Hypertrophic cardiomyopathy (HCM) is a well-recognized and relatively common clinical entity in both cats and humans. Clinical factors associated with poor prognosis in cats with HCM include arterial thromboembolism, congestive heart failure (CHF), increased left atrial size, decreased left atrial and left ventricular systolic function, and left ventricular hypertrophy > 9 mm. Cardiac biomarkers such as N-terminal B-type natriuretic peptide (NT proBNP) and cardiac troponin I (cTnI) have been found to be useful in diagnosing occult and symptomatic feline HCM. They have potential utility as relatively inexpensive and possibly early predictors of cardiac death in cats with HCM. In this prospective investigational study of 41 cats diagnosed with HCM at a veterinary teaching hospital between February 2010 and May 2011, these researchers hypothesized that circulating concentrations of NT proBNP, cTnI, or both would predict cardiac death in cats of any breed with HCM.
Criteria for study inclusion were a diagnosis of HCM on 2-dimensional echocardiography performed by a board-certified veterinary cardiologist or resident under supervision, as well as a plasma sample taken on the same day. Cats with any diagnosis other than HCM, chronic kidney disease, any other possible cause of left ventricular hypertrophy, and cats > 8 years old without a recorded blood pressure or total serum thyroxine (T4) concentration, were excluded. The endpoint of the study was death either from cardiac or noncardiac causes; owners and primary care veterinarians were contacted to obtain this information.
At the end of the study in late 2013, 21 cats had died or been euthanized due to cardiac disease, while 20 were either still alive or had died or been euthanized due to noncardiac causes. Median survival time (MST) to cardiac death was > 946 days. Cats with a cTnI > 0.7 ng/mL had a significantly MST of 40 days. Those with NT proBNP >250 pmol/L also had a significantly shorter MST of 764 days. No additional information regarding survival time was obtained by combining NT proBNP > 250 pmol/L and cTnI > 0.7 ng/mL when compared to a measurement of cTnI > 0.7 ng/mL alone.
The data were used to devise two models that would evaluate the biomarkers as predictors of time to cardiac death in multivariable analysis. The first model evaluated the prognostic usefulness of the biomarkers in patients where the presence or absence of CHF is confirmed but echocardiography is unavailable. This is a common clinical scenario, especially in general and emergency practice, wherein a cat is presented with clinical signs of CHF but has not undergone an echocardiogram for a variety of reasons. In this model, cTnI provided independent predictive value of significantly shorter survival time to death due to cardiac disease, but NT proBNP did not provide additional prognostic value. In the second model, all available data: presence or absence of CHF, echocardiographic measurements, and the biomarkers, were included. In this model, NT proBNP was again not predictive of shorter survival time to death from cardiac disease, but cTnI was still useful in this regard.
In patients with regional left ventricular hypokinesis identified by 2-dimensional echocardiography, cTnI lost its significance as a predictor of shorter survival time to cardiac death. This suggests a close association between left ventricular regional hypokinesis, also predictive of a shorter survival time in cats with HCM, and elevations in cTnI. Both left ventricular regional hypokinesis and elevated cTnI are indicators of myocardial cellular damage, and either can provide additional prognostic information independent of the presence of CHF or left atrial dilatation. As echocardiography may not be available to all patients and clients for a variety of reasons, especially in general practice situations, a cTnI assay may be a more accessible, and if made commercially available, more affordable, prognostic indicator for cats in CHF or with suspected HCM. [PJS)