Pay close attention all of you Miniature Schnauzer lovers! The breed you fancy is prone to a heart condition called Sick Sinus Syndrome (SSS). The sinus involved is not within the respiratory tract. Rather, it is a structure called the sinus node that is located within the heart.
The sinus node is responsible for electronically initiating the normal heartbeat and establishing the normal heart rate.
In dogs with SSS, the sinus node has lapses in which it discharges beats much too slowly, or not at all. As a result, there are long pauses in between heartbeats. Sometimes, an electrical impulse originating from another part of the heart will come to the rescue, particularly if the heart has stopped for several seconds. Such rescue beats can be very rapid.
In most cases, the sinus node will eventually resume its job in which case there will be periods of normal heart rate (60-100 beats per minute). Other dogs with SSS have a constant bradycardia (heart rate is too slow). Even with exercise or excitement, the heart rate remains at less than 40 beats per minute.
What causes it?
The exact cause of the sinus node malfunction is unknown. Although any breed of dog can be affected, a genetic basis is suspect because SSS primarily affects Miniature Schnauzers, Dachshunds, Cocker Spaniels, West Highland White Terriers, and Pugs. Middle-aged to older females are particularly predisposed. The mode of inheritance is unknown, and there is no genetic testing available. Nonetheless, the appearance of SSS in a breeding dog should strongly discourage future breeding.
What are the symptoms?
Dog with SSS becomes symptomatic because of their subnormal heart rate. The most common symptoms include:
- Exercise intolerance
- Fainting episodes (also known as syncopal episodes)
Some dogs with severe, long-standing SSS can develop symptoms of congestive heart failure including weakness, labored breathing, and coughing.
It can sometimes be difficult to differentiate between a fainting episode (syncope) and a seizure. Videotaping such an event at home to then share with the examining veterinarian can be most helpful.
How is it diagnosed?
SSS is strongly suspected based on the dog’s breed, history, and a thorough physical examination. Listening with a stethoscope often reveals a heart rate that is lower than normal and stays this way even when the dog is asked to exercise. Other testing that may be recommended include:
- An electrocardiogram (ECG) looks for abnormalities characteristic of SSS changes.
- Blood testing rules out an underlying metabolic problem. Abnormalities in blood calcium or potassium levels have the potential to mimic SSS changes.
- Holter monitoring provides a 24-hour electrocardiogram (ECG) tracing. The testing equipment is housed within a vest that is worn by the dog at home. This may be necessary to determine if a dog has SSS, particularly if the heart rate is normal at the time of the physical examination.
- An atropine response test can identify dogs with SSS. Atropine is a drug that normally causes the heart rate to escalate. When atropine is given to a dog with SSS, the very low heart rate remains unchanged.
- Chest x-rays document evidence of heart failure.
- Cardiac ultrasound (echocardiogram) looks for changes in the appearance of the heart valves and sizes of the four chambers that can occur secondary to chronic SSS.
For dogs with SSS, the therapeutic goal is to maintain a normal heart rate so as to restore a good quality of life. If SSS is caught quite early during an annual physical exam, and the dog is symptom-free, no treatment other than careful monitoring may be required for the time being.
For dogs who are experiencing symptoms, two forms of therapy can be considered:
Vagolytic drugs: These medications are used in an attempt to maintain a normal heart rate. While it is reasonable to try such drugs, they don’t have a very consistent track record of success. Additionally, side effects are relatively common. Examples of vagolytic drugs are theophylline, terbutaline, and propantheline bromide.
Pacemaker implantation: This is truly the treatment of choice for most dogs with symptoms caused by SSS. When properly placed and monitored, a pacemaker is capable of restoring a normal quality of life for years to come. Veterinarians who specialize in cardiology are the masters of pacemaker implantation. Just as in people, the pacemaker can be placed without a significant surgery involved. Access to pacemaker implantation may be limited depending on where one lives and their ability to pay for such a state-of-the-art procedure.
Have you ever cared for a dog with sick sinus syndrome?
Diplomate, American College of Veterinary Internal Medicine
Author of Speaking for Spot: Be the Advocate Your Dog Needs to Live a Happy, Healthy, Longer Life
Author of Your Dog’s Best Health: A Dozen Reasonable Things to Expect From Your Vet
Recipient, Leo K. Bustad Companion Animal Veterinarian of the Year Award
Recipient, American Animal Hospital Association Animal Welfare and Humane Ethics Award
Recipient, Dog Writers Association of America Award for Best Blog
Recipient, Eukanuba Canine Health Award
Recipient, AKC Club Publication Excellence Award
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