To the Editor:--The use of a 5-mm Mallinckrodt Microlaryngeal tube has been recommended for tracheal intubation through the laryngeal mask. I consider that, for this purpose, the 6-mm Mallinckrodt reinforced tracheal tube (Mallinckrodt, Athlone, Ireland) is a better choice than the 5-mm microlaryngeal tube.
The length of the 6-mm reinforced tracheal tube is 33 cm. When the tracheal tube is inserted as far as possible into the laryngeal mask, the distance between the proximal edge of the tracheal tube cuff and the grill of the mask is 8 cm. Therefore, the cuff of the tracheal tube is unlikely to lie between the vocal cords when the tracheal tube is passed through the laryngeal mask, because the mean distance between the grille of the mask and the glottis is 3.6 cm in males and 3.1 cm in females. .
When a size 5 laryngeal mask is used, a 7-mm internal diameter reinforced tube can be passed through it. The tracheal tube protrudes beyond the grill of the mask for 10 cm, and the distance between the grille and the tracheal tube cuff is 4.5 cm.
The reinforced tube has a greater flexibility than the microlaryngeal tube, and therefore the reinforced tube may be more useful when the glottis is not positioned directly below the aperture of the laryngeal mask. In addition, because these reinforced tubes are larger than the microlaryngeal tube, there may be less concern for the airway resistance of the tube. Therefore, I believe that the 6-mm Mallinckrodt reinforced tracheal tube is more suitable for the technique of tracheal intubation through the laryngeal mask.
There is a factor that we should bear in mind when a longer tracheal tube is passed through the laryngeal mask. When a longer tracheal tube is passed through either a size 3 or 4 laryngeal mask, the tracheal tube may be inserted too deeply into the trachea; therefore, there is a theoretical risk of endobronchial intubation. The position of the tracheal tube should be assessed and the tube fixed to the laryngeal mask at an adequate depth.
Takashi Asai, MD; Department of Anesthesiology, Kansai Medical University, Moriguchi City, Osaka, Japan.
(Accepted for publication May 2, 1996.)