Section 3. Heart and Blood Vessel Disorders

Chapter 27. Abnormal Heart Rhythms

Topics: Introduction | Atrial Premature Beats | Atrial Fibrillation and Atrial Flutter | Paroxysmal Supraventricular Tachycardia | Wolff-Parkinson-White Syndrome | Ventricular Premature Beats | Ventricular Tachycardia | Ventricular Fibrillation | Pacemaker Dysfunction | Heart Block | Bundle Branch Block

Heart Block

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 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.

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. However, the disorder also occurs in people with rheumatic fever, sarcoidosis that affects the heart (see Section 4, Chapter 50), or other structural heart diseases. It may be caused by drugs, particularly those that slow conduction of electrical impulses through the atrioventricular node (such as beta-blockers, diltiazem, verapamil, and amiodarone). This disorder produces no 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.

Treatment

First-degree heart block requires no treatment even when it is caused by heart disease. 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 for the rest of their lives, although heart rhythm may return to normal if the cause of the heart block resolves--for example, after recovery from a heart attack.