American Journal of Emergency Medicine (2012) 30, 1679-1683

Original Contribution

Optimal headrest height for the best laryngoscopic view: by anatomical measurements

Young-Tae Jeon^a, Jung-won Hwang ^a, Kyuseok Kim ^b, Cheol-Kyu Jung^c,

Hee-Pyoung Park ^d, Sang-Heon Park ^a,?

^aDepartment of Anesthesiology and pain medicine, Seoul National University Bundang Hospital, 166, Gumi-ro,

Seongnam City, Kyeonggi-do, 463-802, Korea

^bDepartment of Emergency Medicine, Seoul National University Bundang Hospital, 166, Gumi-ro, Seongnam City,

Kyeonggi-do, 463-802, Korea

^cDepartment of Diagnostic Radiology, Seoul National University Bundang Hospital, 166, Gumi-ro, Seongnam City,

Kyeonggi-do, 463-802, Korea

^dDepartment of Anesthesiology and Pain Medicine, Seoul National University Hospital, 28, Yongon-Dong, Jongno-gu, Seoul

Received 1 December 2011; revised 12 January 2012; accepted 14 January 2012

Abstract

Background: We hypothesized that the oro-pharyngolaryngeal axes, occipito-atlanto-axial extension (OAA) angle and intubation distance would be influenced by the height of headrests.

Methods: Twenty patients were enrolled. The Macintosh 3 blade was used for direct laryngoscopy without a headrest or with the headrest of 6 or 12 cm high in randomized order, whereas a lateral radiograph of the neck was taken when the best laryngoscopic view was obtained. The following measurements were made: (1) the axis of the mouth (MA), the pharyngeal axis (PA), the laryngeal axis (LA), and the line of vision (LV). The various angles between these axes were defined: ? angle between MA and PA, ? angle between PA and LA, and ? angle between LV and LA. (2) Intubation distance, (3) mentovertebral distance, and (4) OAA angle.

Results: Compared with 12-cm and no headrest, the ? angle decreased significantly with 6-cm headrest (19.4?/29.2?/29.2? in 6-cm/12-cm/no headrest, respectively; P b .001), and the intubation distance increased significantly (46.2/37.3/38.7 mm in 6-cm/12-cm/no headrest, respectively; P b .001). Mentovertebral distance was smallest (107.0/106.7/98.5 mm; P b .05) at 12-cm headrest. Occipito- atlanto-axial extension angle was largest significantly (40.7?/35.2?/34.5?; P b .05) at 6-cm headrest. Conclusion: We conclude that compared with no or 12-cm headrest, 6-cm headrest could facilitate more alignment of these axes, increase the OAA angle, and enlarge the intubation distance.

Crown Copyright (C) 2012 Published by

* Corresponding author. Department of Anesthesiology, Seoul National University Bundang Hospital, 166 Gumi-ro, Seongnam City, Kyeonggi-do, 463-802, Korea. Tel.: +82 31 787 7494; fax: +82 31 787 4063.

E-mail address: [email protected] (S.-H. Park).

Introduction

The ability to maintain good visualization of glottis during direct laryngoscopy is probably the major determi- nant of easy tracheal intubation. The aim of direct laryngoscopy is to visualize the vocal cords through the

0735-6757/$ - see front matter. Crown Copyright (C) 2012 Published by http://dx.doi.org/10.1016/j.ajem.2012.01.016

originally curved, oral airway space. Proper head positioning and successful laryngoscopy shifts the craniofacial structure arrangements to make proper visualization of the vocal cords possible. “Line of sight to the larynx” is achieved by retracting the Soft tissues, which is inftuenced by static (head and neck position) and dynamic (displacement of the mandible and submandibular space) components [1].

Anatomical measurements have been compared during head positioning and direct laryngoscopy. Adnet and colleagues [2] demonstrated that the sniffing position does not achieve alignment of the axes of the mouth, pharynx, and larynx in awake patients. On the contrary, a radiologic study showed that the sniffing position provides greater occipito- atlanto-axial (OAA) extension angle than simple extension of the head [1]. An intubation distance (the distance from the midpoint of the anterior margin of the fifth cervical vertebral body to the tips of Laryngoscope blades) is considered as the glottis-visualizing oropharyngeal space to negotiate the endotracheal tube into the larynx [3]. We hypothesized that alignments of the three axes, OAA angle, and intubation distance would be inftuenced by the height of headrests. The main objective of this study was to compare the conse- quences of different headrest height on these anatomical measurements using lateral cervical radiographs.

Methods

Setting and participants

After obtaining Institutional Review Board approval and informed written consent, 20 adult patients with the American Society of Anesthesiologists physical status II to ? were enrolled. They were scheduled for endovascular coil embolization under general anesthesia. Patients with ische- mic heart disease or cerebrovascular disease, loose upper incisors, body mass index greater than 40 (kg/m²), severe kyphosis, cervical spine disease, or with any history of difficult intubation were excluded.

Study design

Preanesthetic airway assessments were performed in the sitting position by an attending anesthesiologist who was not subsequently involved in the airway management of the recruited patients. The following factors were recorded: (A) weight, height, and age; (B) interincisor gap;

(C) classification of the oropharyngeal view according to the Mallampati criteria; and (D) the thyromental distance. Each patient was positioned supine on the operating table and had the measurements done without a headrest or with the incompressible headrest of 6 or 12 cm high in randomized order. Patients were randomized using comput- er-generated numbers that were placed in sealed and opaque envelopes until immediately before induction of anesthesia.

Anesthesia was induced with target-controlled infusion using an Orchestra infusion pump system (Fresenius Kabi, Brezins, France). Patients received propofol 4 ug/mL and remifentanil 4 ng/mL. muscle paralysis was achieved with rocuronium 0.15 mg/kg before tracheal intubation. Anesthesia was maintained using propofol and remifentanil.

Using a no 3. Macintosh curved blade, direct laryngos- copy was performed in the intubating position in which the head of patient was maximally extended on the neck at the atlanto-occipital joint with the addition of the headrest or without. A lateral radiograph of the neck via Neuro-Biplane Angiography System (Integris BN 3000 Neuro; Philips Medical Systems, Best, the Netherlands) was taken when the anesthesiologists told that the best laryngoscopic view was obtained. In laryngoscopy, head and neck positions were fixed, and the occiput remained in contact with the headrest. X-ray films were printed by adjusting the dose of radiation for good visualization of soft tissue and bony tissue. In the same way, the lateral radiograph of the neck was recorded in the other height headrest. Intermittent manual ventilations using a face mask under 100% oxygen were provided to prevent desaturation.

The laryngoscopic view was graded using the five-grade

modified Cormack and Lehane system: grade 1 = full view of the vocal cords; grade 2A = partial view of the vocal cords; grade 2B = only the arytenoids and epiglottis seen; grade 3 = only the epiglottis visible; and grade 4 = neither the epiglottis nor the glottis seen [4]. All the lateral neck radiographs were interpreted by an experienced radiologist who was blinded to the aims of the study.

The following measurements were made on each lateral neck radiograph: (A) the axis of the mouth, defined as a straight line drawn parallel to the hard palate; (B) the pharyngeal axis (PA), defined as a line passing through the anterior portion of the atlas and of C2; (C) the laryngeal axis, defined as a straight line passing through the centers of the inferior (cricoid cartilage) and superior (base of epiglottis) orifices; and (D) the line of vision (LV), defined as a straight line passing through the inferior extremity of the superior incisors and the posterior extremity of the superior portion of the cricoid cartilage [2]. The various angles between these axes were defined: ? angle between the axis of the mouth and PA, ? angle between PA and LA, and ? angle between LV and LA. All of these lines and angles are represented in Fig. 1A.

The positions of the tips of the blades in relation to the Hyoid bone were noted, and the intubation distance (distance from the midpoint of the anterior margin of the fifth cervical [C5] vertebral body to the tips of the blade) was measured (Fig. 1B) as proposed by Mukesh and colleagues [3] in a previous study. The mentovertebral (MV) distance (from the mentum to C5 as described) was also measured to compare the anterior jaw displacement during laryngoscopy. An OAA angle was measured like a previous study [1]. A McGregor line, which connects the most dorsal edge of and caudal portion of the occiput and the dorsal edge of the hard palate, is used as reference for the occiput (C0). A reference line for

Values

Sex (male/female) 11/9 Age, y 57.8 +- 8.4 Mallampati score (1/2/3) 6 /12/2 Body mass index, kg/m2 24.4 +- 3.0 Interincisor gap, cm 4.3 +- 0.6 Thyromental distance, cm 7.2 +- 1.0 C-L grade (1/2a/2b/3) Without headrest 3/10/5/2 With 6-cm headrest ? 8/9/2/1 With 12-cm headrest 2/11/6/1

Data are expressed as means +- standard deviation or number of patients. C-L grade indicates Cormack and Lehane grade. * P b .05, compared with 12-cm or no headrest (Fisher exact test).

Fig. 1 A, Lateral radiograph of the head and neck taken at the best laryngoscopic view with a no. 3 Macintosh curved blade (MCB) displaying the four axes (mouth axis [MA], pharyngeal axis [PA], laryngeal axis [LA], line of vision [LV]) and the ?, ?, and ? angles. B, Larynx visualization space (intubation distance = ID) from C5 to MCB tip marked. Mentovertebral distance (MV) from C5 to mentum to measure anterior displacement of the jaw during laryngoscopy. The McGregor line, which connects the most dorsal edge of and caudal portion of the occiput and the dorsal edge of the hard palate, is used as reference for the occiput (C0). A reference line for the axis (C2) is drawn through the basal plate of the vertebral body. The occipito-atlanto-axial (OAA) extension angle between C0 and C2 is measured.

the axis (C2) is drawn through the basal plate of the vertebral body. The OAA angle between C0 and C2 was measured.

Data analysis

Table 1 Characteristics of patients (n = 20)

Group sizes were determined based on the data in our pilot study (? angle; standard deviation, 5?). We considered the difference of 5? between simple extension and sniffing position to be clinically relevant. Power analysis suggested that a minimum of 18 patients would be required for a ? = .2 and ? = .05. To compensate for potential dropouts, we enrolled 20 patients. The data are expressed as mean +- standard deviation. The angles of anatomical axes, intubation distance, and MV distance with modification of headrest height were analyzed using 1-way analysis of variance, and Scheffe method was used for multiple comparison analysis. The Cormack and Lehane grade depending on the height of headrest was analyzed using Fisher exact test. SPSS software (version 15.0; SPSS Inc, Chicago, Ill) was used. A P value of less than .05 was considered statistically significant.

Results

A total of 20 patients were enrolled in this study. The preanesthetic airway assessments of the patients are shown in Table 1. Each patient had no other predictive factor of difficult intubation. Intubations under direct laryngoscopy were successful for all patients. The laryngoscopic view of the 6-cm headrest was significantly superior to those of other headrest and without a headrest (P b .05; Table 1).

There was no significant difference of the angles measured (? and ?) among the 6-cm, 12-cm headrest, and no headrest (Table 2). Compared with 12-cm and no headrest, however, the ? angle between the LV and the LA decreased significantly with 6-cm headrest (P b .001), and the intubation distance increased significantly (P b .001). Occipito-atlanto-axial extension angle was largest significantly (P b .05) at 6-cm

Height of headrest, cm	?, deg	?, deg	?, deg	ID, mm	OAA angle, deg	MV, mm
0	72.8 +- 9.1	16.4 +- 7.3	29.2 +- 4.9	38.7 +- 7.1	34.5 +- 4.5	107.0 +- 9.9
6	70.7 +- 8.7	14.8 +- 7.1	19.4 +- 5.9 ?	46.2 +- 6.7 ?	40.7 +- 5.3 ?	106.7 +- 7.7
12	70.4 +- 7.3	15.1 +- 6.5	29.2 +- 4.5	37.3 +- 4.7	35.2 +- 5.3	98.5 +- 5.6 +

headrest. The mentovertebral distance was smallest at 12-cm headrest (P b .05).

Table 2 Variability of the angles of anatomical axes and measurements with modification of headrest height

Data are expressed as means +- standard deviation. ID indicates intubation distance from the midpoint of the anterior margin of the fifth cervical (C5) vertebral

body to the tips of the blade; OAA angle, occipito-atlanto-axial extension angle; MV, mentovertebral distance.

* P b .001, compared with 12-cm or no headrest (1-way analysis of variance/Scheffe method).

⁺ P b .05, compared with 6-cm or no headrest (1-way analysis of variance/Scheffe method).

The total time for the 3 laryngoscopic procedures was less than 90 seconds for each patient, and none of the patients showed a Pulse oximeter reading lower than 95% during the intubation attempts.

Discussion

The main result of this study is that 6-cm headrest improves the alignment of the LV of the glottis and the LA (narrowing of ? angle) compared with no headrest or 12-cm headrest. Moreover, our study confirms that 6-cm headrest provides more intubation distance and OAA angle than no headrest or 12-cm headrest.

The reduction of ? angle by 6-cm headrest (19.4?) in this study was greater than those (29?) reported by Adnet and colleagues [5] who found that the oral, pharyngeal, and laryngeal axes were not aligned with the sniffing position (with 7-cm headrest). Considering that these angles were measured under general anesthesia in the present study, this discrepancy may be caused primarily by application of the laryngoscopy. Induction of anesthesia and paralysis cause anterior displacement of epiglottis, hyoid, and thyroid cartilage without laryngoscopy [6]. Thus, a slight decrease of the ? angle can be expected by vertical displacement of LA [2]. With the Macintosh curved blade, moreover, the hyoid was drawn forward and its body was tilted downward during laryngoscopy and intubation [7]. Therefore, the use of laryngoscopy may produce a vertical displacement of LA and more decrease of the ? angle. In contrast, with 12-cm headrest, the LA may be minimally affected by this change. However, the additional ftexion by 12-cm headrest may produce a caudal movement of the inferior extremity of the superior incisors during laryngoscopy; thus, the LV may be much affected and the increase of ? angle can be expected.

Our study showed that 6-cm headrest yielded a larger

intubation distance (46.2 mm) that depicts the glottis- visualizing oropharyngeal space [3]. Although we did not consider the complexity of endotracheal intubation such as intubation difficulty scale [8], the narrower intubation

distance not only obscured vision of the glottis but also might restrict endotracheal tube movement [3]. Isono [9] proposed that 2 groups of obstacles (posterior or anterior to the oral space) between our eyes and the vocal cords can impair the Laryngeal view during direct laryngoscopy. Therefore, proper head positioning should displace these obstacles out of the operator’s view by caudal movement of the mandible and downward movement of the larynx [10]. On the contrary, the additional ftexion by 12-cm headrest showed the reduction of intubation distance and provided poorer Laryngoscopic views compared with 6-cm headrest. The possible explanation is that there was no case of difficult laryngoscopy in the present study, and the additional ftexion of head and neck might produce posterior and cephalad movements of blade tip reducing the intubation distance.

In our study, sniffing position at 6-cm headrest produced an OAA extension angle of 40.7? and that of no headrest or 12-cm headrest was 34.5? or 35.2?. On the contrary, in awake volunteers with normal cervical spine [1], the OAA extension angle in simple Head extension or sniffing position was 20.4? or 24.2?, which is smaller than our results, but the difference between simple extension and sniffing position is comparable. It is assumed that the responsible anesthesiol- ogist always tried to get the best laryngoscopic view at maximum head extension and maximum lift with the laryngoscope on each headrest under general anesthesia. Moreover, the additional ftexion by 12-cm headrest to patients might increase neck ftexion that might induce the smallest MV distance among the headrests. The increased neck ftexion is associated with head extension shown in magnetic resonance imaging [2]. Therefore, there is a possibility that maximal head extension may be limited for patients at 12-cm-high headrest.

Endotracheal intubation using rapid sequence intubation is the cornerstone of Emergency airway management. Although our study was performed under general anesthesia in operation conditions, we suggest that the results can be applicable to emergency airway management (e.g., rapid sequence intubation). Inadequate positioning may result in prolonged or failed tracheal intubation attempts because of the inability to visualize the larynx. The present study showed that anatomical measurements and laryngoscopic views depend on the headrest height, so the optimal height of headrest is recommended to facilitate tracheal intubation.

Our study has some limitations. First, we used no headrest, 6-, or 12-cm headrest for comparison of anatomical measurement depending on the headrest height. Use of a single height headrest does not always provide optimal position for all subjects because of modest variations in weight, head circumferences, neck length, and material of headrest (foamy or incompressible). Because head and neCK elevation beyond the sniffing position improves laryngeal view in cases of difficult direct laryngoscopy [11] and 6- to 7-cm-high headrest is applied in the sniffing position [1,2,5], the 6- and 12-cm-high incompressible headrest were applied in the present study. Second, the crucial clinical point remains larynx visualization, success of intubation, and not anatomical changes. However, the minimization of the ? angle with the increase of intubation distance and OAA angle may explain the benefit obtained in glottis exposure by addition of a headrest under the occiput. Moreover, the laryngoscopic view of the 6-cm headrest was significantly superior to those of other headrest and without a headrest in present study. Last, highly obese patients were not enrolled in this study. Considering that ramping obese patients produces the same alignment of the axis of intubation that the sniff produces in normal weight patients [12], application of a headrest higher than 6 cm may be needed in further studies of obese patients.

In conclusion, compared with no or 12-cm headrest, 6-cm headrest could facilitate more alignment of the oral- pharyngolaryngeal axis, increase the OAA angle, and enlarge the intubation distance. Therefore, it is recommended to use a 6-cm headrest during direct laryngoscopy in the sniffing position other than more than 6-cm (such as 12-cm) headrest.

References

1. Takenaka I, Aoyama K, Iwagaki T, et al. The sniffing position provides greater occipito-atlanto-axial angulation than simple head extension: a radiological study. Can J Anaesth 2007;54:129-33.
2. Adnet F, Borron SW, Dumas JL, et al. Study of the “sniffing position”

by magnetic resonance imaging. Anesthesiology 2001;94:83-6 list.

1. Tripathi M, Pandey M. Short thyromental distance: a predictor of difficult intubation or an indicator for small blade selection? Anesthesiology 2006;104:1131-6.
2. Yentis SM, Lee DJ. Evaluation of an improved scoring system for the grading of direct laryngoscopy. Anaesthesia 1998;53:1041-4.
3. Adnet F, Baillard C, Borron SW, et al. Randomized study comparing the “sniffing position” with simple head extension for laryngoscopic view in elective surgery patients. Anesthesiology 2001;95:836-41.
4. Masato K, Takao A, Yuko H, et al. Effect of head elevation on passive upper airway collapsibility in normal subjects during propofol anesthesia. Anesthesiology 2011;115:273-81.
5. Greenland KB. A proposed model for direct laryngoscopy and tracheal

intubation. Anaesthesia 2008;63:156-61.

1. Adnet F, Borron SW, Racine SX, et al. The intubation difficulty scale (ids): proposal and evaluation of a new score characterizing the complexity of endotracheal intubation. Anesthesiology 1997;87:1290-7.
2. Isono S. Common practice and concepts in anesthesia: time for reassessment: is the sniffing position a “gold standard” for laryngos- copy? Anesthesiology 2001;95:825-7.
3. Kitamura Y, Isono S, Suzuki N, et al. Dynamic interaction of craniofacial structures during head positioning and direct laryngoscopy in anesthetized patients with and without difficult laryngoscopy. Anesthesiology 2007;107:875-83.
4. Schmitt HJ, Mang H. Head and neck elevation beyond the sniffing position improves laryngeal view in cases of difficult direct laryngoscopy. J Clin Anesth 2002;14:335-8.
5. Collins JS, Lemmens HJ, Brodsky JB, et al. Laryngoscopy and morbid obesity: a comparison of the “sniff” and “ramped” positions. Obes Surg 2004;14:1171-5.