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Three Part Question

In [patients needing endo-tracheal intubation] do [head elevation and neck flexion] [improve laryngeal view and the likelihood of successful intubation]?

Clinical Scenario

A short, obese female with pneumonia enters the emergency department. She is tachypnoeic, febrile, and labouring intensely to breath. She has an oxygen saturation of 76% on room air and becomes decreasingly responsive in front of you. You believe her declining respiratory condition merits intubation. You anticipate a difficult intubation and wonder if any simple manoeuvres might be of some help. You have heard that elevating the head and flexing the neck (sniffing the air position) gives you a better view of the vocal cords.

Search Strategy

Medline 1948 to week two June 2011 using the OVID interface
Embase 1980 to June 2011
Cochrane reviews (2011) using the Cochrane Library website
Medline:(Head extension .mp) OR (head position .mp) OR (neck flexion .mp) OR (neck position .mp) OR (sniffing the morning air .mp) AND (intubation .mp) or (endotracheal .mp) OR (intratracheal .mp) OR (laryngoscopy), LIMIT results to Humans, all adults>19 years and English language.

Embase:((head extension) OR (head position) OR (neck flexion) OR (neck position) OR (sniffing the morning air)).ti,ab AND ((endotracheal) OR (intratracheal) OR (intubation) OR (laryngoscopy)).ti,ab, LIMIT results to Humans, all adults >18 years and English language.

Cochrane:(intubation) or (laryngoscopy).

Search Outcome

Medline: 56 papers, 5 were relevant.

Embase: 148 papers, 5 were relevant with four papers found earlier in the Medline search.

Cochrane: No review found. 6 relevancies overall

Relevant Paper(s)

Author, date and country	Patient group	Study type (level of evidence)	Outcomes	Key results	Study Weaknesses
Levitan et al 2003, USA	7 fresh human cadavers undergoing laryngoscopy in three positions (head flat, midposition and neck flexion with head elevation)	Prospective trial	Evaluation of laryngeal view as percentage of glottic opening (POGO) via video laryngoscopy Evaluating physicians blinded to the angle applied	Comparing the three positions, POGO scores increased significantly Significant linear relationship among the three positions (p<0.0001) No difference between raters (p=0.14)	Cadaver-based study Small numbers Only straight laryngoscopy blade and one operator used
Adnet et al, 2001, France	Compared head extension and sniffing positions in 456 consecutive patients undergoing endotracheal intubation under GA Patients acting as their own control	Randomised clinical trial	Glottic exposure assessed using the Cormack scale	Sniffing position improved view in 18% and worsened it in 11% compared to simple extension (p value not significant) Multivariate analysis showed that limited head extension and obesity (BMI>30) were independently associated with good view when sniffing position used OR=2.8 (95% CI 1.2 to 6.8) and 2.4 (95% CI 1.1 to 5.2) respectively	Unblinded trial Theatre conditions Theatre conditions No p value provided Only Macintosh laryngoscope used
Adnet et al, 2001, France	8 normal volunteers undergoing MRI of the airway in three positions (neutral, head extension and sniffing positions)	Prospective observational trial	α=angle of pharyngeal axis (PA) and mouth axis (MA) β=angle of PA and laryngeal axis (LA) δ=angle of line of vision (LV) and LA	Head extension and sniffing positions both reduced δ (p<0.05). β value increased significantly (p<0.05) α value decreased (p<0.05) in the sniffing compared to neutral position Anatomical alignment between LA, PA and MA was impossible in any of the positions examined. No significant difference in the angles measured between the simple extension and the sniffing positions.	Imaging-based study Awake, non-paralysed patients No laryngoscopy performed Patients with potentially difficult airways excluded
Greenland et al, 2010, Australia	42 normal volunteers undergoing MRI imaging in four positions (neutral, head extension, head lift, and the sniffing positions)	Prospective observational trial	Two curved model of the airway as primary (oro-pharyngeal) and secondary (pharyngo-glotto-tracheal) curves. These curves meet in the infliction point (always in the laryngeal vestibule) which can have a tangent line drawn to it.	The sniffing and the head extension caused the tangent to the point of inflection to approximate the horizontal plane (p<0.0001). The head lift (p<0.0075), the head extension (p<0.002) and the sniffing positions (p<0.0001) significantly reduced the area between the line of sight and the airway curve. The changes in these curves supported the use of the sniffing position to facilitate direct laryngoscopy followed by the extension, head lift and neutral positions.	Excluded potentially difficult intubations Awake, non-paralysed patients Imaging based and small study. No laryngoscopy performed
Lee et al, 2008, Australia	20 patients having GA with muscle relaxant had measurement of axial force required for laryngoscopy in two positions (extension-extension and sniffing the air position) acting as their own control.	Randomised clinical trial	Axial force required to visualise the corniculate cartilage during direct laryngoscopy	Mean force required in extension-extension position was less than the sniffing position (p=0.04) Mean difference 4.0N (95% CI 0.3 to 7.6)	Excluded predicted difficult intubations Theatre conditions Proxy endpoint (applied force)
Takenaka et al, 2007, Japan	30 normal volunteers undergoing lateral x-ray imaging of the cervical spine in three positions (neutral, head extension and sniffing positions)	Prospective observational trial	Mean occipito-atlanto-axial (OAA) extension angle	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). Sniffing position provided greater OAA	Imaging based Awake, non-paralysed patients No laryngoscopy or intubation was performed Only considered OAA extension angle and no other anatomic factors

Comment(s)

The ability to obtain a good glottic visualisation during direct laryngoscopy is probably the main determinant of easy tracheal intubation but the assumptions regarding optimal positioning have historically received little scientific scrutiny.

The current short-cut review of the medical literature is limited by the fact that most studies were cadaver or imaging-based studies with small numbers. These studies mostly looked at anatomical angles exclusively, not taking into account any other anatomical factors and their results are therefore difficult to translate into clinical practice.

The only benefit of the so-called sniffing the air position when compared to head extension was found in a sub-group of patients with obesity or limited head extension.

Clinical Bottom Line

There is little evidence to support use of the sniffing the air position compared to simple head extension position to improve laryngeal visualisation.

The sniffing the air position might be useful and should be considered in patients, who have an increased body mass index or limited neck extension.

References

1. Levitan RM, Mechem CC, Ochroch EA et al. Head-elevated laryngoscopy position: improving laryngeal exposure during laryngoscopy by increasing head elevation. Ann Emerg Med. 2003 Mar;41(3):322-30.
2. 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.
3. Adnet F, Borron SW, Dumas JL, et al. Study of the “sniffing position” by magnetic resonance imaging. Anesthesiology 2001;94:83–6.
4. Greenland KB, Edwards MJ, Hutton NJ, et al. Changes in airway configuration with different head and neck positions using magnetic resonance imaging of normal airways: a new concept with possible clinical applications. Br J Anaesth 2010;105:683–90.
5. Lee L, Weightman WM. Laryngoscopy force in the sniffing position compared to the extension-extension position. Anesthesiology 2008;63:375–8.
6. 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.