Middle ear pressure changes after nitrous oxide anesthesia and its effect on postoperative nausea and vomiting.

OBJECTIVES/HYPOTHESIS This study was designed to explore the relationship between changes in middle ear pressure associated with inhalational anesthesia and the incidence of postoperative nausea and vomiting (PONV). STUDY DESIGN Prospective, randomized. METHODS Middle ear compartment pressures were measured by tympanometry in 27 randomly assigned knee arthroscopy patients throughout the surgical procedure as well as into recovery. RESULTS A positive correlation between the maximum positive pressure (MPP) and maximum negative pressure (MNP) gradient and PONV was demonstrated (P <.05). The incidence of PONV in the nitrous oxide (N2O) treatment group was 6 of 16 patients, whereas only 2 of 11 patients in the control group developed nausea, vomiting, and vertigo symptoms. Those patients that did not experience PONV demonstrated a median MPP of 155 with a median MNP of -52. The patients that experienced PONV exhibited a median MPP of 179 with a median MNP of -164. This demonstrates a significant increase in the incidence of PONV in the N2O treatment group. CONCLUSIONS Barometric changes in the middle ear contribute to the incidence of PONV induced by N2O.