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Exit block from a parasystolic focus is recognized when automatic discharges fail to become manifest during the excitable phase of the ventricular cycle. In the present study, an apparently complete exit block and a persistently concealed ventricular parasystole (VP) resulted from an exit refractory period (ExRP) longer than the sinus cycle length. Slowing of the heart rate caused the concealed VP to become apparent in the form of ventricular escapes of variable coupling and as an idioventricular rhythm that failed to show initial "warming up." These features reflect the fact that the automatic focus is protected from activity of the sinus impulses, which, however, can induce a prolonged ExRP. Seven cases of concealed VP are discussed. In two, concealment occurred spontaneously during the follow-up of a typical VP; in three, it was provoked by conduction-depressing drugs; and in two, ventricular escapes of varying coupling were shown to represent the manifestation of a concealed and previously unrecognized VP. In two cases, isoproterenol caused reappearance of typical VP, probably through a shortening of the ExRP. While isoproterenol may be useful for uncovering a concealed VP, conduction-depressing drugs may be used to provoke or increase exit block. Total extinction of the VP seemed to occur in two patients during follow-up studies.

Concealed ventricular parasystole uncovered in the form of ventricular escapes of variable coupling.