AIRWAY SEALING PRESSURES OF THE LARYNGEAL MASK AIRWAY IN PEDIATRIC-PATIENTS

Study Objectives: To determine if the laryngeal mask airway (LMA) seal is maintained during surgery, to evaluate the safety of an LMA leak t est, and to determine the time course of the increase in LMA cuff pres sure in vivo in the presence of nitrous oxide (N2O). Study Design: Des criptive clinical study. Setting: University teaching hospital. Patien ts: 78 ASA Physical Status I and II pediatric patients, aged 3 months to 17 years, undergoing general anesthesia with an LMA; 14 patients we re studied on two occasions approximately 2 months apart. Intervention s: The airway pressure at which the LMA seal was broken (leak pressure ) was determined immediately following insertion of the LMA and al the end of surgery. In 17 patients, the LMA cuff pressure was continuousl y measured during surgery during which 67% N2O was administered. The m ean duration of surgery was 29.5 minutes. Interventions: The airway pr essure 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 durin g surgery during which 67% N2O was administered. The mean duration of surgery was 29.5 minutes. Measurements and Main Results: The LMA leak pressure was determined by closing the circuit popoff valve and record ing 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 elasta nce 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 21/2). 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 c orresponds to a volume increase of 1.4 mb. Conclusions: The LMA leak t est can be safely performed. The airway seal provided by the LMA is we ll maintained during surgery. The increase in LMA cuff pressure during surgery in the presence of N2O is small and probably is not a cause f or clinical concern. If positive pressure is to applied in the presenc e of an LMA, one should limit this pressure to below the LMA leak pres sure so as to lower the risk of inflating the stomach with gas that ma y than leak around the LMA.