Unusual capture phenomena during interaction of dissociated sinus and junctional rhythms: spontaneous expression of intranodal duality and gap phenomenon.

A ten year old boy presented with an irregular pulse which was caused by AV dissociation with capture beats and pauses. Electrophysiologic studies revealed interaction between sinus rhythm and junctional rhythm. Capture beats were of several types. Early captures blocked below the His bundle but reset the junctional pacemaker causing pauses. Earlier sinus beats conducted with marked atrio His delay. Measurement of intervals during spontaneous rhythm demonstrated a type I gap phenomenon due to dual AH pathways. This case illustrates the value of electrophysiologic studies in analysing complex arrhythmias.