Poleszak K, Kutarski A, Koziara D, Baszak J, Olezcak. University Medical Academy; Lublin, Poland. PACE 21(II):176-180, 1998. The aim of the study was to compared the bidirectional DC cardioversion (BOC) with unidirectional transesophageal DC cardioversion (UOC) and to evaluate if the revirsion of the polarity of electrodes alters the effectiveness and the amount of energy during BOC. UCOC was attempted in 300 patients (pts) with atrial fibrillation (AF) and BOC in 241 pts with AF. In UOC mode, shocks were delivered between a 4-ring esophageal electrode (cathode) and a chest pad (anode) positioned in the precordial region. In BOC, shocks were delivered between the same esophageal electrode and 2 chest pads electrically joined with each other and positioned on both sides of the sternum. First 147 pts were cardioverted with the esophageal electrode as a cathode, next 94 with an anode in esophageal position. The effectiveness of both modes (UOC and BOC) was very high, however in pts with chronic AF success rate was higher in BOC approach (82% vs. 100%). BOC, compared with UOC, allowed a significant decrease in the defibrillation threshold: 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 (68.6 J to 50.6 J). The effectiveness of BOC was also very high independently of the polarity of electrodes. The change of polarity also did not affect the minimal and total successful energy of shocks. In pts with esophageal electrode as a cathode defibrillation threshold was 48.4 J and in pts with the anodal electrode 43.7 J. In conclusion, we found BOC as a very effective cardioversion 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 or threshold energy.