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MSH: A cardiac arrhythmia that is caused by interaction of two independently initiated cardiac impulses of different rates from two separate foci. Generally one focus is the SINOATRIAL NODE, the normal pacemaker. The ectopic focus is usually in the HEART VENTRICLE but can be in the HEART ATRIUM or the ATRIOVENTRICULAR NODE. Modulation of the parasystolic rhythm by the sinus rhythm depends on the completeness of entrance block surrounding the parasystolic focus.
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