parasystole [Gk, para + systole, contraction] , an independent ectopic rhythm whose pacemaker cannot be discharged by impulses of the dominant (usually the sinus) rhythm because an area of depressed conduction surrounds the parasystolic focus. In the classic parasystole, the interectopic intervals are exact multiples of a common denominator reflecting the protected status of the parasystolic focus; however, the ectopic focus is usually influenced by the phasic events around its protection zone (modulated parasystole). Thus the sinus rhythm may modulate the parasystolic rhythm so that criteria for absolute, undisturbed regularity are not fulfilled. Fusion beats are common because of the simultaneous discharge of the ventricles by both the sinus and the parasystolic impulses.
