Q: I adopted a pit bull who had lost one ear in the fighting ring, so I named him van Gogh. He is a healthy, sweet, gentle guy who loves to snuggle. When we lie on the couch watching television and my arm drapes across his chest, I sometimes notice that his heart beats in an unsteady rhythm.

It beats fast for a few seconds, then slowly for a few seconds, and then the cycle repeats. I am concerned that van Gogh may have heart disease because he is relaxed and unstressed when I feel this irregular heart rhythm.

Should I take him to his veterinarian or a veterinary cardiologist?

A: Without examining van Gogh, I can’t say for sure. But, my guess is that when he is calm, he develops respiratory sinus arrhythmia, a normal condition that commonly occurs when a dog’s heart rate decreases with relaxation.

Not all arrhythmias, or irregular heart rhythms, indicate heart disease.

In respiratory sinus arrhythmia, the heart rate quickens during inspiration and slows when the dog exhales and pauses between breaths. The term “sinus” refers not to the nose but to the sinoatrial node, also called the sinus node, the heart’s natural pacemaker located in the right atrium.

To convince yourself that van Gogh’s changes in heart rhythm constitute the normal sinus arrhythmia that occurs when a dog is relaxed, take him for a run.

While his heart rate is still elevated from the exertion, feel his chest. You should feel steady, evenly spaced heart beats.

There’s an old adage in veterinary medicine that “cats are not small dogs,” because the two species differ in significant ways.

This is but one example. When a cat relaxes, the heart continues to beat in a steady pattern. Any arrhythmia, even a respiratory sinus arrhythmia, is abnormal in a cat.

If you are in doubt about van Gogh’s arrhythmia, ask his regular veterinarian to listen to his heart and perhaps perform an electrocardiogram, or ECG.

•        •        •

Q: The liquid flea and tick product I apply to my six barn cats’ skin every month is expensive. Each cat weighs about 10 pounds.

I want to save money by buying a liquid flea and tick product for a 60-pound dog and dividing it into smaller doses for my cats using some little syringes I have.

Will this plan work?

A: As I said, cats are not small dogs. Therefore, a flea-tick product labeled for dogs should never be applied to a cat. Doing so could prove fatal.

Most canine topical flea-tick products contain ingredients that are safe for dogs but toxic to cats — so toxic, in fact, that a cat can be poisoned by rubbing up against a dog whose product hasn’t dried yet.

The most common of these ingredients are permethrin and other chemicals that end in “-thrin.” They are related to pyrethrins, natural plant chemicals that repel insects.

Many canine flea-tick products are classified as pyrethroids, which means they have been chemically modified to enhance and prolong their effectiveness.

While pyrethroids, permethrin and related chemicals are safe for dogs, cats are exquisitely sensitive to them. Toxic signs in exposed cats include drooling, vomiting, weakness, loss of coordination, hyperactivity, tremors, seizures and death.

Please continue to use flea and tick products labeled as safe for cats

Lee Pickett DVM practices companion animal medicine in North Carolina. Click here to ask her questions for her weekly column. The opinions expressed are her own.