Cardiac Arrhythmias in Dogs

Normal heart rhythms begin in the sinoatrial (SA or sinus) node, which is located in the right upper chamber (atrium) 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 a type of arrhythmia known as sick sinus syndrome. Other arrhythmias arising outside of the SA node may occur. 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. Some arrhythmias worsen to the point of fibrillation and eventually the absence of any heartbeat (asystole).

Cardiac arrhythmias can lead to a very slow heart rate (potentially as slow at 40 beats per minute), termed bradycardia; very fast heart rate (potentially over 200 beats per minute in a dog), termed tachycardia; or an erratic heart beat. Numerous different 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 disease, 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

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