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An Ultrasound Evaluation of Laryngeal Mask Airway Position in Pediatric Patients: An Observational Study

Kim, Jeongmin MD*; Kim, Ji Young MD, PhD; Kim, Won Oak MD*; Kil, Hae Keum MD*

doi: 10.1213/ANE.0000000000000551
Pediatric Anesthesiology: Research Report
Free

BACKGROUND: In children, the laryngeal mask airway (LMA) is frequently displaced within the hypopharynx, resulting in repositioning of the device. When the tip of the LMA is placed in the esophageal inlet, the arytenoids are moved ventrally. When the LMA is rotated or deviated, the ventral movement of the arytenoids may result in asymmetric elevation of an arytenoid cartilage, which can be detected with ultrasound (US). In this study, we sought to estimate the incidence of LMA malposition detected with US in pediatric patients. The primary end point was to compare the incidence of LMA malposition between US and fiber optic bronchoscopy (FOB). The secondary end points were to find the interrelationship between US-detected and FOB-detected malposition of the LMA and to locate the diagnostic performance of US in detecting LMA malposition.

METHODS: In this observational study, 100 consecutive children were included. After anesthetic induction, US evaluation was performed before and after LMA insertion to obtain the glottic image on the anterior neck. FOB was performed to assess LMA position (FOB LMA grade and LMA rotation grade). With a post-LMA US image, the symmetry of the arytenoid cartilages was evaluated. Asymmetrical elevation of an arytenoid cartilage in reference to the glottic midline and the opposite arytenoid cartilage was graded as 0 to 3 (US arytenoid grade). The interrelationships between US arytenoid grade and FOB LMA grade or LMA rotation grade were assessed.

RESULTS: The incidence of asymmetrical elevation of an arytenoid was 50% (95% confidence interval [CI], 40%–60%). On FOB, the incidence of LMA malposition was 78% (95% CI, 69%–86%), and that of LMA rotation was 43% (95% CI, 33%–53%). The incidence of LMA malposition was higher with FOB (P < 0.0001), but the incidence of rotation was similar (P = 0.395). US arytenoid grade did not correlate with FOB LMA grade (P = 0.611) but showed a significant correlation with LMA rotation grade (P < 0.0001; 95% CI, 60%–83%). To detect a rotated LMA, US had a sensitivity of 93% (95% CI, 81%–98%) and a specificity of 82% (95% CI, 70%–91%). The positive and negative predictive values were 80% (95% CI, 66%–90%) and 94% (95% CI, 83%–99%), respectively. The accuracy was 87% (95% CI, 79%–93%).

CONCLUSIONS: Although US could not detect the suboptimal depth of an LMA, US has promise of being an accurate tool in detecting a rotated LMA.

From the *Department of Anesthesiology and Pain Medicine, Severance Hospital, Anesthesia and Pain Research Institute, Yonsei University College of Medicine, Seoul, Republic of Korea; and Department of Anesthesiology and Pain Medicine, Kangnam Severance Hospital, Anesthesia and Pain Research Institute, Yonsei University College of Medicine, Seoul, Republic of Korea.

Accepted for publication September 24, 2014.

Funding: None.

The authors declare no conflicts of interest.

Reprints will not be available from the authors.

Address correspondence to Hae Keum Kil, MD, Department of Anesthesiology and Pain Medicine, Severance Hospital, Anesthesia and Pain Research Institute, Yonsei University College of Medicine, Yonsei-ro, Seodaemun-gu, Seoul 120-752, Republic of Korea. Address e-mail to hkkil@yuhs.ac.

In modern anesthetic practice, the use of laryngeal mask airway (LMA) in children undergoing minor surgery has become popular. The LMA fits against the periglottis and occupies the posterior hypopharyngeal space to form a seal above the glottis.1 In pediatric patients, the overall success rate of LMA insertion (within 3 attempts) varies from 67% to 100%.2–5 This variable success rate can be explained both by differences in defining successful insertion and the variety of insertion techniques.6 Several studies on LMA position using fiber optic bronchoscopy (FOB) reported that a large proportion of children (12.8%–49%) required repositioning of the LMA to an optimal position.3,7–10 The pediatric LMA is a smaller version of the original prototype, which was cast from an adult cadaveric mold. However, children have large tongues and a floppy epiglottis. In addition, their larynx is higher and more anterior compared with that of adults. This difference in airway anatomy may influence the correct placement of the LMA.2–6,11

In clinical practice, it may be necessary to reposition the LMA during surgery. Although the relationship between poor initial LMA position and the frequency of later displacement has not been studied, it is desirable to have the LMA optimally positioned from the start. Successful insertion is usually assessed clinically with a capnogram, appropriate chest excursion, and the absence of an audible leak at a peak inspiratory pressure of 20 cm H2O. However, even if all of these clinical signs are reassuring, one cannot be certain that correct positioning has been achieved. FOB studies have shown that although ventilation may be judged as adequate, smaller-sized pediatric LMAs are more commonly associated with suboptimal anatomical positioning with partial or complete obstruction of the view of the glottic apertures.12 It is important to place a properly sized LMA in the optimal position to provide adequate ventilation and to prevent complications such as mucosal injury, glossoptosis, and gastric insufflation with the potential for aspiration.12,13

Ultrasound (US) is an emerging technology for airway assessment. Although the laryngeal cartilage is poorly calcified in children, the arytenoid and thyroid cartilages are easily recognizable as hypoechoic structures surrounded by a hyperechoic border on US.14–16 The LMA cuff forms a seal with the periglottic area to provide a continuity of the airway. Three-dimensional radiologic reconstruction of the airway with the LMA showed that within the hypopharynx, the tip of the LMA is placed at the esophageal inlet just below the glottis, and the inflated cuff may elevate the arytenoid and thyroid cartilages to some extent (Fig. 1).17 Although US was unable to identify the air-filled cuff of the LMA, it could detect the anatomical change of the cartilage section before and after LMA insertion. We hypothesized that US would detect a malpositioned LMA within the hypopharynx through the positional change of the arytenoid cartilage. The aim of this study was to estimate the incidence of LMA malposition detected with US in pediatric patients. The primary end point was to compare the incidence of LMA malposition between US and FOB. The secondary end points were to find the interrelationship between US-detected and FOB-detected malposition of the LMA and to determine the diagnostic performance of US in detecting LMA malposition.

Figure 1

Figure 1

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METHODS

The study was performed at our university hospital from January 2012 to July 2012. This study was approved by the IRB of our institution (ref. number: 4-2011-0800) and registered at www.ClinicalTrials.gov (ref. number: NCT01655459). Written informed consent was obtained from the parents of children to be enrolled. A total of 100 children (from 3 to 69 months of age) with ASA Physical Status I or II undergoing ambulatory infraumbilical urologic surgery were enrolled. Children with upper respiratory tract infections, restricted mouth opening, airway malformation, congenital heart disease, and those at risk for aspiration were excluded.

No premedication was given. Children were transferred to the operating room with their parents. Children were laid in the supine position, with the neck extended slightly using a rolled surgical cotton pad. Under standard monitoring, inhaled induction was performed with 6% to 7% sevoflurane in oxygen. After loss of consciousness, IV access was secured, and the inspiratory concentration of sevoflurane was reduced. A neuromuscular blocking drug (atracurium 0.4 mg/kg) was given for smooth LMA insertion and prevention of swallowing or coughing during the US and FOB examination. Three minutes after atracurium administration, highly experienced anesthesiologists performed a B-mode US on the anterior neck using an 8- to 13-MHz linear probe (LOGIQeTM, GE Healthcare, Wauwatosa, WI) while the practitioner stopped mask ventilation. Transverse scans were started from the hyoid bone (a superficial hyperechoic inverted U-shaped linear structure with posterior acoustic shadowing) (Fig. 2A). Then the probe was moved caudally with a slight cephalad angle to localize the vocal cords (paired hypoechoic and hyperechoic linear structures) and arytenoids (hypoechoic structures surrounded by a hyperechoic border) (Fig. 2B). The glottic image was saved for later analysis.

Figure 2

Figure 2

After US, a single anesthesiologist who had >3 years of experience in pediatric anesthesia inserted a reusable classical LMA (LMATM, The Laryngeal Mask Company Ltd., Henley-on-Thames, UK) with a partially inflated cuff using the standard method.8,17,18 The LMA was held like a pen and inserted using the index finger, with pressure against the palate and posterior pharyngeal wall, and advanced until the mask tip reached the oropharynx. LMA insertion was performed while the assistant thrust the patient’s jaw with 2 hands. The cuff was inflated with air to a pressure of 60 cm H2O using a cuff inflator pressure manometer. The LMA was selected according to patient weight as recommended by the manufacturer (LMA 1.5 for 5–10 kg; 2 for 10–20 kg; 2.5 for 20–30 kg). Successful insertion was judged by clinical tests (chest and bag movement with manual ventilation, no audible leak at 20 cm H2O of airway pressure, and squared wave of capnogram). Repeated attempts were made if LMA insertion was not successful on the first attempt. A maximum of 3 attempts was allowed. Controlled ventilation was started with 9 mL/kg of tidal volume, and anesthesia was maintained with sevoflurane in 50% oxygen with air. The peak airway pressure was recorded for 5 ventilatory cycles. After 5 ventilator cycles, US was repeated on the anterior neck in the same manner as described previously. To assess LMA position, FOB (LF-DPTM, 3.1 mm OD of 0°, AIZU Olympus Co., Ltd., Fukushima, Japan) was performed with the aid of an assistant holding the mask in place. FOB evaluation of the LMA position was made just proximal to the inner aperture of the LMA. Another FOB image was obtained to visualize the LMA grille for measurement of the angle of LMA rotation at approximately 1 cm above the LMA grille. If the LMA was rotated from the midline, FOB was advanced beyond the grille to confirm the rotated status.

In a pilot study, the tip of the LMA was frequently rotated or shifted from the glottic midline. Thus, the angle of LMA rotation from the glottic midline was also evaluated during FOB. The angle of rotation (to the right or left side) was determined as the angle between the midline of the LMA grille and midline of the larynx (Fig. 3). The midline of the larynx was defined as the vertical line of the anterior–posterior commissure when both commissures were seen. If one or both commissures were not seen, the vertical line between the midpoints of the epiglottis and arytenoid fold was regarded as the midline. LMA rotation was graded as 0 (none), 1 (5°–10°), 2 (11°–20°), or 3 (>20°).

Figure 3

Figure 3

The FOB LMA grade was divided into a 4-point scale (1, larynx only seen; 2, larynx and epiglottis seen; 3, epiglottis impinging on grille, but larynx seen; 4, epiglottis downfolded, and larynx not seen), as described in previous studies.7,9,10 The best view of the larynx (FOB LMA grade 1) is seen when the epiglottis is not downfolded, allowing a clear view of the anterior and posterior commissures of the larynx. The tip of the LMA should lie below the larynx, occluding the upper esophageal sphincter. If the LMA showed an FOB LMA grade of ≥3 or an LMA rotation grade of 3, the LMA was repositioned to an FOB LMA grade of 1. If the LMA showed FOB LMA grade 4 with peak airway pressure ≥15 cm H2O, the LMA was replaced with another size.

A US specialist who was not involved in this study evaluated the US data. The positional change of arytenoids was evaluated with the comparison of pre-LMA and post-LMA US images. On the pre-LMA image, a vertical line was drawn from the anterior to posterior commissures just above both arytenoids, which is surrounded by the hyperechoic densities. The vertical line was evenly divided into 3 parts, with 4 horizontal lines starting from the line joining both arytenoids (Fig. 4). On the post-LMA image, the asymmetrical elevation of an arytenoid (US arytenoid grade) was graded 0 to 3 depending on the location of the elevated arytenoid along the divided vertical line (0, horizontal arytenoids; 1, elevation limited to lower one third of the vertical line; 2, middle one third of the vertical line; 3, upper one third of the vertical line).

Figure 4

Figure 4

Because there is no previous study on the difference in the incidence of LMA malposition between US and FOB, a pilot study of 29 patients was performed. The incidence of LMA malposition with the FOB was 69% (20 of 29), and with US, it was 48% (14 of 29). Assuming a 5% 2-tailed significance level (α = 0.05) and power of 80% (β = 0.20), to detect 21% absolute difference in the incidence of LMA malposition with US and FOB, a sample size of 95 patients was needed. Sample size was calculated based on the McNemar test.

The diagnostic performance was tested with sensitivity, specificity, positive and negative predictive value, and accuracy. Sensitivity is the percentage of US arytenoid grades 1 to 3, which are correctly identified as LMA rotation grades 1 to 3 with FOB. The specificity is the percentage of US arytenoid grade 0, which is correctly identified as LMA rotation grade 0. The positive predictive value is the percentage of LMA rotation grade 1 to 3 when US arytenoid grade is 1 to 3. The negative predictive value is the percentage of LMA rotation 0 when the US arytenoid grade is 0. Accuracy is the percentage of concordance between 2 devices in terms of grades. The confidence interval (CI) of the above-mentioned parameters was calculated with Clopper-Pearson method. The interrelationship of US arytenoid grade and FOB LMA grade or LMA rotation grade was analyzed with Spearman correlation coefficient, and the 95% CI was calculated using Fisher z transform. Data are presented as number (%) and 95% CI. Statistical analysis was performed using IBM SPSS Statistics 19 (SPSS Inc., Chicago, IL). P < 0.05 was considered statistically significant.

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RESULTS

A total of 100 children (males/females = 89/11; mean age = 28.6 months [SD = 19.6]; mean weight = 13.0 kg [SD = 4.0]; mean height = 87.0 cm [SD = 16.4]) were included in the study. The LMA was successfully inserted at the first attempt in 84 children. In 16 children, the LMA was placed at the second attempt. After completion of the US and FOB, the LMA was repositioned in 54 patients with an FOB grade of ≥3 and in 1 patient with an LMA rotation grade of 3. The LMA was replaced in 8 children with an FOB grade of 4 and peak airway pressure of ≥15 cm H2O (smaller LMA in 6; larger LMA in 2). Mean peak airway pressures ranged from minimum 10 mm Hg to maximum 17 mm Hg during 5 consecutive breaths. There were no significant differences of peak airway pressures between FOB LMA grades. (P = 0.369)

Table 1 presents the fiber optic grade of LMA position. The number of patients with rotated LMA and US arytenoid grade with respect to FOB LMA grade is also shown. The position of LMA was suboptimal in 78% (95% CI, 69%−86%) of children. Asymmetrical elevation of 1 arytenoid was detected in 50% (95% CI, 40%−60%) of children. There was no child with a US arytenoid grade of 3 in this study. The incidence of LMA malposition was higher with FOB (P < 0.0001), but the incidence of rotation was similar (P = 0.395). FOB LMA grade showed no correlation with US arytenoid grade (P = 0.611).

Table 1

Table 1

Table 2 presents US arytenoid grade of LMA. The LMA rotation grade with respect to US arytenoid grade is also shown. The LMA was rotated at varying degrees in 43% (95% CI, 33%−53%) of children. The US arytenoid grade had a significant correlation with LMA rotation grade (P < 0.0001; 95% CI, 60%−83%). With regard to test performance, US was found to have a sensitivity of 93% (95% CI, 80%−98%), with a positive predictive value of 80% (95% CI, 66%−90%) to detect a rotated LMA of any number of degrees. The specificity was 82% (95% CI, 70%−91%), with a negative predictive value of 94% (95% CI, 83%−99%). The accuracy was 87% (95% CI, 79%−93%).

Table 2

Table 2

There were no significant correlations of airway pressure and US arytenoid grade (P = 0.756).

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DISCUSSION

This observational study was designed to estimate the incidence of LMA malposition detected with US in pediatric patients. The incidence of LMA malposition was higher with FOB (P < 0.0001), but the incidence of rotation was similar (P = 0.395). US arytenoid grade did not correlate with FOB LMA grade (P = 0.611) but showed a significant correlation with LMA rotation grade (P < 0.0001; 95% CI, 60%−83%).

An appropriately placed LMA is usually confirmed by several clinical tests, such as: (1) meeting resistance on insertion of the LMA; (2) ventral movement of LMA upon cuff inflation; (3) effective ventilation as evidenced by chest movement, PETCO2, expired gas volume, and volume changes of the reservoir bag; and (4) positive airway pressure.19–21 In adults, the ability to effectively ventilate, and the airway pressure generated from ventilation, showed a good correlation with correct LMA positioning.22 However, several studies have demonstrated that the frequency of suboptimal LMA positioning was 12.8% to 49% in children despite clinically successful ventilation.3,6,10 A suboptimally positioned LMA can maintain an airway to some extent, but the LMA may be at increased risk for further displacement during the course of the operation. Therefore, it is important to ensure optimal positioning of the LMA at the time of insertion in children.

In this study, the arytenoid and thyroid cartilages were landmarks on US assessment of LMA position. On a transverse view of US, the thyroid cartilages have an inverted V-shape, within which the true and false vocal cords are visible.14–16 Laryngeal cartilage is still poorly calcified in children, but the thyroid and greater part of the arytenoid cartilage consist of hyaline cartilage that undergoes calcification throughout the aging process.14 Therefore, the thyroid cartilage and arytenoids are easily recognizable as hypoechoic structures surrounded by hyperechoic margins.15,16 If the LMA is not properly positioned in the esophageal inlet and is rotated from the midline, then the asymmetrical morphological changes can be detected on US (Fig. 4B). Gupta et al.23 reported that the US grade of LMA position closely correlated with the FOB LMA grade in adult patients. Although they graded LMA positioning as a degree of structural indentation by the LMA cuff relative to the preprocedural image on an airway manikin, they did not present any US image changes after LMA placement. In this study, we could not define the grade of LMA position with US compared to FOB findings. However, the presence of arytenoid asymmetry on US showed a relatively high sensitivity (93%) and positive predictive value (80%) for rotated LMAs (Table 1). As US arytenoid grade increased, the angle of LMA rotation increased (P < 0.0001; 95% CI, 60%−83%). This suggests the value of US in detecting a rotated LMA from the laryngeal midline in children.

This study demonstrated that the LMA is rotated within the hypopharynx in 43% of pediatric patients. Although the initial peak airway pressure showed no differences between rotated and nonrotated patients, rotated LMAs may become increasingly displaced over time. Several studies using FOB, computed tomography, and magnetic resonance imaging have reported that a suboptimal LMA position can be clinically acceptable, but there are data to show that inappropriate positioning of an LMA is associated with airway complications.24,25 In this study, 20% of size 1.5, 22.7% of size 2, and 35.5% of size 2.5 LMAs (overall, 26%) were found to have an FOB LMA grade of 4 (epiglottis was completely downfolded, obstructing view of the glottis aperture), which is a higher incidence compared with other studies.6,8,12,26 Although some authors suggested that smaller LMAs are more commonly associated with suboptimal positioning than larger LMAs,12,27 this discrepancy may be related to the ethnic differences in the shape of the upper airway. Pediatric LMAs are a smaller version of adult LMAs, which are manufactured based on prototypes obtained from cadavers of Western individuals. Thus, these pediatric LMAs do not account for Asian children. Asian males have a shorter cranial base, shorter maxilla, more obtuse angle between the mandibular ramus and the posterior skull base, and larger posterior airway spaces than Caucasian males.25,28,29 These anatomical differences may extend to children, although the ethnic differences of cephalometric variables have not yet been clarified in children. A well-positioned LMA should provide a direct view only of the vocal cords with the retroflexed epiglottis. If the LMA size is too large for the patient, the LMA tip may not advance into the esophageal inlet, and the anterior surface of the epiglottis can be seen on FOB, obstructing the LMA grille. FOB grade might be influenced by the insertion technique and maneuvers. In this study, we used a standard technique using a partially inflated LMA with jaw thrust. In several studies, a rotational technique had a better clinical success rate in classical LMA insertion in children.4,7,20 One FOB study reported better LMA position with the rotational technique than the standard technique.7 To clarify the effect of the insertion techniques, further FOB study is required.

Despite various FOB LMA grades, controlled ventilation was maintained in a range of appropriate airway pressures for 5 consecutive breaths. Previous studies suggest that positive-pressure ventilation can push the epiglottis anteriorly with inspiration if the distal end of the LMA is wedged well into the esophageal entrance.22,24 However, in a spontaneously breathing child, an obstructed glottic aperture by the epiglottis might result in profoundly increased work of breathing. Moreover, a rotated LMA may become increasingly displaced as time passes. In this situation, portable US may be a useful device to assist in repositioning of an LMA to the midline. Taken together, the results of this study further our understanding of the role of US in rapid and easy evaluation of LMA position, especially in children having certain comorbidities such as juvenile rheumatoid arthritis. Prolonged asymmetrical elevation of the arytenoids by LMA rotation may result in significant consequence in these children.30

FOB LMA grade was considered as the “gold standard” of LMA malposition that results in ventilator difficulty. Actually, US arytenoid grade that shows LMA rotation may not predict a truly malpositioned LMA. However, a rotated LMA is also a malpositioned LMA, and it may cause ventilator difficulty. Most studies in the field of the FOB in LMAs have only focused on the degree of exposure. In other words, the depth of the LMA was the main concern for optimal position of an LMA by FOB. However, far too little attention has been paid to rotation or deviation of the inserted LMA. This study examined the emerging role of US in the context of bedside real-time airway evaluation of the LMA. US could detect the rotated or deviated LMA easily in pediatric patients despite inadequate evaluation of the depth of the device.

There are several limitations to this study. The first and main weakness of this study is the use of only the transverse US view. If the sagittal and parasagittal scans had been used, morphological changes of the thyrohyoid ligament, anteriorly lifted epiglottis, and the relationship between pre-epiglottic and epiglottis could have been identified after LMA insertion.16 Thus, the degree of LMA rotation on transverse image may reflect the degree of relative LMA position, not the solid position within the hypopharynx. The second limitation is that the location of the US probe was not standardized. The probe location and angle may have accentuated the difference in the cartilage height unrelated to the mask. However, this is unlikely because the US arytenoid grade correlated with the LMA rotation grade. The third limitation is that we evaluated LMA position after administration of the neuromuscular blocking drug. Thus, we cannot extrapolate the results of this study to children under spontaneous respiration because hypopharyngeal muscular tension may affect LMA positioning.

In conclusion, although US could not detect the suboptimal depth of the LMA, it has the promise of being an accurate tool for detecting a rotated LMA. US detects rotation of the LMA even if it is positioned at optimal depth. If the LMA deviates from the midline position intraoperatively, US could assist in appropriate repositioning of the device. US may help us better interpret optimal LMA positioning in pediatric patients.

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DISCLOSURES

Name: Jeongmin Kim, MD.

Contribution: This author conducted the study, analyzed the data, and wrote the manuscript.

Attestation: Jeongmin Kim has conducted the original study, analyzed the data, and wrote the final manuscript.

Name: Ji Young Kim, MD, PhD.

Contribution: This author helped the analysis of the data and description of the result.

Attestation: Ji Young Kim has seen the original data and approved the final manuscript.

Name: Won Oak Kim, MD.

Contribution: This author helped the study design and description of the data.

Attestation: Won Oak Kim has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Hae Keum Kil, MD.

Contribution: This author helped design the study, conduct the study, and write the manuscript.

Attestation: Hae Keum Kil has seen the original study data, reviewed the analysis of the data, and approved the final manuscript. This author is responsible for archiving the study files.

This manuscript was handled by: Peter J. Davis, MD.

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