Difficulty in assessing the correct position of the laryngeal mask airway.

Sir,—Accurate assessment of the position of the laryngeal mask airway (LMA) is important during controlled trials involving the LMA. Radiography or magnetic resonance imaging (MRI) show the position of the LMA precisely [1, 2]. A fibreoptic bronchoscope is used frequently for assessment in the clinical setting [3-5]. I have noticed recently that fibreoscopy cannot confirm the correct position of the LMA, although it provides valuable information. Brain designed the LMA to enclose the larynx. He decided that the internal length of the long axis of the LMA aperture should always be greater than the distance between the upper border of the thyroid cartilage and the lower border of the cricoid cartilage and he confirmed this in adult cadavers [6]. Thus because of individual variation in the size of the larynx, the LMA sometimes encloses the epiglottis even when the tip of the LMA cuff correctly occupies the hypopharynx. Therefore, it is difficult to determine if the LMA is positioned correctly based on the view of the epiglottis through the bronchoscope. Thus we may need to revise the results of studies on the positioning of the LMA. For example, Dr Wilson stated in his Editorial [7] that "one clear difference between adults and paediatric use of the LMA is the incidence of an imperfect position diagnosed by fibreoscopy". This statement was based on the statements of Rowbottom, Simpson and Grubb [3] and Mizushima, Wardall and Simpson [4] that the position of the LMA was "perfect" in 49 % of children and 44 % of infants; these figures are smaller than those in adults [5]. They considered that the position of the LMA was perfect when only the glottis was seen through the fibreoptic bronchoscope. However, as described above, the LMA may be in a correct position even when the epiglottis is seen also in the LMA aperture. If we arbitrarily consider that the position of the LMA is correct when only the glottis, or the glottis and posterior surface of the epiglottis are seen in the LMA aperture, the incidence of a suboptimal position is similar (about 20-35 %) in infants, children and adults. Therefore, the true incidence of a suboptimal position of the LMA in adults and children may not be as different as was thought previously.