PATIENT PREPARATION – POSITIONING AND PRE-OXYGENATION
The importance of patient preparation, time allowing, cannot be overemphasised. Correct positioning of the patient, in the sniffing position, facilitates tracheal intubation by optimally aligning the primary and secondary curves. However, there is also good evidence that patient positioning has beneficial effects on pre-oxygenation. Several studies have now shown that pre-oxygenating patients in a 20° head-up or sitting position increases time to desaturation, therefore giving more time for tracheal intubation. This holds true for obese and non-obese patients, and for the sitting and reverse Trendelenberg positions, which can be used, for example, when the patient has been immobilised for suspected spinal injury.8
Pre-oxygenation should occur for at least 3 minutes, time allowing, and ideally should be enhanced with expiratory +/- inspiratory pressure if oxygen saturations are not maintained with facemask oxygen at a high FiO2.8
Continued, ‘apnoeic’ oxygenation during the intubation attempt is now gaining traction as a routine intervention. This is most simply delivered via standard nasal cannulae, and has been shown to increase the time to desaturation in two anaesthetic studies at a flow rate of 5L.min-1.9,10 A more recent study using humidified oxygen at a flow rate of 70L.min-1, termed Transnasal Humidified Rapid-Insufflation Ventilatory Exchange (THRIVE), achieved complete maintenance of oxygen saturations for very long periods (median time 14 minutes, range 5-65 minutes) in 25 ENT patients, 12 of whom were obese, and nine had stridor.11 Current recommendations suggest a flow rate of 15L.min-1 via standard nasal cannulae,8 a practice that appears to be well tolerated, even in awake patients.12
LEVITAN’S THREE-STAGE APPROACH TO TRACHEAL INTUBATION
Thinking about the primary and secondary curves helps to explain difficulties with visualisation of the glottis (due to a pronounced primary curve and/or an up sloping vestibule axis) and with delivery of the tracheal tube (due to a pronounced secondary curve). Understanding this can therefore help us trying to overcome these difficulties, for example, with correct positioning, external laryngeal manipulation, and the use of stylets and bougies.
When managing these difficulties during the intubation attempt, it can be helpful to break tracheal intubation down into three stages as described by Richard Levitan:13
- Laryngeal exposure
- Delivering the tube or bougie to the glottic opening
- Advancing the tube or bougie into the trachea
It is essential to understand these three stages and that how we achieve them differs between airway devices.
STAGE 1: LARYNGEAL EXPOSURE
Levitan suggests that Stage 1 should be a slow, methodical process, where the first objective is visualisation of the epiglottis (‘epiglottoscopy’). This structure will almost always be in the midline, and signals our first view of the laryngeal structures. Levitan advises having a Yankauer sucker in the right hand during this process, to clear secretions, which might be masking the tip of the epiglottis from view. How the epiglottis is reached depends on the device used, the difference being how the tongue (and therefore the primary curve) is negotiated. For example, the straight blade is directed lateral to the tongue and bypasses the primary curve, the standard Macintosh blade displaces the tongue to the left and flattens the primary curve, and angulated videolaryngoscopes follow the tongue in the midline and therefore follow the primary curve. In all cases, the next step is to gain a view of the glottis by either lifting the epiglottis directly (straight blade laryngoscopy) or indirectly, by deploying the hyo-epiglottic ligament with pressure in the vallecula (all other devices). In all cases, the next step is to lift the tongue and other tissues of the anterior column with upward force in the axis of the handle of the laryngoscope, hopefully resulting in a full view of the glottic opening. If not, Levitan advocates the use of bimanual laryngoscopy, whereby the intubator uses their right hand to manipulate the larynx into view. An assistant can then hold the larynx in the same position while the intubator moves on to Stage 2. Bimanual laryngoscopy has been found to improve the view of the glottis more frequently than cricoid pressure, BURP14 (Backward Upward Rightward Pressure on the thyroid cartilage) or no manipulation in a cadaver model.15 Another manoeuvre, which can be used in difficult cases, is to lift the head further, either with the right hand or with the help of an assistant. This is contraindicated if there is cervical spine instability.
STAGE 2: DELIVERY OF THE TUBE OR BOUGIE TO THE GLOTTIC OPENING
This stage is also dependent on the device used. Laryngoscopes that bypass or flatten the primary curve, by definition, result in a straighter line of sight to the glottis than those that follow the primary curve. As a result, a tracheal tube can be advanced towards the glottis at a straighter angle than when using a non-channelled angulated videolaryngoscope. This explains why angulated videolaryngoscopes require a channel to guide the tracheal tube, or the use of introducers to aid tube delivery. If an angulated device is being used, it is important not to advance too close to the glottis – the larynx should not fill the entire monitor screen. In addition, at this point, angulated devices should be tilted slightly downwards, so that the glottic opening is seen in the upper half of the monitor screen, leaving the lower half for visualisation of the approaching tube or bougie. This has two main advantages. Firstly, the tube or bougie can be seen approaching the glottis and adjustments can be made to allow easy passage. Secondly, tilting the device results in a decreased angle between the laryngoscope blade and trachea, so that the tube or bougie will pass more naturally into the glottis.
The use of different laryngoscopes during Stages 1 and 2 is summarised in the table below.