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A 15-year-old boy with congenital heart disease showed two types of wide QRS complexes in the limb leads. One type appeared during sinus rhythm with a QRS axis of ca. -45°, and the other during isorhythmic A-V dissociation with a QRS axis of ca. +110°. Transitions between the two rhythms were gradual, and several intermediate QRS complexes intervened. Possibility of parasystolic fusion of upper and lower pacemakers during transitions was discussed.
Kimiaki Nakamoto (Sat,) studied this question.