During your pet’s physical exam, you are sure to notice your veterinarian donning his or her stethoscope to listen to your pet’s chest. They do this to listen for abnormalities in your pet’s lung sounds as well as detect the rate, rhythm, and quality of your pet’s heart beats.
While the heart rate can vary depending on your pet’s species and level of excitement, it should still be within a normal range and should have a nice, steady rhythm. Abnormalities in the rhythm are called cardiac arrhythmias, and include slow heart rates, fast heart rates, and everything in between.
The normal heart rate of a dog ranges from 60 to 170 beats per minute in adults and can be a bit quicker (up to 220 beats per minute) in dogs under a year old. Cats have heart rates in the range of 80 to 160 beats per minute, but can have heart rates well over 200 if they are stressed (as they often are in the clinic) or if they have cardiomyopathy.
When the heart rate exceeds 180 in adult dogs and 240 in adult cats, they are said to be tachycardic (or have tachycardia). This simply means that their heart rate is increased above the normal range. Similarly, heart rates below the normal range (slower) are called bradycardias.
There are many different kinds of heart rate abnormalities (or cardiac arrhythmias) in veterinary medicine. Too many, in fact, to cover in one short blog. Instead, let’s focus on the top five most commonly seen arrhythmias in dogs and cats.
I think I will always remember the case of two young paramedics who rushed their dog in, claiming that he had something terribly wrong with his heart; they had been listening to his chest with their stethoscope and heard a very unusual heart rhythm. I took a listen and was able to set their minds immediately at ease. Their dog had an arrhythmia because the heart was not beating with a regular beat. Instead, his heart rate varied with the dog’s breathing—when he breathed in, his heart rate sped up. And when he exhaled, his heart rate dropped. This is called a sinus arrhythmia, but we commonly see it in dogs, where it is a normal finding.
The four-chambered heart beats as two units, with the upper chambers (called the atria) sending the signal to lower ventricular chambers. In the condition called atrial fibrillation, the atrium rapidly “misfires,” sending way too many signals to the ventricles to contract. These unorganized signals mean that the heart chambers don’t function as a unit the way they should. In giant breed dogs, atrial fibrillation can exist with no other concurrent cardiac disease, but in other breeds of dogs and almost exclusively in cats, atrial fibrillation occurs because of serious underlying cardiac conditions.
You know from above that tachycardia is a heart rate faster than the normal range. This rapid heart rate originates in the ventricles, or the lower chambers of the heart. Ventricular tachycardia is life threatening and sometimes leads to sudden death. It can occur in patients with underlying cardiac disease, as is the case in Boxers with arrhythmogenic right ventricular cardiomyopathy and Doberman Pinschers with dilated cardiomyopathy. It can also be an inherited condition in pets with structurally normal hearts.
A normal heart has four chambers but should function as one smooth unit, with the atrial chambers working in conjunction with the ventricular chambers to produce the “lub-dub” sound of the heartbeat. Premature beats occur when either the atrium or the ventricle beats out of turn. They can occur for any number of reasons, from system disease to electrolyte disturbances, to cardiac disease, and they tend to increase in frequency with age.
Atrioventricular block (AV block)
AV block occurs when the signal between the atria and the ventricles is impaired or fails. There are three types of AV block, depending on the totality of the signal impairment. Higher grade AV block is more serious and is generally pathologic in nature. Patients with AV block may be asymptomatic, or may suffer from weakness, fainting, and even sudden death. A pacemaker can be used to get the heart back on track.