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An ectopic pacemaker or ectopic focus is an excitable group of cells that causes a premature heart beat outside the normally functioning SA node of the human heart. Acute occurrence is usually non-life-threatening, but chronic occurrence can progress into tachycardia, bradycardia or ventricular fibrillation. In a normal heart beat rhythm, the SA node usually suppresses the ectopic pacemaker activity due to the higher impulse rate of the SA node. However, in the instance of either a malfunctioning SA node or an ectopic focus bearing an intrinsic rate superior to SA node rate, ectopic pacemaker activity may take over the natural heart rhythm. This phenomenon (an intrinsically slower pacemaker activity being unmasked by failure of faster pacemaker tissue 'upstream') is called an escape rhythm, the lower rhythm having escaped from the dominance of the upper rhythm. As a rule, premature ectopic beats (i.e. with a shorter than the prevailing preceding R-R' interval) indicate increased myocyte or conducting tissue excitability, whereas late ectopic beats (i.e. with a prolonged preceding R-R' interval) indicate proximal pacemaker or conduction failure with an escape 'ectopic' beat. ==Etiology== Ectopic pacemakers can occur within healthy hearts in response to various stimulating events, such as: *Increased local parasympathetic nervous system activity 〔 *Elevated sympathetic nervous system output 〔 *Overstimulation from drugs such as caffeine, digitalis and catecholamines. *Cardiac Ischemia (particularly ventricular ischemia) - The membranes of apoptotic (dying) cells become "leaky" and cause surrounding tissue to become hyperkalemic or hypercalcemic (high concentration of potassium/calcium), causing random excitation.〔Keller-Wood, Maureen. "Electrical Activity in the Heart." Lecture at the University of Florida College of Pharmacy, 23-Oct-2007.〕 They can also occur within unhealthy hearts, caused by: *Infection *Disease, such as sinus venosis and atrial defects. *SA node dysfunction, (1st degree block) which can cause the rate of impulse to slow.〔 *SA node blockage so that impulses never leave the atria.〔 * AV node blockage (3rd degree block) prevents normal conduction across ventricles.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「ectopic pacemaker」の詳細全文を読む スポンサード リンク
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