Heart Disease of the Sinus Node in Cats

Sick Sinus Syndrome in Cats

The sinoatrial node (SA Node, or SAN), also called the sinus node, is the initiator of electrical impulses within the heart, triggering the heart's contractions by firing off electrical surges. One of the disorders that can affect the heart’s electrical impulse formation within the sinus node is called sick sinus syndrome (SSS).

This disorder complicates conduction of the electrical impulse out of the sinus node and the specialized conduction system of the heart. Secondary pacemakers, such as the muscle fibers of the sinus node, will also be affected by sick sinus syndrome. (Note: the body's natural pacemakers are responsible for setting the pace for the heart's rhythm and generating the electrical impulses within the muscle tissue.)

Any irregular contraction of the heart (arrhythmia) will be visible on an electrocardiogram (ECG). 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 cats will become apparent when organs begin to dysfunction because they are not receiving a normal amount of blood supply.

Symptoms and Types

If your cat tends to be fairly inactive under normal circumstances, it will take longer for symptoms of sick sinus syndrome to become apparent. When symptoms do show, those that will generally present are:

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

Causes

Some of the suspected relationships to SSS are genetic, however, the causes for this condition are mostly unknown. One possible cause is when there is a heart disease that is cutting off the blood supply to or from the heart, disrupting normal heart function and electrical functionality. Cancer in the thoracic or pulmonary (both refer to the chest) region may also lead to SSS.

Diagnosis

A complete physical exam will be performed by your veterinarian. It will include a blood chemical profile, a complete blood count, a urinalysis, and an electrolyte panel to verify proper organ function. You will be asked to provide a thorough history of your cat'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 give your veterinarian may provide clues as to which organs are being affected secondarily.

In order to to assess sinus node function, a provocative atropine response test may be done. This test uses the drug atropine to stimulate the firing action (output of electrical impulses) of the SA Node.

An ECG may be indicated in certain breeds that 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.

Treatment

Only cats showing clinical signs need treatment, and only cats requiring electrophysiologic testing of the heart, or implantation of an artificial pacemaker will need to be hospitalized. Attempts to manage an abnormally fast or abnormally slow heart rate, without prior pacemaker implantation, carries a significant risk of worsening the extremes of the abnormal heart rate syndrome.

Living and Management

You will need to keep your cat's physical activity to a minimum while it is healing from this condition. Encourage rest in a quiet, non-stressful environment as much as possible, away from other pets or active children. Cage rest may be recommended for the interim. Although therapy for SSS may appear to work at the beginning of treatment, medical therapy commonly does not have long term benefits. The only solution in these instances is surgical correction.