CARDIAC ARRHYTHMIA

variation from the normal rate or regularity of the heartbeat, usually resulting from irregularities within the cardiac conduction system. Arrhythmias occur in both normal and diseased hearts and have no medical significance in and of themselves, though they may endanger heart function when coupled with other cardiac abnormalities. The most common arrhythmias are sinus arrhythmia, in which heart rate accelerates with the intake of breath and decelerates with expiration; tachycardia (q.v.), which is a regular acceleration of the heart rate; bradycardia (q.v.), a regular slowing of the heart rate; and premature atrial or ventricular beats, which are extra contractions within otherwise normal heart rhythm. While occasional irregularities are normal, prolonged or chronic arrhythmias associated with some forms of heart disease may reduce cardiac output, lowering blood pressure and affecting the perfusion of vital organs with blood, and can precipitate heart failure. Severe arrhythmias can trigger atrial or ventricular fibrillation, in which the heart beats ineffectively at many times its normal rate (see atrial fibrillation; ventricular fibrillation). Cardiac arrhythmias reflect the failure of the sinus node, the normal cardiac pacemaker, to maintain a regular heartbeat, usually because of defects in the various pathways by which electrical impulses are carried to different areas of the heart. Anatomical defects or disease can slow down or speed up the propagation of electrical impulses, causing them to arrive out of the normal rhythm, or can turn the impulses back on their path, short-circuiting the pacemaker. Many arrhythmias can be corrected through cardioversion, in which an electric shock is applied to override abnormal impulses, or by drugs such as quinidine or digitalis, which directly affect the sensitivity of the heart muscle and the conduction rate of electrical impulses.