Sinus Arrest and Sinoatrial Block
Persistent sinus arrest not attributed to drug use typically signals sick sinus syndrome (SSS), a condition affecting the heart’s electrical impulse generation within the sinus node. The sinus node, also known as the sinoatrial node (SA Node or SAN), is responsible for initiating electrical impulses in the heart, prompting its contraction by emitting electrical surges. Sinus arrest occurs when there’s a disruption in the heart’s impulse formation due to a slowdown or halt in spontaneous sinus nodal automaticity, which is the inherent behavior of tissues setting the heart’s rhythm. It manifests as the failure of the sinoatrial (SA) node to generate an impulse at the expected time.
Sinoatrial block, on the other hand, pertains to a disorder in impulse conduction. It occurs when an impulse generated within the sinus node fails to propagate through the atria (the heart’s interior) or experiences a delay in doing so. Typically, the fundamental rhythm of the sinus node remains undisturbed even when there are issues with impulse conduction.
Symptoms and Types
-
- Typically asymptomatic
- Weakness
- Fainting
- Pale gums
- Very slow heart rate, may be detectable
Sinoatrial block is categorized into first, second, and third-degree SA block, akin to degrees of atrioventricular (AV) block. Diagnosing first and third-degree SA block solely from an electrocardiogram (ECG) reading is challenging.
Second-degree SA block is the most common type and the only degree identifiable on a surface ECG. There are two types of second-degree SA blocks: Mobitz type I (also known as Wenckebach periodicity) and Mobitz type II.
First-degree sinoatrial block:
- Conduction slows
Second-degree sinoatrial block:
- Intermittent failure to conduct
- Two types:
- Mobitz type I/Wenckebach periodicity: Conduction gradually slows until impulses fail to reach the atria
- Mobitz type II: Conduction is all or none until complete failure occurs
- Both types indistinguishable on a surface ECG
Third-degree sinoatrial block:
- Complete failure to conduct
Causes
Physiological
- Vagal stimulation, such as coughing, and irritation of the pharynx (the back of the mouth/throat)
- Elevated pressure in the eye or carotid artery sinus (which carries blood from the heart to the brain)
- Surgical interventions
Pathological
- Degenerative heart disease: Heart becomes tougher and less flexible
- Dilated heart disease: Heart enlarges and experiences failure
- Acute inflammation of the heart
- Cardiac cancer
- Sick sinus syndrome (SSS): Characterized by intermittent rapid and slow supraventricular arrhythmias
- Irritation of the vagus nerve due to neck or chest cancer
- Electrolyte imbalance: Abnormal potassium levels in the blood
- Drug toxicity (e.g., digoxin)
Diagnosis
Your veterinarian will conduct a comprehensive physical examination of your dog, along with blood tests including a chemical profile, complete blood count, electrolyte panel, and urinalysis. The electrolyte panel can reveal hyperkalemia, abnormal potassium levels in the blood, which may contribute to arrhythmias. Providing a detailed history of your dog’s health, including symptom onset, is crucial.
To confirm or rule out heart disease and abnormal tissue growth (neoplasia), your veterinarian may perform thoracic (chest) x-rays and/or cardiac ultrasound imaging.
A provocative atropine response test may be administered to evaluate sinus node function. This test involves the use of 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 dogs can receive treatment on an outpatient basis. Hospitalization is reserved for patients displaying clinical signs of illness. Fluid therapy will be administered to patients requiring it. For severely ill dogs unresponsive to medical therapy, implantation of an artificial pacemaker may be necessary, necessitating hospitalization before surgery preparation. If a dog becomes excessively weak or shows signs of losing consciousness or fainting, activity restriction is imperative.
Living and Management
Post-treatment care will vary based on whether your dog has an underlying disease in addition to the SA block. Your veterinarian will arrange follow-up appointments as needed, with an ECG reading conducted at each visit to monitor your dog’s progress. If your dog experiences weakness or loses consciousness, promptly contact your veterinarian for guidance.
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