Modulated ventricular parasystole as a mechanism for concealed bigeminy.

Concealed extrasystolic ventricular bigeminy reflects a distributional pattern of ventricular extrasystoles where intervening sinus beats are always in odd numbers. This has been explained on bigeminal rhythm associated with exit block. This presentation reflects a modification of this concept in that the distributional pattern may be explained on the basis of complex concealed modulation of ventricular parasystole, the concealment being due to impulses falling within the refractory period rather than suffering true exit block.