Cardiac biomarkers in the cat

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.

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.

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.

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 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.

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.

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.

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.