Heart Block or Conduction Delay (Right Bundle) in Dogs

Electrical impulses that coordinate heart muscle contraction pass through a number of different nodes and bundles of specialized muscle tissue. The atrioventricular (AV) node is located in the interatrial septum at the top of the heart. It receives electrical impulses from the atria and passes them on to the ventricles, ensuring a long enough delay that all the blood will have time move from the atria into the ventricles before they in turn contract. From the AV node, the impulse passes through the bundle of His before dividing into the left and right branch bundles which control a network of purkinje fibers throughout the two ventricles. The right bundle branch is located on the interventricular septum where it coordinates contraction in the right ventricle. If the impulse is delayed when passing through the right bundle branch, called a right bundle branch block (RBBB), then the contraction of the right ventricle will also be delayed, and the simultaneous rhythm of the heart will be lost. This is called an aberrant conduction. The muscles on the right side of your dog’s heart are naturally less developed than the left side, so a small delay in right ventricle contraction is often an inconsequential finding that is asymptomatic. More severe RBBB can be a sign of underlying cardiac disease located on the right side of the heart, especially a heartworm infection but also cancer, fibrosis, or a congenital abnormality.

A conduction delay in one of the bundle branches in the heart, means that the ventricles on either side of the heart will not beat in unison. In dogs, a delay in the right bundle branch called a right bundle branch block, slows down the contraction of the right ventricle.

RBBB is very often asymptomatic in dogs. Severe cases show the typical symptoms of heart disease.

• Exercise intolerance
• Weakness
• Lethargy
• Fainting (syncope)
• Increased breathing rate (tachypnea)
• Fluid in the abdomen (ascites)

Types

RBBB can be defined as either complete or incomplete depending on the length of the delay.

• Complete – the entire activation of the left ventricle begins and is completed before the right ventricle contracts
• Incomplete – there is a smaller time delay between the activation of the two ventricles; as little as 60-65 milliseconds

These are some of the factors that could be related to RBBB.

• Increased heart rate (usually incomplete RBBB)
• Cancerous neoplasm
• Fibrosis
• Congenital heart abnormality
• Heartworm infection
• Chagas disease (disease caused by a tropical protozoan parasite)
• Acute pulmonary thromboembolism (blood clot)
• Electrolyte imbalance (hypokalemia)
• Post-surgery complication

With RBBB that is asymptomatic, the abnormal rhythm may only be noted by an electrocardiogram, a device that tracks and measures the rhythms of the heart. An electrocardiogram may be ordered because the veterinarian noticed an abnormal rhythm on a physical exam, or for another reason such as a pre-surgery evaluation. If your dog is showing symptoms of heart disease, an electrocardiogram will usually be ordered to help diagnose the problem. With RBBB, the veterinarian may note a wide QRS complex and a wide S wave. Depending on the degree of aberrancy, the veterinarian may order further tests to diagnose underlying heart disease, including x rays, an echocardiogram, and a heartworm test.

If RBBB is asymptomatic and no underlying cardiac disease is found, no treatment will be necessary. This is frequently the case with incomplete RBBB. Dogs that are experiencing severe symptoms of congestive heart failure may need to be stabilized with supplementary oxygen and medication to reduce fluid retention. Abdominocentesis, insertion of a hollow tube to syphon off fluid, may also be necessary.

If heart disease is found, treatment will focus on resolving the underlying cause of RBBB. Heartworm infection can be treated medically once your dog is stabilized. Medication to reduce blood coagulation levels may be necessary if your dog has a blood clot in the lungs. A few types of cancer may be operable, but this is rare. The same is true of congenital heart conditions especially those that affect the lower ventricle portion of the heart. In many cases, the veterinarian may only be able to treat your dog symptomatically. There is also no cure for Chagas infection which can cause fatal heart disease.

Many mild forms of RBBB are merely due to the right side of the heart being slightly weaker than the left. There is no cure for this condition, but if it asymptomatic it will not affect your dog’s life much. The veterinarian may want to monitor the condition with regular check-ups to make sure it isn’t getting worse and may advise only light to moderate exercise.

If RBBB is caused by underlying cardiac disease, the prognosis will depend on the disease. Heartworm can be treated in dogs, as long it is still in the early stages. Many other heart conditions can be managed medically for a period of time, but most will eventually affect the length of your dog’s life. Many different types of conditions can cause RBBB, so the chances of recovery can vary considerably.