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Objective To compare the effect of inhaled nitrous oxide (INO) on pain control during in‐office hysteroscopy with 1% lidocaine paracervical infiltration and no analgesic. Design Single‐blind stratified randomised clinical trial with masked assessment by a third party. Setting Department of Obstetrics and Gynaecology in a Spanish hospital. Population Women who underwent hysteroscopy. Methods Patients were stratified into three groups according to the purpose of the hysteroscopy (biopsy, polypectomy or tubal sterilisation) and then assigned to different treatment groups through a permuted‐blocks randomisation within strata. Pain scale was provided by a gynaecologist totally blinded to procedures and treatments. Effects were assessed using a one‐way analysis of variance following an intention‐to‐treat approach. Main outcome measures Visual analogue scale (VAS) from 0 to 100 mm. Results A total of 314 women were included: 105 to INO, 104 to 1% lidocaine and 105 to no analgesic. Baseline characteristics were comparable. Mean VAS score after the procedure was 34.7 ± 25.8 mm, 36.1 ± 22.9 mm (P = 1.0) and 47.3 ± 28.2 mm (P = 0.001) for INO, 1% lidocaine and no analgesic, respectively. No adverse events were reported in 91 (86.7%) patients in the INO group compared with 79 (76%) in the 1%‐lidocaine group (P = 0.04) and 85 (81%) in the no‐analgesic group (P = 0.26). Conclusion INO was as effective as 1% lidocaine in pain control for in‐office hysteroscopy and was better tolerated. The no‐analgesic group presented the poorer results, so was the least recommended clinical option.

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