Also called: Third Degree Heart Block, Atrioventricular Block, First Degree Heart Block, Complete Heart Block, Movitz AV Block, Second Degree Heart Block, AV Block, Complete AV Block
Heart block (or “AV block”) is a condition in which electrical impulses have been slowed or blocked in their normal path through the heart’s conduction system. The result is a delayed or complete lack of electrical communication between the upper chambers of the heart (atria) and the lower chambers of the heart (ventricles).
Heart block may be caused by a variety of factors, including many medical conditions such as past heart attacks, myocarditis or electrolyte imbalances. It can also be caused by certain medications (e.g., beta blockers, calcium-channel blockers or digitalis).
Both the symptoms and treatments for heart block depend on its degree of severity. Mild forms of heart block usually produce no symptoms and may not require treatment. However, more severe forms can lead to serious symptoms such as weakness, dizziness, fainting or even sudden cardiac death. Some people with second-degree heart block and virtually all people with third-degree or complete heart block will require a permanent pacemaker that can better regulate the heart rhythm.
In rare cases, physicians may create complete heart block to treat the abnormal heartbeats of atrial fibrillation. Medications are often tried first, but if they are unsuccessful, a physician can destroy an area of the heart’s conduction system called the atrioventricular (AV) junction, blocking the errant signals. As with natural cases of complete heart block, these patients require a pacemaker to regulate the heart’s rhythm.
About heart block
Heart block is the delayed or complete lack of electrical communication between the upper chambers of the heart (atria) and the lower chambers of the heart (ventricles). Normally, the ventricles are stimulated to contract by electrical impulses that travel through the conduction system from the upper-right chamber of the heart to the ventricles. If these impulses are delayed as they travel to the ventricles, an abnormally slow heart rhythm (bradycardia) could result. If these impulses are completely blocked from reaching the ventricles, they will fail to stimulate a heartbeat at all. In this case, secondary impulses may arise in the ventricles (producing ventricular escape beats). However, if no secondary beats arise, this situation will result in death without immediate treatment.
Certain factors may contribute to heart block. They include:
Lack of oxygen-rich blood to the heart (cardiac ischemia), due to either blockages in the coronary arteries (coronary artery disease) or damage to the heart from a past heart attack.
Electrolyte imbalances, such as high (hyperkalemia) or low (hypokalemia) levels of potassium.
Disease or normal aging of the heart’s conduction system.
Myocarditis.
Use of certain heart medications, such as beta blockers, calcium-channel blockers or digitalis (associated with first–degree heart block).
Heart surgery.
Rheumatic fever.
Congenital heart condition.
Excellent physical conditioning. Well-conditioned athletes sometimes develop first-degree heart block.
In some cases, physicians may surgically induce heart block to treat the abnormal heartbeat known as atrial fibrillation. When atrial fibrillation cannot be controlled by other means, a cardiac electrophysiologist is able to ablate (destroy) the atrioventricular (AV) junction and induce complete heart block. The patient would then become dependent on a permanent pacemaker to govern the heartbeat in the lower chambers of the heart. Although this procedure is not a cure for atrial fibrillation, it prevents the rapid and irregular pulse associated with the condition and can be quite effective in reducing symptoms.
Types and differences of heart block
Heart block is classified according to the extent to which electrical activity is disrupted, as described below:
First-degree heart block. The electrical impulses are slowed as they pass through the conduction system, but all of them successfully reach the ventricles. First-degree heart block rarely causes any symptoms or problems. In most cases, the source of the delay is in the atrioventricular node. Approximately one in every 1,000 people has first-degree heart block. Well-trained athletes are particularly likely to have it because their hearts pump more efficiently, requiring fewer contractions than the hearts of the general population. Heart block may also be caused by certain medications, such as beta blockers, calcium-channel blockers or digitalis (a type of inotropic drug). A change of medication often resolves the condition. No other treatments are generally necessary.
Type I second-degree heart block (also known as Mobitz Type I second-degree AV block or Wenckebach AV block). The electrical impulses are delayed longer with each heartbeat until a beat is skipped entirely. The source of the interference is usually within the AV node. The condition may sometimes cause dizziness.
Type II second-degree heart block (also known as Mobitz Type II second-degree AV block). Some of the electrical impulses are unable to reach the ventricles because of interference from somewhere below the AV node (e.g., the bundle of His). In contrast with type I second-degree heart block, this condition is generally less common and carries a higher risk of developing into complete (third-degree) heart block. A physician may recommend an artificial pacemaker for the abnormally slow heartbeat (bradycardia) associated with this condition.
Third-degree heart block (also known as complete heart block or complete AV block). None of the electrical impulses can reach the ventricles, due to a problem that may lie anywhere between the atrioventricular node and the bundle branches, although the latter is more common. In the absence of any electrical impulses from the atria, the ventricles may generate some impulses on their own (called ventricular escape beats) via secondary impulse generators. However, these natural “backups” are usually very slow and are generally unable to sustain the full functioning of the heart muscle. Therefore, complete heart block poses a medical emergency with potentially severe symptoms and a serious risk of the heartbeat stopping completely (cardiac arrest). If a pacemaker cannot be implanted immediately, then a temporary pacemaker wire might be used to keep the heart pumping until surgery can be performed.
Signs and symptoms of heart block
The symptoms associated with heart block depend on the degree of heart block present:
First-degree heart block generally produces no symptoms, and in otherwise healthy individuals it usually does not progress to more serious forms. A notable exception to this is patients with heart failure, in whom first-degree heart block may further worsen the pumping function of the heart.
Type I second-degree heart block involves regularly occurring skipped beats, which patients may or may not notice. No other symptoms are generally reported, and the condition tends to resolve on its own.
Type II second-degree heart block may produce symptoms if the heart’s output of blood begins to decrease. These symptoms include dizziness or even fainting (syncope). Some people may feel confused or unsteady, or find that they are becoming easily fatigued. If any of these signs or symptoms are present, then treatment is warranted.
Third-degree heart block may produce any of the signs or symptoms associated with type II second-degree heart block, as well as symptoms somewhat like those preceding a heart attack. A patient is also at risk of convulsions and/or collapsing (due to cardiac arrest). This is the most severe form of heart block and the most likely form to be treated with a permanent pacemaker.
Diagnosis methods for heart block
The first step in diagnosing heart block is for a physician to obtain a patient’s complete medical history and to give the patient a complete physical examination. Blood tests may also be ordered to rule out electrolyte imbalances, or detect cardiac enzymes associated with a past heart attack or abnormally high levels of prescribed medications in the bloodstream.
Next, the physician will order a common, painless test called an electrocardiogram (EKG). This test measures the heart’s electrical activity at rest and under stress (stress test). The information obtained from the test will help the physician determine if the electrical impulses are being delayed or blocked as they travel through the heart’s normal conduction system. By examining the results of an EKG, a physician will likely be able to identify the distinctive heart rhythms characteristic of a heart block and to prescribe appropriate treatments, if necessary.
If heart block occurs only infrequently, it might not be picked up by tests during a scheduled office visit. Therefore, a portable EKG (ambulatory electrocardiogram) may be ordered. A portable EKG measures heart activity while the patient is “on the move” instead of in a physician’s office. The heart’s electrical activity is recorded on a portable tape inside the machine. There are two categories of AECG (ambulatory electrocardiography) recorders:
Continuous recorders (such as a Holter monitor) are set to monitor heart activity continuously for about 24 hours.
Event recorders (such as a loop monitor) can record specific symptoms and over a longer period of time (weeks or even months) when prompted by the patient.
Depending on the results of the EKG and/or portable EKG, a physician may then order a test called an electrophysiology study. This test requires the insertion of several thin tubes (catheters) into veins (usually in the groin) from where they are guided to the heart. This test enables physicians to perform specific measurements of the heart’s electrical activity and pathways.
Other tests used to diagnose heart block include:
Echcardiogram. Uses sound waves to create pictures of the heart and its blood vessels. A moving image of the patient’s beating heart is played on a video screen, where a physician can study the heart’s thickness, size and function. This test may be ordered when a problem is suspected in the heart muscle or one the heart’s valves.
Tilt table test. Conducted on a pivoting table, this test involves securing the patient on his or her back, and then tilting the table upright (head up and feet down). This test may be used to determine the cause of fainting spells (syncope). This is not a direct test for heart block. It is used to rule out conditions with similar symptoms.
Treatment and prevention for heart block
Treatment generally depends on the degree of heart block that is present:
First-degree heart block generally requires no treatment, although some people may need to be treated for an underlying electrolyte imbalance. If medications are causing the condition, a physician may reevaluate the dosage or advise the patient to stop taking it. Medications should not be adjusted without consulting a medical professional. A physician may recommend follow-up with a cardiologist to be sure that the patient does not develop a more severe form of heart block. Follow-up is particularly important if first-degree heart block developed as a result of a past heart attack or myocarditis. In patients with severe heart failure and first-degree heart block, implantation of a pacemaker may sometimes improve cardiac performance. Otherwise, further treatment is generally unnecessary and prognosis is excellent.
Type I second-degree heart block is generally treated by addressing any underlying conditions that are contributing to it. Temporary pacing and/or medication (e.g., atropine) may be required if the heartbeat is too slow, but a permanent pacemaker is generally not necessary unless the condition worsens.
Type II second-degree heart block often produces noticeable symptoms and carries a significant risk of potentially life-threatening complications. Therefore, treatment is very important. A physician may administer medications (e.g., atropine) and recommend the implantation of an artificial pacemaker. If the condition worsens to third-degree heart block, then a temporary pacemaker wire may be needed during the medical crisis until a permanent pacemaker can be inserted.
Third-degree heart block patients almost always require an artificial pacemaker to better regulate the electrical activity of the heart. If a medical crisis occurs before the pacemaker can be implanted, then a temporary pacemaker wire may be used to keep the heart beating. Most patients who are diagnosed with complete heart block will require placement of a permanent pacemaker, unless a treatable cause is identified and corrected.
Researchers are exploring various gene therapies as a way of transforming certain tissue cells into those that can pace the heart.
The best strategy for preventing heart block is to prevent the heart conditions that contribute to heart block.
Inducing heart block to treat atrial fibrillation
Although physicians generally recommend medications to treat atrial fibrillation, other procedures may be necessary to control the condition if the medications are unsuccessful. One alternative is catheter ablation, which is a minimally invasive procedure that destroys the tiny, specific areas of the heart that were causing the arrhythmia.
In patients whose atrial fibrillation cannot be controlled by other means, a cardiac electrophysiologist is able to ablate (destroy) the atrioventricular (AV) junction and induce complete heart block. The patient would then become dependent on a permanent pacemaker to govern the heartbeat in the lower chambers of the heart.
Although this procedure is not a cure for atrial fibrillation, it prevents the rapid and irregular pulse associated with the condition and can be quite effective in reducing symptoms.
Questions for your doctor
Preparing questions in advance can help patients have more meaningful discussions with their physicians about their conditions. Patients may wish to ask their doctors the following questions related to heart block:
What type of heart block do I have?
I have no symptoms. How can you tell I have heart block?
How long can it take to diagnose heart block if I do not have symptoms all the time?
Will my heart block worsen to another type, or get better?
How long does heart block take to worsen?
If I get a pacemaker, will that eliminate my symptoms?
Will I notice the pacemaker’s effect on my heart?
Can I take daily medication to treat heart block?
If my heart block worsens can it cause a heart attack?
Will a pacemaker affect other conditions I have or medications I take?
