Nitrous oxide and the middle ear

Abstract

A gradual increase in the volume of the induced gas space follows artificial pneumoperitoneum in dogs and cats with sulphur hexafluoride (SF,).' The volume change occurs because the rate of diffusion of nitrogen from the blood in to the pneumoperitoneum exceeds the rate of loss of SF, from the pneumoperitoneum to blood. The volume increases in artificial pneumothoraces during nitrous oxide anaesthesia are well-known.2 In these cases the rate of diffusion of nitrous oxide inwards exceeds the rate of diffusion of nitrogen outwards. The effect of similar anaesthesia on a non-compliant gas space causes rapid increases in pressure in the air-filled cisternal spaces of dogs following N 2 0 admini~tration.~ Similar effects occur in the middle ear during N 2 0 anaesthesia in patients4 Measurements in middle ear gas pressure and composition in cats and in man have confirmed these finding^.^." The effect is independent of airway pressure, and temporary or permanent hearing loss has occurred in several patients following N 2 0 anaesthesia.'. * Tympanic membrane graft displacement is commonly seen in middle ear surgery under N20 anaesthesia, yet the agent is used almost universally in this work. The possible effects of N 2 0 excretion into the middle ear cavity suggest that the gas should be withdrawn some minutes before the middle ear is ~l o s e d .~*~-" It has also been suggested that N 2 0 anaesthesia may be hazardous for the hearing of patients with polyethylene struts or surgical reconstructions for chronic ear disease.8The present study was undertaken because of the occurrence of a marked hearing loss following N 2 0 anaesthesia in one such patient. Its purpose was to confirm the effects of the gas on the normal middle ear, to compare the effect with that of non-N20 anaesthesia and to explore the time course of the events observed. Case historyThe patient, a 43-year-old woman, had a left stapedectomy for otosclerosis about 10 years ago and a standard 4.5-mm Schucknecht gelfoam and stainless steel prosthesis was inserted. Figure l(a) shows her audiogram at postoperative review; the upper trace is her (normal) sensorineural level and the lower is her hearing level to air conduction. The functional result of surgery was good with complete closure of the conductive gap.A year later she had a hysterectomy under general anaesthesia and immediately after operation complained of severe deterioration in her hearing. Her complaint was confirmed by audiometry which showed a 20-25 dB conductive loss- Figure I(b)-which has persisted to the present. The anaesthetic sequence was thiopentone, alcuronium, intubation and ventilation with N 2 0 and O2 to which 0.5% halothane was added.

Avaliação de alterações timpanométricas em pacientes submetidos à anestesia geral com óxido nitroso

“…As to tympanoplasties, displacement of graft is a common finding in otological surgery with protoxide 8,11,13,33 , caused by changes in intratympanic pressure during and right after the anesthesia 8,16,33 , which may cause significant implications in stability of ME reconstituted components 11 . For this reason, its use in this type of surgery and in patients that were submitted to ossicle reconstruction is questionable.…”

“…Its exclusion is preferable, recommending oxygen combined with air as a vehicle for inhaling anesthetic agents during the anesthesia for conservative mastoidectomy, tympanoplasty and other ME surgeries 11,33 .…”

“…In cases of auditory tube dysfunction, the pressure may continue to be high even 20 minutes after the end of nitrous oxide inhalation 11,25 , because there is no pressure balance through the auditory tube [1][2][3] .…”

“…However, protoxide input speed is 34 times faster than the output of nitrogen in the body [5][6][7][8][9] . Owing to this imbalance, there is increase in pressure of little complacent cavities, such as the middle ear, in addition to increase in volume of complacent cavities, such as intestinal loop, pulmonary or renal cysts, up to the point that balance is restored [10][11][12] .…”

“…There are reports that state these pressure modifications and their consequences may affect most of the population with auditory tube dysfunction, owing to the inability the auditory tube has to ventilate the ear, hindering rebalance of atmospheric pressure 1,3,11,14 .…”