Heart Block or Conduction Delay (Left Bundle) in Dogs

Contractions in the heart are initiated by electrical impulses that act on the muscle cells. As these impulses move through nodes and specialized muscle tissue in the heart, they coordinate the timing of contractions in all four chambers. As the atria contract, a message is passed to the atrioventricular (AV) node at the top of the heart which delays the impulse long enough to allow time for the blood to move from the atria into the ventricles. After leaving the AV node, the impulse moves down the heart, through the bundle of His, and separates into the left and right bundle branches which initiate a simultaneous contraction of the left and right ventricles. The left bundle branch is located on the interventricular septum. After passing through the left bundle branch, the signal divides further to move through the left anterior and left posterior fascicles which control a network of Purkinje fibers in the left ventricle. If there is a block in the left bundle branch (LBBB) the contraction of the left ventricle will be delayed in comparison with the right ventricle causing an uneven heartbeat. This is known as an aberrant conduction. Although LBBB does not directly affect the function of the heart, it is usually a sign of significant heart disease. The left side of the heart, which receives oxygen-rich blood from the lungs and transports it throughout the body, is stronger and more developed than the right side, so blocks on this side rarely happen without a significant cause, unlike the right ventricle which can sometimes become delayed just through strenuous exercise. Any disease which affects the left side of the heart can cause LBBB, including cancer, congenital abnormalities, or cardiomyopathy. LBBB often appears on an electrocardiogram as an early sign of heart disease, in which case your dog may still be asymptomatic. If the aberrancy is discovered in the later stages, dogs will often show symptoms of congestive heart failure on the left side.

A conduction delay in the left bundle branch in the heart means that the contraction of the left ventricle will be delayed. This is called a left bundle branch block. In dogs, this is usually a sign of underlying heart disease.

These are some of the signs you might notice in a dog with LBBB.

• Exercise intolerance
• Weakness
• Lethargy
• Fainting (syncope)
• Increased breathing rate (tachypnea)
• Coughing
• Difficulty breathing
• Bluish mucus membranes (cyanosis)

Types

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

• Complete – the entire activation of the right ventricle begins and is completed before the left ventricle contracts
• Incomplete – there is a smaller time delay between the activation of the two ventricles; as little as 60-65 milliseconds
• Left anterior fascicular block – occurs below the left bundle branch, where the impulse has divided further

These are some of the heart conditions that could cause LBBB.

• Cardiomyopathy
• Congenital abnormalities
• Cancerous neoplasm in the heart
• Trauma
• Scarring after heart surgery
• Fibrosis

If your dog is showing symptoms of heart disease, the veterinarian will physically examine him, listen to the heart through a stethoscope, and order a number of tests to determine what is causing the problem. One of these tests will be an electrocardiogram, a device that records and analyses the heart rhythms. This will specifically show LBBB which usually has a widened and notched QRS complex (a combination of several factors that record depolarization between the right and left ventricles). An aberrant conduction on an electrocardiogram can diagnose that the problem is located in the left ventricle, but the veterinarian will need to order further tests to determine what is causing LBBB. This usually includes x rays and an ultrasound of the heart, called an echocardiogram. 

If your dog is not yet showing symptoms, LBBB may be discovered on an electrocardiogram ordered for another purpose, such as a surgical evaluation or a routine check-up. In this case, the aberrant conduction will still warrant further testing since it is often a sign of developing heart disease.

Since LBBB is a symptom rather than a disease itself, treatment will depend on the underlying cause. Dogs that are experiencing severe symptoms of congestive heart failure may need to be stabilized with supplementary oxygen and intravenous medications to support heart function and electrolyte balance. In dogs, heart surgery is much less common than in humans, so supportive treatment for the symptoms of congestive heart failure may be all the veterinarian recommends. Some experiments have succeeded in using radio frequency ablation to stabilize the heart rhythms in dogs, so this may be one of the treatments your veterinarian recommends. A few congenital abnormalities, such as a defect in the atrioventricular septum, can be treated surgically, and some cancerous tumors may be operable depending on the placement of the tumor. In this case, the veterinarian may refer your dog to a specialist surgeon.

Full recovery from a serious heart condition is rare in dogs, but symptoms of congestive heart failure may be manageable with medication and periodic treatment for an extended period of time. LBBB is often a sign of heart disease in the early stages when it may be more treatable, so discovering this abnormal rhythm early may help with treatment and give your dog a longer survival time. Prognosis with LBBB can vary greatly depending on the underlying condition. Your veterinarian will be able to give you a better idea of your dog’s chances upon diagnosis.