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Like in humans, a dog’s heart is extremely important. It is a strong muscle that is constantly working to pump oxygen-rich blood throughout the body and to return oxygen-poor blood back to the lungs for more.
The nervous system plays an important role in this process because the electrical signal that is sent through the heart’s tissues is responsible for telling the heart to contract so that it may pump. But what happens when this signal changes, and what can be done to correct it?
A heart that is pumping at a normal speed usually has a normalrhythm or a regular repeated pattern. When a heart pumps too quickly or too slowly, there can be a problem with the rhythm of the heart. This is the definition of anarrhythmia. Similarly, the time between heartbeats is also a factor. A rhythm with equal pacing in between the beats is called asinus rhythm. Anything that causes unequal pacing between beats will cause an arrhythmia.
Your dog’s heart rate (the number of beats per minute) can increase due to stress or exercise, but this is usually temporary. A healthy heart will return to a normal resting heart rate. For small dogs, this might be around 120 to 160 beats per minute. For large breed dogs, a normal resting rate is around 60 to 120 beats per minute. If your dog’s heart rate remains too slow or too fast, he may have an arrhythmia or will have an increased risk of developing one.
There are many different kinds of arrhythmias in dogs. Some will primarily affect large breed dogs while others might affect specific breeds. Boxer dogs are most likely to develop arrhythmogenic right ventricular cardiomyopathy or ARVC, a kind of arrhythmia that can be fatal if left untreated. Boxers are also at risk of developing atrial fibrillation, as are Great Danes, Doberman Pinschers, Irish Wolfhounds, and Newfoundlands. ARVC and sometimes atrial fibrillation can be treated with medications that slow down the heart rate. Smaller breeds like Cocker Spaniels, Dachshunds, Miniature Schnauzers, and Westies are at a higher risk of developing sick sinus syndrome, which causes such a slow heart rate that surgical implantation of a pacemaker is necessary.
Structural changes to the heart can cause arrhythmias. For example, in dogs with dilated cardiomyopathy (DCM), the walls of the heart become very thin and literally dilate outwards. Because of this, the heart cannot pump blood efficiently, and the body becomes deprived of oxygen. As a result, the heart is asked to pump harder, and the heart rate becomes so fast that arrhythmias like supraventricular tachycardia (SVT) and atrial fibrillation can occur.
Infection or inflammation of the heart, akamyocarditis can increase the risk of ventricular arrhythmia or a heart block. Aheart block is when the electrical signal that moves through the heart to make it contract ends up being stopped. Most dogs can overcome this stop in signal thanks to a natural “secondary pacemaker” but this still results in a slow heart rate. Dogs with heart blocks, therefore, can appear very lethargic. Neurologic disease, respiratory disease, gastrointestinal diseases, and certain medications (especially those associated with general anesthesia) can increase the risk of heart blocks in dogs.
Dogs with an arrhythmia can show subtle signs at first. They may seem slightly more tired than normal or they may try to avoid exercise. For example, you may notice that your pup is easily fatigued and cannot walk as far as he used to do. When trying to sleep, he may seem more restless, but a more telltale sign is when the breathing rate orrespiratoryrate is more than 40 breaths per minute while asleep. This is common with underlying heart disease.
Dogs with heart disease tend to cough a lot, especially at nighttime. As heart problems progress, your dog may have trouble breathing all throughout the day. Severe heart disease can cause fluid to fill up in the abdomen and can cause weight loss due to generalized muscle wasting orcachexia.
In some cases, you may be able to detect an arrhythmia yourself. In order for you to check your dog’s heart rate, you need to place your hand on his chest or you can listen with a stethoscope if you have one. You will then count the number of beats in a fifteen-second window. Once you have that total, you multiply it by four to obtain the number of beats per minute. While palpating or listening, you should hear regularly repeated beats. If the beats are occurring in an abnormal pattern, or if you are hearing more than two heart sounds (e.g. more than the typical “lub dub” sound), contact your veterinarian right away!
Anelectrocardiogram (EKG) is one of the best ways to look for an arrhythmia. By using small metal electrodes, your vet can trace the electrical activity through your dog’s heart. Variations in speed, pattern, and the overall height and width of the waveforms from the signal can indicate a number of heart abnormalities. Additional heart testing like x-rays or anechocardiogram (heart ultrasound) is also recommended to rule out heart disease.
In some cases, an arrhythmia may not repeat frequently enough to be detected via stethoscope or a quick EKG reading. For these dogs, aHolter monitor is recommended. It comes in the form of a small vest that your dog can wear, and it tracks his EKG readings for a full 24 hours.
Treatment will always be recommended based on the cause. Dogs with very slow arrhythmias orbradyarrhythmias usually require medications that help speed up the heartbeats while dogs with very fast arrhythmias (tachyarrhythmias) require medications that slow it down.
In cases where medication cannot speed up the heart sufficiently, a pacemaker can be surgically implanted by a veterinary cardiologist. Dogs with 1st degree and 2nd-degree heart blocks typically respond to medications or the avoidance of whichever drug caused the block to occur, but dogs with 3rd-degree heart blocks almost always require a pacemaker.
Dogs with severe structural heart disease are likely to have the severe respiratory disease as a result, and patients with acute respiratory distress require immediate medical attention. Heart medications are typically started while the pet is hospitalized and receiving oxygen supplementation.
There are instances where arrhythmias can be caused by other organ systems. Ventricular premature contractions or VPCs can occur in dogs with illnesses such as gastric dilatation-volvulus or a ruptured splenic tumor. Anti-arrhythmic medications like lidocaine are necessary to help slow the heart, but ultimately, the main underlying illness will need to be addressed to truly resolve the arrhythmia.
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There are numerous types and causes of heart arrhythmias in dogs. It can be difficult to detect when the arrhythmia occurs sporadically, but dogs with fulminant arrhythmias or heart disease are more likely to have severe clinical signs such as respiratory distress. It is important to contact your vet right away when this kind of emergency arises. If your vet ever recommends an EKG based on exam findings, make this a priority so that you can help find a cause early before it becomes a bigger problem for your pup!
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Meet The Author
Dr. Erica Irish
Erica has worked in the veterinary field since 2006, starting out as a veterinary technician before graduating from the UF College of Veterinary Medicine in 2013. As a general practitioner in an animal hospital, she has many interests and is especially interested in dermatology, cardiology, internal and integrative medicine.
