Heart block, AV bundle, or bundle branch block affects the electrical system of the heart. It is different from coronary artery disease, which affects the heart's blood vessels.
In heart block, the heart beats irregularly and more slowly than usual, potentially stopping for up to 20 seconds at a time.
This is due to a delay, obstruction, or disruption along the pathway that electrical impulses travel through to make the heart beat. It can result from injury or damage to the heart muscle or heart valves.
Heart block itself does not usually need direct treatment, but related underlying health conditions do.
A healthy human heart beats at about 60 to 100 times a minute. A heartbeat is one contraction of the heart muscles, which pushes blood around the body.
Normally, every heart muscle contraction is controlled by electrical signals that travel from the atria, or the upper chambers of the heart, to the ventricles, or the lower chambers.
A partial heart block happens when the electrical impulses are delayed or stopped, preventing the heart from beating regularly.
A complete heart block is when the electrical signals stop completely. The heartbeat will drop to about 40 times per minute.
Even changes to impulses that last only a fraction of a second can cause heart block.
Sometimes, a heart block makes it difficult for the heart to pump blood properly through the circulatory system, so the muscles and organs, including the brain, do not get enough oxygen to function properly.
Heart block typically causes lightheadedness, fainting, and palpitations. Depending on the severity of the heart block, this can be dangerous. For example, a third-degree heart block can worsen pre-existing conditions, such as heart failure. It can cause loss of consciousness and even sudden cardiac arrest. There can also be chest pain.
Coronary heart disease, on the other hand, occurs when a waxy substance, called plaque, builds up in the coronary arteries. It can cause chest pain, known as angina, or heart attack, also called myocardial infarction (MI).
There are three types of heart block.
First-degree heart block involves minor heartbeat disruptions, such as skipped beats. It is the least serious type of heart block, and it does not generally require treatment.
Second-degree heart block occurs when some electrical signals never reach the heart, causing dropped or skipped beats. The patient may feel dizzy, and they may need a pacemaker. The ventricle may not contract, as the atrial impulse did not reach the ventricles.
Third-degree or complete heart block is when electrical signals do not travel between the upper and lower chambers of the heart. It is more common in patients with heart disease. Without a pacemaker, there is a serious risk of heart attack.
In a healthy heart, electrical impulses that travel inside a heart muscle instruct it to contract, or beat. The impulses move along a pathway, from the upper heart chambers, through the atrioventricular (AV) node, to the lower chambers.
Along this pathway is a cluster of cardiac fibers. These are called the bundle of His, the "bundle branch block" or the "AV bundle." This bundle divides into two branches, the right and left bundles. The bundles conduct the electrical impulses to the heart ventricles. Each ventricle has a branch.
Damage to one of the branch bundles can cause uncoordinated ventricular contractions, and an abnormal heart beat can result.
A blocked signal on the right side of the heart is not usually serious, but a block on the left side can indicate a higher risk of coronary artery disease, or some other heart problem.
If a person has a heart block, they may experience:
- slow or irregular heartbeats, or palpitations
- shortness of breath
- lightheadedness and fainting
- pain or discomfort in the chest
- difficulty in doing exercise, due to the lack of blood being pumped around the body
People with a heart block may appear healthy, but they may have an underlying heart problem.
The most common cause of heart block is scarring of the heart tissue as people get older. Some people are born with heart block, but older people with a history of heart disease or smoking are most at risk.
The following conditions increase the risk:
- coronary thrombosis
- myocarditis, or inflammation of the heart muscle
- endocarditis, or inflammation of the heart valves
- scar tissue in the heart, following surgery or a heart attack.
Acute, or sudden, heart block may also occur after a heart attack or a heart operation. It can also occur as a complication of Lyme disease.
A physician will discuss symptoms with the patient and listen to their heart. Depending on age and medical history, the doctor may suspect heart disease, and will refer the patient to a cardiologist, or a heart specialist.
There are a number of diagnostic tests for heart block.
An electrocardiogram (ECG) is the most common test. It records heart activity. Probes placed on the skin of the chest show the electric impulses through the heart as wave patterns.
Wave abnormalities may indicate heart block. An ECG can also reveal whether the left or right branch is affected.
Holter tape is a portable device which records all the patient's heartbeats. The patient wears it under their clothing, and it records information about the electrical activity of the heart while the person carries out their normal activities for 1 to 2 days.
When symptoms occur, the patient presses a button. This creates a record of the heart rhythms that are present at that moment.
An echocardiogram is an ultrasound scan that allows the doctor to see the heart muscles and valves.
An electrophysiology test uses tiny electrical shocks to determine the cause of the abnormal rhythm, and where in the heart it is.
In a tilt-table test, the patient lies on a bed that changes position. This can provoke arrhythmia, or abnormal heartbeats.
There is no heart-block-specific treatment. Most people with bundle branch block have no symptoms, and they do not require treatment. However, any underlying causes, such as hypertension, will need treatment.
If a person with left bundle branch block has a heart attack, reperfusion therapy may be given to restore blood flow through blocked arteries.
This can be done by using an anti-clotting agent, such as streptokinase, to dissolve blood clots and increase blood flow to the heart. However, anti-clotting drugs increase the risk of bleeding.
An artificial pacemaker, a small, battery-operated device, may be implanted under the skin in a patient with a history of fainting. It is placed near the collarbone during a surgical procedure lasting 1 to 2 hours, under a local anesthetic.
Many pacemakers can be set to produce an electrical impulse only when needed. Some can sense if the heart stops beating, and produce an electrical impulse to restart it. The battery can last many years.
Pacemakers are not affected by cell phones, personal stereos, or household appliances, but a person with a pacemaker should not undergo a magnetic resonance imaging (MRI) scan.
People with a left side bundle branch block have a higher risk of complications than those with a right side block.
Possible complications include:
- arrhythmia, or irregular heart beat
- bradycardia, or low heart rate
- insufficient contraction
- cardiac arrest and circulatory failure
- sudden cardiac death, which can be fatal within one hour of symptoms starting
Heart block is not always avoidable, but the risk of heart disease can be reduced by consuming a healthy diet, exercising regularly, minimizing alcohol consumption, and avoiding tobacco.