on serum or EDTA plasma.

The SNAP Feline proBNP uses an ELISA (enzyme-linked immunosorbent assay) technique to detect normal or abnormally raised levels of proBNP in serum or EDTA plasma

Figure 3. The SNAP® Feline proBNP uses an ELISA (enzyme-linked immunosorbent assay) technique to detect normal or abnormally raised levels of proBNP in serum or EDTA plasma. It includes a bidirectional flow method that improves sensitivity (as the sample gets two chances to bind to the antibodies) and a wash step to maximize specificity by removing debris from the result window. The test kit is stored in a refrigerator, but all components should sit for 30 minutes at room temperature before running the test. The result can be read 10 minutes after completing the procedure. 
Credit: Laboratoire Idexx

The main drawback of the quantitative NT-proBNP measurement assay is the time needed to send the sample to the laboratory and to obtain results, which can limit its use in an emergency situation. Nevertheless, it is a reliable test to differentiate between cardiac and non-cardiac respiratory distress 5. The various studies conducted report sensitivities of 86.4-100% and specificities ranging from 82.4-88.9% on blood samples. The sensitivities reported on undiluted pleural effusion are all 100% (no false negatives) with a specificity of 76.5-94.4%, depending on the threshold value used 7,8,9.

The TnI assay can also help differentiate between a patient with cardiac respiratory distress and a patient with respiratory distress from other causes (Figure 4). In fact, cats with dyspnea resulting from a cardiomyopathy have significantly higher blood TnI concentrations than healthy cats or cats where the dyspnea is not cardiac in origin 5,10. However, as with the quantitative NT-proBNP assay, this analysis is usually performed in an external laboratory, which delays the results and can slow the diagnostic process, which can be problematic for patients requiring urgent care.

The TnI assay is useful in helping determine if a dyspneic cat has a primary cardiac lesion or not

Figure 4. The TnI assay is useful in helping determine if a dyspneic cat has a primary cardiac lesion or not.
Credit: Shutterstock

Biomarkers for suspected subclinical cardiomyopathy

Subclinical cardiomyopathy is defined as any form of cardiomyopathy that does not result in detectable clinical signs in the patient. It should be remembered that in some cases a cat may have an advanced cardiomyopathy before signs appear, and that affected cats can remain asymptomatic for a long time before suddenly deteriorating. In most cases the appearance of respiratory distress will prompt a consultation and subsequent investigations. However, in certain individuals at risk, or where auscultation may suggest an abnormality, early detection of a cardiomyopathy enables appropriate treatment which can delay the onset of decompensation. Therefore, non-invasive techniques that can be carried out in general practice are very useful, using biomarkers such as NT-proBNP or TnI in the first instance.

The rapid qualitative point-of-care tests are of less useful in situations where the objective is a screen to determine if the patient can be considered healthy or as having subclinical cardiomyopathy. In a subclinical setting the main value of biomarker tests is to differentiate between cats with advanced cardiomyopathy and cats that are healthy or that have mild cardiomyopathy 5. In one study of apparently healthy cats coming for screening, the SNAP® Feline proBNP showed a sensitivity of 43% (i.e., many false negatives) and a specificity of 96% (few false positives). Sensitivity increased to 71%, with a specificity of 92%, when only asymptomatic cats with a heart murmur were included. These observations suggest that a positive SNAP® is likely to be associated with cardiomyopathy, warranting further investigation such as echocardiography, whereas a negative result does not exclude the presence of cardiomyopathy 11.

Clémence Peyron

A negative SNAP® test will suggest, with reasonable reliability, that the dyspnea is most probably non-cardiac in origin.

Clémence Peyron

Quantitative NT-proBNP measurement is useful in investigating potential cardiomyopathy, especially when echocardiography is not available. However, this test is not recommended if the objective is to differentiate between a healthy cat and a cat with early stage HCM. Although plasma NT-proBNP concentration increases with the severity of cardiac injury, the overlap in values between different groups does not allow grading of cardiomyopathies (mild, moderate, severe) on the basis of NT-proBNP results alone 5. In addition, a result that is within the usual reference interval is not sufficient to conclude that the patient is healthy, especially when early-stage cardiomyopathy is present, nor does such a result indicate that the patient will never develop cardiomyopathy. If there is a strong clinical suspicion of disease a “normal” result does not obviate the need for other complementary examinations, such as echocardiography. On the other hand, an abnormal result should always prompt further investigation 5. However, one study showed that quantitative NT-proBNP measurement could distinguish cats with occult HCM from healthy cats and cats with obstructive vs. non-obstructive HCM. The same study demonstrated a correlation between NT-proBNP and TnI, as well as a correlation between NT-proBNP and various echocardiographic severity markers 12.

The TnI assay can be considered for detection of cats with subclinical HCM provided that an assay with very high sensitivity is used 5. Affected cats have a higher serum TnI concentration than healthy cats, and this concentration correlates with the measured left ventricular free wall thickness. Interestingly, serum TnI concentration differs significantly between different groups (healthy cats, mild HCM, moderate HCM, severe HCM, presence or absence of arterial thrombus, decompensated vs. compensated stage, etc.) and increases with the severity of HCM 13,14. One study identified that a threshold TnI value of 0.06 ng/mL could differentiate healthy cats from cats with subclinical HCM, with a sensitivity of 87.8% and a specificity of 95.4%. Sensitivity reaches 100% (with the same specificity of 95.4%) when using this threshold to differentiate healthy cats from cats with advanced subclinical HCM 15. Note, however, that the reported sensitivity and specificity vary according to the studies, their objectives, and the threshold value used.

Biomarkers for monitoring feline cardiomyopathies

Few studies exist on the use of these two biomarkers in the follow-up of feline cardiomyopathies, and their use is described mostly as one of the steps in the diagnostic process. TnI has been described as having a prognostic value, with increased plasma concentrations being associated with an increased risk of cardiovascular death, particularly in cats with HCM, regardless of the presence of congestive heart failure or left atrial dilatation 5,16,17. However, regular measurements of TnI are of only moderate value in follow-up, and provide little additional prognostic information 17.

Fanny Bernardin

TnI has been described as having a prognostic value, with raised plasma concentrations being associated with an increased risk of cardiovascular death, particularly in cats with HCM.

Fanny Bernardin

Biomarkers in non-cardiac conditions

Because TnI is eliminated via the kidneys, it has been shown that cats with chronic kidney disease (CKD) can have elevated TnI values even in the absence of a cardiomyopathy 15,18. Similarly, severe respiratory or renal impairment may result in elevated plasma NT-proBNP concentration, thereby increasing the probability of false positives 19. In cats with CKD, NT-proBNP concentrations are higher in hypertensive cats compared with healthy cats or CKD cats without an associated hypertension. Similarly, NT-proBNP concentrations are higher in cats with advanced CKD compared to other stages. It would therefore appear that NT-proBNP is a potential marker of elevated systemic blood pressure. However, in normotensive cats with mild to moderately advanced CKD, NT-proBNP concentrations were not significantly different from those of healthy cats.

TnI has been studied in dogs for its prognostic value in critical situations requiring intensive care such as gastric torsion, systemic inflammatory response syndrome (SIRS), multi-organ failure, or piroplasmosis (with detection of secondary myocarditis). However similar studies and data are still lacking in cats, especially for conditions such as SIRS, toxin exposure, hypoxia, anemia, and hyperthermia 4.

Conclusion

NT-proBNP and TnI are an integral part of the set of complementary examinations proposed for the exploration of feline cardiac function and possible lesions. It is important not to jump to hasty conclusions when the results are obtained, but rather to consider them as pieces of a puzzle and systematically associate them as soon as possible with other additional tests, the first of which is echocardiography. Performing complementary tests, and correlating the results with the clinical findings of physical examination and the medical history, are likely to guide an informed diagnosis. 

References

  1. Bomassi E. L’intérêt des marqueurs biologiques en cardiologie : le NT-proBNP et la Troponine I. Prat. Vet. 2009;44:647-649.

  2. de Lima GV, Ferreira FS. N-terminal-pro brain natriuretic peptides in dogs and cats: A technical and clinical review. Vet. World 2017;10(9):1072-1082.

  3. Linde A. Biochemical cardiac markers – Cardiac troponin I. In Proceedings. WSAVA World Congress 2005.

  4. Langhorn R, Willesen JL. Cardiac troponins in dogs and cats. J. Vet. Intern. Med. 2016;30:36-50.

  5. Luis Fuentes V, Abbott J, Chetboul V, et al. ACVIM consensus statement guidelines for the classification, diagnosis, and management of cardiomyopathies in cats. J. Vet. Intern. Med. 2020;34:1062-1077.

  6. Ward JL, Lisciandro GR, Ware WA, et al. Evaluation of point-of-care thoracic ultrasound and NT-proBNP for the diagnosis of congestive heart failure in cats with respiratory distress. J. Vet. Intern. Med. 2018;32:1530-1540.

  7. Hezzell MJ, Rush JE, Humm K, et al. Differentiation of cardiac from non-cardiac pleural effusions in cats using second-generation quantitative and point-of-care NT-proBNP measurements. J. Vet. Intern. Med. 2016;30:536-542.

  8. Wurtinger G, Henrich E, Hildebrandt N, et al. Assessment of a bedside test for N-terminal pro-B type natriuretic peptide (NT-proBNP) to differentiate cardiac from non-cardiac causes of pleural effusion in cats. BMC Vet. Res. 2017;13:394-400.

  9. Humm K, Hezzell MJ, Sargent J, et al. Differentiating between feline pleural effusions of cardiac and non-cardiac origin using pleural fluid NT-proBNP concentrations. J. Small Anim. Pract. 2013;54:656-661.

  10. Wells SM, Shofer FS, Walters PC, et al. Evaluation of blood cardiac troponin I concentrations obtained with a cage-side analyser to differentiate cats with cardiac and noncardiac causes of dyspnea. J. Am. Vet. Med. Assoc. 2014;244:425-430.

  11. Lu TL, Côté E, Kuo YW, et al. Point-of-care N-terminal pro B-type natriuretic peptide assay to screen apparently healthy cats for cardiac disease in general practice. J. Vet. Intern. Med. 2021;35:1663-1672.

  12. Fox PR, Rush JE, Reynolds CA, et al. Multicenter evaluation of plasma N-terminal probrain natriuretic peptide (NT-proBNP) as a biochemical screening test for asymptomatic (occult) cardiomyopathy in cats. J. Vet. Intern. Med. 2011;25:1010-1016. 

  13. Hori Y, Iguchi M, Heishima Y, et al. Diagnostic utility of cardiac troponin I in cats with hypertrophic cardiomyopathy. J. Vet. Intern. Med. 2018;32:922-929. 

  14. Herndon WE, Kittleson MD, Sanderson K, et al. Cardiac troponin I in feline hypertrophic cardiomyopathy. J. Vet. Intern. Med. 2002;16:558-564.

  15. Hertzsch S, Roos A, Wess G. Evaluation of a sensitive cardiac troponin I assay as a screening test for the diagnosis of hypertrophic cardiomyopathy in cats. J. Vet. Intern. Med. 2019;33:1242-1250.

  16. Borgeat K, Sherwood K, Payne JR, et al. Plasma cardiac troponin I concentration and cardiac death in cats with hypertrophic cardiomyopathy. J. Vet. Intern. Med. 2014;28:1731-1737.

  17. Langhorn R, Tarnow I, Willesen JL, et al. Cardiac troponin I and T as prognostic markers in cats with hypertrophic cardiomyopathy. J. Vet. Intern. Med. 2014;28:1485-1491.

  18. Langhorn R, Jessen LR, Kloster AS, et al. Cardiac troponin I in cats with compromised renal function. J. Feline Med. Surg. 2019;21:985-991.

  19. Borgeat K, Connolly D, Luis Fuentes V. Cardiac biomarkers in cats. J. Vet. Cardiol. 2015;Suppl 1:74-86.

Clémence Peyron

Clémence Peyron

Dr. Peyron graduated from the National Veterinary School of Lyon in 2007 and had a year of advanced training at Aquivet Veterinary Hospital near Bordeaux before completing a European College residency in Companion Animal Internal Medicine, finishing the program in 2013 Read more

Fanny Bernardin

Fanny Bernardin

Dr. Bernardin graduated from the National Veterinary School of Maison Alfort in 2007 and followed an internship in the USA before undertaking an internship in companion animal medicine and surgery at the University of Montreal Read more

Other articles in this issue

Issue number 32.2 Published 23/11/2022

Feline allergic asthma

Asthma is a common disease in cats, but it can mimic many other pathologies, and the diagnosis and treatment of the condition can be challenging, as this paper explains.

By Carol Reinero

Issue number 32.2 Published 16/11/2022

Feline mediastinal lymphoma

Lymphoma is one of the most common neoplasms of cats, and the mediastinal form can be a major differential for a cat that presents with respiratory signs, as James Elliott describes.

By James Elliott

Issue number 32.2 Published 09/11/2022

Feline systemic hypertension

Hypertension in the cat is now recognized as a significant contributor to ill health; Alice Rădulescu offers an overview of its underlying causes and the aids to diagnosis.

By Alice M. Rădulescu

Issue number 32.2 Published 03/11/2022

Management of the cat with heart failure

Cats with cardiomyopathies other than hypertrophic cardiomyopathy often first present with severe or even life-threatening clinical signs; this article focuses on the diagnosis of the underlying cardiac condition and the options for treatment.

By Joanna Dukes-McEwan and Catheryn Partington