Evaluation of tympanometric alterations in patients subject to general anesthesia with nitrous oxide

Nitrous oxide is an inhaling gas that can increase intratympanic pressure during the anesthetic act and cause negative pressure after it is discontinued, mainly in patients with Eustachian tube dysfunction. These pressure variations may come up with clinical implications such as tympanic membrane rupture, ossicular system disarticulation, haemotympanum, barotraumas, prosthesis displacement stapaedotomy and tympanic graft lateralization after tympanoplasty, in addition to serous fluid entrance into the middle ear during the negative pressure phase. Aim: To evaluate the nitrous oxide influence on the middle ear pressure in a population without tube malfunction performing pre and postoperative tympanometry. Study design: Transversal cohort. Material and Method: A prospective study was carried out with Universitário Clementino Fraga Filho Hospital- UFRJ inpatients submitted to general anesthesia with the use of 50% nitrous oxide from April to June 2003. It was also evaluated whether the duration of surgery, associated anesthetics, presence of allergic rhinitis and nasal septal deviation could contribute to the onset of intratympanic pressure alteration. Results: The sample was made up of 50 patients and in almost half of them (48%), postoperative tympanometry alterations (type C curve) were found when comparing to preoperative tympanometric control (type A curve). Neither gender nor age interfered in the onset of postoperative tympanometry alterations, similarly to surgery duration. The associated volatile anesthetic type, nasal septal deviation and allergic rhinitis were not able to influence postoperative middle ear pressure. Conclusion: Nitrous oxide modifies intratympanic pressure during the anesthetic act and after its discontinuation.