Arrhythmias in Dogs

What is an arrhythmia?

An arrhythmia is an abnormal heart rate or rhythm. It occurs when the electrical signals that tell your dog’s heart to beat are not working properly. The heart may beat too fast or too slowly, skip beats, or beat irregularly. 

Normally, electrical signals tell the heart muscles to contract (squeeze) in a coordinated way to produce a heartbeat. Each signal starts in the sinoatrial (SA) node, the heart’s natural pacemaker. 

The signal first travels through the atria, the heart’s two upper chambers, causing them to contract. Then it passes through the atrioventricular (AV) node and travels to the ventricles, the two lower chambers, causing them to contract. Damage or disease anywhere along this pathway can lead to an arrhythmia. 

Are all arrhythmias dangerous?

No, occasional irregular heartbeats may not be dangerous, but the cause should always be investigated. Sinus arrhythmia is a normal variation in heart rhythm that can occur in dogs. The heart rate changes with breathing: it speeds up as the dog breathes in and slows down as the dog breathes out. Sinus arrhythmia can occur when dogs are resting, especially in very athletic dogs.

What causes arrhythmia in dogs?

The primary cause of arrhythmia is underlying heart disease.

Disease of the heart muscle: Cardiomyopathy is a disease of the heart muscle. The most common type in dogs is dilated cardiomyopathy (DCM). The muscles lose their ability to contract normally, leading to stretching of the muscle and enlargement of the heart chambers. 

There is a genetic predisposition to this disorder in some large and giant breeds such as Dobermans, Boxers, Great Danes and Giant Schnauzers. Other causes include toxicity from certain chemotherapeutic drugs, nutritional deficiencies, and infectious diseases such as parvovirus. 

Some studies suggest an association between grain-free, legume-rich diets and development of DCM (diet-related DCM) in dogs. However, the definitive cause of dr-DCM has not been proven, and it is thought to be influenced by more than one factor. Research is ongoing.

Disease of the heart valves: The heart valves control the forward flow of blood through the heart. If these valves are abnormal at birth, or wear down over time, they become leaky. Blood can then flow backward, making the heart work harder. Older, small-breed dogs are more likely to develop leaky heart valves. Bacterial infection of the heart valves (endocarditis) can also occur when bacteria spread from dental disease or other severe infections.

Other causes of arrhythmia include:

• Electrolyte imbalances (imbalances in minerals in the blood)
• Trauma (injury)
• Systemic inflammation (inflammation throughout the body) or sepsis (a life-threatening response to infection)
• Diseases of the spleen or stomach, such as tumors or gastric dilation and volvulus (GDV)
• Toxins such as chocolate or certain drugs like amphetamines
• Anesthetic drugs
• Endocrine disorders
• Tumors in or near the heart

What types of arrhythmias occur in dogs?

Arrhythmias can be either tachyarrhythmias, in which the heart beats too fast, or bradyarrhythmias, in which the heart beats too slowly. Some arrhythmias can alternate between fast and slow.

Examples of tachyarrhythmias include:

• Atrial fibrillation (A-fib). The atria beat rapidly and out of sync with the ventricles. The most common cause in dogs is dilated cardiomyopathy, although it can be idiopathic (no known medical cause) in some large and giant breed dogs.
• Ventricular premature contractions (VPCs), ventricular tachycardia (V-tach) and supraventricular tachycardia (SVT). A heartbeat originates somewhere other than the normal conduction pathway and can be caused by a heart condition or be a sign of another disease.

Examples of bradyarrhythmias include:

• Heart block. One or more signals are blocked at the AV node, interrupting the signals that tell the ventricles to beat. Anesthetics, certain pain medications, and heart disease can cause heart block. 
• Sick sinus syndrome. The SA node does not work properly. This disorder is more common in certain breeds, such as the Miniature Schnauzer, Cocker Spaniel, and West Highland White Terrier. It can be associated with damage to or gradual wearing down of the heart’s electrical system , but in most cases the cause is unknown.

What are the signs of an arrhythmia?

There may be no outward signs of an arrhythmia, especially if heart disease is not advanced. With more severe disease, some or all of the following symptoms may occur:

• Breathing faster or with more effort
• Coughing
• Exercise intolerance
• Weakness or lethargy (unusual tiredness)
• Collapse or fainting (syncope)
• Weight loss
• Poor appetite
• A swollen abdomen (belly) due to fluid buildup (ascites)
• Pale or bluish gums

Rarely, sudden death can occur without any warning signs.

How is an arrhythmia diagnosed?

An arrhythmia is often first suspected when your veterinarian listens to your dog’s heart with a stethoscope. They may also detect an irregular pulse. If an irregularity is found, your veterinarian may recommend additional tests to identify the arrhythmia and its cause. These may include:

• ECG (electrocardiogram). This test records the electrical signals running through your pet’s heart and displays them on a screen or paper. It can measure heart rate and detect abnormal rhythms. A holter monitor records a continuous ECG over a longer period of time and can be fitted to your dog.
• Echocardiogram (heart ultrasound). This test looks at the valves and chambers of the heart in more detail. It can identify structural heart disease, such as abnormal valves or cardiomyopathy.
• X-rays. Chest X-rays may show an enlarged heart associated with heart disease. Fluid build-up in or around the lungs may be noted with advanced heart disease. Abdominal X-rays may detect disorders of the spleen or stomach, as well as ascites.
• Blood and urine testing. Screening tests such as a complete blood count, a biochemistry profile, and a urinalysis help rule out other causes of arrhythmias, such as electrolyte abnormalities, infectious diseases, endocrine disorders, and toxicities.
• ProBNP test. This blood test measures a substance released by stretched or overworked heart muscle.

If your veterinarian suspects or diagnoses heart disease, they may refer you to a veterinary cardiologist (heart doctor) for further testing and management.

How is an arrhythmia treated?

Treatment depends on the underlying cause of the arrhythmia and whether it is associated with a slow or fast heart rate. If the underlying cause can be identified and removed or treated (such as electrolyte abnormalities), then the arrhythmia will often correct itself. 

Treatment to control the heart rate may include:

• Fast rhythms (tachyarrhythmias) may be treated with anti-arrhythmic drugs given intravenously (IV) or by mouth to control heart rate (e.g., lidocaine, procainamide, sotalol, and diltiazem).
• Slow rhythms (bradyarrhythmias) may be managed in the short term with medications to increase heart rate (atropine, glycopyrrolate) or treated with a surgically implanted pacemaker.

Other medications may be needed to manage underlying heart disease. These may include medications to improve the heart’s ability to contract (pimobendan), diuretics to prevent fluid buildup (furosemide), and medications to decrease blood pressure and the heart’s workload (benazepril).

A special diet may be recommended for your dog. If your dog is experiencing congestive heart failure, then a mild to moderate restriction of sodium (salt) in the diet may be advised. Omega-3 fatty acids (DHA and EPA) may also be recommended for their anti-inflammatory effects and other benefits.

When should I seek immediate veterinary care?

Contact your veterinarian if you notice any sudden change in your dog’s condition, including:

• Breathing changes: increased breathing rate or effort, or a new or worsening cough
• Weakness or severe lethargy
• Collapse, especially combined with loss of consciousness
• Pale or bluish gums
• A swollen abdomen

What is the long-term outlook for dogs with arrhythmias?

The outlook depends on the underlying cause. If the cause can be eliminated, then the outlook is good. If heart disease is present, the outlook varies with its type and severity. With appropriate treatment, your dog may have a good quality of life for months or even years. However, if your dog does not respond well to treatment, or if symptoms are advanced, then the outlook is poorer.

Regular monitoring helps your veterinarian adjust treatment as needed. Depending on your dog’s condition, this may include repeat ECGs, echocardiograms, and chest X-rays.