cTnI = cardiac troponin I; NT-proBNP = N-terminal pro–B-type natriuretic peptide
Hypertrophic Cardiomyopathy
Hypertrophic cardiomyopathy is another important cause of an elevated cTnI. Several studies have shown that cTnI is elevated in both symptomatic and asymptomatic hypertrophic cardiomyopathy.7,8 One study evaluated the utility of cTnI in distinguishing congestive heart failure from respiratory disease in cats presented for dyspnea.7 Unfortunately, although cTnI was indeed higher in the cats with heart failure, levels were also somewhat elevated in cats with respiratory disease. As a result, cTnI is not as useful as once hoped. NT-proBNP appears to be a better test to distinguish between congestive heart failure and noncardiac causes of respiratory signs in both dogs and cats.
Myocardial Infarction & Ischemic Injury
In humans, the most common cause of an increased cTnI is myocardial infarction or acute coronary syndromes. This biomarker has become the new gold standard for diagnosis of myocardial infarction. Coronary artery disease and ischemic injury to the heart are not common in the dog (unlike in humans); however, the measurement of cTnI in various canine heart diseases other than myocarditis and suspected neoplastic pericardial effusion has helped better define the pathophysiology and diagnose certain types of heart disease.
For example, in Oyama and Sisson’s study of dogs with various types and severities of heart disease, approximately 50% of dogs had some elevation in cTnI.6 This finding indicates that although ischemia was not the primary myocardial insult, there appeared to be low-grade, yet appreciable, myocardial death and remodeling. cTnI has not proven to be a good test to detect occult cardiomyopathies in dogs but may help predict clinical outcome.
Gastric Dilatation-Volvulus
Another cause of an elevated cTnI in dogs is gastric dilatation-volvulus, suggesting that myocardial injury from ischemia and reperfusion is the primary mechanism of the arrhythmias common in postoperative dogs with this condition.3 Schober showed that the severity and frequency of the arrhythmias present in postoperative dogs with gastric dilatation-volvulus correlated well to cTnI concentrations.
Cardiotoxicity
Serial monitoring of cTn1 may also help detect cardiotoxicity in dogs undergoing high-dose doxorubicin administration. Doxorubicin cardiotoxicity causes multifocal necrosis of the heart that is more common with higher cumulative doses. This cardiotoxicity usually manifests as diminished left ventricular systolic function and arrhythmias. Other forms of toxic insults to the heart will cause elevations in cTnI, such as myocardial injury due to rattlesnake envenomation, oleander toxicity, and blunt thoracic trauma.
See Table 1 for a list of causes.
Causes of Increased NT-proBNP
Plasma NT-proBNP concentrations are elevated in dogs and cats with congestive heart failure and those with asymptomatic heart disease (Figure 2, page 18). Several studies have shown plasma BNP or NT-proBNP to be sensitive and specific for the diagnosis of heart failure in dogs and cats presenting with cough or dyspnea (Table 1).9–13
cTnI = cardiac troponin I; NT-proBNP = N-terminal pro–B-type natriuretic peptide
Severity of Disease
The magnitude of NT-proBNP increase typically correlates with the severity of disease, which suggests its possible utility as an objective tool to predict clinical outcome. Many studies are investigating the best cutoff values between normal, mild, or moderate heart disease and severe heart disease with congestive heart failure. These cutoff values are quite different in the dog than in the cat.
Figure 2: Lateral thoracic radiograph from a 10-year-old castrated male domestic shorthair cat with vague clinical signs of lethargy and loss of appetite of 1 day’s duration. No murmur or gallop was noted. The only physical examination abnormality was an elevated respiratory rate (60 breaths/min). Thoracic radiographs were suspicious for congestive heart failure, but the NT-proBNP level (1126 pmol/L) added confidence to the diagnosis.