The maintenance of a clear airway in an anaesthetized patient can often be achieved by a simple elevation of the jaw (jaw thrust) and/or extension of the head on the cervical spine. These movements tend to separate the tongue, epiglottis and soft palate from one another and away from the posterior pharyngeal wall (Fig. 6.1). However, in many patients maintenance of the airway in this manner is either ineffective or impractical for surgery. Patients with anatomical reasons for such manoeuvres to fail include those whose ‘pharyngeal spaces’ are absolutely or relatively small (e.g. a large tongue, small lower jaw, large diameter neck, obesity and also those with large adenoid or palatine tonsils). In such patients the obstruction must be relieved and the easiest way to achieve this is by inserting a device that separates these structures and thus creates an artificial airway. These airway adjuncts may be inserted via the mouth (oropharyngeal airway) or via the nose (nasopharyngeal airway) (Fig. 6.2).

Figure 6.1 A. The obstructed airway following the administration of a general anaesthetic if the jaw is unsupported. The airway may be obstructed by (i) epiglottis and/or (ii) tongue pressing on the posterior pharyngeal wall, or by (iii) the soft palate, occluding the oral or nasal airways. B. The effect of simple elevation of the jaw in a patient with normal anatomy.

Figure 6.2 A. An unobstructed air passage produced with an oropharyngeal airway. B. An unobstructed air passage produced with a nasopharyngeal airway.

Oropharyngeal airway

These devices are shaped to emulate and so restore the space present in the pharynx during consciousness by pushing the tongue and epiglottis away from the posterior pharyngeal wall. Oropharyngeal airways are usually oval-shaped, occasionally circular, in cross-section and are produced in varying lengths and diameters to suit different patient sizes from premature neonate to large adult. The proximal end has a flange to limit the depth of insertion and prevent its loss into the pharynx. Devices are sufficiently rigid to prevent collapse should the patient bite down and indeed they may be used as a ‘bite block’ in an intubated patient (to prevent airway obstruction as a result of the tracheal tube being obstructed by biting). When inserted, the distal end should lie at the posterior of the tongue but above the epiglottis. Insertion too deep may in itself lead to airway obstruction either by mechanically pushing the epiglottis back or by irritating the sensitive laryngeal inlet leading to laryngospasm. There is a standard colour and number coding for size. The most popular oropharyngeal airway type is the Guedel pattern (Fig. 6.3). To select the correct size of Guedel airway, the distance from the flange to the distal tip of the airway should be about the same as from the patient’s lips to the tragus of the ear.

Figure 6.3 Guedel airways in sizes 00, 0, 1, 2, 3, 4 (a smaller size 000 is not shown here).

Nasopharyngeal airway

The nasopharyngeal airway is designed to be passed through the nares and along the floor of the nose to deliver the tip beyond the soft palate to lie in the oropharynx, above the epiglottis. It can bypass nasal, soft palate and tongue base obstruction. The tubes have either a fixed or adjustable flange (Fig. 6.6) at the proximal end to prevent loss of the device into the nose and to limit insertion of an excessive length. The tip is bevelled to make its passage through the nose less traumatic. Some designs have a hole cut in the wall opposite the bevel to maintain patency if the tip becomes obstructed. They are produced in a range of sizes with the length of the airway governed by the internal diameter of the tube.

Figure 6.6 Nasopharyngeal airways (from top to bottom) Portex and Rusch designs, the lower with an adjustable flange.

Nasopharyngeal airways are often indicated where a patient has limited jaw opening, awkward or fragile dentition or where an oropharyngeal airway is frequently displaced by a marked overbite. They are well tolerated by patients during relatively light levels of anaesthesia and during emergence. To avoid traumatic insertion and heavy bleeding nasopharyngeal airways are ideally soft (plastic, polyurethane or latex rubber). Avoiding excessively large tubes also minimizes complications during use, but the tube must be long enough to extend well into the oropharynx. An estimate of appropriate length is the distance from the tip of the nose to the tragus of the ear.

The use of nasopharyngeal airways has reduced dramatically with the rise in popularity of the LMA. They retain an important role in patients in whom the oropharynx cannot be accessed and in sedated patients who would not tolerate an oropharyngeal airway.

Complications

The commonest complication is trauma causing bleeding, which can be extensive and may compromise the airway further. They should not be used where there is evidence of coagulopathy or other potential causes of epistaxis. Inappropriate sizes may either fail to clear the airway (too short) or lead to coughing, laryngospasm or retching (too long).

Facemasks

Anaesthetic facemasks are designed to fit over the patient’s nose and mouth and enable the creation of a low pressure seal. This should not require excessive force. The facemask (Fig. 6.7) consists of three parts: the mount, the body and the edge. A snug fit is achieved by incorporating one or more of the following features into the design: by anatomically shaping the body, by the use of an air-filled cuff at the edge (Figs 6.8B, D and E) that has a soft cushioning effect or by a soft pliable flap (Figs 6.8A and C) that takes up the contour of the face.

Figure 6.7 The parts of an anaesthetic facemask: A, the mount; B, the body; C, the edge.

Figure 6.8 A selection of facemasks: A. black rubber ‘Everseal’ mask with soft pliable flap (designed by MIE Ltd in 1953 for the British Everest expedition); B. ‘anatomical’ facemask in black rubber with air filled cushion; C. single-use facemask with flap; D. single-use facemask with adjustable air filled cushion; E. single-use facemask with pre-filled air cushion. Note: A, D and E carry attachments for use with a harness (see text).

The mount should be a 22 mm female taper if made to the standards of ISO (International Organization for Standardization) or BS (British Standards). It is usually constructed of hard synthetic rubber but may be plastic or metal. The former two wear more easily with repeated use and eventually produce a leak or potential for accidental disconnection.

The body may be made from black rubber, neoprene, plastic, polycarbonate or silicone rubber. In some cases, a malleable wire stiffener or wire gauze is incorporated in the body so that the shape may be altered to fit the patient’s face. The transparent body of a polycarbonate or plastic facemask permits continuous inspection of the airway and respiration to be monitored by the appearance of condensation during exhalation. This is useful during anaesthesia and particularly during resuscitation. A transparent facemask also affords the possibility of seeing regurgitant matter or vomitus emerging from the mouth. Such masks are perhaps less threatening to children and anxious adults (Figs 6.8C, D and E).

The internal volume (apparatus dead space) within the body of the facemask is relatively unimportant in adults but may assume significance in neonates and infants where it can constitute 30% or more of their tidal volume. Several designs shape the paediatric facemask to minimize the apparatus dead space (Fig. 6.9). In spite of this, good fit may be more important than theoretical apparatus dead space: paradoxically in paediatrics, a larger facemask particularly of the Rendell-Baker or Laerdal type, may allow the face to fit further into the mask with improved fit and reduced effective apparatus dead space.

Figure 6.9 The full range of Rendell-Baker paediatric facemasks, sizes 00, 0, 1, 2, 3.

The edge may be anatomically shaped and fitted with a cuff or flap. A good fit is essential to prevent dilution of administered gasses by room air during spontaneous respiration and to allow positive pressure ventilation without gas leak. A variety of types and sizes of facemask must be available, since none will be a good fit for every face. Edentulous or bearded patients may be especially difficult. The former are best managed by leaving any dentures in place to prevent the cheeks from falling away from the mask and by using a smaller mask. Beards often prevent a good seal around the edge of the mask and a leak-free fit may sometimes be achieved with a bigger mask held firmly with two hands. Anaesthetists have often had to go to bizarre lengths to achieve a useful seal in heavily bearded patients (e.g. using a pierced defibrillator gel pad on the face ³ or even wrapping the entire head in clingfilm⁴). Reusable masks with a cuff have a small filling tube fitted with a plug to enable the degree of inflation to be regulated. The plug must be removed to allow the cuff to deflate if the mask is to be autoclaved.

Whereas some facemasks withstand the high temperatures of autoclaving, others do not. Since these are not easily distinguished, many adopt uniform policies of disinfection. Care must be taken if chemical disinfection is used as some chemicals, e.g. chloroxylenol (Dettol), are known to have been absorbed by the material of the facemask and have resulted in injury to the patient’s skin.

The shift towards disposable single-use facemasks avoids the cost of sterilization (increasingly performed off-site for many hospitals) packaging, and mandatory tracking and trace systems, but risks a reduction in quality. The same principles of design nevertheless apply to plastic disposable facemasks (Figs 6.8C, D and E). Materials and components are cheaper and of lower quality, but more importantly there is a much more limited range of designs and sizes. Most, even those for paediatric use, are essentially based on the one design of air-filled cuff and cone-shaped body. As they are reproduced with differing quality and materials by numerous manufacturers, uniformity of performance cannot be assumed. They may be advertised with bold claims of high performance and popularity but formal clinical evaluation is not a pre-requisite to bringing them to market. Several new designs have been found to be substandard and it is wise to assess performance before changing devices. Sizes do not necessarily equate between manufacturers. Some single-use masks do not allow adjustment of the volume of the air-filled cuffs and these may be both under filled and of less pliable plastic leading to a poor-quality seal. Poor design or poor fit, even in the absence of a leak, can cause areas of high pressure on the skin, which if unchecked, could lead to ulceration. Single-use masks do, however, have a number of advantages beyond sterility. These include use of inert plastics (eliminating risk of allergy to latex), transparency and even the potential to add scent to the plastic making the products more readily accepted by patients.

Many anaesthesia masks are supplied with a ring device that has several lugs protruding to allow attachment of a head harness. Older black rubber masks have a similar metal harness ring (Figs 6.8A, D and E). A head harness may be used to allow facemask anaesthesia while allowing the anaesthetist to keep both hands free. Since the introduction of supraglottic airways head harnesses are rarely used, if ever, except during non-invasive ventilation (see below).

Masks for some dental anaesthetic techniques are designed to fit the nose only, so that the dentist has unimpeded access to the mouth (Fig. 6.10). They are also known as nasal inhalers.

Figure 6.10 A McKesson comfort cushion nasal inhaler.

Facemasks are also used for non-invasive ventilation (NIV) or continuous positive airway pressure (CPAP) in differing scenarios, both in and out of hospital (see Chapter 7, Fig. 7.17). These are similar to anaesthesia facemasks and may cover the mouth and nose (conventional CPAP) or just the nose (for nasal CPAP). Such masks tend to be single-patient use devices made of plastics and soft silicone rubber. They are usually of a higher quality than single-use anaesthesia masks as comfort and tolerability is the key to treatment compliance and success. CPAP masks often incorporate additional ports for valves and airway manometry and have attachments for a retention harness to keep the mask comfortably on the patient’s face.

Supraglottic airways

History

Prior to 1988 maintaining a clear airway in a non-intubated, anaesthetized patient, might well have involved elevating and protruding the lower jaw, supporting it in this position with a Guedel airway, placing a facemask over the nose and mouth and securing this with a Clausen’s (head) harness to provide a gas-tight fit: all essentially dextrous tasks requiring practised performance. Although preceded by numerous devices that were designed to control the airway without tracheal intubation, it was not until the advent of the Laryngeal Mask Airway (LMA), first described in 1983,⁵ and introduced commercially in 1988, that such devices achieved success in anaesthesia. It soon became evident that the LMA allowed all the tasks described above to be replaced with a single device requiring minimal training in its use, but with an astonishing success rate in achieving a clear leak-free airway. Within a year of its introduction all UK hospitals had placed orders: the anaesthetic ‘supraglottic revolution’ had begun.

Since that time more than 40 novel, and less novel, supraglottic airway devices (SADs) have been brought to market. Since 2003 numerous manufacturers have produced copies (or near copies) of the LMA, most of them disposable and plastic. Many new supraglottic devices are based on the general design of the LMA and several are based on the Combitube (see below), which, although in origin pre-dates the LMA,^6,⁷ was (and remains) more a device for airway rescue than for routine anaesthetic use. Other novel approaches to SAD design have met with varying degrees of success. No design has so far proven as popular as the LMA, in spite of the occasional publications claiming similar or better performance.^8,⁹

Major advances since the original LMA have been rather few but the recent evolution of devices designed with increased safety in mind, is a significant step.

Features

SADs generally have the following features in common:

• An inflatable cuff that sits over or above the level of the larynx. A minority of devices are cuff-less. Crucially, in all SADs, function relies on an open laryngeal inlet.

• ‘Blind’ insertion is possible, obviating the need for laryngeal visualization.

• They rely, for correct positioning, on a soft distal tip that lodges in or just above the upper oesophagus behind the cricoid ring. A minority sit well above this.

• There is an inability to guarantee tracheal isolation from gastric contents in the event of reflux or vomiting. This issue is increasingly being solved.

• The use of wide bore tubing for the airway tube (as this is not limited by the size of the larynx and trachea), and which can therefore be used as a conduit for other instruments such as a tracheal tube or a flexible endoscope.

• An ISO 15 mm male connector proximally enables connection to an angle piece, catheter mount or breathing system.

Terminology

The term originally coined to describe the generic group of LMAs and other similar devices is supraglottic airway devices (SADs). The term extraglottic has been suggested as more accurate by some authors.¹⁰ The term SAD has the advantage of primacy and widespread use and is adopted here. Several authors have attempted to classify SADs on the basis of patient or device anatomy but the classifications are cumbersome and not widely adopted.¹¹ The author of the previous incarnation of this chapter attempted (to no avail!) to introduce the term ‘augmented supraglottic airway’ to delineate simple oral and nasopharyngeal airways from those supraglottic devices that aim to provide a gas-tight seal to the airway.

This author has coined the phrase first-generation and second-generation SADs to demarcate those devices which are simple airway tubes (first generation), from those which are designed with the intention of reducing the risk of pulmonary aspiration of gastric contents (second generation). The proposed advantage of this classification is that it is simple, is not dependant on patient/device anatomy, but is ‘patient centred’. Second-generation devices may be designed to improve patient safety, but the effectiveness of this design feature is not proven in all cases. Second-generation devices comprise the ProSeal LMA, the Combitube and Easytube, the Laryngeal Tube Suction mark II, the Streamlined Liner of the Pharynx Airway, the i-gel and the Supreme LMA.

Pharyngeal seal and efficacy vs oesophageal seal and safety

Functionality of an SAD depends on several factors including insertion ease and success rate, manipulations required during anaesthesia to maintain airway position and patient tolerance of the device during emergence. During controlled ventilation efficacy is dependent on factors such as whether the device orifice sits over the larynx and the quality of the device seal with the laryngopharynx (pharyngeal seal). The pharyngeal or airway seal pressure is usually assessed by allowing a fresh gas flow of 3–5 l min^-1 into the closed breathing system of an apnoeic non-paralyzed patient and noting the maximal airway pressure generated or the pressure when a leak can be detected.

The safety of a SAD reflects the likelihood and severity of complications occurring at all stages of anaesthesia and afterwards. Complications include failures, displacements, airway obstruction and sequelae such as sore throat, pharyngeal trauma and nerve injuries from pressure effects. The risk of aspiration is a major concern with SADs. Moderation of this risk requires a good-quality seal with the hypopharynx and/or oesophagus (oesophageal seal) to prevent gas leaking into the oesophagus and stomach and also to prevent regurgitant matter passing from the oesophagus into the airway. Ideally oesophageal seal pressures are assessed in terms of hydrostatic pressure needed in the oesophagus to cause liquid regurgitation (hence needing cadaver studies). A correctly functioning drain tube should enable regurgitant matter to bypass the larynx and be vented externally. This protects the airway and gives an early indication of the presence of regurgitation.

Several recent studies indicate that the extent of oesophageal seal varies considerably between different SADs.¹²^–¹⁵ They also demonstrate that under experimental conditions in cadavers, those with a drain tube will usually effectively vent regurgitant fluid provided the drain tube is not occluded. Particulate matter has not been studied. These are important findings, though their clinical correlates are not in all cases confirmed.

As its name implies the classic laryngeal mask airway (cLMA) (Intavent Direct, Maidenhead, UK) is designed to act as a mask that fits over the larynx. It consists of an oval soft silicone mask that sits over the larynx with an integrated stem that extends through the oral cavity to allow attachment to the anaesthetic circuit or other appropriate equipment.

The mask (distal) end of the cLMA is made of medical grade silicone and consists of a shallow bowl resembling a small facemask, which is surrounded by an inflatable tubular cuff (Fig. 6.11). The latter, when inflated, fits around the laryngeal inlet and supports it in a position away from the posterior pharyngeal wall. The back of the bowl leads into a semi-flexible tube which passes out of the pharynx and mouth and has a 15 mm ISO male connector so that it can be attached to a breathing system. At the point the tube enters the mask, there are two thick silicone rubber strands (grilles, or bars) designed to prevent the epiglottis falling into it and occluding the lumen. The mask cuff is inflated via a pilot tube that terminates in a small ‘pilot balloon’ giving an indication of cuff inflation/deflation. A self-sealing valve prevents deflation of the cuff.