THE CHANGE OF INTRATYMPANIC PRESSURE DURING GENERAL ANESTHESIA WITH LARYNGEAL MASK

Abstract

S230 It is known that nitrous oxide(N2 O) causes either a volume or a pressure increase, due to diffusion, inclosed body spaces. In the intratympanic cavity both pressure(ITP) and volume increase because of its semicompliant boundaries, the tympanic membrane and the eustachian tube. This results in bulging of the intact drum which may be important during operations such as tympanoplasty. This problem is avoided if N2 O is discontinued 30min before the anticipated placement of the graft. [1] Our purpose is to examine the effects of N2 O on ITP during general anesthesia with LM compared to tracheal intubation. METHODS: Eleven patients with normal healthy ears were divided into two groups either use of LM(n=6) or intubation(n=5). All patients were induced with propofol /vecuronium intravenously. We measured MEP by using tympanometer and the time which take ITP reached beyond 200 dePa level after inspiration of N2 O. Anesthesia was maintained with sevoflurane 2% and N2 O 66% in oxygen. Moreover, ITP were measured in oxygen 100% after elimination of N2 O and after extubation in both groups. Patients were maintained ETCO2 between 30mmHg and 35mmHg. RESULTS: With inspiration of N2 O, ITP increased progressively in both groups. It took 16 +/- 5 min in the intubation group and 42 +/- 18 min in LM group that ITP reached to peak level (>or=to200dePa) from after inspiration of N (2) O. After cessation of inhalation of N2 O, ITP decreased progressively. It took 68 +/- 24 min in LM group that ITP recovered to about 100 dePa(125 +/- 17 dePa), which never recovered to normal level and was much longer than that in intubation group (33 +/- 14 min, p<0.05).After removal of LM, ITP recovered to normal level. DISCUSSION: An increase in ITP depends on the following factors: a) duration of N2 O; b) Concentration of N2 O or another gas used; c) Presence of a blocked or narrow eustachian tube; d) control ventilation (swallowing reflexes). [2] Consequently, it is suggested that as the one of the reasons of significant delay in recovery of ITP is due to compression to the eustachian tube by using LM.