Heart disease and respiratory distress due to heart failure can be a challenge to diagnose in cats. Unfortunately, cats with heart murmurs may have completely normal hearts and cats without heart murmurs could very possibly have significant heart disease.² Furthermore, chest radiographs can show various combinations of bronchial, interstitial and alveolar disease with heart disease in cats (not to mention pleural effusion!) and may not be definitive for the diagnosis of heart failure.² Recent data evaluating the vertebral heart score (VHS) has shown some promise in determining if heart disease is the cause of dyspnea in cats. A normal VHS in cats has been previously identified as <8.0 with a mean around 7.3-7.5.² A recent manuscript, however, evaluated the VHS of cats in respiratory distress with the specific goal of finding a cutoff point over which a cat's respiratory disease was definitively due to heart disease. The authors found that a cutoff point of 8.0 was very sensitive for the diagnosis of heart disease as the cause of dyspnea and a cutoff point of 9.3 was very specific for heart disease.² In other words, if a cat has a VHS< 8.0, it likely is dyspneic due to a disease process other than heart disease and if the VHS is >9.3, you can be certain that the dyspnea is due to heart disease. For those cases with VHS' between 8.0 and 9.3, further testing such as echocardiography, is necessary to definitively determine if congestive heart failure is present.²

The natriuretic peptides are a group of structurally related proteins which have been evaluated extensively in experimental, human clinical and veterinary clinical subjects as indicators of heart disease as well as markers in a myriad of respiratory, central nervous system and inflammatory diseases.^a There are multiple natriuretic peptides with various physiologic functions in the body. All natriuretic peptides are synthesized as preprohormones and are rapidly processed to form prohormones.a Prohormones are further cleaved into two parts creating the biologically inert amino-terminal prohormone (eg.NT-proBNP) and the biologically active C-terminal hormone (eg. C-BNP). Once in circulation, the active hormone is degraded into fragments. The amino-terminal prohormones have a longer half-life than the active hormones, and, in general, these have been found to be more useful to measure clinically as the timing of sampling to capture peak hormone levels is less pivotal.a Amino terminal-proBNP is the most extensively studied natriuretic peptide in veterinary medicine and has been looked at in both dogs and cats as an indicator of whether or not heart disease is the cause of dyspnea in animals with respiratory compromise. Amino terminal proBNP levels were elevated in dogs with CHF over levels in dogs with other causes of respiratory distress including pneumonia, pulmonary neoplasia, neoplastic pleural effusion, laryngeal paralysis, chronic bronchitis, collapsing trachea, lung lobe torsion, non-cardiogenic pulmonary edema, and eosinophilic bronchopneumopathy.^a In addition, NT-proBNP has been found to be significantly higher in dogs with CHF compared to those with primary pulmonary disease and concurrent asymptomatic heart disease.^a,3 It was found that a NT-proBNP value greater than 1158 pmol/L was 85.5% sensitive and 81.3% specific for the diagnosis of CHF in dogs.³ Similarly, NT-proBNP has been found to be significantly higher in cats with CHF compared to those with non-cardiac related respiratory distress.^4,5 Different researchers found NT-proBNP values in the range of 220-265pmol/L and higher were relatively sensitive and specific for the diagnosis of CHF as the cause of dyspnea in cats with respiratory distress.^4,5 It is important to note that there are cases which do not read the book and have elevations of NT-proBNP suggestive of CHF when heart disease, in fact, is not present.   As clinicians, we must remember to look at the whole picture and take each diagnostic result as part of the puzzle rather than the entire picture. Amino terminal-proBNP is a diagnostic test available through IDEXXb which has been validated in dogs and cats and has a turnaround time of 1-2 business days. Sample handling can drastically affect the measurement and special sampling tubes that contain a protease inhibitor to minimize NT-proBNP degradation in plasma are supplied by IDEXX. IDEXX is currently undertaking a multi-center, prospective study to evaluate a point-of-care test for NT-proBNP in cats. 

Some initial veterinary investigation of NT- proCNP indicates that this marker may be helpful in distinguishing infection (bacterial or fungal) from non-infectious inflammation.⁶ A recent study found NT-proCNP elevations to be sensitive and specific for an underlying septic disease process as the cause of the systemic inflammatory response syndrome in dogs with the exception of peritoneal infections.⁶ Although not specifically looked at in respiratory disease, this early research suggests that NT-proCNP may one day help us distinguish infectious pneumonia from other causes of pulmonary infiltrates or non-infectious pneumonitis. Currently, however, NT-proCNP analysis is not readily available to the majority of veterinary practitioners.

The use of high resolution computed tomography to diagnose respiratory disease in veterinary medicine has increased dramatically over the last ten years. It is now considered standard of care in critical care by experts in the field of small animal respiratory disease.c Computed tomography results in cross sectional images of slices of variable (1-10mm) thickness of the lungs. Helical CT scanners have increased the speed at which images are obtained, minimizing motion artifact from the heart and respirations. High definition CT is comprise of thin (1-2mm) slices that allows for fine detail and markedly increased sensitivity for parenchymal and vascular changes one can't detect using plain radiography.^c,7 Recent studies have confirmed that CT is more sensitive for the detection of metastasis than plan radiography.^8-10 High resolution CT with angiography can be used to detect pulmonary thromboembolism, a pathologic process which is thought to be involved in clinical respiratory disease more often than we
previously thought due to limitations in diagnosis.^c,d A recent preliminary study (not yet published) demonstrated the use of the pulmonary artery in comparison to the aorta and bronchial lumen to detect pulmonary hypertension.^c Figures 2 and 3 depict thoracic radiographs and CT respectively of a case seen recently at CCVS. The dog had progressive, severe dyspnea with relatively underwhelming radiographs. The CT demonstrated nodular interstitial lung disease which was diagnosed as pulmonary carcinoma on endotracheal wash.

The requirement of absolute stillness to render images without motion artifact has dictated that CT scans have traditionally been performed under general anesthesia in veterinary medicine. Some animals can be scanned with heavy sedation and those with significant intracranial disease may even be scanned awake.c The recent development of a chamber, the "mousetrap", by a group at the University of Illinois has started to standardize a method of restraining cats for CT (see figure 4).

The area of diagnostics for respiratory disease in veterinary medicine is one of intense research right now. Hopefully this research will help us to determine the most sensitive and specific modalities as well as develop bed side tests to help categorize disease processes in the emergent setting.