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An Easy Formula to Remember the Laryngeal Mask Airway Size–Patient Weight Relationship

Kagawa, Tetsuro M.D.; Obara, Hidefumi M.D.

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To the Editor:—
Table 1
Table 1
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A laryngeal mask airway (LMA) is selected based on patient weight, especially for pediatric patients. However, the LMA size–patient weight relation is difficult to memorize because there are many sizes of LMA (table 1).
Equation U1
Equation U1
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We developed an easy single formula to determine LMA size based on patient weight as follows:MATH With this formula, it is easy to calculate a suitable LMA size for a patient of known weight. In addition, there is little difference between the calculated LMA size–patient weight relation between this formula and the recommended formula. 1 For example, if the patient weighs 15 kg, the calculation would be as follows: LMA size ≥ (15/5)0.5 = (3)0.5 = 1.7. In this case, a No. 2 LMA would be used.
Although some suggest that adult sizes of LMA (No. 4 and No. 5) can be selected based on sex, 2 this formula may also be applicable to these adult sizes. This formula is easy to remember and, we hope, helpful in clinical situations.
Tetsuro Kagawa M.D.
Hidefumi Obara M.D.
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1. 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

2. Brimacombe JR, Berry AM, Campbell RC, Verghese RC: Selection of proper size of laryngeal mask airway in adults. Anesth Analg 1996; 83:664

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