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Heart block is a delay in the conduction of electrical current as it passes through the atrioventricular node, bundle of His, or both bundle branches, all of which are located between the atria and the ventricles.
Heart blocks can be divided into
Heart block is classified as first-degree when electrical conduction to the ventricles is slightly delayed, second-degree when conduction is intermittently blocked, or third-degree (complete) when conduction is completely blocked.
Most types of heart block are more common among older people. The most common causes are fibrous tissue developing in the heart's conduction system and coronary artery disease. However, a few cases of heart block are caused by drugs, particularly those that slow conduction of electrical impulses through the atrioventricular node (such as beta-blockers, diltiazem, verapamil, digoxin, and amiodarone), rheumatic heart disease, or sarcoidosis that affects the heart (see Sarcoidosis).
In first-degree heart block, every electrical impulse from the atria reaches the ventricles, but each is slowed for a fraction of a second as it moves through the atrioventricular node. First-degree heart block is common among well-trained athletes, teenagers, young adults, and people with a highly active vagus nerve. This disorder rarely causes symptoms and can be detected only by electrocardiography (ECG), which shows the conduction delay.
In second-degree heart block, only some electrical impulses reach the ventricles. The heart may beat slowly, irregularly, or both. Some forms of second-degree heart block progress to third-degree heart block.
In third-degree heart block, no impulses from the atria reach the ventricles, and the ventricular rate and rhythm are controlled by the atrioventricular node, bundle of His, or the ventricles themselves. These substitute pacemakers are slower than the heart’s normal pacemaker (sinus or sinoatrial node) and are often irregular and unreliable. Thus, the ventricles beat very slowly—less than 50 beats per minute and sometimes as slowly as 30 beats per minute. Third-degree heart block is a serious arrhythmia that can affect the heart’s pumping ability. Fatigue, dizziness, and fainting are common. When the ventricles beat faster than 40 beats per minute, symptoms are less severe.
First-degree heart block requires no treatment even when it is caused by a heart disorder.
Some people with second-degree heart block require an artificial pacemaker. Almost all people with third-degree heart block require an artificial pacemaker.
A temporary pacemaker may be used in an emergency until a permanent one can be implanted. Most people need an artificial pacemaker (see Keeping the Beat: Artificial Pacemakers) for the rest of their lives, although heart rhythm may return to normal if the cause of the heart block resolves—for example, after the drug that caused the heart block is stopped or after recovery from a heart attack.
Bundle branch block is a type of conduction block involving partial or complete interruption of the flow of electrical impulses through the right or left bundle branches.
The bundle of His is a group of fibers that conducts electrical impulses from the atrioventricular node. The bundle of His divides into two bundle branches. The left bundle branch conducts impulses to the left ventricle, and the right bundle branch conducts impulses to the right ventricle. Conduction may be blocked in the left or right bundle branch.
Bundle branch block usually causes no symptoms. Right bundle branch block is not serious in itself and may occur in apparently healthy people. However, it may also indicate significant heart damage due to, for example, a previous heart attack. Left bundle branch block tends to be more serious. In older people, it often indicates coronary artery disease due to atherosclerosis.
Bundle branch block can be detected by electrocardiography (ECG—see Electrocardiography). Each type of block produces a characteristic pattern.
Usually, no treatment is needed for either type. However, an artificial pacemaker (see Keeping the Beat: Artificial Pacemakers) may be implanted in people who are at high risk of complete heart block (such as people with certain types of second-degree heart block) to maintain the heart rate if complete heart block occurs.
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