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Heart Impulse Block in Dogs

Cardiac Electrical Failure in Dogs

The sinus node, also known as the sinoatrial node (SA Node or SAN), is responsible for generating electrical impulses that initiate heart contractions. Sinus arrest occurs when there is a disruption in the normal rhythm of the heart due to a slowing down or complete cessation of electrical activity originating from the sinus node. This failure of the sinoatrial (SA) node to produce impulses at the expected intervals leads to sinus arrest. When sinus arrest persists without the influence of drugs, it often indicates the presence of sick sinus syndrome (SSS), a condition characterized by irregular electrical impulse formation within the sinus node.

Sinoatrial block, on the other hand, pertains to a disorder involving the conduction of impulses. In this condition, impulses generated by the sinus node may fail to travel through the atria of the heart or may experience delays in doing so. Typically, the underlying rhythm of the sinus node remains unaffected even when impulse conduction is impaired.

Symptoms and Types

  • Typically asymptomatic
  • Weakness
  • Fainting
  • Pale gums
  • Very slow heart rate may be detectable

Sinoatrial block is categorized into three degrees: first, second, and third-degree SA block, akin to the degrees of atrioventricular (AV) block. First and third-degree SA block can be challenging to diagnose solely from an electrocardiogram (ECG) reading.

Second-degree SA block is the most prevalent type and is identifiable on a surface ECG. Additionally, there are two subtypes of second-degree SA block: Mobitz type I (also known as Wenckebach periodicity) and Mobitz type II.

First-degree sinoatrial block:

  • Slowed conduction

Second-degree sinoatrial block:

  • Intermittent failure to conduct impulses
  • Two subtypes:
    • Mobitz type I/Wenckebach periodicity: Conduction slows gradually until impulses fail to reach the atria
    • Mobitz type II: Block occurs in an all-or-none fashion until complete conduction failure
    • The two subtypes cannot be distinguished on a surface ECG.

Third-degree sinoatrial block:

  • Complete failure to conduct

Causes

Physiological:

  • Vagal stimulation, such as from coughing or irritation of the pharynx (back of the mouth/throat)
  • Increased pressure in the eye or carotid artery sinus (which carries blood from the heart to the brain)
  • Surgical procedures

Pathological:

  • Degenerative heart disease: where the heart becomes tougher and less flexible
  • Dilated cardiomyopathy: characterized by enlargement and failure of the heart
  • Acute myocarditis: sudden inflammation of the heart
  • Cardiac neoplasia: cancer affecting the heart
  • Sick sinus syndrome (SSS): marked by intermittent rapid and slow supraventricular arrhythmias
  • Vagus nerve irritation due to neck or chest tumors
  • Electrolyte imbalance, particularly abnormal potassium levels in the blood
  • Drug toxicity, such as from digoxin

Diagnosis

Your veterinarian will conduct a comprehensive physical examination along with various diagnostic tests, including a blood chemical profile, complete blood count, electrolyte panel, and urinalysis. The electrolyte panel is crucial for detecting hyperkalemia, abnormal potassium levels in the blood, which can contribute to arrhythmias. Providing a detailed history of your pet’s health, including symptom onset, is essential.

Thoracic (chest) x-rays and/or cardiac ultrasound imaging may be performed by your veterinarian to confirm or rule out heart disease and identify any abnormal tissue growth (neoplasia).

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.

Treatment

The majority of patients will receive treatment as outpatients. Hospitalization is reserved for patients displaying clinical signs of illness. Fluid therapy will be administered to patients requiring it. In severe cases where medical therapy is ineffective, implantation of an artificial pacemaker may be necessary. These patients will be hospitalized prior to surgery to prepare for the procedure. If your pet experiences excessive weakness or shows signs of losing consciousness or fainting, activity restriction will be necessary.

Living and Management

Post-care will vary based on whether your pet has an underlying disease in addition to the SA block. Your veterinarian will arrange follow-up appointments as needed, and an ECG reading will be conducted at each visit to monitor your pet’s condition. Should your pet experience weakness or loss of consciousness, promptly contact your veterinarian for guidance.

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