The purpose of this study was to determine if the Entonox gas could cause adequate analgesic and sedative effects in patients who need cardioversion. In this randomized not blinded clinical trial, the sedative and analgesic effects of midazolam and fentanyl were compared with Entonox. Eligible patients who need synchronized cardioversion because of dysrhythmia were randomly divided into two groups. The first group received intravenous midazolam and fentanyl; the second group received Entonox through a blower-dependent mask. Onset and end of sedation, sedation level, and pain score were recorded. There were nonsignificant differences between the two groups (22 patients in each group) regarding age, gender, weight, sedation level, and frequency and level of shock. The pain score recorded in the first group was 5.05 ± 1.32, and 3.9 ± 0.7 in the second group (P=0.002). Furthermore, sedation duration and time to full recovery consciousness were shorter in the second group (P<0.001). In the first group, seven patients needed additional doses to induce and maintain sedation. In addition, as a result of apnoea, four patients required airway support. None of them occurred in the second group. Entonox is a suitable medication in rapid cardioversion, as it has minimal side effects and adequate analgesic and sedative effects.