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Airway management

A novel method for laryngeal mask airway size selection in paediatric patients

Zahoor, Abdul; Ahmad, Nauman; Sereche, Getu; Riad, Waleed

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European Journal of Anaesthesiology: August 2012 - Volume 29 - Issue 8 - p 386-390
doi: 10.1097/EJA.0b013e32835562f4

Abstract

Introduction

The successful use of the laryngeal mask airway (LMA) depends partly on the appropriate selection of its size, the method of insertion and inflation of the air cuff. The LMA is available in different sizes ranging from 1 to 6. Sizes over 4 are designed for the adult population, with LMA size 4 being appropriate for an adult female and size 5 for an adult male with a body weight not exceeding 100 kg. Sizes of 3 or less are targeted at the paediatric population. However, the selection of the correct size in children remains tenuous due to a lack of standardisation of weight-based and age-based methods for sizing.1–3 In clinical practice, the most commonly used method for size estimation is the weight-based calculation4 which may not be suitable in many patients because of the wide range for each category of weight. Additionally, overweight and underweight children may be excluded from the range defined by the weight-based table.5 The development of the oropharyngeal cavity and the tissues surrounding the upper airway (bone and soft tissues) is linearly related to the age and the height independently of the sex or weight of a child.6,7

Many anaesthesiologists choose an orotracheal tube approximately equal to the size of the little finger of a child.8 Although this estimation may be difficult and unreliable,9,10 it provides a rough approximation of the size of the tube required. No analogous method exists for rough estimation of the required LMA size.

The aim of the current study was to determine whether the size of the external ear (pinna), which also resembles the shape of LMA, could be used as a proxy for the appropriate size of LMA in children.

Methods

Ethics board approval for this study (Ethical Committee Number 0909-P) was provided by the Ethical Committee of King Khaled Eye Specialist Hospital, Riyadh, Saudi Arabia (Chairperson Dr Hassan Al-Dhibi) on 22 February 2010. Written parental informed consent was obtained for all patients. Children with American Society of Anesthesiologists’ (ASA) physical status 1–2, scheduled for routine ophthalmological surgery and in whom a LMA was indicated for anaesthesia were included in the present study. Children were excluded if they required tracheal intubation for their surgery (e.g. dacrocystorhinostomy), were under 6 months or above 15 years of age or had a history of an oropharyngeal lesion other than tonsillar hypertrophy. Emergency cases and patients with a full stomach, a history of hiatus hernia or the presence of decreased pulmonary or chest wall compliance, were excluded from the study. All patients were assessed 1 day before surgery and were pre-medicated according to the hospital protocol with oral midazolam 0.5 mg kg−1.

As part of the pilot study, the external ear was measured with a ruler in the vertical and horizontal dimensions in the first 30 participants and the closest corresponding size of a classic reusable LMA was chosen for insertion (Fig. 1). Following accomplishment of this first part of the study, the selection of the LMA was performed in all remaining patients based on visual observation rather than measurement in order to have the LMA that most closely approximated the size of the external ear (Fig. 2). If the external ear fell between two sizes of LMA, we opted for the larger one. LMAs one size larger and one size smaller were available for exchange if the approximation was incorrect.

Fig. 1
Fig. 1:
No captions available.
Fig. 2
Fig. 2:
No captions available.

Induction of anaesthesia was performed in all children with sevoflurane in a mixture of 40% oxygen and 60% nitrous oxide. When the proper depth of anaesthesia had been achieved with an end-tidal sevoflurane concentration of 1.5 to 2%, the intended size of a lubricated, partially inflated LMA was inserted using the recommended insertion technique.11 After insertion, the air cuff of the LMA was inflated further to obtain a cuff pressure of 60 cmH2O as measured by a manometer. Volume-controlled ventilation was applied using a Datex Ohmeda Aestiva/5 (Madison, Wisconsin, USA) anaesthesia machine and the tidal volume was set at 10 ml kg−1 body weight while the respiratory rate was adjusted to keep the end-tidal carbon dioxide tension between 4.7 and 5.3 kPa.

The primary outcome was the number of patients with successful insertion of the LMA after the first attempt. Successful insertion of the LMA was defined using the following five criteria: correct placement of the LMA was verified by slight outward movement of the tube upon full cuff inflation, the presence of a small oval swelling in the neck around the thyroid and cricoid area, no cuff visibility in the oral cavity on visual inspection, expansion of the chest wall on hand-bag compression and correct visualisation of the exhaled carbon dioxide trace on the capnograph.

The secondary outcomes were the number and the causes of insertion failure. Failure of insertion of the LMA was defined as the presence of at least one of the three following events: an audible leak, peak airway pressure higher than 20 cmH2O, or a difference between inspiratory and expiratory volumes of more than 5%. In the case of insertion failure, the LMA was removed and a half-size larger was inserted. If there was still failure of either correct placement (cuff visibility in the oral cavity or a rotated cuff) or ventilation after the second attempt, the trachea was intubated with an orotracheal tube.

The LMA was removed at the end of surgery after the child returned to an appropriate spontaneous breathing pattern and was fully awake. Any postoperative complication which could be related to the use of the LMA was recorded in the recovery room, namely laryngospasm, bronchospasm, postoperative sore throat, hypoglossal nerve injury, hypoxaemia and aspiration. The participants were discharged to their surgical wards after they fulfilled the criteria for discharge from the recovery room.

Statistical analysis

The numbers of successful LMA insertions after the first attempt are expressed as numbers and percentages of the total number of patients. The number of failures of LMA insertion and their causes are expressed as numbers and percentages of the total number of patients. Other numerical data are presented as mean and SD, whereas categorical data are expressed as numbers and percentages. The required LMA sizes based on two methods (weight-based and ear size-based) are tabulated and agreement between the two methods were computed using the κ-statistic. Data entry and analysis were performed using SPSS version 14 (IBM Corp., Somers, New York, USA).

Results

The study included a total of 210 children [112 (53%) males and 98 (47%) females]. The mean (SD) age was 6.3 years (3.8) and mean (SD) weight was 21.6 kg (12.9). The sizes of LMA corresponded closely with the external ear by vertical and horizontal dimensions after measuring with a ruler (Fig. 1).

Insertion and adequate ventilation were achieved in 196 (93.3%) patients on the first attempt. Fourteen participants (6.7%) required a second attempt (Table 1). The main reason for failure of the first attempt was presence of an audible leak around the cuff after participants were put on controlled mode ventilation. For 11 of the 14 failures, the ear size-based estimation led to an underestimation of the required LMA size and a half-size larger LMA was finally chosen. Tonsillar hypertrophy and light anaesthesia were the other causes of failure. Insertion of the LMA and ventilation of the lungs were completely unsuccessful in only one (0.5%) participant with Marfan's syndrome whose trachea was subsequently intubated with an orotracheal tube (Table 1).

Table 1
Table 1:
Success and causes of failure of insertion of laryngeal mask airway

The most commonly used LMA size was 2.5, inserted in 94 (45%) participants, followed by LMA size 2, inserted in 64 (30%). LMA sizes 1, 1.5, 3 and 4 were used in six (3%), five (2%), 38 (18%) and three (1.5%) patients, respectively.

Table 2 shows the number of patients according to the two methods of LMA size selection (weight-based and ear size-based). In 100 (47%) patients, the size was found to be in the range of recommended weight for that size. In 110 participants, ear size-based estimation led to the selection of a LMA size lower than would have been expected based on weight. In no participant did ear size-based estimation lead to the selection of an LMA size larger than would have been estimated based on weight (Table 2). Agreement between the two methods of LMA size selection was moderate using κ-statistics (κ=0.50).

Table 2
Table 2:
Laryngeal mask airway sizes based on weight and ear size

Discussion

The results of the present study demonstrate that the proposed ear size-based sizing method is effective in determining the required size of LMA in children. In addition, we demonstrated that there was no need to adjust the size to the sex, as the ear size depends on the age and to a lesser extent on the body size.

We evaluated a very simple hypothesis to determine the chosen size of LMA for children between 6 months and 15 years of age. The success rate for insertion with adequate ventilation obtained in the present study is comparable with the reported success rates with the LMA.12 Therefore, we can say with confidence that the size of the external ear of the child may be used as a proxy for the required size of LMA in children. This method would be an addition to the existing methods and may help clinicians to avoid memorising weight/age-based tables and formulae. In addition, this technique may be highly relevant for emergency physicians, ambulance staff and emergency medical technicians because they occasionally use LMAs in children and often do not have time to look at weight-based tables or other methods prior to insertion. The ear size-based method may also be useful in a failed intubation drill. However, although we could not detect any particular pitfall to this method of LMA size selection, we are aware that this technique was used in healthy children and may not be applicable to children with congenital abnormalities.

We used a partially inflated cuffed LMA for the purpose of matching the LMA size to that of the external ear which has an additional advantage for ease of insertion.13 We elected to use a standard technique for cuff inflation, measuring cuff pressure instead of volume because filling to the maximum volume has been shown to result in cuff pressures exceeding 60 cmH2O in paediatric LMAs.14 The intention was to avoid exceeding the maximum pressure because increased cuff pressures in LMAs are closely related to the likelihood of postoperative pharyngolaryngeal adverse effects such as sore throat.15,16

The LMA is used widely by anaesthesiologists for a variety of elective cases and is part of the difficult airway algorithm of the ASA.17 It is now a standard device in many ambulances and emergency departments and estimating the correct size especially for children can be clinically beneficial. Furthermore, senior anaesthesiologists often cannot remember the correct LMA size based on a child's weight or the weight-based table itself. Although weight is a useful measure, choosing the correct size can become difficult when a patient's weight borders two sizes. Similarly, remembering various formulae3 for the correct LMA size is difficult and confusing. Inserting an improperly sized LMA may cause malposition;18–20 hence, an easy alternative sizing method is desirable. However, by implementing this insertion technique, our results may have been biased because, occasionally, two sizes may fit the same patient. Further analysis is also required to test the feasibility of this method in adults or infants under the age of 6 months.

In summary, we have demonstrated that the success rate of LMA size selection by the ear size-based method is comparable to that of the weight-based method. This method is simple, does not require memorising different tables or formulae and can be applied routinely in clinical practice, particularly by physicians who use the LMA only occasionally.

Acknowledgements

This work was supported only by the Department of Anesthesia, King Khaled Eye Specialist Hospital, Riyadh, Saudi Arabia.

None of the authors has any conflict of interest.

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Keywords:

ear size; laryngeal mask airway size; paediatrics

© 2012 European Society of Anaesthesiology