Poleszak K, Kutarski A, Koziara D, Baszak J, Oleszczak K Department of Cardiology, University Medical Academy, Lublin, Poland. Pacing Clin Electrophysiol 1998 Jan;21(1 Pt 2):176-80. The aim of the study was to compare the bidirectional transoesophageal DC cardioversion (BOC) with unidirectional transoesophageal DC cardioversion (UOC) and to evaluate, if the reversion of the polarity of electrodes alters the effectiveness and the amount of energy during BOC. UOC was attempted in 300 patients (pts) with atrial fibrillation (AF) and BOC in 241 pts with AF. In UOC mode shocks were delivered between the 4-ring oesophageal electrode (cathode) and the chest pad (anode) positioned in the precordial region. In BOC shocks were delivered between the same oesophageal electrode and two chest pads joined with each other, positioned on both sides of the sternum. First 147 pts were cardioverted with the oesophageal electrode as a cathode, next 94 with an anode in oesophageal position. The effectiveness of both modes (UOC and BOC) was very high, however in pts with chronic AF success rate was better in BOC approach (82% vs 100%). BOC, compared with UOC, allowed to decrease the threshold defibrillation significantly: in pts with recent onset of AF from 61.5 J to 33.3 J and in pts with chronic AF from 99.8 J to 75.2 J. In pts with long standing AF the reduction of the defibrillation threshold was statistically not significant (from 68.6 J to 50.6 J). The effectiveness of BOC was also very high independently of the polarity of electrodes. The change of the polarity did not affect the minimal and total successful energy of shocks, too. In pts with oesophageal electrode as a cathode defibrillation threshold was 48.4 J and in pts with the anodal electrode 43.7 J. In conclusions we found BOC as a very effective method in pts with AF. Defibrillation threshold in BOC is lower than in UOC and the polarity of electrodes does not influence the success rate and successful energy.