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 N ₂O treatment group. Conclusions: Barometric changes in the middle ear contribute to the incidence of PONV induced by N₂O.