Symmetry, broken symmetry, and restored symmetry of apparent pure ventricular parasystole

Previous reports dealing with the dynamics of "pure" parasystole using mathematic and electronic (pacing-induced) models, respectively, showed that a number of predictions emerged naturally from mathematic analysis.^1,2 Therefore, it appeared of interest to determine whether they would also apply to a series of patients with clinical (apparently) "pure" ventricular parasystole who were monitored for relatively long periods of time. In contrast to previous studies, Glass et al¹ had the ingenious idea of analyzing the number of sinus beats interposed between 2 consecutive manifest parasytolic beats. They observed that, with the rates being practically constant, a detailed mathematic description could be made using 2 parameters: the ratio of ectopic cycle length to sinus cycle length and the ratio of ventricular refractory period to sinus cycle length. From a correlation of these parameters 3 basic rules emerged: (1) There are at most 3 different values for the number of sinus beats in between parasystolic beats. (2) Only 1 of these values is odd. (3) The sum of the 2 lesser values is 1 less than the larger value.