Modulation of ventricular parasystole by extraneous depolarizations.

The ectopic cycle length of a ventricular parasystole was influenced in a predictable fashion (modulated) by nonparasystolic beats. Although the interectopic intervals were not multiples of the ectopic cycle length (as in "classical" parasystole), the diagnosis could be established because: a) nonparasystolic beats falling during the first half of the cycle produced an 8-12% shortening of the ectopic cycle length; and b) nonparasystolic beats occurring during the second half of the cycle decreased the ectopic cycle length by 9-14%. The events occurring in this case are further proof that the classical criteria for the diagnosis of parasystole have to be reevaluated.