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Heart Disease of the Sinus Node in Dogs

Sick Sinus Syndrome in Dogs

Sick sinus syndrome (SSS) is a condition affecting the sinoatrial node (SA Node or SAN) in dogs, also known as the sinus node, which is responsible for initiating electrical impulses within the heart. These impulses prompt the heart to contract and beat rhythmically. SSS involves a disruption in the heart’s electrical impulse formation and conduction within the sinus node, as well as affecting subsidiary pacemakers and the heart’s specialized conduction system.

This disorder can be observed on an electrocardiogram (ECG) by irregular heart contractions (arrhythmia). A variant of SSS is tachycardia-bradycardia syndrome, characterized by alternating episodes of slow and fast heart rates. Clinical signs of SSS become evident as affected organs suffer from inadequate blood supply due to abnormal heart rhythms. Sick sinus syndrome can occur in both dogs and cats, posing a significant health concern for affected animals.

Symptoms and Types

Symptoms of sick sinus syndrome in dogs may not always be apparent, especially in cases where the dog is typically inactive. However, common symptoms include weakness, fainting, fatigue, collapse, seizures, abnormal heart rates (either too fast or too slow), pauses in the heart rate, and, in rare instances, sudden death.


The exact causes of sick sinus syndrome (SSS) are largely unknown. However, there are several suspected factors associated with SSS. Genetic predisposition may play a role, as certain breeds such as the miniature schnauzer seem to have a higher likelihood of developing the condition. Additionally, heart disease that obstructs blood flow to or from the heart, disrupting normal cardiac function including electrical activity, can contribute to SSS. Cancer located in the thoracic or pulmonary regions (both referring to the chest) may also lead to the development of SSS.


To diagnose sick sinus syndrome (SSS) in your dog, your veterinarian will conduct a thorough physical examination, which may include blood chemical profiling, a complete blood count, urinalysis, and electrolyte panel to assess organ function. Providing your veterinarian with a detailed history of your dog’s health, including symptom onset and any relevant incidents or recent health conditions, is crucial for diagnosis. This information may help identify secondary organ involvement.

A provocative atropine response test may be administered to evaluate sinus node function. This test involves using the drug atropine to stimulate the firing action of the SA Node. Dogs with SSS typically exhibit either no response or an incomplete response to atropine.

An electrocardiogram (ECG) may be recommended for breeds predisposed to SSS, as these breeds often have a higher risk of other heart diseases such as valvular diseases. Therefore, if a heart murmur is present, it’s important to rule out any valve-related heart diseases first.


Treatment for sick sinus syndrome (SSS) in dogs is typically reserved for patients exhibiting clinical signs. Hospitalization may be required only for patients needing electrophysiologic testing of the heart or implantation of an artificial pacemaker.

Dogs that do not respond to medical therapy or experience adverse effects from medication, as well as those with abnormal heart rate syndromes, may require artificial pacemaker implantation. Attempting to manage abnormal heart rate syndromes medically without prior pacemaker implantation carries a significant risk of exacerbating these conditions.

Living and Management

During your dog’s recovery from sick sinus syndrome, it’s important to limit physical activity. Encourage rest in a calm, stress-free environment, away from other pets or energetic children. While medical therapy may initially appear effective, it often does not provide lasting improvement. In such cases, surgical correction may be the only viable alternative.

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