Irregular ventricular tachycardia: a possible manifestation of longitudinal dissociation within the reentry pathway

Sustained monomorphic ventricular tachycardia is usually regular; that is, it is associated with constant R-R intervals. In several cases, however, the cycles of ventricular tachycardia are more or less variable. Fifty-four cases of sustained monomorphic ventricular tachycardia were evaluated in order to assess whether tachycardia was regular. Nine cases were defined as irregular (i.e., the R-R cycles varied by more than 40 msec throughout a 1-minute recording). In five cases tachycardia was "regularly irregular," since the R-R cycles could be divided into two separate groups: the group of long cycles and that of short cycles. In these cases the variability manifested according to a defined and constant pattern: bigeminal pattern (alternation of short and long cycles), trigeminal pattern (two short cycles followed by a long cycle), and so on. The regular variability of tachycardia cycle length suggests one of the following possibilities. (1) There are two alternative circuits (a short circuit and a long circuit) that share the same exit pathway. Whenever the reciprocating impulse runs through the short circuit, the R-R cycle is short; but if a block in the short circuit occurs, the impulse runs through the long circuit, resulting in a long R-R cycle. (2) There is a longitudinal dissociation within the reentry circuit; two separate pathways with different inherent conduction velocities are present. When the impulse runs through the fast pathway, the R-R cycle is short; whereas when a block in the fast pathway occurs, the impulse traverses the slow pathway, resulting in a long R-R cycle.