An MRI Study of the Sniffing, Neutral and Extended Head and Neck Positions

Overview

The traditional method of tracheal intubation requires the insertion of a laryngoscope into the oral cavity and directly visualisation of the glottis. The traditional head position called the sniffing the morning air position; consisting of lower cervical flexion and extension of the atlanto-occipital joint is commonly used to facilitate tracheal intubation in this manner by aligning the oral axes and providing the operator a view of the glottis. With the advancement of technology, has seen videolaryngoscopes (VL) developed to ease tracheal intubation. With VL, the glottis is seen through a camera connected to a viewing screen either attached to the top of the laryngoscope or connected to a camera screen by cable. One of the main drawbacks with VL is that a good view of the glottis is seen on the screen, but it can be difficult to pass the tracheal tube through the glottis in comparison to the traditional intubation technique described above. One of the main factors the authors suggest is that VL-assisted intubation requires a head and neck position that is different to the sniffing the morning air position. This study aims to assess the oral axes in three different head and neck positions under MRI to assess which positions improve axial alignment the most.

The traditional method of tracheal intubation requires the insertion of a laryngoscope with the left hand into the oral cavity to displace the tongue and directly visualise the glottis. The sniffing the morning air (or sniffing) position has been traditionally used for tracheal intubation using direct laryngoscopy. This comprises of flexion of the lower cervical spine, extension at the atlanto-occiptal joint and a horizontal level between the tragus of the ear and sternum. The theoretical purpose of this position is to improve the glottic visualisation by aligning the oral, pharyngeal and tracheal axes. In recent years with the advancement of technology, videolaryngoscopy has become popular and many centres are now using only videolaryngoscopes for tracheal intubation. Visualisation occurs indirectly through fibreoptic or digital channels. Most videolaryngoscopes use two types of blades. The Macintosh type for normal airway and hyper-angulated blade for difficult airway. Hyperangulated blades such as the Glidescope LoPro (Verathon Inc, Bothwell, WA, USA), McGrath Series 5 X blade (Medtronic, Minneapolis, MN, USA) and Storz C-Mac D-blade (Karl Storz Endoscoke, Tuttlingen, Germany) are more curved than the traditional standard geometry used with direct laryngoscopy and are thought to improve glottis visualisation without significant change in head position. However, visualisation of glottis is one of the three steps involved in videolaryngoscopy assisted tracheal intubation. The other two steps include aligning the tube with the glottic inlet and advancing the tube through the glottis into the trachea. One of the main drawbacks in videolaryngoscopy is that despite good view of glottis on the screen, it can be difficult to successfully advance the tracheal tube into the trachea. Anterior impingement of the tracheal tube at the sub-glottic region is a recognised problem with hyperangulated and channelled videolaryngoscopes. VL directs the pre-formed tracheal tube anteriorly but the trachea descends posteriorly into the thorax creating an acute angle impeding passage of the tube. This can result in difficult intubation or a failed intubation. Manoeuvres such as tube rotation and rotation of bougie can rectify this problem to some extent. A previous study found that laryngoscopy was more difficult with hyper-angulated blades in the sniffing position as compared to neutral position, contradictory to traditional laryngoscopy teaching. Intermediary positions between sniffing and supine should be studied as these may balance the needs of maintaining sufficient mouth opening and not hindering angles affecting the axial alignment. Both studies recommend the benefits of a ramped position as it does reduce the time to desaturation and reduces aspiration risk compared to supine neutral positions and should be considered when using hyperangulated blades. . Using magnetic resonance imaging in awake and healthy patients, Adnet, et al. measured the oral, pharyngeal and tracheal axes using the neutral position, sniffing position and simple head extension and found neither position resulted in perfect alignment of the 3 axes. However, they did not study the effect of position on angle between tracheal and laryngeal axes. The search for further evidence is necessary for VL as there is a paucity of research in with regard to best head and neck position for videolaryngoscopy.