Determination of the appropriate oropharyngeal airway size in adults: Assessment using ventilation and an endoscopic view
a b s t r a c t
Introduction: Size 9 and 8 airways for men and women, respectively, have been proposed as most appropriate based on endoscopy. However, a limitation of this guideline is that ventilation was not assessed.
Methods: In this retrospective review of prospectively collected data, 149 patients requiring tracheal intubation for general anesthesia were included. The adequacy for manual and pressure-controlled mechanical ventilation and views at the distal end of each airway was assessed using a fiber-optic bronchoscope with various airway sizes (7, 8, 9, 10, and 11).
Results: For men, size 9, 10, and 11 airways permitted clear manual and adequate mechanical ventilation; size 7 and 8 airways caused partially obstructed manual and inadequate mechanical ventilation. On endoscopy, size 7 and 8 airways caused complete obstruction by the tongue; size 10 and 11 airways either touched or passed beyond the tip of the epiglottis. For women, the size 7 airway caused partially obstructed manual and inadequate mechanical ventilation; size 9 and 10 airways provided clear manual and adequate mechanical ventilation. The size 8 airway permitted clear manual ventilation, though mechanical ventilation was inadequate in one patient. On endoscopy, the size 7 airway caused complete obstruction in N 50% of women; size 9, 10, and 11 airways either touched or passed beyond the tip of the epiglottis.
Conclusions: With respect to adequate ventilation in conjunction with an acceptable endoscopic view, size 9 and size 8 oropharyngeal airways appear to be the most appropriate sizes for clinical use in men and women, respectively.
(C) 2017
Introduction
Oropharyngeal airways are frequently employed as important air- way adjuncts in emergency care [1] and for short-term airway manage- ment during the peri-anesthetic period [2]. The use of an airway is simple, but it is crucial to select the proper size. If the airway is too small, the distal end of airway will be obstructed by the tongue, resulting in inadequate ventilation. If it is oversized, there is a risk of laryngospasm or traumatic injury to the laryngeal structures. Therefore, selection of the appropriate airway size is important.
To select an appropriate airway size, traditionally, external facial mea- surements have been used based on the distances between the Maxillary incisors and the angle of the mandible (IM distance) [3] and between the corner of the mouth and the angle of the mandible (MM distance) [4]. In this regard, our study [5] demonstrated that the airway size selected based on the IM distance, rather than the MM distance, was more appro- priate for achieving adequate ventilation and an acceptable fiber-optic view. However, the airway size selected based on the IM distance did not fulfil the requirement for the appropriate size of an airway, as defined
E-mail addresses: [email protected] (H.J. Kim), [email protected] (S.H. Kim), [email protected] (J.Y. Min), [email protected] (W.K. Park).
as the positioning of the distal end of the airway as close as possible to the tip of the epiglottis without complete obstruction by the tongue while allowing effective mask ventilation. This is because the distal ends of the airways were located beyond the tip of the epiglottis in 25% of patients. However, in a previous study conducted to assess the appropriate sizes of airways based on fiber-optic viewing [6], we found that a size 9 airway for men and a size 8 airway for women was most appropriate and completely fulfilled the requirement for appropriateness regarding the lo- cation of the distal end of the airway. However, the limitation of this pre- vious study was that ventilation was not assessed.
Therefore, the purposes of the present study were to investigate the adequacy of ventilation for a size 9 airway in men and a size 8 airway in women and, thereby, provide a simple guideline for choosing an appro- priate airway size for men and women without using traditional guide- lines for facial measurements.
Methods
Study design and recruitment
To assess the adequacy of ventilation in previous work [6], we per- formed a study (Study I) to assess both ventilation and position for
http://dx.doi.org/10.1016/j.ajem.2017.04.029
0735-6757/(C) 2017
several sizes of airways (men: sizes 7, 8, and 9; women: sizes 8 and 9) chosen based on the results of the previous study. In addition, to assess the adequacy of ventilation for various airway sizes, we analyzed the re- sults of ventilation and position status for various airway sizes used in a previous study [5] in which the airway sizes were chosen based on ex- ternal facial distances (Study II). We combined all data in these two studies and assessed the appropriateness of various sizes of airways, es- pecially for a size 9 airway for men and a size 8 airway for women, based on both effective ventilation and positioning. Enrollment included 149 patients (66 men and 83 women).
Approval was obtained from the Institutional Review Board of the hospital. The clinical trials were registered before patient enrollment. Written informed consent was obtained from all patients.
Selection of participants and airway assessments
All patients were aged 20-75 years, with American Society of Anes- thesiologists Class I-II physical status and had scheduled elective ear, nose or throat surgeries under general anesthesia. Patients with the fol- lowing characteristics were excluded: an abnormal airway anatomy in preoperative radiologic studies; cervical spine pathology; a history of difficult intubation, neurologic disease, cardiovascular disease, or dental problems; or a mouth opening smaller than 2 cm. Airway assessments included the modified Mallampati classification [7], thyromental dis- tance and inter-incisor gap, all of which were evaluated before induc- tion of anesthesia.
Anesthesia
On arrival in the operating room, the patients were laid supine with the head in a neutral position, 3-4 cm above the plane of the table using a ring support. All patients were given 0.2 mg glycopyrrolate intrave- nously. Standard monitors for electrocardiography, pulse oximetry and non-invasive arterial blood pressure were applied. Anesthesia was induced with 1.5 mg/kg propofol, 1.0 ug/kg remifentanil and
0.5 mg/kg atracurium. The patients’ lungs were manually ventilated
with oxygen at 4-6 L/min and 4-5% sevoflurane via a face mask with the head held in an extended position by pulling the mental symphysis in the anterocephalic direction. Complete Muscle relaxation was assessed at the adductor pollicis muscle by supramaximal train-of- four stimulus applied to the ulnar nerve using a peripheral nerve stim- ulator (Innervator 252; Fisher & Paykel Healthcare, Auckland, New Zealand).
Data collection in study I
After induction, when anesthesia was considered to be deep enough, the airway was inserted into the mouth with the tip positioned upwards and rotated 180? and was then advanced until the flange of the airway made contact with the upper incisors. The adequacy of manual and me- chanical ventilation was assessed after airway insertion. The adequacy of manual ventilation was classified into three grades: clear, partial ob- struction or complete obstruction. Partial obstruction was defined by the presence of adventitious sounds (snoring) with insufficient chest wall movement. Complete obstruction was defined by the absence of chest expansion despite the application of positive pressure ventilation. Because we could not deliver the inspired tidal volume and pressure consistently during manual ventilation, we applied mechanical ventila- tion using the pressure-controlled mode to provide a consistent inspira- tory pressure and tidal volume after evaluating the adequacy of manual ventilation. To evaluate the adequacy of mechanical ventilation, the an- aesthesiologist held the face mask with both hands. Then, with the head in extension and the mandible in a forward position, we mechanically ventilated the lungs with 100% oxygen using the pressure-controlled mode of the anesthesia machine (Aisys; GE Datex-Ohmeda, Munich, Germany). The fresh gas flow of oxygen was set to 2 L/min. The peak
inspiratory pressure, respiratory rate and inspiration-to-expiration ratio were set to 15 cm H2O, 15 breaths per min, and 1:2, respectively. Mechanical ventilation was classified as either adequate or inadequate. Inadequate ventilation was defined as when the mean expired tidal vol- ume from the sixth to tenth breaths was b 5 mL/kg.
Following the evaluation of mechanical ventilation, the curvilinear distance from the incisors to the tip of the epiglottis was measured with a flexible fiber-optic bronchoscope (Olympus LF-GP, Olympus Op- tical Co., Tokyo, Japan) after passing the fiberscope through the lumen of the airway (Fig. 1). The fiber-optic views at the distal end of the airway were classified as follows: epiglottis visible (clear or partial obstruction by the tongue), complete obstruction by the tongue, contact with the epiglottis tip or passing beyond the epiglottis tip. The classification of passing beyond the epiglottis tip included impaction in the vallecula, entry of the epiglottis into the lumen of the airway and proximity to the vocal cords. During insertion of each airway and measurement, the face mask was removed, and ventilation was ceased. After measure- ments were performed, the airway was removed, and mask ventilation resumed until the next different-sized airway was inserted. Three dif- ferent Guedel airway sizes (Hudson RCI, Teleflex Medical, Research Tri- angle Park, NC), [size 7 (7 cm, white), 8 (8 cm, green), and 9 (9 cm, yellow)] in men (n = 20) and 2 different sizes (size 7 and 8) in women (n = 16) were examined. During the study, as we found that the fiber-optic views for each airway size in men and women were quite similar to the results of a previous study [6], we did not evaluate further; thus, we only observed 20 and 16 male and female patients, respectively.
Data collection in study II
The methods have been described previously [5]. Briefly, before in- duction of anesthesia, two facial distances were measured: the MM dis- tance and the IM distance. After induction, when anesthesia was considered to be sufficient, the two different-sized airways selected ac- cording to the measured MM or IM distances were inserted in random order into each patient. The size of the airway was determined by the measured distance rounded down to the nearest whole number. For ex- ample, if the measured MM distance was 9.6 cm, a size 9 airway was chosen. Five different Guedel airway sizes (Hudson RCI, Teleflex Medi- cal, Research Triangle Park, NC), namely, 7 (7 cm, white), 8 (8 cm,
green), 9 (9 cm, yellow), 10 (10 cm, red) and 11 (11 cm, orange), were chosen based on the MM or IM distance. The adequacy of manual and mechanical ventilation was assessed after airway insertion. The fiber-optic view was examined using a flexible fiber-optic broncho- scope. The procedures to measure manual or mechanical ventilation as well as the assessment of fiber-optic views were the same as in Study I. The classification of adequacy of manual and mechanical
Fig. 1. Schematic representation for assessing the position of the distal oropharyngeal airway using fiber-optic bronchoscopy. FOB: fiber-optic bronchoscope, OPA: oropharyngeal airway.
Patient characteristics and airway assessments.
Patient characteristics Values
|
Men/women |
66/83 |
|
Age, years |
47 +- 13 |
|
Weight, kg Men |
74 +- 11 |
|
Women Height, cm |
57 +- 8 |
Men 172 +- 7
Women 158 +- 5
BMI, kg/m2 24 +- 3
Airway assessment
Modified Mallampati classification, 1/2/3/4 18/63/68/0
Thyromental distance, cm 7.2 +- 1.0
Inter-incisor gap, cm 4.2 +- 0.7
Cormack and Lehane grade, 1/2/3/4 129/15/5/0 Oropharyngeal airway size, 7/8/9/10/11
Men 29/52/44/25/2
Women 45/58/40/23/0
Values are expressed as mean +- SD or number of patients.
ventilation as well as the fiber-optic views at the distal ends of the air- ways was also the same as in Study I.
After all measurements were obtained, orotracheal intubation was performed using an appropriately sized endotracheal tube and anesthesia was maintained with sevoflurane and remifentanil. During laryngoscopy, the glottic view was graded according to the Cormack and Lehane grading system [8].
Results
One hundred and forty-nine patients (66 men and 83 women) were enrolled in the study. Their characteristics, airway assessment findings, and the number of patients using various airway sizes are shown in Table 1.
Ventilatory status“>Ventilatory status
Ventilatory status during manual and mechanical ventilation is shown in Table 2.
For men, while clear manual ventilation was observed in all patients for whom size 9, 10 and 11 airways were used, partially obstructed ven- tilation occurred in 31% and 15% of patients for whom size 7 and 8 air- ways were inserted, respectively. Mechanical ventilation was adequate in patients with size 9, 10 and 11 airways, but inadequate ven- tilation was observed in 14% and 6% of patients using size 7 and 8 air- ways, respectively. Out of the 4 patients of the size 7 inserted airway group, the expired tidal volume during mechanical ventilation appeared to be 4 mL/kg in one patient, and expired tidal volumes b 2 mL/kg, indi- cating near complete obstruction, were observed in the other 3 patients. For 3 patients in the size 8 inserted airway group, near complete ob- struction (expired tidal volume less than one mL/kg) and expired tidal volumes between 3 and 4 mL/kg were observed in 1 and 2 patients,
respectively. In contrast, both manual and mechanical ventilation ap- peared to be clear and adequate in airways above size 8. The average ex- pired tidal volume with adequate mechanical ventilation using size 9, 10 and 11 airways was 851 +- 238 mL (n = 71).
In women, while clear manual ventilation was observed in all pa- tients for whom airways above size 7 were used, partially obstructed ventilation was observed in 7% of patients for whom size 7 airways were inserted. Mechanical ventilation was adequate in patients for whom airways above size 8 were used. However, inadequate ventilation was observed in 7% and 2% of patients using size 7 and 8 airways, re- spectively. In 3 patients using size 7 airways, the expired tidal volumes appeared to be between 2 and 4 mL/kg. In a patient using a size 8 air- way, the expired tidal volume was close to 5 mL/kg. In contrast, manual ventilation was clear for patients using airways above size 7 and me- chanical ventilation was adequate in patients using airways above size
8. The average expired tidal volume with adequate mechanical ventila- tion using size 9 and 10 airways was 686 +- 229 mL (n = 63).
Fiber-optic findings
Fiber-optic findings at the distal ends of the different-sized airways are shown in Table 3. In men, the distal ends of the airways appeared to be completely obstructed by the tongue in 33% of patients in whom the size 8 airway was inserted. When the size 7 airway was used, com- plete obstruction was observed in the majority of patients (86%). In con- trast, complete obstruction was negligible or absent when airways above size 8 were used, while the distal ends of the airways either touched or passed beyond the epiglottis tip in 8% of patients with size 10 airways. There were no patients in whom airway contact with the epiglottis tip or an airway passing beyond the epiglottis tip was ob- served when size 9 airways were used.
In women, the distal ends of the airways appeared to be completely obstructed by the tongue in 64% and 26% of patients for whom size 7 or 8 airways were inserted, respectively. While no patients showed air- ways either touching or passing beyond the epiglottis tip when size 7 or 8 airways were used, this was observed in 25% of patients for whom size 9 airways were used. This finding was observed in the ap- proximately half of the patients when the size 10 airways were used (52%).
Impaction of the distal end of the airway in the vallecula, entry of the epiglottis into the airway lumen and proximity to the vocal cords were all categorized as passing beyond the tip of the epiglottis. Interestingly, we frequently found that the epiglottis entered into the airway lumen in most men and women (85%). The epiglottis entered the airway by N 1 cm in 3 women, and impaction in the vallecula or proximity to the vocal cords was observed in 2 women.
In both men and women, the mean lengths of the size 7, 8, 9 and 10 airways were shorter than the distance from the upper incisors to the epiglottis tip (Fig. 2). For size 9 airways in men and size 8 airways in women, the mean differences in distance between the distal end of the airway and the epiglottis tip were similar, at 2.0 +- 0.6 cm. None of
Ventilation status during manual and mechanical ventilation.
|
Men |
Women |
||||||||||
|
Airway sizes |
7 |
8 |
9 |
10 |
11 |
7 |
8 |
9 |
10 |
||
|
Number |
29 |
52 |
44 |
25 |
2 |
45 |
58 |
40 |
23 |
||
|
Manual ventilation Clear |
20 (69) |
44 (85) |
44 (100) |
25 (100) |
2 (100) |
42 (93) |
58 (100) |
40 (100) |
23 (100) |
||
|
Partial obstruction |
9 (31) |
8 (15) |
0 |
0 |
0 |
3 (7) |
0 |
0 |
0 |
||
|
Complete obstruction Mechanical ventilation |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
||
|
Adequate |
25 (86) |
49 (94) |
44 (100) |
25 (100) |
2 (100) |
42 (93) |
57 (98) |
40 (100) |
23 (100) |
||
|
Inadequate |
4 (14) |
3 (6) |
0 |
0 |
0 |
3 (7) |
1 (2) |
0 |
0 |
||
Values are expressed as numbers (percentages). Inadequate ventilation was defined when the mean expired tidal volume from the sixth to tenth breaths was b5 mL/kg.
Fiber-optic views at the distal ends of the different sizes of oropharyngeal airways.
|
Men |
Women |
||||||||||
|
Airway sizes |
7 |
8 |
9 |
10 |
11 |
7 |
8 |
9 |
10 |
||
|
Number |
29 |
52 |
44 |
25 |
2 |
45 |
58 |
40 |
23 |
||
|
Epiglottis visible |
4 (14) |
35 (67) |
43 (98) |
23 (92) |
1 (50) |
16 (36) |
43 (74) |
30 (75) |
11 (48) |
||
|
Complete obstruction by the tongue |
25 (86) |
17 (33) |
1 (2) |
0 |
0 |
29 (64) |
15 (26) |
0 |
0 |
||
|
Contact with the epiglottis tip |
0 |
0 |
0 |
1 (4) |
0 |
0 |
0 |
3 (8) |
7 (30) |
||
|
Passing beyond the epiglottis tip |
0 |
0 |
0 |
1 (4) |
1 (50) |
0 |
0 |
7 (17) |
5 (22) |
||
|
Impaction in the vallecula |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
1 |
0 |
||
|
Entry of the epiglottis into the lumen of the airway |
0 |
0 |
0 |
1 |
1 |
0 |
0 |
6 |
4 |
||
|
Close proximity to the vocal cords |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
1 |
||
Values are expressed as numbers (percentages). Passing beyond the tip of the epiglottis includes impaction in the vallecula, entry of the epiglottis into the lumen of the airway and close proximity to the vocal cords.
the patients developed desaturation, laryngeal injury or dental acci- dents during the measurement period. All patients completed the study. To further assess the appropriateness of positioning of the size 9 air- way in men and the size 8 airway in women using fiber-optic views, we combined all data in these studies together with the results of a previous study [6] and performed an analysis (Table 4). These results were in ac- cordance with those of a previous study [6], indicating that a size 9 air- way in men and a size 8 airway in women were the most appropriate.
Discussion
In the present study, size 9 and size 8 airways appeared to be most appropriate for men and women, respectively, considering the risk of inadequate ventilation or traumatic injury that may occur with inappro- priately sized oropharyngeal airways.
Clinically, the appropriate size of an oropharyngeal airway would be one that provides a patent airway, allows effective mask ventilation and does not cause traumatic injury to the laryngeal structures. Based on these requirements, the appropriately fitting airway for an individual can be defined as one that is positioned at the distal end of the airway as close as possible to the tip of the epiglottis without complete obstruc- tion by the tongue and that permits effective mask ventilation. Based on the average differences of distances between the distal end of the air- way and the tip of the epiglottis, the most appropriate airway sizes in our results appeared to be 10 and 9 for men and women, respectively (Fig. 2). However, for these sizes, the distal ends of the airways touched or passed beyond the tip of the epiglottis in 8% of men and 25% of women, indicating that these airways may not be appropriate. In this re- gard, entry of the epiglottis into the lumen of the airway was found in most men and women (85%) for whom the distal end of the airway passed beyond the tip of the epiglottis. These findings suggest that the possibility of injury to the epiglottis and its base during insertion of over-sized airways should not be ignored. Additionally, impaction in the vallecula or proximity to the vocal cords may also cause traumatic injury to laryngeal structures.
Interestingly, despite complete obstruction by the tongue in 26% of women in whom a size 8 airway was used, both manual and mechanical ventilation appeared to be effective in all patients. Although mechanical ventilation was inadequate in one female patient when using a size 8 airway, even the expired tidal volume of 287 mL was close to 5 mL/kg (320 mL), indicating possible effective ventilation. This is likely due to the closer location of the distal tip of the size 8 airways to the base of the tongue. However, when the size 9 and size 8 airways were used for men and women, respectively, we found that the distal end of the airway did not touch or pass beyond the tip of the epiglottis.
When the best fitting airway for an individual is defined by permit- ting effective mask ventilation while also positioning the distal end of airway as close as possible to the tip of the epiglottis without any ob- struction by the tongue, size 9 airways for men and size 8 airways for women are not the best fitting but are appropriate for use based on ef- fective ventilation and avoidance of possible injury to the laryngeal
structures due to passage of the distal end of the airway beyond the tip of the epiglottis. Based on the increased incidence of either partial obstruction during manual ventilation or inadequate mechanical venti- lation with smaller airway sizes for men and women as well as the higher incidence of passing beyond the tip of the epiglottis with larger airway sizes, our results indicate that the most appropriate airway size is 9 for men and 8 for women.
In the present study, when the size 7 airway was used for men, none of the patients showed complete obstruction during manual ventilation despite inadequate mechanical ventilation in four patients (14%), for whom the average tidal volume was 140 +- 46 mL, indicating near com- plete obstruction. Similar results were obtained when the size 7 airway was used for women. Among these women, inadequate mechanical ventilation occurred in three patients (7%), for whom the average tidal volume was 171 +- 56 mL. These findings were probably due to much higher inspiratory pressures applied during manual ventilation than during mechanical ventilation. In this regard, in the previous study [5], we demonstrated that manual application of high positive inspiratory pressure pushed the obstructed tongue backward, creating a small space between the tongue and the distal end of the airway during inspi- ration, which in turn allowed lateral channelling of oxygenated airflow around the obstruction. Because of this tongue displacement mecha- nism, manual ventilation would be possible in most patients despite a high incidence of complete obstruction of the distal end of the airway by the tongue in a fiber-optic view when airway sizes smaller than 9 for men and 8 for women were used. However, it is not desirable to
Fig. 2. Distances between the distal ends of oropharyngeal airways (OPAs) and epiglottis tips for each airway size in men (?) and women (?). The size marked on the airways indicates the length (cm) measured horizontally from the flanged end to the distal end. The error bars indicate SD.
Fiber-optic views at the distal ends of the different oropharyngeal airway sizes in Studies I and II and Reference [6].
|
Men |
Women |
|||||||||||
|
Airway sizes |
7 |
8 |
9 |
10 |
11 |
7 |
8 |
9 |
10 |
11 |
||
|
Number |
29 |
124 |
116 |
97 |
74 |
45 |
135 |
117 |
100 |
77 |
||
|
Epiglottis visible |
4 (14) |
100 (81) |
114 (98) |
75 (77) |
8 (11) |
16 (36) |
118 (87) |
101 (86) |
24 (24) |
1 (1) |
||
|
Complete obstruction by the tongue |
25 (86) |
24 (19) |
2 (2) |
0 |
0 |
29 (64) |
17 (13) |
0 |
0 |
0 |
||
|
Contact with the epiglottis tip |
0 |
0 |
0 |
7 (7) |
3 (4) |
0 |
0 |
6 (5) |
14 (14) |
1 (1) |
||
|
Passing beyond the epiglottis tip |
0 |
0 |
0 |
15 (16) |
63 (85) |
0 |
0 |
10 (9) |
62 (62) |
75 (98) |
||
Values are expressed as numbers (percentage). Passing beyond the epiglottis tip includes impaction in the vallecula, entry of the epiglottis into the lumen of the airway, and close prox- imity to the vocal cords.
apply a high inspiratory pressure N 15-20 cm H2O because it can cause gastro-esophageal insufflation, resulting in gastric distention [9,10].
The limitations of this study were as follows. First, we performed
manual ventilation with the head extended. However, mechanical ven- tilation was performed with the head in extension with forward dis- placement of the mandible. The position of Head extension with or without forward displacement of the mandible (jaw thrust) can influ- ence the patency of the airway [11,12]. If we kept the head extended during mechanical ventilation, a more accurate comparison of ventila- tory status between manual and mechanical ventilation would have been possible. Second, in our study population, 70 of 138 men were taller than 170 cm, and 63 of 160 women were taller than 160 cm. The mean heights of men and women in our population were 172 +- 7 cm (range: 158-193) and 158 +- 5 cm (range: 143-172), respectively. Considering the limited distribution of heights in this racial background, our results may not be generalizable to those who are taller than 180 cm.
In conclusion, the present results suggest the simplest method for choosing an appropriate airway size without using traditional guide- lines for facial measurements can be based on effective ventilation in conjunction with an acceptable endoscopic view. Size 9 and size 8 oro- pharyngeal airways are the most appropriate sizes for clinical use for men and women, respectively. If signs of airway obstruction occur when these sizes are used, an airway one size larger should be considered.
Declaration of interest“>Declaration of interest
The authors have no conflicts of interest to declare.
This research did not receive any specific grant from funding agen- cies in the public, commercial, or not-for-profit sectors.
This research did not receive any specific grant.
Acknowledgements
The authors are grateful to Dong-Su Jang, MFA (Medical Illustrator, Medical Research Support Section, Yonsei University College of Medi- cine, Seoul, Republic of Korea) for his help with the illustrations.
References
- Roberts K, Allison KP, Porter KM. A review of emergency equipment carried and pro- cedures performed by UK front line paramedics. Resuscitation 2003;58:153-8.
- Dob DP, Shannon CN, Bailey PM. Efficacy and safety of the laryngeal mask airway vs Guedel airway following tracheal extubation. Can J Anaesth 1999;46:179-81.
- Deakin CD, Nolan JP, Soar J, Sunde K, Koster RW, Smith GB, et al. European resusci- tation council guidelines for resuscitation 2010 section 4. Adult advanced life sup- port. Resuscitation 2010;81:1305-52.
- Neumar RW, Otto CW, Link MS, Kronick SL, Shuster M, Callaway CW, et al. Part 8: adult advanced cardiovascular life support. 2010 American heart association guide- lines for cardiopulmonary resuscitation and emergency cardiovascular care. Circula- tion 2010;122:S729-67.
- Kim HJ, Kim SH, Min NH, Park WK. Determination of the appropriate sizes of oro- pharyngeal airways in adults: correlation with external facial measurements: a randomised crossover study. Eur J Anaesthesiol 2016;33:936-42.
- Kim SH, Kim JE, Kim YH, Kang BC, Heo SB, Kim CK, et al. An assessment of oropha- ryngeal airway position using a fibreoptic bronchoscope. Anesthesia 2014;69:53-7.
- Samsoon GL, Young JR. Difficult tracheal intubation: a retrospective study. Anesthe- sia 1987;42:487-90.
- Cormack RS, Lehane J. Difficult tracheal intubation in obstetrics. Anesthesia 1984;39:
Devitt JH, Wenstone R, Noel AG, O’Donnell MP. The laryngeal mask airway and pos- itive-pressure ventilation. Anesthesiology 1994;80:550-5.
- Bouvet L, Albert ML, Augris C, Boselli E, Ecochard R, Rabilloud M, et al. Real-time de- tection of gastric insufflation related to facemask pressure-controlled ventilation using ultrasonography of the antrum and epigastric auscultation in nonparalyzed patients: a prospective, randomized, double-blind study. Anesthesiology 2014;120: 326-34.
- Safar P, Escarraga LA, Chang F. upper airway obstruction in the unconscious patient. J Appl Physiol 1959;14:760-4.
- Morikawa S, Safar P, Decarlo J. Influence of the head-jaw position upon upper airway