Objectives This study investigated whether pain catastrophizing is associated with distress and perceived disability in patients with atrial fibrillation, whether pain catastrophizing predicts pain and fear during a medical procedure of atrial cardioversion, and whether pain catastrophizing influences the effects of an opioid analgesic during internal cardioversion. Methods A secondary analysis is performed upon data from a double-blind placebo-controlled trial during which the effect of intranasal butorphanol tartrate (INB) was evaluated in patients with atrial fibrillation using a step-up internal atrial cardioversion protocol. Before the procedure, patients completed measures of pain catastrophizing, mood, distress and perceived disability. After each shock, patients completed measures of pain and fear. Results We found that pain catastrophizing predicted the affective pain rating of the first shock, and the fear increase during subsequent shocks. There was no effect of INB. However, when controlling for the differences in pain catastrophizing, INB significantly reduced fear as compared to placebo. This suggests that the effects of INB during atrial cardioversion were overshadowed by the effects of pain catastrophizing. Conclusions It is recommended that in future atrial cardioversion trials, stratification based on pain catastrophizing be performed. Reducing catastrophizing thinking about pain through cognitive–behavioral techniques are likely to reduce levels of fear during internal atrial cardioversion and may increase the effectiveness of concomitant analgesics.