Common Causes of Ventricular Tachycardia in Dogs and Cats

Ventricular tachycardia can develop with or without structural disease of the heart. Disorders that cause VT may be cardiac in origin (primary), or it can be secondary to a generalized problem. It can also be hereditary. In a structurally normal heart, VT may occur as hereditary arrhythmias (irregular heartbeats).

When VT is associated with abnormalities of the heart muscle, the most common causes include cardiomyopathy (heart muscle disease), myocarditis (inflammation of the heart), or heart valve disease. In dogs, VT is a common cause of arrhythmias.

If ventricular tachycardia is not hereditary, all age groups can develop the condition.

Common Causes of VT include:

• Cardiomyopathy (heart muscle disease)
• Congenital heart defects
• Chronic disease of the heart valves
• Gastric dilatation (caused by a buildup of gas and/or fluid in the stomach), and subsequently volvulus (the stomach rotates on its axis) - the condition is commonly known as “bloat”
• Myocarditis (inflammation of the heart muscle usually caused by an infection)
• Trauma to the heart
• Toxicity from heart medication (such as Digitalis)
• Hyperthyroidism (increased thyroid hormone levels) in cats
• Heart tumors (such as hemangiosarcoma)
• Pancreatitis (inflammation of the pancreas)

Risk Factors

• Hemangiosarcoma (a type of malignant cancer) of the heart or spleen
• Acid-base imbalances
• Hypoxemia (low oxygen levels in the blood)
• Hypokalemia (low potassium levels in the blood) or hyperkalemia (high potassium levels in the blood)
• Hypomagnesemia (low magnesium levels in the blood)

Breed Predilections

Ventricular tachycardia can occur in dogs and cats with primary heart disease. It is commonly seen in large-breed dogs with cardiomyopathy, such as arrhythmogenic right ventricular cardiomyopathy (common in Boxers) or dilated cardiomyopathy (common in Doberman Pinschers).

The average age that these breeds typically start to develop ventricular arrhythmias (irregular heartbeats) can vary:

• Boxers with right ventricle muscle disease - 4 to 6 years of age with frequency and severity of the arrhythmia typically increasing over time.
• Doberman Pinschers with occult cardiomyopathy (also called “hidden” heart muscle disease) - 3 to 6 years old with frequency and severity of the arrhythmia typically increasing over time.
• German Shepherds with ventricular arrhythmias - 12-16 weeks of age; the frequency and severity of ventricular arrhythmias tend to increase until the dogs are 24-30 weeks old. However, the severity starts to decrease after 8 months of age. If the dog reaches 18 months of age, there is a decreased probability of sudden death.

Understanding How the Heart Functions

The normal heart rate for dogs depends on the size of the dog; the range is between 60-180 beats per minute. Generally, smaller dogs have faster heart rates. In cats, the normal heart rate ranges from 120-240 beats per minute.

The heart of cats and dogs has four chambers. The upper chambers are the right and left atria and the bottom chambers are the right and left ventricles. Between the right atrium and the right ventricle is the tricuspid valve; between the left atrium and the left ventricle is the mitral valve. Between the right ventricle to the pulmonary artery is the pulmonary valve, and from the left ventricle to the aorta is the aortic valve.

The heart works in a coordinated fashion in order to pump blood to the lungs and throughout the body. To achieve this, the heart is controlled by a pacemaker, called the sino-atrial (SA) node, from where the electrical impulse originates to start the coordinated muscular contraction of the heart. It’s the electrical impulse that starts the heart’s coordinated contraction.

The electrical impulse from the SA node causes the atria to contract so blood can be pumped into the ventricles. As the electrical impulse moves through the AV node and into the ventricles, it causes the ventricles to contract and pump the blood to the lungs (right ventricle) and the body circulation (left ventricle).

Symptoms of Ventricular Arrhythmia in Dogs and Cats

• Excessive panting
• Shortness of breath
• Poor exercise tolerance
• Rapid heart rate (tachycardia)
• Femoral pulse tends to be weak
• Fainting
• Collapse
• Sudden death

If congestive heart failure (CHF) is present, the dog or cat may also exhibit the following symptoms:

• Coughing
• Difficulty breathing
• Bluish discoloration of the skin and mucus membranes as a result of inadequate oxygen levels in the circulating red blood cells

Diagnosis of Ventricular Tachycardia in Dogs and Cats

During the physical exam, one of the prominent signs that veterinarians detect when listening to a dog’s or cat’s heart with a stethoscope is a heart murmur.

Diagnostic procedures that are necessary to confirm the initial diagnosis include the following:

• Electrocardiogram - records the electrical activity of the heart
• Echocardiogram - a special type of ultrasound that can help evaluate the function and structure of the heart and major blood vessels.
• Bloodwork
• Holter monitoring - a special vest with a battery-powered ECG monitor that is worn by the dog or cat for 24-48 hours to establish an overall picture of the heart rate and rhythm.

Treatment of Ventricular Tachycardia in Dogs and Cats

The treatment regimen will depend on the underlying cause. Hypokalemia or hypomagnesemia may need to be corrected before commencing medical therapy. There is a need to bring the irregular heartbeat under control and make sure that the patient is stable.

There is no effective treatment that could prevent sudden death in patients with VT. In breeds that are predisposed to sudden death or exhibiting symptoms, medications may be prescribed.

A dog or cat with ventricular tachycardia will require regular checkups for close monitoring of symptoms and progress associated with the treatment regimen.

If the cause of ventricular tachycardia is metabolic (such as hypokalemia or hypomagnesemia), the condition may resolve once corrected and the prognosis is generally good. If ventricular tachycardia is associated with heart disease, the prognosis is usually guarded because the underlying heart disease is likely to be chronic and progressive. As time goes on, VT may worsen and there is an increased risk of sudden death.

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