Effects of postshock atrial pacing on atrial defibrillation outcome in the isolated sheep heart

Abstract

Background-Failed atrial defibrillation shocks are associated with organization of postshock activity and a substantial postshock electrical quiescence. We investigated the ability of a train of pacing stimuli to capture or locally entrain atrial myocardium during the quiescent period after low-energy shocks and to alter defibrillation outcome. Methods and Results-High-resolution video imaging of near-defibrillation-threshold atrial shocks was performed in 12 Langendorff-perfused sheep hearts. A train of 10 pacing stimuli (10-ms pulse width, 200-ms cycle length) was coupled to the shock at various delays in 7 hearts. Coupling intervals of 40 to 130 ms were investigated for feasibility of capture of the first pacing stimulus. The success rate of capture was 0, 0.08±0.08, 0.43±0.13, 0.73±0.13, and 0.11±0.1 for 40-, 60-, 80-, 100-, and 120-ms coupling intervals, respectively (P<0.001). In 5 experiments, the coupling interval was fixed at 100 ms (highest success, see above), and the pacing stimulus amplitude was varied between 1.0, 2.0, and 4.0 V. Successful capture rates were 0.38±0.08, 0.75±0.08, and 0.64±0.08, respectively (P<0.003 for 1.0 versus 2.0 V, P=0.2 for 2.0 versus 4.0 V). Rates of successful defibrillation for the groups without and with pacing were 0.56±0.07 and 0.64±0.04, respectively (P=0.3). With capture of the first pacing stimulus, the rate of successful defibrillation rose to 0.75±0.05 (P<0.01); it remained unchanged without capture (0.48±0.07 versus 0.56±0.07 for no pacing). Conclusions-Pacing during the quiescent period that follows defibrillation shocks is feasible. A pacing train whose first pacing stimulus successfully captures during the quiescent period of near-defibrillation-threshold shocks appears to alter the outcome.