Atrio-ventricular junctional parasystole: modulation by sinus impulses.

A rare case of atrio-ventricular (A-V) junctional parasystole with a quinquageminy and a trigeminy in the presence of a right bundle branch block (RBBB) is reported, in which the ectopic beats showed the same QRS complex as that of the conducted sinus beats. After taking a bath and after intravenous administration of 1.0 mg of atropine, viz. the maneuver to block the vagal effects, the short interectopic interval shortened from control of 2.65-2.70 sec to 2.08 sec, when the shortest interval of 0.98 sec was observed. Thus, the short interectopic intervals resulted from a 2:1 exit block. When the intrinsic ectopic cycle length was about 1.52 sec, the second of four intervening sinus beats in quinquageminy reset the parasystolic pacemaker and a 2:1 exit block ensued, which was attributed to entrance block failure due to a supernormal phase of excitability and conductivity in the His bundle.