Cardiac versus Respiratory Disease – A Diagnostic Challenge

Published Nov. 9, 2011

Pet owners like quick answers from their veterinarians. This is understandable, but unfortunately not always possible. If your dog or cat presents with some combination of weakness, lethargy, coughing, rapid breathing, and/or an increased respiratory effort, you might have to be patient when waiting for a diagnosis.

All of these signs are classic for some common types of heart disease. They are also exactly what you see with many conditions that adversely affect the respiratory tract. So, when a veterinarian is faced with an animal having some or all of these symptoms, the first thing he or she has to do is figure out which organ system is to blame.

Sometimes, a vet can practically make the diagnosis based on a pet’s signalment (i.e., age, sex and breed) and/or past medical history. An 8-year-old Cavalier King Charles Spaniel — it is heart disease until proven differently. A cat previously diagnosed with asthma — bet on a flare-up. But usually, veterinary cases are not so clear cut.

The first step is the physical exam. Finding a heart murmur, arrhythmia, weak pulses, or ascites (a build-up of fluid in the abdomen) will point your veterinarian in the direction of heart disease. Some types of abnormal lung sounds, like wheezes, are heard more frequently with primary respiratory disease. But, these findings are not fool-proof. For example, a pet may have a murmur that does not significantly affect cardiac function. In these cases, a vet can falsely assume that the murmur is related to the pet’s current condition and start down the wrong path.

So, it’s on to the diagnostic testing. Most doctors would want a fairly comprehensive database under these circumstances — probably a complete blood count, chemistry panel, urinalysis, fecal exam, heartworm testing (unless your pet is current on prevention and screening), perhaps a blood pressure check, and for cats, a thyroid level and FeLV/FIV test. This gives us a picture of the pet’s overall condition, the potential risks of using certain types of medications, and helps to rule out concurrent or underlying diseases.

Chest X-rays would also be included because they are a relatively inexpensive and easy way to get a peek at the heart and lungs. X-rays have some limitations, however. Radiographic images do a good job of revealing the overall shape of the heart (i.e., too big or with an abnormal bulge in a particular area). Where they come up short, though, is at looking inside the heart and assessing heart function. An electrocardiogram (EKG) can help fill in some of these gaps.

Seeing changes in the appearance of the lungs on an X-ray can be very useful. Certain patterns and their locations often correspond to particular diseases. For example, an alveolar pattern in the dorsal (i.e., upper) lung fields is typically seen with left-sided heart failure, while a similar pattern located lower down in the lungs often develops with bronchopneumonia or aspiration pneumonia.

If a definitive diagnosis is still not possible, a new blood test for something called N-terminal pro-brain natriuretic peptide (NT-proBNP) has recently become available for the veterinary market. NT-proBNP is a biomarker that "leaks" into the blood stream when heart muscle is stretched and therefore helps differentiate primary cardiac disease from respiratory tract disease. The blood sample has to be sent to a commercial laboratory, which limits its usefulness when a patient is gasping for air on the exam table, but if it ever becomes available as a patient-side test, it could prove to be very helpful in these types of cases.

Hopefully after all this, your veterinarian is able to tell you whether heart or respiratory tract disease is responsible for your pet’s symptoms. If expense is a concern (when is not?), your veterinarian can rank the tests in order of which is most likely to provide critical information and proceed accordingly.