Mechanisms of concealed ventricular bigeminy: The concept of concealed conduction in the reentrant pathway

Summary To clarify the presence of concealed conduction in the reentrant pathway of extrasystoles, 20 patients with ventricular extrasystoles were studied in whom two forms of interectopic periods were found in the same recording. One form is the XS 1 S 2 X period, in which two sinus QRS complexes (S 1 and S 2 ) intervene between an interpolated extrasystole (the first X) and the next extrasystole (the second X). The other is the XS 2 X or XS 1 X period, in which one sinus QRS complex intervenes between two extrasystoles. In all patients except one, the XX interval in the XS 1 S 2 X period was longer than that in the XS 2 X or XS 1 X period though shorter than twice the latter XX interval. This strongly suggests the presence of two-level block in the reentrant pathway of the extrasystoles. It appears that the sinus impulse S 1 in the XS 1 S 2 X period invaded a large portion of the reentrant pathway and then was blocked at a distal site of the pathway; namely, that concealed conduction of the impulse S 1 occurred in the pathway. It is suggested that such concealed conduction prolonged the conduction time of the following sinus impulse, S 2 , in the reentrant pathway, resulting in lengthening of the XX interval. The presence of three- or four-level block is also suggested. By the use of such multilevel block, mechanisms of concealed ventricular bigeminy are explained.