ORDER IN DISORDER - EFFECT OF BARIUM ON VENTRICULAR-FIBRILLATION

BACKGROUND: Drugs that prolong cardiac refractoriness can decrease def ibrillation energy requirements. In particular, barium, a relatively s elective blocker of cardiac Ik1 channels, produces marked decreases in defibrillation energy. The mechanism of this effect is unknown, and m ay relate to modulation of the effect of defibrillatory shocks, or an alternation of the pattern of ventricular fibrillation (VF) by the dru g. METHODS AND RESULTS: Accordingly, the effect of barium chloride was examined, 1.1 mg/kg followed by 0.1 mg/kg/min intravenously, or salin e control, on the pattern of unipolar electrograms using a 120 electro de array, during 73 episodes of VF (37 after saline, 36 after barium). For each episode of VF, peak -dV/dt associated with local activations and mean activation-activation (ACT-ACT) intervals for the last 2 s o f a 10 s episode of VF were measured for each electrodes. 'Organizatio n' in VF was measured by the variability in ACT-ACT intervals, their v isually assessed pattern, and the relation between local activations o n adjacent electrodes. Voltage gradients were measured at each of 40 e picardial sites for each defibrillation shock, delivered at voltages, ranging from 20% to 100% successful in defibrillation. At identical vo ltage shocks (400 V), mean voltage gradients before and after barium w ere similar: 18+/-9 and 19+/-1.2 V/cm, respectively. Mean peak -dV/dt for all activations was -8.7+/-0.5 V/s before and -7.7+/-2.8 V/s after barium, suggesting no apparent change in local conduction velocity. W hen the lowest voltage gradient at any site was less than 3.5 V/cm, de fibrillation was successful 1.4% of the time (two of 14) during contro l, but 88% of the time (14 of 16) after barium infusion (P<0.01). Mean ACT-ACT intervals after barium for all episodes over all electrodes w as 107.5+/-14.1 ms, significantly longer than 89.7+/-3.9 ms after sali ne, indicating a 20% increase in the cycle length of fibrillation. Dur ing saline control, local epicardial electrogram patterns showed irreg ular, variable morphology electrograms and a mean lowest SD of ACT-ACT intervals over any electrode of 5.1+/-1.5 ms, compared with 1.2+/-0.7 ms after barium (P<0.0001). Following barium, most unipolar epicardia l electrograms showed regular, phasic activations that appear to refle ct an organized, uniformly repetitive local activation pattern, sugges ting spatially homogeneous and temporally regular activation wavefront s. CONCLUSIONS: During VF after barium, despite and apparently disorga nized surface electrocardiographic pattern, epicardial electrogram pat terns are altered and reflect a more 'ordered', homogeneous and regula r local activation. This increased order may be in part responsible fo r the decreased defibrillation energy requirements observed after bari um.