Abstract: Objective To compare the effect of laryngeal mask airway (LMA) size selection based on ideal and actual body mass on the success rate of first insertion in patients with low body mass index (BMI). Methods This study was performed in 137 patients aged 18-60 years with BMI below 18.5 kg/m2, in whom discrepancies occurred in the selection of LMA size based on their actual body mass and the ideal body mass. The patients were randomized divided into ideal body mass group and actual body mass group, in which the size of LMA was selected based on the ideal body mass and their actual body mass, respectively. The success rate of first LMA insertion, overall success rate, fiberoptic visual field grade, leakage pressure, and LMA-related complications of the patients were recorded during the maintenance and recovery of anesthesia. Results The success rate of first LMA insertion was significantly higher in ideal body mass group than in the actual body mass group (86.8% vs 68.1%, P=0.016). Compared with those in the actual body mass group, the patients in the ideal body mass group used larger LMA (P<0.005) and had better fiberoptic field scores (P=0.001) and higher airway seal pressure (P<0.005). The peak inspiratory pressure (P=0.154) or the incidence of LMA-related complications during anesthesia maintenance and recovery did not differ significantly between the two groups (P>0.05). Conclusion The size selection of LMA based on the ideal body mass of the patients, determined according to their height and sex, can significantly improve the success rate of first LMA insertion in patients with low BMI.

Key words: low body mass index; size of laryngeal mask airway; ideal mass; success rate of first insertion