Mechanisms of ventricular parasystole.

Eight cases of ventricular parasystole are reported. In all these cases, regardless of whether parasystole seems intermittent or "continuous," the presence of second degree entrance block of the Mobitz type I was suggested. Parasystole alternated with concealed extrasystolic bigeminy showing occasional reentrant extrasystoles. Such intermittent parasystole appears to originate in the reentrant path of extrasystoles. Reentrant extrasystolic bigeminy was seen in a comparatively rapid sinus rhythm, whereas parasystolic bigeminy was seen in a comparatively slow sinus rhythm. The difference between the interectopic intervals during parasystolic bigeminy and during (manifest or concealed) extrasystolic bigeminy was comparatively small so that occasionally the difference was not distinct; on such an occasion the case showed a seemingly "continuous" parasystole. These observations strongly suggest the possibility that most cases of parasystole, whether intermittent or "continuous," may be governed by incomplete entrance block of the second degree.