Bigeminy occurs when an extrasystole follows every sinus beat. Conversely, trigeminy occurs when every third beat is extrasystolic. The pattern can start or stop abruptly but is very regular. It is theorized that these rhythms occur due to focal myocardial ischemia. As a depolarizing wave-front moves through the ventricular mass, it travels at a set speed. When the wave-front comes upon a focal ischemic zone of myocardium, the hopoxia of the region causes cell-to-cell depolarization in that region to be slower. However, the wave-front traveling through the normal myocardium continues to travel at a normal speed and spreads around the ischemic zone rapidly. If the wave-front reaches the other side of the ischemic zone by the time the healthy myocardium has already depolarized and is out of its refractory period, a new complex is initiated with the edge of the ischemic zone as its point of origin. Since the lesion size is fixed as the territory of the blocked coronary branch, the RR’ distance between the first and second complex of the doublet is also fixed. The next complex arising in the SA node finds the twice depolarized ventricular myocardium out of its refractory period and starts the cycle over again.
See examples of bigeminy