Airway management

Face-masks

Face-masks are designed to fit the face so that no leak of gas occurs, but without applying excessive pressure to the skin. An appropriate size of face-mask must be used to ensure a proper fit. Most modern masks consist of a hard plastic cone with a soft deformable gas-filled cuff to place against the face. Traditional rubber masks have been replaced by transparent single-use devices ( Fig. 23.1 ). These are preferred by patients and enable the anaesthetist to observe the airway for vomitus or secretions during use.

Oropharyngeal and nasopharyngeal airways

The Guedel oropharyngeal airway ( Fig. 23.2 ) consists of a rigid plastic flattened tube that is straight in its proximal section and curved distally. It is designed to be placed into the mouth to reverse the effects of gravity on the tongue (which falls back and can occlude the oral cavity and posterior pharynx) after induction of anaesthesia and thus maintain a patent airway. Size can be estimated from laying the airway on the cheek and it should be approximately the distance from the angle of the mandible to the midline of the lips.

The nasopharyngeal airway is a small plastic tube (6.0–7.0 mm ID) designed to be placed through the nose so the tip lies in the upper pharynx ( Fig. 23.3 ). It is designed to lift the soft palate off the pharyngeal wall and so eliminate this mode of airway obstruction. The proximal end of the airway has a flange to prevent loss of the device into the nose. Complications include bleeding (common) and submucosal passage (very uncommon).

Connectors and catheter mounts

The anaesthetic circuit is constructed from plastic tubes joined together by connectors. The distal end of the anaesthetic circuit is a plastic connector of 22-mm external diameter. Between the distal end of the anaesthetic circuit and the proximal end of the airway device sits the catheter mount with a 22-mm connector at the proximal end and a 15-mm connector distally. These push fit connectors must be actively pushed and twisted to connect reliably; failure to do this is a common cause of circuit disconnection. The catheter mount may be straight or angled, fixed length or concertinaed and may serve additional functions (e.g. including a self-sealing diaphragm at the angled end to facilitate endoscopic inspection through the airway device). The catheter mount enables positioning of the anaesthetic circuit such that there is no excessive traction on the airway device that may lead to displacement. Proximal to the catheter mount a heat and moisture exchange filter (HMEF) is usually placed ( Fig. 23.4 ). This helps maintain airway gases temperature and humidity and isolates the anaesthetic circuit from the patent so that it may be used for multiple patients.

First-generation SADs

Classic LMA

The cLMA is the forerunner of many modern SADs and consists of a silicone tube with an elliptical distal mask, which has a cuff, inflated through a pilot tube ( Fig. 23.5A ). The cuff, which resembles a miniature face mask, is designed to seal around the posterior perimeter of the larynx. In the middle of the bowl two soft bars run longitudinally to prevent the epiglottis entering the airway tube and occluding the airway. When correctly positioned, the bowl of the cLMA surrounds the laryngeal inlet and epiglottis, and the cuff of the mask extends from the base of the tongue to the upper oesophagus. Seven sizes are routinely available, enabling use in all patients from neonates to large adults ( Table 23.1 ).

Table 23.1

Laryngeal mask airway (LMA) sizes

Mask size	Patient weight (kg)	Cuff volume (ml)
1	<5	2–5
1.5	5–10	5–7
2	10–20	7–10
2.5	20–30	12–14
3	>30	15–20
4	n/a	25–30
5	n/a	35–40

The cLMA (in common with most other SADs) is reusable up to 40 times. There are numerous single-use devices based on the cLMA, collectively referred to as laryngeal masks. Performance of laryngeal masks may be similar to the cLMA, but in many cases this is unproven, and some devices perform less well.

Second-generation SADs

i-gel

The i-gel is a single-use SAD with a non-inflatable mask made of a soft thermoplastic elastomer ( Fig. 23.6 ). The mask is a preformed soft mould which fits into the perilaryngeal structures. It has a narrow-bore drain tube, short wide-bore airway tube and integral bite block. The mask portion is shorter than other laryngeal masks, and it therefore sits less deeply, and seals less well, in the upper oesophagus. It has now been used for many millions of anaesthetics and has a good efficacy and safety record. Sore throats and dysphagia may occur less commonly after i-gel use than with other SADs. It is well suited to guided tracheal intubation during difficult airway management, and because it is extremely simple to insert, it has become widely used for airway management during cardiac arrest.

Intubating LMA

The ILMA is a specialised LMA designed to facilitate tracheal intubation ( Figs 23.5B and 23.7 ). It is described in the section on Difficult Airway Management later in this chapter.

Macintosh laryngoscope

The most commonly used adult laryngoscope blade is the Macintosh blade ( Fig. 23.10 ), which is manufactured in several sizes. Similar to other laryngoscopes, the handle contains batteries, and clicking the blade into place (90 degrees to the handle) turns the light on. Older laryngoscopes had bulbs in the blade, but these have largely been replaced by a light on the handle that is transmitted along the blade via cold fibreoptics.

Specialised blades

McCoy (levering) laryngoscope

The blade of the McCoy laryngoscope ( Fig. 23.11 ) is hinged in its distal portion and can be flexed by squeezing a lever on the handle. This moves the fulcrum of laryngoscopy from the teeth to the end of the blade. When deployed, the tip lifts the epiglottis and often improves laryngeal view by one Cormack-Lehane grade (see Airway Assessment section). However, the device is somewhat cumbersome and has largely been replaced by videolaryngoscopes.

Videolaryngoscopes

Videolaryngoscopes use high-resolution electronic video cameras incorporated in a laryngoscope blade. The image is relayed to a dedicated video display which may be on the laryngoscope handle or separately. Videolaryngoscopy enables the anaesthetist to ‘see around the corner’ and effectively gain a view from the blade. This converts the very narrow angle view obtained with direct laryngoscopy (≈15 degrees) into a wider angle view (≈60 degrees) ( Fig. 23.12 ). An exception to this electronic design is the Airtraq ( Fig. 23.13 ), which projects its light and image through a series of prisms.

There are three main designs of videolaryngoscopes:

• Bladed

  • Macintosh-shaped blade, such as C-MAC ( Fig. 23.14 ), GlideScope Titanium, McGRATH MAC (may be used for both direct and videolaryngoscopy)

    

  • Hyperangulated blade, such as C-MAC D blade, GlideScope original ( Fig. 23.15 )

    

• Conduited, such as Airtraq, Pentax AWS

• Optical stylets, such as Bonfils, Shikani optical stylet, C-MAC VS videostylet.

Optical stylets are metal rods with an internal fibreoptic or video system which enables the user to view the image from the distal end directly from the viewing port or on a remote screen ( Fig. 23.16 ). Most stylets are rigid with a fixed angle (e.g. Bonfils, Levitan). The Shikani stylet is semimalleable and the SensaScope has a flexible fibreoptic tip which allows some manipulation. Optical stylets are placed within the lumen of the tracheal tube and then directed into the larynx before the tracheal tube is advanced into the airway. The main advantage of stylets is that they require minimal mouth opening (as little as 1 cm) and can be advanced with negligible tissue disruption. Their main disadvantage is the inability to manipulate and displace airway structures in the manner that a bladed instrument can. Bladed videolaryngoscopes are increasingly popular, with optical stylet use restricted to expert local use.

The potential advantages of videolaryngoscopy compared with direct laryngoscopy include:

• ability to ‘see round corners’;

• improved view at laryngoscopy (especially when there is difficulty)

• reduced head and neck movement to achieve a view of the larynx, which is important in patients with potentially unstable cervical spines;

• improved ease of tracheal intubation and reduced failed intubation;

• reduced tissue distortion and haemodynamic responses to laryngoscopy;

• reduced airway trauma;

• reduced postoperative hoarseness;

• ability to display the image on a remote screen, which permits:

  • improved supervision by trainers of tracheal intubation performed by trainees;

  • an assistant to observe what the intubator sees (e.g. guiding cricoid force);

  • improved skill acquisition of direct laryngoscopy by trainees;

  • recording for medical notes and medicolegal reasons; and

  • recording of an objective image or video for presentation, teaching or research purposes.

These benefits are particularly apparent to those experienced with videolaryngoscopy.

When laryngoscopy is easy, use of a hyperangulated or conduited, but not a Macintosh-type videolaryngoscope may overcomplicate tracheal intubation and, therefore, slow it down and increase the number of attempts.

Use of a hyperangulated blade prevents direct laryngoscopy as the blade is too curved to enable direct vision of its distal end. To overcome this problem, many manufacturers advise the use of a rigid or semirigid stylet to pre-form the shape of the tracheal tube before insertion. There is a risk of damage to other tissues in the airway as the tracheal tube/stylet assembly is introduced (blindly). This is minimised if the tip of the tracheal tube is advanced for as long as possible under direct vision and then along the blade of the videolaryngoscope.

The 2015 DAS guidelines for difficult tracheal intubation state that ‘videolaryngoscopy should be rapidly available (in any location where tracheal intubation is performed) and that all anaesthetists should be trained and expert in the use of videolaryngoscopy’. This makes it a mainstream technique that should be taught during routine anaesthetic care. As videolaryngoscopy becomes more widely available, there is a strong argument for initially selecting a device with a Macintosh type blade (suitable for direct or videolaryngoscopy), reserving a specialised (i.e. hyperangulated, conduited or stylet) device only for when there is difficulty.

Flexible intubating laryngoscope and bronchoscope

The term ‘fibreoptic scope’ or ‘fibrescope’ ( Fig. 23.17 ) describes a flexible fibreoptic endoscope suitable for introduction into the airway via the nose or mouth to guide tracheal intubation. Traditionally fibrescopes contain tiny fibres (20 µm glass fibres) in bundles that either transmitted an external light to the tip of the device to illuminate the subject or transmitted an image from the tip of the device to the eyepiece, or a connected screen. The fibres transmitting light (light guide) are arranged in a random fashion but those returning the image (image guide) are precisely located relative to each other to ensure integrity of the transmitted image. The flexible fibrescope consists of a long flexible cord, the distal tip of which is manipulated by controllers operated the proximal end. Various working channels are incorporated in the cord, their size varying with device diameter and function; this enables suction, instrumentation and drug or oxygen administration. Proximally there are directional controls and an eyepiece. The light source may be externally mains- or battery-powered. The eyepiece consists of the viewing lens and dioptre adjustment ring. A camera can be attached to the eyepiece either to take photographs or to transmit the pictures to a monitor.

The term ‘fibrescope’ is increasingly redundant as many modern devices (flexible videoscopes) no longer contain fibreoptic bundles but rather a distal electronic camera that captures a high-definition video image and transmits it electronically to a screen. This is a rapidly developing technology. Disposable flexible videoscopes (e.g. the aScope Fig. 23.18 ) are now available.

The size of flexible fibrescopes varies from 1.8–6.4 mm to fit inside tracheal tubes of 3.0–7.0 mm ID. Most intubating videoscopes are ≈3–5 mm diameter.

Tracheal tube size

Choice of size of tracheal tube is a matter for debate. Where the patient's lungs are ventilated there is little to be gained from using a large tracheal tube, and a smaller tube (e.g. 6.0–6.5 mm ID for women, 6.5–7.0 mm ID for men) may make tracheal intubation less traumatic, especially where there is an awkward view at laryngoscopy. If the patient will breathe spontaneously whilst the trachea is intubated (e.g. planned postoperative ICU admission), a larger tracheal tube (e.g. 7.0–7.5 mm ID for women, 8.0–8.5 mm ID for men) may lead to less airway resistance and improved access for passage of suction catheters.

Plain and cuffed tracheal tubes

Uncuffed tracheal tubes are generally only used in children (see Chapter 33 ). In adults, as the larynx is the narrowest part of the airway, a cuff is necessary to seal with the trachea (as inserting a tube large enough to seal with the larynx would cause trauma). Use of a cuffed tube facilitates positive pressure ventilation and (largely) protects the airway from soiling with secretions, regurgitated gastric contents, blood, pus and so on.

Cuff design, volume and pressure

Tracheal tube cuffs are generally described as low-volume/high-pressure or high-volume/low-pressure. A tracheal tube with a low-volume cuff needs almost complete inflation (high pressure) to create a seal within the trachea, whereas a large-volume cuff needs only partial inflation (low pressure). Most tracheal tube cuffs are now high-volume/low-pressure as, although not all pressure within the cuff is transmitted to the tracheal mucosa, these have a reduced likelihood of mucosal ischaemia. The volume of air inserted into the cuff need only be enough to create a seal and leak-free ventilation. For prolonged periods of tracheal intubation, manometry may be used to ensure the cuff pressure remains less than 30 cmH ₂ O. Some tracheal tubes now incorporate a pilot tube on which the pilot balloon is replaced by an integrated ‘traffic light’ pressure indicator to assist in maintaining safe pressures. If nitrous oxide is used, this can diffuse into the cuff as inspired concentration rises (and out as it falls), and cuff pressure should be checked 20 min after any increase in fractional concentration of nitrous oxide. Alternatively, the cuff volume may be inflated with fluid to avoid this problem.

High-volume/low-pressure plastic cuffs are incompletely inflated, and as a result small folds occur longitudinally, leading to microchannels which may enable fluid to bypass the cuff. Silicone cuffs (e.g. on ILMA tracheal tubes) inflate without microchannels and are generally low-volume/high-pressure (reusable ILMA tracheal tubes) or intermediate-volume/intermediate-pressure (single-use ILMA tracheal tubes). Herniation of an overinflated cuff may occlude the distal end of the tracheal tube and cause partial or total airway obstruction, but this is extremely rare.

Tube shape

A curved tracheal tube is used in most settings. Preformed tubes in shapes which either fit the pharyngeal contour or move the proximal end of the tracheal tube away from the mouth are used particularly for head and neck surgery (see Chapter 37 ). The preformed shape means bronchial intubation is more common, especially if the head is extended.

Specialised tracheal tubes

A flexible tube made of softer plastic or silicone and reinforced with an internal spiral of nylon or wire may be useful if there is a danger of the tube kinking during surgery, such as during head and neck surgery or prone positioning ( Fig. 23.20 ). Flexible tubes are often straight and are more awkward to place at laryngoscopy, so use of a bougie or stylet is recommended. The internal spiral means they cannot be cut and care needs to be taken to avoid bronchial intubation.

The tip of many tracheal tubes is a rigid lateral bevel, and when used to railroad over a bougie, stylet or fibrescope, this can catch on the glottic tissues and cause hold-up and trauma. The ILMA tracheal tube is a wire-reinforced straight tube with a soft silicone bullet tip. The tip is designed to deviate when it hits tissue and to wrap around a fibrescope ( Fig. 23.21A ). This makes for atraumatic, smooth insertion when used for fibreoptic intubation. The Parker and GlideRite tracheal tubes ( Fig. 23.21B and C ), which have anterior-posterior bevels and softer tips, may also be useful when railroading.

Other specialised tubes include laser, micolaryngeal, double-lumen and laryngectomy tracheal tubes. These are discussed in Chapters 37 and 41 .

Cricothyroidotomy devices

Cricothyroidotomy is the creation of an opening in the cricothyroid membrane to gain access to the airway either as an elective procedure in an anticipated difficult airway or as an emergency to rescue a lost airway (e.g. in the ‘cannot intubate, cannot oxygenate’ (CICO) situation).

Cricothyroidotomy can be carried out using a:

• small cannula (≤ 2-mm ID);

• large-bore cannula (≥ 4.0-mm ID); or

• tracheal tube (5–6 mm ID).

If a small cannula is used (cannula over needle technique), there is a risk of failure to insert, kinking or device failure. A specifically designed device, such as the Ravussin cannula ( Figs 23.22 and 23.23 ), is recommended over the use of a standard i.v. cannula to ensure it is long enough, to reduce kinking and to improve ventilation. After insertion, ventilation needs to be provided by a high-pressure source (e.g. gas cylinder or wall oxygen) during which there is a risk of barotrauma. Devices such as the VBM Manujet III ( Chapter 16 , Fig. 16.37 ) and Rapid-O ₂ insufflation device assist ventilation in these settings and are preferred to ad hoc equipment. Technical failure and complications of ventilation are much more common in an emergency. The Ventrain device ( Fig. 23.24 ) facilitates ventilation through a narrow-bore cricothyroidotomy device (or a narrow tracheal tube) by providing high-flow gas for inspiration and using the Bernoulli principle to provide active expiration. Using this manual device, normal minute ventilation can be achieved through a 2-mm ID cannula.

Large-bore cannulae are inserted either with a cannula-over-needle technique (e.g. VBM Quicktrach) or with a Seldinger technique (e.g. Cook Melker cuffed and uncuffed devices, Fig. 23.25 ). They provide a tracheal tube through which an adult can breathe spontaneously (≥ 4.5-mm ID). The cannula-over-needle designs may be too short to reach the trachea in obese patients. The Portex cricothyroidotomy kit is designed for emergency use and has a spring-loaded Veress needle with a blunt stylet to aid insertion of a 6.0-mm ID tracheal tube. The Portex Minitrach II is a wire-guided kit which provides a 4-mm ID uncuffed airway designed for tracheal suction and specifically not for use in an emergency situation.

Insertion of a tracheal tube via the cricothyroid membrane can be achieved with a scalpel and bougie ( Fig. 23.26 ). It is a versatile and quick technique and is described later in the chapter. It is currently the technique recommended in the DAS guidelines for both non-obese and obese patients. Unfamiliarity and bleeding are the main concerns with this technique.

Bougie

If the larynx cannot be seen adequately during laryngoscopy, or if the tracheal tube cannot be manoeuvred into the laryngeal inlet, a bougie (diameter ≈5 mm, length ≈70 cm; Fig. 23.27 ) may facilitate this. The lubricated bougie is inserted into the trachea to act as a guide for the tracheal tube. The bougie's tip is bent distally (Coude tip) to aid insertion into the glottis. The bougie should never be inserted beyond the carina (maximum insertion distance 25 cm). The tracheal tube should be rotated as it is advanced over the bougie so that the bevel does not become lodged against the aryepiglottic fold or the vocal cord. Single-use disposable bougies are now available, but they may be rigid (increasing risk of trauma), have poor memory (i.e. they unfurl too rapidly after curling) or lack the Coude tip. They have not been shown to be better than the reusable Eschmann (gum elastic) in practice.

Stylet

An (often malleable) metal stylet may be used to adjust the degree of curvature of a tracheal tube as an aid to its insertion ( Fig. 23.28 ). The stylet must not protrude from the distal end of the tube, in order to prevent trauma. Stylets have traditionally been used much more in North America, with the bougie more commonly used in the UK; recently this has changed as stylets are particularly suited to aiding tracheal intubation with hyperangulated videolaryngoscope blades. Stylets for use with videolaryngoscopes may be device specific and may be rigid or malleable. The Parker Flex-It stylet is a single-use plastic stylet that can be used to increase the curvature of the tracheal tube and so facilitate tracheal intubation ( Fig. 23.29 ).

Aintree intubating catheter

The Aintree Intubation Catheter (AIC) is a 56-cm hollow catheter designed to facilitate tracheal intubation via a SAD ( Fig. 23.30 ). It is supplied with special (Rapi-Fit) adapters that connect to either a 15-mm connector for use with conventional anaesthetic circuits or a Luer-Lok for use with a high-pressure oxygen source ( Fig. 23.31 ). Although the connectors enable oxygenation via the AIC, this is rarely necessary or effective and risks barotrauma if a high-pressure source is used. The catheter has a 4.8-mm ID, which enables it to be preloaded over a videoscope. A standard 7.0-mm ID tube (or a 6.5-mm ID ILMA tube) fits over the AIC.

Airway exchange catheters

Airway exchange catheters (AECs) are long (45–140 cm), narrow (external diameter 2.67–6.33 mm) catheters that are used to exchange one tracheal tube for another. The AEC is inserted through the tracheal tube, which is then removed and a new tracheal tube railroaded into place. Like the AIC, AECs may be hollow and be provided with adaptors enabling administration of oxygen, including from a high-pressure source. Airway exchange catheters have been associated with barotrauma, which may be serious or even fatal, and this is best avoided by: (a) never inserting the tip of the AEC beyond the carina (approximately 25 cm); and (b) not using the AEC for oxygen administration, but only for tracheal tube exchange. Some AECs have soft tips, but it is not known if this reduces complications.

Airway exchange catheters may also be used to facilitate high-risk tracheal extubation. In this setting either the AEC or a firm wire (over which the AEC can be railroaded) is left in the trachea after tracheal extubation until the airway is stable. The AEC should be fixed in place to avoid displacement or inward migration. Oxygen should be delivered by alternative routes.

Preoxygenation

At the core of safe anaesthesia and airway management is maintenance of oxygenation. After induction of anaesthesia, apnoea is common, and a degree of atelectasis (worsened by supine position, obesity, abdominal distension, etc.) means that hypoxia will occur rapidly if this is not prevented by administration of oxygen via a patent airway. The time until critical hypoxia occurs (the safe apnoea time) is dramatically prolonged by preoxygenation (administering oxygen to replace the nitrogen in the lungs with oxygen – also termed denitrogenation) and by per-oxygenation (administering oxygen after loss of consciousness).

Safe apnoea time can be prolonged by:

• sitting the patient upright;

• preoxygenation: administering 100% oxygen for 3–5 min via a closely fitting face-mask during tidal ventilation;

• per-oxygenation:

  • oxygen delivered by standard nasal cannulae at up to 15 L min ^–1 ;

  • oxygen delivered via a buccal or pharyngeal cannula or tube;

  • high-flow humidified nasal oxygen (HFNO) at 60–70 L min ^–1 ;

• mask ventilation after induction of anaesthesia; and

• CPAP.

Importantly, each of these techniques relies entirely on a patent airway to transmit oxygen from the upper airway to the alveoli.

Use of the face-mask

Face-mask ventilation is part of most anaesthetics but is now rarely used throughout surgery, having been replaced by the use of a SAD. In most settings the patient will be breathing spontaneously via a face-mask and anaesthetic circuit until the point of loss of consciousness; the airway must then be maintained and ventilation of the lungs assisted manually.

If the patient is breathing spontaneously, the anaesthetist need only maintain a clear airway. Soft tissue indrawing in the suprasternal and supraclavicular areas is evidence of upper airway obstruction, as is noisy ventilation or inspiratory stridor. Capnography should be used to confirm the airway is clear and ventilation adequate.

Face-mask ventilation is a core anaesthetic skill, but its difficulty should not be underestimated: honing the skill requires training and practice. The patient's head and neck should be maintained with the lower cervical spine flexed and the upper cervical spine extended (variously termed ‘sniffing the morning air’, ‘first sip of beer’ or ‘flextension’) ( Fig. 23.32 ). Face-mask ventilation requires the combination of:

• establishing a seal between the mask and the face;

• maintaining a clear upper airway; and

• ventilation of the lungs.

Using a C -grip, the thumb and first finger are used to hold the mask, pushing downwards, while the remaining three fingers pull the chin and jaw (with fingers holding the bony elements of the mandible) and soft tissues up into the mask and also maintain head and neck positions. Manual ventilation is performed with the anaesthetist's other hand ( Fig. 23.33 ). Where there is difficulty (novice anaesthetist, wrong face-mask size, obese or hirsute patient) it may be necessary to use two people (one maintaining position and face-mask seal and one manually ventilating) ( Fig. 23.34 ) or three people (two maintaining the airway and seal and one manually ventilating) ( Fig. 23.35 ). Patients for whom obtaining an adequate mask seal is often problematic include those who are edentulous bearded and/or require high ventilation pressures, such as the morbidly obese.

If there is airway obstruction, an oropharyngeal, nasopharyngeal airway or SAD may be of benefit. A nasopharyngeal airway is tolerated at the lightest plane of anaesthesia followed by an oropharyngeal airway and then a SAD. Insertion at an inadequate depth of anaesthesia can precipitate coughing, breath holding, retching, vomiting or laryngospasm.