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Sick Sinus Syndrome in Dogs

The sinoatrial node (SA Node, or SAN), also called the sinus node, is the initiator of electrical impulses within the heart, triggering the heart to beat, or contract, by firing off electrical surges. Sick sinus syndrome (SSS) is a disorder of the heart’s electrical impulse formation within the sinus node. It is also a disorder of the conduction of the electrical impulse out of the sinus node. Sick sinus syndrome will also affect subsidiary (backup) pacemakers and the specialized conduction system of the heart. Pacemaker refers to the generation of electrical impulses within the muscle tissue, which set the pace for the heart's rhythm.

On an electrocardiogram (ECG), the irregular contraction of the heart (arrhythmia) will be visible. Tachycardia-bradycardia syndrome, in which the heart beats too slowly, and then too quickly, is a variant of sick sinus syndrome. Clinical signs of sick sinus syndrome in animals will become apparent when organs begin to dysfunction because they are not receiving a normal amount of blood supply.

This syndrome can affect both dogs and cats. If you would like to learn more about how this disease affects cats, please visit this page in the PetMD health library.

Symptoms and Types

Some dogs will not show any symptoms of sick sinus syndrome, especially if they tend to be fairly inactive under normal circumstances. Generally, the symptoms that will present are:

• Weakness
• Fainting
• Fatigue
• Collapse
• Seizure
• Abnormally fast, or abnormally slow heart rate
• Pauses in the heart rate
• Rarely, sudden death

Causes

The causes for this condition are mostly unknown. Some of the suspected relationships to SSS are genetic, since some breeds, like the miniature schnauzer, appear to be predisposed; another cause is heart disease that is cutting off the blood supply to or from the heart and disrupting normal heart function, including the electrical functionality; and, cancer in the thoracic or pulmonary (both refer to the chest) region may also lead to SSS.

Diagnosis

Your veterinarian will perform a complete physical exam, including a blood chemical profile, a complete blood count, a urinalysis, and an electrolyte panel to verify proper organ function. You will need to give your doctor a thorough history of your dog's health, including a background history and onset of symptoms, and possible incidents or recent health conditions that might have precipitated this condition. The history you provide may give your veterinarian clues as to which organs are being affected secondarily.

A provocative atropine response test may be done to assess sinus node function. This test uses the drug atropine to stimulate the firing action (sending electrical impulses out) of the SA Node. Dogs with SSS generally will have no response, or will have an incomplete response to the atropine.

An ECG may be indicated in certain breeds which are predisposed to SSS, as these same breeds are often predisposed to other diseases of the heart valves (the valves that separate the four chambers of the heart). Hence, if there is a heart murmur, disease of any of the heart’s valves should first be ruled out.