Second degree heart block describes a disorder of the conduction pathways in the heart. Normally, an electrical impulse initiates at the sinus node located in the right atrium. Cells carry the impulse to the atrioventricular node or AV node located in the center of the heart between the atria and ventricles. The electrical impulse is delayed at the AV node. Once the atrium contracts, the electrical impulse passes to the ventricles. With second degree heart block, some electrical impulses are not conducted from the atria to the ventricle and skipped heart beats occur. The causes are acquired, congenital or unknown.
Acquired Second Degree Heart Block
Partial interruption in the conduction of electrical impulses results from swelling and scarring of heart tissue. Acquired second degree heart block develops because of swelling and scarring of cardiac tissue. According to the Merck Manuals website, idiopathic fibrosis and sclerosis of the cardiac conductive system are the most common causes of acquired heart block. Idiopathic fibrosis refers to the spontaneous development of excessive fibrous tissue in the heart. Sclerosis describes the scarring or hardening of heart tissue. Ischemic heart disease is responsible for 40 percent of all cases of acquired heart block. Cholesterol deposits which block the coronary arteries lead to ischemic heart disease. The blockage prevents adequate blood flow to the heart muscle. Citing the American College of Cardiology, about 14 million Americans have ischemic heart disease. Within this group, 4 million are asymptomatic and are not aware of their increased risk for angina, heart attack, or sudden death. Referring back to the Merck Manuals website, the remaining 10 percent of all cases of acquired heart block result from medications, such as calcium channel blockers, digoxin and amiodarone. Calcium channel blockers are used to treat high blood pressure. Digoxin is prescribed for controlling atrial fibrillation and the treatment of congestive heart failure. Amiodarone corrects irregular heart rhythms in the ventricles of the heart.
Congenital Second Degree Heart Block
Congenital heart block is present at birth. According to the National Heart Lung and Blood Institute, infants who are born to women with autoimmune diseases such as lupus will have a form of congenital heart block. The mother produces antibodies in response to the autoimmune disease. These antibodies cross the placenta and damage the unborn child's heart. Congenital heart block is a possible consequence.
Wenckebach Second Degree Heart Block
As cited on the website of Heart Rhythm Society, some healthy individuals will have Wenckebach second degree heart block. Also referred to as Mobitz type 1 heart block, this disorder causes dizziness and sometimes occurs during sleep. The cause is not documented.
