Laryngoscopy force in the sniffing position compared to the extension‐extension position

Abstract

SummaryLaryngoscopy is sometimes easier with the patient's head and neck in the extension-extension position (head extension with the neck extended by the head section of the table bent down at 30°) rather than the classical 'sniffing the morning air' position. We therefore tested the hypothesis that the axial force required for laryngoscopy is less in the extension-extension than the sniffing position. We measured the force axial to the handle of a Macintosh 3 laryngoscope in 20 subjects under general anaesthesia who had been given neuromuscular blocking drugs. Measurement of force was made in the sniffing position and the extension-extension position. The mean (SD) axial force required in the extension-extension position was lower than in the sniffing position (19.6 (7.8) N versus 23.6 (8.6) N, p = 0.04). In the setting of routine tracheal intubation, less force is required when the patient is in the extension-extension position than in the sniffing position. The 'sniffing the morning air' position has been recommended for oral tracheal intubation by direct laryngoscopy [1]. The sniffing position can be achieved using a small pillow behind the patient's head, resulting in atlanto-occipital extension and cervical vertebral flexion. However, intubation is sometimes easier with cervical extension, e.g. by extending the head section of the operating table (Fig. 1), or by removing the pillow from beneath the patient's head and placing it behind the shoulders. These manoeuvres result in atlanto-occipital extension and extension of the cervical vertebrae -'extension-extension'. There are few scientific data comparing the extension-extension position (as opposed to simple head extension on a flat table) to the sniffing position in laryngoscopy and intubation. A published study [2] that used an otorhinolaryngological laryngoscope to visualise the anterior vocal cords showed that less force was required in the extension-extension position than in the sniffing position. It is uncertain how relevant this is to anaesthetic practice.The force that is applied to the laryngoscope is one of the factors that relates to the ease of intubation, as the laryngoscopist needs to visualise at least the corniculate cartilages of the larynx in order to intubate the trachea safely. We therefore tested the hypothesis that the laryngoscopy force required to view the corniculate cartilages is less with the patient's head in the extensionextension position than in the sniffing position. MethodsWith Local Research Ethics Committee approval and written, informed consent, we recruited male and female patients aged between 18 and 80 years presenting for elective surgery under general anaesthesia with neuromuscular blockade at the Sir Charles Gairdner Hospital. Exclusion criteria included predicted or actual difficult laryngoscopy, previous neck injury, neck surgery or any anatomical deficiency of the neck, and any patients in whom cricoid pressure was to be used at induction and laryngoscopy.In order to estimate the sample size required, we con...

A randomised comparative study on customised versus fixed sized pillow for tracheal intubation in the sniffing position by Macintosh laryngoscopy

“…[ 6 ] Similar findings were also observed in the studies conducted in Australia and Japan. [ 7 10 ] In contrast, the SP and head elevation have been found to improve the glottic view in numerous studies. A study conducted on human cadavers recorded a significant increase in percentage of glottic opening score with progressive elevation of the head to as much as possible, increasing neck flexion.…”

“…[ 4 5 ] Others found no distinct advantage compared to simple extension of the head in routine practice. [ 6 7 ] Horizontal alignment of the sternal notch and the external auditory meatus (EAM) has been used as a marker of appropriate positioning, in terms of the head elevation. This is also the aim in 'ramping' which has an established role in obese patients.…”

The sniffing and extension‐extension position: the need to develop the clinical relevance

“…I congratulate Lee and Weightman [1] on their article which supports previous work by Hochman [2] and Adnet [3]. The original Three Axes Alignment Theory [4] and the sniffing position [5] have been the corner stone of successful tracheal intubation for several decades.…”

Evaluating the Mechanics of Laryngoscopy

“…Some of the studies did not factor in the forces applied to the maxillary incisor teeth, which have a force vector in the opposite direction of the tongue. 18,23,24 This could be attributed to variation in experimental setup. 14 Manikins tend to result in higher applied forces because of the stiff plastic material used in the manikin's tongue.…”