EFFECTS OF POSTSHOCK ATRIAL-PACING ON ATRIAL DEFIBRILLATION OUTCOME IN THE ISOLATED SHEEP HEART

Background-Failed atrial defibrillation shocks are associated with org anization 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 p eriod after low-energy shocks and to alter defibrillation outcome. Met hods and Results-High-resolution video imaging of near-defibrillation- threshold atrial shacks 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. Coupl ing intervals of 40 to 130 ms were investigated for feasibility of cap ture 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 e xperiments, 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 gro ups without and with pacing were 0.56+/-0.07 and 0.64+/-0.04, respecti vely (P=0.3). With capture of the first pacing stimulus, the rate of s uccessful defibrillation rose to 0.75+/-0.05 (P<0.01); it remained unc hanged without capture (0.48+/-0.07 versus 0.56+/-0.07 for no pacing). Conclusions-Pacing during the quiescent period that follows defibrill ation shocks is feasible. A pacing train whose first pacing stimulus s uccessfully captures during the quiescent period of near-defibrillatio n-threshold shocks appears to alter the outcome.