Response of fibrillating atria to successful low probability of success shocks

The mechanism of very low probability of success (PS) atrial defibrillation (AF) is not understood. We studied the activation patterns of the atria following low energy/low probability of success shocks to determine if the pattern of activation following these shocks was different than for higher PS shocks. Five adult sheep were conditioned with rapid right atrial (RA) pacing into chronic AF for a minimum of 15 weeks. These sheep were then mapped with left and RA plaques. Biphasic shocks were delivered though a coronary sinus to superior vena cava lead configuration. Forty-five successful defibrillation episodes were analyzed: 25 below and 20 above the 25% success voltage. The number of post shock non-sinus beats (PSNSB) were counted and the activation patterns during those beats was identified as: (I) one wavefront, (II) two wavefronts or (III) greater than two wavefronts for both the right and left atrial plaques. The number of PSNSBs was not different for the two groups however the complexity of the early post shock beats was higher in the RA than in the LA and for lower PS shocks. Low PS shocks leave a greater number of RA wavefronts that appear to organize and then break.