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Original Article

Simple method to determine the size of the laryngeal mask airway in children

Gallart, L.*; Mases, A.*; Martinez, J.; Montes, A.*; Fernandez-Galinski, S.*; Puig, M. M.*

Author Information
European Journal of Anaesthesiology: July 2003 - Volume 20 - Issue 7 - p 570-574

Abstract

The laryngeal mask airway is a device frequently used for airway management in anaesthesia and for cardiopulmonary resuscitation [1-4]. In children, the appropriate size of the laryngeal mask airway to be used is usually determined, as recommended by the manufacturer, by the patient's weight (Table 1). Up to now, this is the gold standard method, but the weight-related technique is not always possible. The patient's weight is sometimes unknown (e.g. in immobilized or unconscious patients) or medical staff do not remember the relationship between weight and size. Moreover, there are no constant correlations between laryngeal airways and body indices such as height and weight [5], so that the standard method could cause an inappropriate size of laryngeal mask airway to be chosen.

Table 1
Table 1:
Choice of the size of the laryngeal mask according to the manufacturer's guidance related to body weight.

That is why it would be useful to have an alternative method to determine the size of the laryngeal mask airway in children. This technique should be easy to perform, easy to remember and accurate in determining the size of the laryngeal mask airway, and applicable at the patient's bedside.

We propose a new method related to the anatomical features of the patient that fulfils all these requirements. To validate this procedure, the sizes of the laryngeal mask airway determined with this method were compared with the sizes according to the patient's weight.

Methods

Our hospital Committee approved the study, and informed consent was obtained from parents or guardians.

One hundred and sixty-three children, aged from birth to 14 yr, who were consecutively admitted to the Paediatrics Department of our hospital, were enrolled into the study. Exclusion criteria were: anatomical abnormalities of the patients, especially those related to the head, neck or limbs, or weight and/or height outside the range of the 5th to 95th growth centiles for children up to 14 yr of age.

In each patient, the hand was extended with the palm side facing up. The hand was opened separating the thumb and the little finger, keeping the second, third and fourth fingers extended but together. Laryngeal masks (Laryngeal Mask Co., Henley-on-Thames, UK) of different sizes were inflated with the maximum standard suggested volumes (www.lmaco.com). The anterior aspect of every laryngeal mask airway was then placed against the palmar side of the patient's fingers, keeping the widest part of the laryngeal mask airway in line with the widest part of the three fingers. The investigator selected the mask that best matched the width of these three fingers (Fig. 1).

Figure 1
Figure 1:
Technique used to determine the size of the laryngeal mask airway, which is related to the anatomy of the patient.

The patients' characteristics data were obtained afterwards in order to avoid any bias when choosing the size of the laryngeal mask airway. For each patient, the size chosen with our method was compared with the size determined by the patient's weight, according to the manufacturer's guidelines (Table 1). Once the sizes of the laryngeal masks were obtained from both techniques, the patients were then distributed into different groups and classified on a grid.

Statistics

To evaluate the agreement between the anatomical- and weight-related methods, the kappa coefficient (κ) was calculated [6]. The mean and SD of the patient characteristics were determined for every group of patients.

Results

Table 2 shows the distribution of the 183 patients with the sizes of the laryngeal masks determined by the patient's weight and by the three-finger sizing method. In 142 children (78%), the size of the mask coincided for both methods. In the remaining 41 individuals, the disagreement between the mask sizes in both methods concerned only one size. In no patient was a two-size difference observed. The calculated κ coefficient was 0.81, which shows an 'excellent agreement' [6] between both techniques.

Table 2
Table 2:
Distribution of the patients according to the sizes of mask obtained from the standard (weight) and three finger-related methods.

Tables 3 and 4, respectively, show the mean and SD of body weight and height of the patients for each combination of size of laryngeal mask. For those patients on the diagonal line, in which the mask size coincides for both techniques, their weight was within the appropriate range for the standard technique (weight related). In those patients in whom there was a disagreement between the two methods, patients' weights were borderline values that indicate the choice of a different size of laryngeal mask airway.

Table 3
Table 3:
Weight in each group of patients.
Table 4
Table 4:
Height in each group of patients.

Discussion

This study demonstrates that the proposed three-finger sizing method to determine the size of the laryngeal mask is useful compared with the weight-related method. The following matters need to be analysed further: the need for an alternative method to choose the appropriate mask size, the accuracy of this method and the reason why fingers are used as a selection technique; the viability of this method for patients who are overweight, or have anatomical anomalies; and whether the use of this new method is adequate for adults.

The choice of a wrongly sized laryngeal mask has been suggested as a cause of malposition of the mask [7-9]. Therefore, when the standard method cannot be performed or we think the weight-related technique might be imprecise, an alternative method would be desirable.

In children, the standard method is not suitable if the true weight of the patient is unknown and cannot be easily checked, e.g. in immobilized individuals, cardiopulmonary resuscitation and emergency situations. In such circumstances, the child's weight may be incorrectly estimated [10], which could cause the wrong size of mask to be selected. Sometimes, although the weight is known, medical staff do not remember the relationship between weight and size, especially in emergency situations when a laryngeal mask might be needed (e.g. resuscitation, failed orotracheal intubation [11]).

The weight-related choice may occasionally fail to indicate the correct size. Airway anatomy differs in adults and children. Laryngeal mask airways for children were initially scaled down versions of the adult version, but later anatomical studies were undertaken to confirm their suitability and accuracy of fit [12]. However, in some instances, such as obesity or malnutrition, the weight of the patient varies but the size of the airway anatomy does not, so the standard method is unreliable. Moreover, for those patients whose body weight is at the borderline between two sizes, the choice of the size of mask can be doubtful. Hence, weight is a useful measure when choosing the size of mask for most patients, but in some cases, an alternative anatomically related method could be useful.

Therefore, we looked for an alternative method that had to be anatomically related if possible. Paediatric orotracheal tubes are chosen according to age rather than to body weight [13]. Measurement of the size of the index [14] or little [15] fingers has been advocated, although these methods have been shown to be less effective than the age-related choice [12]. For most of our patients, the size chosen was the same according to both methods. In the remaining patients, a difference of only one size was observed, and in these patients, their weight was borderline. For such patients, the use of the three-finger method could be useful.

In patients with nutritional disorders who were excluded from this study, there would probably be discrepancies in both methods when determining the size of the laryngeal mask. If in subjects of normal weight both methods are adequate, in those with obesity or malnutrition the finger-related method might be more reliable because the size of the hand appears to change less than other parts of the anatomy (obese surgeons do not usually need larger gloves). These are just hypotheses, and further studies are needed to establish which method is the more precise in those patients where there is disagreement between the techniques.

In adults, predicting the optimal size of the laryngeal mask for individual adult patients can be complex [16,17]. According to the manufacturer's instructions, the size is determined by the patient's weight (Table 1), although alternative methods have been proposed, according to weight [7,18], gender [17,18] or height [16](Table 5). It has also been suggested that body mass index is a better indication of pharyngeal size than a patient's height or weight [19], but no factors have yet been identified that give an easy measurement of potential pharyngeal volume directly correlating with the appropriate size of laryngeal mask [18,20,21]. Our experience with the finger-related method is that it is appropriate in adults as well as in children, but needs to be formally evaluated.

Table 5
Table 5:
Alternative methods used to determine the size of the laryngeal mask in adults.

In conclusion, we have introduced a new technique to determine the size of the laryngeal mask in children without anatomical or nutritional disorders. This method is accurate, easy and feasible at the bedside. Further studies are needed to validate this method for adults and patients with nutritional disorders.

Acknowledgements

This study received financial support (FIS 01/1586) from the Ministerio de Sanidad y Consumo, Spain. This work was presented in part at the European Society of Anaesthesiologists Annual Congress, Barcelona, Spain, 28 April 1998 (Br J Anaesth 1998; 80 (Suppl 1): A486). We thank Drs Montserrat Martin and Ferran Sanz for statistical support, and José Antonio Pereira for the assessment in human anatomy.

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Keywords:

ANAESTHESIA, GENERAL; INTUBATION, INTRATRACHEAL, laryngeal masks; PAEDIATRICS

© 2003 European Society of Anaesthesiology