| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
From the Department of Anesthesia, Nippon Steel Yawata Memorial Hospital, Kitakyushu, Japan.
Address correspondence to: Dr. Ichiro Takenaka, Department of Anesthesia, Nippon Steel Yawata Memorial Hospital, 1-1-1 Harunomachi, Yahatahigashi-ku, Kitakyushu 805-8508, Japan. Phone: 81-93-671-9420; Fax: 81-93-671-9605; E-mail: takenaka.i{at}ns.yawata-mhp.or.jp
 |
Abstract |
|---|
 TOP Abstract IntroductionMethodsResultsDiscussionReferences |
|---|
Methods: Thirty volunteers with normal cervical spines were studied. Radiological examinations of the lateral cervical spine were taken and compared in each of the following three positions for each subject: neutral position (flat on the table with no pillow and without head extension or flexion); simple head extension (head maximally extended without a pillow); and the sniffing position (head extension with cervical flexion obtained by 7 cm occipital elevation).
Results: Mean angles of the occipito-atlanto-axial extension in simple head extension and the sniffing position were 20.4°± 5.1° and 24.2°± 5.6°, respectively (P < 0.01).
Conclusion: The anatomic sniffing position provides greater occipito-atlanto-axial extension compared to simple head extension. These findings should be taken into consideration when optimizing patient positioning for laryngoscopy.
|
Introduction |
|---|
|
TOPAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Direct laryngoscopy requires movement of the cervical spine for optimal visualization of the glottis. Among the cervical spine segments, extension at the occipito-atlanto-axial (OAA) complex is pivotal for several reasons. First, most extension created by direct laryngoscopy occurs at the OAA complex and the sub-axial cervical segments are only minimally displaced during laryngoscopy.7 Second, Calder et al.8 have demonstrated that in patients with cervical spine disease involving the OAA complex that there is a higher prevalence of difficult laryngoscopy in comparison to patients with disease below the level of the axis. Third, OAA extension enhances mouth opening ability.9 Finally, aligning the pharyngeal axis with the axis of the mouth is the main factor for improving laryngoscopic view, and depends entirely on the degree of the OAA extension (Figure 1
).10 Thus, greater OAA extension can produce a good laryngeal view during laryngoscopy.
|
|
Methods |
|---|
|
TOPAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Experimental protocol
The subjects lay on a flat table with no pillow, looking straight without head extension or flexion, which was defined as the neutral position.5 The subject was then asked to extend her/his head maximally without a pillow (defined as simple head extension) and with a pillow (the sniffing position) in randomized order (Figure 2).5 We used a solid platform 7 cm in height as the pillow to keep the degree of cervical flexion constant (Figure 2). The observer (I.T.) was careful not to move the subject’s shoulders when the head was extended. Simple head extension and the sniffing position were determined by actively positioning with the subject’s effort. Lateral cervical radiographs were taken in all three positions.
|
). This line connected the most dorsal edge of, and caudal portion of the occiput and the dorsal edge of the hard palate. The reference line for the axis (C2) was drawn through the basal plate of the vertebral body (Figure 3). Because reference lines did not intersect on the radiograph in some subjects, the C0 or C2 angle was defined as the difference in angle between the C0 or C2 reference line and the common line which was the ventral vertical edge on each radiograph, respectively (Figure 3). The C0-2 angle was calculated by the difference between C0 and C2 angles. The OAA extension angle was defined as the difference in the C0-2 angle between the neutral position and simple head extension or the sniffing position.
|
|
Results |
|---|
|
TOPAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Occipito-atlanto-axial extension angle
Mean angles of the OAA extension in simple head extension and sniffing positions were 20.4°± 5.1° and 24.2°± 5.6°, respectively (P < 0.01). The sniffing position provided greater OAA extension compared with simple head extension. The difference in the OAA extension angle between both positions was 3.8° ± 3.7°. Lateral radiographs showed that near maximal extension of the OAA complex occurred in both positions in all subjects. The variation in the radiographic angle measurement was less than 1.2°, demonstrating low interobserver variability.
|
Discussion |
|---|
|
TOPAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
The three-axes alignment theory has been used to explain the steps in dynamic structural interactions during laryngoscopy, but Adnet et al.5 have clarified a contradiction of the theory using magnetic resonance imaging. In contrast, the obstacle theory proposed by Isono14 is useful in understanding this discrepancy, but does not fully explain why the sniffing position confers an advantage for improving the laryngoscopic view.14 Our results suggest that there are two kinds of obstacles between the laryngoscopist’s visual field and the glottis. One obstacle is located anterior to the oral airway, and the other is located posteriorly (Figure 4A). To visualize the glottis, the space between both obstacles must be enlarged (Figure 4B). In simple head extension, extension of the head (OAA complex) enlarges this space by moving the posterior obstacle downward, ensuring a posterior field of vision (Figure 4C). In the sniffing position, flexion of the neck (lower cervical spine) first moves both obstacles upwards without changing their relative positions (Figure 4D). Because the cervical flexion produces a greater degree of the OAA extension compared with simple head extension, a larger space between both obstacles is established (Figure 4E). Our results clarify part of the anatomic advantage of the sniffing position for laryngoscopy compared to simple head extension.
|
There are several potential limitations of our experimental design. First, we did not examine the relationship between the OAA extension angle and visualization of the larynx during direct laryngoscopy. Second, the maximal OAA extension angle would be underestimated because we did not use continuous fluoroscopy to assess the extension angle. Intra-subject variability may have existed in part, from the fact that the endpoint for extending the head maximally depended on the voluntary participation of each subject. We did not examine subjects to document maximal extension, because near maximal extension at the OAA complex was confirmed radiographically in both the simple head extension and sniffing positions. Third, we used a 7-cm high rigid platform as a "pillow" to avoid the variable degree of cervical flexion caused by the compressibility of a regular pillow. However, use of a single standard pillow size did not always provide optimal cervical flexion for all subjects, because of modest variations in weight, head circumference, and length of the neck. Finally, we performed this study in volunteers with normal cervical spines and not in patients with limited head extension. Hence, our findings may not be generalizable to patients with disease of the cervical spine.
In conclusion, in comparison to simple head extension, the sniffing position provides greater OAA extension in awake volunteers, indicating that the sniffing position may confer a distinct anatomic advantage for laryngoscopy. These findings should be taken into consideration when optimizing patient positioning for laryngoscopy.
|
Footnotes |
|---|
Accepted for publication September 18, 2006. Revision accepted November 8, 2006.
|
References |
|---|
|
TOPAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
2 Benumof JL. Difficult laryngoscopy: obtaining the best view. (Editorial) Can J Anaesth 1994; 41: 361–5.
3 Benumof JL. Conventional (laryngoscopic) orotracheal and nasotracheal intubation (single-lumen tube). In: Benumof JL (Ed.). Airway Management: Principles and Practice, 1st ed. St. Louis: Mosby; 1996: 261–76.
4 Gal TJ. Airway management. In: Miller RD (Ed.). Anesthesia, 6th ed. Philadelphia: Elsevier Churchill Livingstone; 2005: 1637–5.
5 Adnet F, Borron SW, Dumas JL, Lapostolle F, Cupa M, Lapandry C. Study of the "sniffing position" by magnetic resonance imaging. Anesthesiology 2001; 94: 83–6.[Medline]
6 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.[Medline]
7 Sawin PD, Todd MM, Traynelis VC, et al. Cervical spine motion with direct laryngoscopy and orotracheal intubation. An in vivo cinefluoroscopic study of subjects without cervical abnormality. Anesthesiology 1996; 85: 26–36.[Medline]
8 Calder I, Calder J, Crockard HA. Difficult direct laryngoscopy in patients with cervical spine disease. Anaesthesia 1995; 50: 756–63.[Medline]
9 Calder I, Picard J, Chapman M, O’Sullivan C, Crockard HA. Mouth opening: a new angle. Anesthesiology 2003; 99: 799–801.[Medline]
10 Urakami Y, Takenaka I, Nakamura M, Fukuyama H, Aoyama K, Kadoya T. The reliability of the Bellhouse test for evaluating extension capacity of the occipitoatlantoaxial complex. Anesth Analg 2002; 95: 1437–41.
11 Ordway NR, Seymour RJ, Donelson RG, Hojnowski LS, Edwards WT. Cervical flexion, extension, protrusion, and retraction. A radiographic segmental analysis. Spine 1999; 24: 240–7.[Medline]
12 Horton WA, Fahy L, Charters P. Disposition of cervical vertebrae, atlanto-axial joint, hyoid and mandible during x-ray laryngoscopy. Br J Anaesth 1989; 63: 435–8.
13 Panjabi M, Dvorak J, Duranceau J, et al. Three-dimensional movements of the upper cervical spine. Spine 1988; 13: 726–30.[Medline]
14 Isono S. Common practice and concepts in anesthesia: time for reassessment: is the sniffing position a "gold standard" for laryngoscopy? (Editorial). Anesthesiology 2001; 95: 825–7.[Medline]
Read all eLetters
eLetters:
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
), therefore, depends entirely upon the angle between C0 and C2 (
), as follows: 
) moves both obstacles upward without a change in their relative position (D). Because neck flexion produces a greater OAA extension (ß’, ß < ß’), a larger space between both obstacles is established (E). OA = obstacle located anterior to the oral air way (tongue, mandible, and others); OP = obstacle located posterior to the air way (maxilla, teeth, and others); G = glottis;