THE EFFECTS OF VENTRICULAR-FIBRILLATION DURATION AND SITE OF INITIATION ON THE DEFIBRILLATION THRESHOLD DURING EARLY VENTRICULAR-FIBRILLATION

Objectives. The purpose of this study was to determine if the defibril lation threshold (DFT) is lower during the first few cycles of ventric ular fibrillation (VF) than after 10 s of VF and, if so, if the effect is caused by local or global factors. Background. The DFT may be low very early during VF because: (1) for the first few cycles VF arises f rom a localized region close to a defibrillation electrode where the s hock field is strong (local factors), or (2) during early VF the effec ts of ischemia and sympathetic discharge have not yet fully developed and the heart has not yet completely dilated (global factors). Methods . Protocol 1 included seven pigs in which a defibrillation electrode a nd a pacing catheter were both placed in the right ventricular apex, V F was induced by delivering a high current premature stimulus from the pacing catheter that should have caused reentry confined to the right ventricular apex for the first few cycles of VF. A bipolar electrogra m was recorded from the tip of the defibrillation catheter. Using a th ree reversal up-down protocol, the DFT was determined for biphasic sho cks delivered after 1, 2, 3, 4, 5, 7, 10, 15, 20 and 25 activations in this electrogram and after 10 s (control). Protocol 2 included seven pigs undergoing the same procedure as in protocol 1 except that an add itional pacing catheter was placed in the left ventricle. Defibrillati on thresholds were determined after 1, 2, 3, 4 and 5 VF activations fo llowing VF induction from the right ventricle (RV) or the left ventric le (LV) and after 10 s (control). Results. In protocol 1, the mean +/- SD DFTs were lower during the first three cycles than after 10 s of V F (3.0 +/- 4.1 J for the first VF cycle vs 15.8 +/- 6.6 J after 10 s o f VF, p < 0.05). In protocol 2, the DFT for the first few cycles of VF induced away from the defibrillation electrode in the LV (6.9 +/- 1.4 J for the first VF cycle) was significantly lower than that after 10 s of VF (16.0 +/- 2.2 J), whereas the DFT for the first few cycles ind uced near the defibrillation electrode in the right ventricular apex w as significantly lower (2.3 +/- 2.7 J for the first VF cycle) than tha t induced from the LV. Conclusions. This study demonstrates that the D FT is significantly lower during the first few VF cycles of VF than af ter 10 s of VF and that this decrease may be caused by both local fact ors and global factors. These results provide an impetus far exploring earlier shock delivery in implantable devices.