A subvariant of concealed bigeminy.

A subvariant of the "even variant" of concealed bigeminy was studied in two patients. The typical pattern consisted of two types of sequences of conducted sinus beats between extrasystoles. One pattern was bigeminal, i.e., alternating sinus beats and extrasystoles. Between bigeminal sequences were intervals in which there were more than one conducted sinus beat between extrasystoles. In such longer sequences, the numbers of sinus beats were almost invariably even. In the bigeminal sequences, the coupling intervals progressively diminished for each successive extrasystole in the sequence. The proposed explanation for the subvariant was based on a reentry loop in which there were three sites of block: proximal, intermediate, and distal. Block was postulated to take place at the proximal site after those extrasystoles with the shortest coupling intervals, at the intermediate site after odd-numbered conducted sinus beats in the non-bigeminal sequences, and at the distal site ("concealment") after the even-numbered sinus beats in these longer sequences.