The dose-response of nitrous oxide in postoperative nausea in patients undergoing gynecologic laparoscopic surgery: a preliminary study.

BACKGROUND Whether nitrous oxide (N(2)O) increases the incidence of postoperative nausea and vomiting (PONV) after laparoscopic gynecologic surgery is still controversial, which may be due to the administration of different concentrations of inspired N(2)O. We investigated whether N(2)O results in a dose-response increase in PONV. METHODS Patients undergoing gynecologic laparoscopic surgery were randomized to receive 30% oxygen with air (G0, n = 46), 50% N(2)O with oxygen (G50, n = 46), or 70% N(2)O with oxygen (G70, n = 45). A standardized general anesthetic was used with no PONV prophylaxis. Known risk factors for PONV were controlled. Metoclopramide was used as a rescue antiemetic. The incidence of nausea, vomiting, use of rescue antiemetic, and pain visual analog scale (VAS) score was measured at 2 and 24 h postoperatively. RESULTS Patient demographics were comparable, and there were no differences among groups regarding factors that may influence PONV. The incidence of PONV at 24 h was 33% (15 of 46) in the G0 group, 46% (21 of 46) in the G50 group, and 62% (28 of 45) in the G70 group (P = 0.018). Subgroup analysis revealed a difference between G0 versus G70 groups (P = 0.018), but no significant difference between G0 versus G50 groups and G50 versus G70 groups. The incidence of nausea showed a similar difference (G0 = 26%, G50 = 35%, and G70 = 56%; P = 0.012), but the incidence of vomiting was not different among the groups although there was a trend (G0 = 28%, G50 = 35%, and G70 = 42%; P = 0.377). The severity of nausea (measured by VAS 100 mm) was significantly increased with increasing N(2)O concentration (G0 = 10.9, G50 = 12.7, and G70 = 20.5; P = 0.027). The highest VAS score during 24 h was used for the analysis. There was no difference in the use of a rescue antiemetic among groups. Pain VAS scores and opioids consumption were not different among groups (at 2 and 24 h after surgery). CONCLUSIONS N(2)O increases the incidence of postoperative nausea after gynecologic laparoscopic surgery. This preliminary finding indicates that N(2)O may increase PONV in a dose-dependent fashion. A study with a sample size of >400 patients in each group would be necessary to demonstrate a statistically significant difference among each of these three groups. We do not recommend using a high concentration of N(2)O in this clinical setting.