• R H Epstein, F Ferouz, and M A Jenkins.
• Department of Anesthesiology, Jefferson Medical College, Thomas Jefferson University, Philadelphia, PA 19107, USA.
• J Clin Anesth. 1996 Mar 1;8(2):93-8.

Study ObjectivesTo determine if the laryngeal mask airway (LMA) seal is maintained during surgery, to evaluate the safety of an LMA leak test, and to determine the time course of the increase in LMA cuff pressure in vivo in the presence of nitrous oxide (N2O).Study DesignDescriptive clinical study.SettingUniversity teaching hospital.Patients78 ASA Physical Status I and II pediatric patients, aged 3 months to 17 years, undergoing general anesthesia with an LMA; 14 patients were studied on two occasions approximately 2 months apart.InterventionsThe airway pressure at which the LMA seal was broken (leak pressure) was determined immediately following insertion of the LMA and at the end of surgery. In 17 patients, the LMA cuff pressure was continuously measured during surgery, during which 67% N2O was administered. The mean duration of surgery was 29.5 minutes.Measurements And Main ResultsThe LMA leak pressure was determined by closing the circuit popoff valve and recording the pressure at which gas was first heard to escape around the LMA at the mouth. The LMA cuff pressure was determined by connecting the check valve of the LMA pilot balloon to a sphygmomanometer. The elastance of the LMA was determined from the slopes of the regression lines of pressure versus volume for the various sized LMAs in vitro (Sizes 1-4) and in vivo (sizes 2 and 2 1/2). The initial LMA leak pressure was 25.9 cm H2O, and it increased to 31.2 cm H2O during surgery (p < 0.001). 146 leak tests were performed without causing gastric dilatation or LMA dislodgement. The mean LMA cuff pressure increased during surgery from 106.2 mmHg to 132.8 mmHg (p < 0.001), a pressure increase that corresponds to a volume increase of 1.4 ml.ConclusionsThe LMA leak test can be safely performed. The airway seal provided by the LMA is well maintained during surgery. The increase in LMA cuff pressure during surgery in the presence of N2O is small and probably is not a cause for clinical concern. If positive pressure is to applied in the presence of an LMA, one should limit this pressure to below the LMA leak pressure so as to lower the risk of inflating the stomach with gas that may than leak around the LMA.

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