The junctional premature beat: an instructional exercise in modes of concealment

?The relative brevity of the main His bundle refractory period compared with that of the A-V node above, and the trifascicular system below, makes it likely that premature beats originating in the His bundle will encounter physiologic delay, or block in both antegrade and retrograde modes. Two clinical cases of junctional premature beats are presented, which demonstrate many facets of concealment (antegrade, retrograde and bidirectional). Hitherto unreported is a ventricular echo which was induced by a junctional premature beat, the antegrade concealment of which was due to functional trifascicular block.