Arrhythmia, cardiac

An abnormality of the rhythm or rate of the heartbeat. Arrhythmias, which are caused by a disturbance in the electrical impulses in the heart, can be divided into 2 main groups: tachycardias, in which the rate is faster than normal, and bradycardias, in which the rate is slower. In sinus tachycardia, the rate is raised, the rhythm is regular, and the beat originates in the sinoatrial node (see pacemaker). Supraventricular tachycardia is faster and the rhythm is regular. It may be caused by an abnormal electrical pathway that allows an impulse to circulate continuously in the heart and take over from the sinoatrial node. Rapid, irregular beats that originate in the ventricles are called ventricular tachycardia. In atrial flutter, the atria (see atrium) beat regularly and very rapidly, but not every impulse reaches the ventricles, which beat at a slower rate. Uncoordinated, fast beating of the atria is called atrial fibrillation and produces totally irregular ventricular beats. Ventricular fibrillation is a form of cardiac arrest in which the ventricles twitch very rapidly in a disorganized manner. Sinus bradycardia is a slow, regular beat. In heart block, the conduction of electrical impulses through the heart muscle is partially or completely blocked, leading to a slow, irregular heartbeat. Periods of bradycardia may alternate with periods of tachycardia due to a fault in impulse generation (see sick sinus syndrome). A common cause of arrhythmia is coronary artery disease, particularly after myocardial infarction. Some tachycardias are due to a congenital defect in the heart's conducting system. Caffeine can cause tachycardia in some people. Amitriptyline and some other antidepressant drugs can cause serious arrhythmias if they are taken in high doses. An arrhythmia may be felt as palpitations, but in some cases arrhythmias can cause fainting, dizziness, chest pain, and breathlessness, which may be the 1st symptoms. Arrhythmias are diagnosed by an ECG. If they are intermittent, a continuous recording may need to be made using an ambulatory ECG. Treatments for arrhythmias include antiarrhythmic drugs, which prevent or slow tachycardias. With an arrhythmia that has developed suddenly, it may be possible to restore normal heart rhythm by using electric shock to the heart (see defibrillation). Abnormal conduction pathways in the heart can be treated using radio frequency ablation during cardiac catheterization (see catheterization, cardiac). In some cases, a pacemaker can be fitted to restore normal heartbeat by overriding the heart’s abnormal rhythm.