Anesth Analg 2002;94:1023-1027
© 2002 International Anesthesia Research Society
GENERAL ARTICLES
Intubating Laryngeal Mask Airway Size Selection: A Randomized Triple Crossover Study in Paralyzed, Anesthetized Male and Female Adult Patients
S. Kihara, MD*,
Y. Yaguchi, MD*,
J. Brimacombe, FRCA, MD
,
S. Watanabe, MD, PhD*,
N. Taguchi, MD*, and
N. Hosoya, MD*
*Department of Anaesthesia, Pain Clinic, and Clinical Toxicology, Mito Saiseikai General Hospital, Ibaraki, Japan; and University of Queensland, Department of Anaesthesia and Intensive Care, Cairns Base Hospital, The Esplanade, Cairns, Australia
Address correspondence and reprint requests to J. Brimacombe, FRCA, MD, Department of Anaesthesia and Intensive Care, Cairns Base Hospital, The Esplanade, Cairns 4870, Australia. Address e-mail to jbrimacombe{at}austarnet.com.au
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Abstract
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We determined the optimal size of intubating laryngeal mask airway (ILM) for ventilation and blind tracheal intubation in men and women. We also determined the distance the tracheal tube needs to protrude beyond the distal aperture to ensure that the cuff is through the vocal cords. Fifty male and 50 female anesthetized, paralyzed patients (ASA physical status I or II, aged 18–80 yr) were studied. Three operators (A, B, and C) were involved for the purposes of blinding. The size 3, 4, or 5 ILM was inserted into each patient in random order by Operator A, and the quality of ventilation was scored (adequate, suboptimal, or failed) by Operator B. The fiberoptic position (correct, too shallow, or too deep) and the distance between the distal aperture and the vocal cords was determined by Operator B. A single attempt at blind intubation was made by Operator C. Operators B and C were blinded to the size of the ILM. Operator C was also blinded to the information recorded by Operator B. All ILMs were inserted into the laryngopharynx at the first attempt. For men and women, the ventilation score was smaller for the Size 3 than the Size 4 or 5 (all: P < 0.002). For men, correct positioning was less common with the Size 3 than the Size 4 or 5 (both: P < 0.02). For women, correct positioning was similar among sizes. For men, tracheal intubation was successful less frequently with the Size 3 (84%) than the Size 4 (100%) or 5 (98%) (both: P 
0.01). For women, tracheal intubation success was similar among sizes (Size 3, 4, and 5: 86%, 96%, and 92%, respectively). Intubation was always successful if the ILM was correctly positioned and always failed if it was too shallow or deep. In both male and female patients, the distance between the distal aperture and the vocal cords increased with increasing ILM size (all: P < 0.04) and patient height (P < 0.0001) and was always longer for men (all: P < 0.0001). The overall mean distance (95% confidence interval) that the tracheal tube needed to protrude was 10–12 cm (8–13 cm) in men and 8–11 cm (8–12 cm) in women. We conclude that for men, the Size 4 and 5 ILMs are better than the Size 3 for ventilation and blind intubation. For women, the Size 4 and 5 ILMs are better than the Size 3 for ventilation, but there is no difference among sizes for blind intubation. The length the tracheal tube must protrude from the distal aperture to ensure that the cuff is completely through the vocal cords is 8–13 cm, depending on ILM size, the tracheal tube size, and the sex and height of the patient.
IMPLICATIONS: For men, the Size 4 and 5 intubating laryngeal mask airways are better than the Size 3 for ventilation and blind tracheal intubation. For women, the Size 4 and 5 are better than the Size 3 for ventilation, but there is no difference among sizes for blind intubation. The length the tracheal tube must protrude from the distal aperture of the intubating laryngeal mask airway to ensure that the cuff is completely through the vocal cords is 8–13 cm, depending on the size of the mask and tracheal tube and on the sex and height of the patient.
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Introduction
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The intubating laryngeal mask airway (ILM) functions as a ventilatory device, airway intubator, or both (1,2). It is currently available in three adult sizes. The dimensions of the tube portion are identical among sizes, but the inflated Size 4 and 5 cuffs are approximately 15% and 37% larger than the Size 3, respectively. Size selection is clinically important when the classic laryngeal mask airway (LMA) is used as a ventilatory device (3), and there are comparative data supporting some size-selection criteria (3), but differences in design and primary function mean that these findings may not apply to the ILM. A variety of size-selection criteria have been used with the ILM, including weight-based criteria (2), sex-based criteria (4), height-based criteria (5), and criteria based on nose-chin distance (6). However, these criteria are unsupported by comparative data. In addition, Asai et al. (7) found that the tracheal tube needed to protrude 95 mm beyond the mask aperture bars of the LMA to ensure safe tracheal intubation, but no such information is available for the ILM. In the following randomized, triple cross-over study, we compare the Size 3, 4, and 5 ILMs for ventilation and blind tracheal intubation in men and women. We also determine the distance the tracheal tube needs to protrude to ensure safe tracheal intubation.
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Methods
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With ethics committee approval and written informed consent, 50 male and 50 female ASA physical status I or II patients requiring tracheal intubation for elective surgery participated in the study. Patients were excluded if they were <18 yr old, had cardiorespiratory or cerebrovascular disease, were at risk of aspiration, required head and neck surgery, had a known difficult airway, were Mallampati Grade 4 (8), or were considered otherwise unsuitable for the ILM. Three operators (A, B, and C) were involved for the purposes of blinding. The operators who performed the insertion, intubation, and fiberoptic assessment were experienced ILM users (>200 uses).
Mallampati score, nose-chin distance (tip of the nose to tip of the chin with mouth closed), and thyromental and sternomental distances (with head in extension) were measured preoperatively. Patients were premedicated with diazepam 5 mg orally and roxatidine 75 mg orally 1.5 h before the induction. Routine monitoring was applied before the induction and included an electrocardiograph, pulse oximeter, gas analyzer, noninvasive blood pressure monitor, and peripheral nerve stimulator. The patient was placed in the supine position with the head and neck on a standard pillow 7 cm in height. Oxygen was administered via a face mask for 5 min. Lidocaine 0.5 mg/kg was given IV with the tourniquet inflated for 30 s to prevent propofol pain. Anesthesia was induced with fentanyl 2 µg/kg and propofol 2.5 mg/kg and maintained with sevoflurane 2% in oxygen 100% until intubation was attempted. Muscle paralysis was induced by vecuronium 0.1 mg/kg. Patients were ventilated via a face mask until the train-of-four count from peripheral nerve stimulation was zero.
The Size 3, 4, and 5 ILMs were inserted into each patient in random order by using a single-handed rotational technique by Operator A. The cuff was inflated with air (Size 3, 20 mL; Size 4, 30 mL; Size 5, 40 mL), and the position was maintained by holding the handle firmly. No attempt was made to maneuver the cuff in the pharynx. The quality of ventilation via the ILM was scored by Operator B as adequate (no oropharyngeal air leakage with peak airway pressures 
20 cm H2O and a square wave capnograph trace), possible (oropharyngeal air leakage with peak airway pressures <20 cm H2O and a square wave capnograph trace), or failure (no square wave capnograph trace). The anatomic position of the tip of the epiglottic elevating bar relative to the vocal cords and the tip of the epiglottis was determined fiberoptically by Operator B and scored as follows: correct position (between the vocal cords and the tip of the epiglottis), too deep (inferior to the vocal cords), or too shallow (superior to the tip of the epiglottis). If the position was correct, the distance between the distal aperture of the tube and the anterior commissure of the vocal cords was mea-sured with the fiberoptic scope. Blind intubation was attempted by Operator C with the 7.5-mm-inner-diameter wire-reinforced flexometallic silicone tracheal tube with a central bevel recommended by the manufacturer (ILM endotracheal tube; Euromedical Industries, Kedah, Malaysia). Intubation was successful if the tube passed 10 cm beyond the distal aperture and there was a square wave capnograph trace. Only one attempt at intubation was permitted. Intubation was considered a failure if tactile resistance was encountered or the tube passed 10 cm beyond the distal aperture with no square wave capnograph tract (esophageal intubation). Care was taken to avoid movement of the ILM once it was inserted. Between ILM insertions, patients were ventilated via a face mask. If the ILM failed as a ventilatory device, fiberoptic assessment and intubation were still performed, but if the oxygen saturation decreased to <95%, the ILM was removed and the patient face mask was ventilated. In addition, the distance from the distal tip of the tracheal tube to the proximal margin of the tracheal tube cuff was measured in vitro for the Size 7.0-, 7.5-, and 8.0-cm tracheal tube reinforced flexometallic silicone tracheal tube with a central bevel. Three of each size of tracheal tube were measured.
Operators B and C were blinded to the size of ILM. Operator C was also blinded to the information recorded by the operator. Sample size was selected to detect a 20% difference in first-attempt blind intubation success rates among sizes for a Type I error of 0.05 and a power of 0.8. The calculation was based on the first-time blind intubation success rates from a previous study by our group (5). Nonparametric data were analyzed with the 
2 test, and parametric data were analyzed with factorial analysis of variance and the Bonferroni-Dunn test. The relationship between demographic data and the distance between the distal aperture and vocal cords was analyzed by multivariate analysis. Unless otherwise stated, data are presented as mean ± SD. Significance was taken as P < 0.05.
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Results
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Demographic data are presented in Table 1. Ventilation scores, anatomic position, and intubation success are presented in Table 2. All ILMs were inserted at the first attempt. For men, the ventilation score was smaller for the Size 3 than the Size 4 or 5 (both: P < 0.001). For women, the ventilation score was smaller for the Size 3 than the Size 4 or 5 (both: P < 0.002). For men, correct positioning was less common with the Size 3 than the Size 4 or 5 (both: P < 0.02). For women, correct positioning was similar among sizes. For men, intubation was successful less frequently with the Size 3 than the Size 4 or 5 (both: P 0.01). For women, intubation success was similar among sizes. Intubation was successful for all patients in whom the ILM was correctly positioned and none in whom it was too shallow or deep. In male and female patients, the distance between the distal aperture and the vocal cords increased with increasing ILM size and was always larger for men (all: P < 0.0001). The distance between the distal aperture and the vocal cords had a positive correlation with patient height (all sizes: P < 0.0001), nose-chin distance (all sizes: P < 0.001), thyromental distance (all sizes: P < 0.03), and sternomental distance (all sizes: P < 0.03) (Table 3), but there was no correlation with weight and age. The distance from the distal tip of the tracheal tube to the proximal margin of the tracheal tube cuff for Size 7.0-, 7.5-, and 8.0-cm tubes was exactly 5.5, 6.0, and 6.5 cm, respectively. All failures were due to tactile resistance and none to esophageal intubation. SpO2 was 95% in all patients.
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Table 3. The Distance (mean ± sd) Between the Distal Aperture of the Tube and the Anterior Commissure of the Vocal Cords if the Fiberoptic Position is Correct (DA-VC distance) and the Calculated Mean (95% confidence interval) Distance the Tracheal Tube Must Protrude to Ensure the Cuff is Through the Vocal Cords
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Discussion
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We found that insertion success rates for the ILM were similar among sizes, but ventilation was more frequently successful with the Size 4 and 5 ILMs than the Size 3. These findings are similar to the classic LMA for both insertion (9–12) and ventilation (9,10). We found that the Size 4 and 5 ILMs have a higher success rate for blind tracheal intubation than the Size 3 in men. This is related to the more frequent incidence of incorrect placement of the Size 3 ILM compared with the Size 4 and 5 in men. Blind intubation success rates were similar among sizes in women because the incidence of incorrect positioning was similar. There are no data about blind intubation success rates among the LMA sizes, but fiberoptic position is better for the Size 4 and 5 compared with the Size 3 (9). We found that if the epiglottic elevating bar tip was between the vocal cords and the tip of the epiglottis, intubation was always successful at the first attempt, but if the epiglottic elevating bar tip was inferior to the vocal cords or superior to the tip of the epiglottis, intubation failed. This is because if the epiglottic elevating bar tip is inferior to the vocal cords, the tracheal tube will enter the hypopharynx, and if it is superior to the tip of the epiglottis, it will enter the vallecula.
We found that the distance between the distal aperture and vocal cords was longer for men than women for all sizes of ILM. This may be related to the larger size of the male larynx and pharynx and suggests that the tracheal tube needs to penetrate to a greater depth to ensure passage of the cuff through the vocal cords in these patients. There was also a positive correlation with height, nose-chin distance, and sternomental and thyromental distance. Asai et al. (7) found that the mean distance between the distal aperture and the vocal cords for the classic LMA was 36 ± 5 mm in men for the Size 4 and 31 ± 5 mm in women for the Size 3. Our data for the Size 4 ILM in men and the Size 3 in women were similar, at 48 ± 9 mm and 29 ± 4 mm, respectively. We also found that the distance between the distal aperture and vocal cords increased with increasing size. This may be because the distal aperture of a larger cuff is more proximally located in the pharynx. The instruction manual for the ILM recommends that if the ILM is inserted too deep, a smaller size should be used, and if it is inserted too shallow, a larger size should be used (13). Our findings do not support these recommendations, but rather suggest that the opposite strategy is required: if the ILM is inserted too deep, use a larger size, and if it is inserted too shallow, use a smaller size.
The instruction manual for the ILM recommends that size selection be based on weight (Size 3, 30–50 kg; Size 4, 50–70 kg; and Size 5, 70–100 kg). Most of our study population weighed between 50 and 70 kg, and the Size 5 performed equally as well as the Size 4. This suggests that weight-based criteria are only approximate guidelines, as admitted in the instruction manual. A variety of size-selection criteria have been used for the ILM in published studies: weight-based criteria (Size 3, <50 kg; Size 4, 50–70 kg; Size 5, >70 kg) (2), sex-based criteria (Size 4, women; Size 5, men) (4), height-based cri-teria (Size 3, 160 cm; Size 4, 160–170 cm; Size 5, >170 cm) (5), and criteria based on nose-chin distance (<6.5 cm, Size 3; 6.5–7.5 cm, Size 4; >7.5, Size 5) (6). Our data suggest that using either the Size 4 or 5 in adults is a suitable strategy.
We found that the Size 7.0-, 7.5-, and 8.0-mm wire-reinforced silicone tracheal tubes need to protrude by an additional 0.5 cm per tracheal tube size increase to ensure that the cuff has penetrated the vocal cords. We have also shown that the protrusion length is longer for men and for larger sizes of ILM. The length of the ILM tube is 155 mm, and the length of all three sizes of wire-reinforced silicone tracheal tube is 315 mm, so the maximal protrusion is 160 mm. This length would have been adequate to ensure that the cuff penetrated the vocal cords in all patients in our study.
Our study has several limitations. First, we inflated the cuff to the maximal recommended volume, and our findings might not apply to different cuff volumes. Laboratory testing of five ILMs of each size showed that the mean ± SD cuff width of the Size 3, 4, and 5 was 14.1 ± 0.4 mm, 16.2 ± 0.2 mm, and 18.3 ± 0.3 mm, respectively. In principle, the more air added to the cuff, the more proximal the distal aperture will be in the pharynx and the greater the distance will be to the vocal cords. This is indirectly supported by Asai et al. (7), who found that the tube moves proximally out of the mouth by 7 mm during full cuff inflation. However, there is evidence that the fiberoptic position for the ILM does not vary with cuff volume (14,15). Second, we did not adjust the position of the ILM after insertion. Perhaps the position of the ILM can be optimized before tracheal intubation is attempted, by moving the cuff around the pharynx until the best seal is obtained (2). However, there is only anecdotal evidence supporting this strategy. Third, our study population was predominantly Japanese, and our results may not necessarily apply to other populations. Fourth, our study did not include difficult-airway patients, and our findings may not necessarily apply to this subpopulation. Fifth, we used the manufacturer’s recommended tracheal tube. Our data for protrusion length will need to be adjusted for other tracheal tubes if the distance between the tip and proximal margin of the cuff is different. Sixth, although observers were blinded to the size of ILM, it may be possible for some to correctly guess which size was being used, on the basis of performance. However, the tube and handle portions protruding from the mouth are identical among the three sizes, and we consider that blinding was preserved in our study.
We conclude that, for men, the Size 4 and 5 ILMs are better than the Size 3 for ventilation and blind intubation. For women, the Size 4 and 5 are better than the Size 3 for ventilation, but there is no difference among sizes for blind intubation. The length the tracheal tube must protrude from the distal aperture to ensure that the cuff is completely through the vocal cords is 8–13 cm, depending on the ILM size, the tracheal tube size, and the sex and height of the patient.
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Footnotes
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Presented in part at the 75th International Anesthesia Research Society Meeting, Ft. Lauderdale, FL, March, 2001.
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References
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Accepted for publication December 17, 2001.
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Abstract
Introduction
