To the Editor:—

Size selection of the Laryngeal Mask Airway ™ (LMA ™ Laryngeal Mask Company Limited, San Diego, CA) is important for avoiding complications and is based on patient body weight. 1The previously proposed formula for determining the appropriate LMA ™ size 2is impractical because it contains the square root of the patient’s body weight. We advocate here an alternative formula:LMA ™ size = 1 + BWru/20, where BWru indicates body weight (in kilograms) rounded up at the first digit. For example, if a patient’s body weight of 14 kg (BWru) is 20 because 14 can be rounded up to 20 at the first digit, the calculation would be as follows:LMA ™ size = 1 + 20/20 = 1 + 1 = 2.

If the calculation of LMA ™ size shows 3.5 or 4.5, we choose LMA ™#3 or #4. This manner of size selection agrees with the recommendation by Brimacombe and Brain. 1Alternatively, if the calculated LMA ™ size is 3.5 or 4.5, we can also use LMA ™#4 or #5. This size selection is similar to the sex-based LMA ™ size recommendation for adult patients. 3 

Our formula is convenient for pediatric patients, although it cannot be used in infants weighing less than 5 kg. In addition, the formula does not require a pocket calculator.

Brimacombe JR, Brain AIJ: Preparation for use, The Laryngeal Mask Airway: A Review and Practical Guide. Edited by Brimacombe JR, Brain AIJ, London, Saunders, 1997, pp 52–64
Kagawa T, Obara H: An easy formula to remember the laryngeal mask airway size-patient weight relationship. A nesthesiology 2000; 92: 631–2
Brimacombe JR, Berry AM, Campbell RC, Verghese RC: Selection of proper size of laryngeal mask airway in adults. Anesth Analg 1996; 83: 664