Cardiac Arrhythmias in Dogs

Overview of Cardiac Arrhythmias in Dogs

Cardiac arrhythmias are abnormal heart rhythms that can occur in dogs. These disorders are classified based on the area of the heart in which they originate—either the upper chambers, the lower chambers, the area of the heart responsible for creating the heartbeat, or the electrical conduction system within the heart.

Each heartbeat originates as an electrical impulse in the upper right chamber of the heart (sinoatrial [SA] node). The impulse then travels across the upper chambers of the heart (atria), to an intermediate station (atrioventricular [AV] node), and finally to the lower chambers of the heart (ventricles).

The electrical impulse generates the typical pattern seen on an electrocardiogram (EKG or ECG). Disturbance in the generation or transmission of the electrical impulse in the heart causes a cardiac arrhythmia. Some are temporary and do not cause illness, but others are serious and may be life-threatening.

Cardiac arrhythmias may affect dogs of any age or sex, and may also affect any breed. However, some breeds are more at risk of developing arrhythmias than others.

Giant breeds of dog are more prone to a type of arrhythmia known as atrial fibrillation, which is a rapid abnormal beat originating in the atria. Labrador Retrievers are prone to supraventricular tachycardia, which is a rapid heart rate originating above the ventricles. Doberman Pinschers and Boxers are prone to ventricular tachycardia, which is a rapid abnormal heart beat originating in ventricles.

Sick sinus syndrome is an abnormality that affects the SA node: It most commonly occurs in Miniature Schnauzers, Dachshunds, Cocker Spaniels, and West Highland White Terriers. Spaniels, German Shepherds, and Labrador Retrievers are predisposed to certain types of heart block.

The prognosis for animals with cardiac arrhythmias depends on the type of arrhythmia, the underlying cause of the arrhythmia, and the type and extent of any existing heart disease.
Symptoms of Cardiac Arrhythmia include:

• Weakness
• Collapse
• Slow heart rate
• Fast heart rate
• Erratic heart rate
• Difficulty breathing
• Lack of appetite

Diagnosis of Cardiac Arrhythmias in Dogs

Blood work, including a complete blood count and biochemical profile, should be performed to look for any underlying abnormalities. Some dogs may be anemic, have an elevated white blood cell count, or organ dysfunction. Some diseases, such as hypothyroidism, may be the cause of cardiac arrhythmias.

Cardiac arrhythmias are diagnosed with an EKG. The type of arrhythmia can be diagnosed from an ECG oscilloscope, or from a printout of the trace. A chest x-ray may help determine if heart disease or heart failure are present.

Home Care and Prevention

There is no home care for abnormal heart problems in dogs, except that you should administer any medications your veterinarian has prescribed. If you suspect that your dog has an abnormal heart rate or rhythm, you should contact your veterinarian immediately.

Cardiac arrhythmias are difficult to prevent, but early diagnosis and treatment of predisposing causes can reduce the risk of arrhythmias developing.

Scientific Explanation of Canine Arrhythmias

Normal heart rhythms begin in the sinus node, which is located in the right upper chamber of the heart. While abnormalities of the sinus node are typically a consequence of a systemic disorder, such as hypo- or hyperthyroidism, primary sinus disease is common and can lead to an arrhythmia known as sick sinus syndrome.

Other arrhythmias arising outside of the SA node may occur, and among the most serious of these is atrial fibrillation. Arrhythmias arising from the ventricles include premature ventricular contractions and ventricular tachycardia. More serious arrhythmias sometimes lead to cardiac decompensation and acute or chronic heart failure.

Cardiac arrhythmias can lead to a very slow heart rate (potentially as slow at 40 beats per minute, known as bradycardia), a very fast heart rate, potentially over 200 beats per minute in a dog (tachycardia), or an erratic heart beat. Numerous types of arrhythmias may occur. Some of the more common ones include:

• Atrial fibrillation
• Atrial tachycardia
• Ventricular escape rhythm
• Ventricular premature complex
• Ventricular tachycardia
• Ventricular fibrillation
• First-degree heart block
• Second-degree heart block
• Third-degree heart block

Often, cardiac arrhythmias are associated with underlying heart problems in dogs, such as dilated cardiomyopathy, congestive cardiac failure, or cardiac birth defects. In addition, a variety of other diseases or events may cause cardiac arrhythmias, including:

• Hypothyroidism (under active thyroid gland)
• Chronic lung disease
• Anemia
• An overdose of certain medications, such as digoxin, narcotics, xylazine
• Administration of anesthetic agents
• High or low blood potassium
• Tumors of the heart
• Trauma
• Toxicity, such as chocolate poisoning
• Hypoadrenocorticism (Addison’s disease – a disease of the adrenal glands)
• Urinary obstruction
• Lyme disease
• Smoke inhalation
• Head trauma
• Hypothermia
• Fear
• Excitement
• Pain
• Low blood pressure (hypotension)
• Gastric dilatation-volvulus
• Diseases of the spleen
• Severe infections

Treatment Options

Some arrhythmias do not require treatment, and they spontaneously convert to normal. Other arrhythmias are serious and require treatment, options of which include:

• Medications to control the arrhythmia, treat underlying heart disease in dogs, or normalize cardiac function. Medications used include digoxin, diltiazem, propranolol, enalapril, procainamide, lidocaine, and atropine.
• Certain types of cardiac arrhythmias require the placement of a pacemaker to control the arrhythmia. Severe heart blocks often require a pacemaker to keep the heart rate and rhythm normal.
• Dogs in congestive heart failure may need treatment with a diuretic (e.g. furosemide) and possibly nitroglycerine.
• Dogs with underlying medical diseases often require additional medication. For example, dogs with hypoadrenocorticism may require steroid supplementation.

Follow-Up Care for Dogs with Cardiac Arrhythmias

Optimal treatment for your dog requires a combination of home and professional veterinary care. Follow-up can be critical and may include the following:

• Administration of any prescribed veterinary medications as directed. Be certain to contact your veterinarian if you are experiencing problems treating your dog.
• Frequent veterinary evaluation (including listening to the heart with a stethoscope [cardiac auscultation]).
• Repeat EKGs to monitor your dog’s response to therapy.