Chapter 13 – Supraglottic Airways

Abstract

Supraglottic airways (SGAs) have been part of core anaesthetic practice for approximately 30 years, since the introduction of the classic laryngeal mask which is described in some detail. In many countries an SGA is now used for airway management for the majority of anaesthetics. Optimal use and insertion technique are described. The range of SGAs available and their roles have expanded dramatically in this time. Second generation SGAs are those designed to reduce the risk of aspiration and are emphasised in this chapter. With the wide range of SGAs available it can be problematic to decide which device to use and this is especially so as the boundaries of acceptable use are ever widened. This chapter describes those devices with the greatest versatility and utility – arguably also with the greatest safety profile too – and some newer devices. SGAs have a major role to play in advanced and difficult airway management. The use of an SGA as a rescue device and as a conduit for intubation during management of the difficult airway is described in detail.

Chapter 13 Supraglottic Airways

Tim Cook

The term supraglottic airway device (SGA) describes a group of airway devices designed to establish and maintain a clear airway during anaesthesia. SGAs’ wider roles include use during airway rescue in or out of hospital, during cardiopulmonary resuscitation and as a conduit to assist (difficult) tracheal intubation. For this reason, a full understanding of the currently available devices and techniques is fundamental to modern anaesthetic practice. SGAs’ roles are described in many other chapters in this book: this chapter aims to fill the gap of information not provided elsewhere, including areas of controversy.

Prior to 1998 almost all general anaesthesia was conducted with face mask or tracheal tube. The classic laryngeal mask airway (cLMA) was introduced in 1988 and rapidly transformed the way in which anaesthesia was practised through much of the world. In many countries, SGAs are used to maintain the airway in the majority of anaesthetics, but usage varies considerably by country. There has subsequently been an explosion of new SGAs designed to compete with the cLMA. These devices vary considerably in the degree to which they have been evaluated before and since marketing. Several devices have been modified multiple times since introduction so interpretation of literature on device performance must be made with great care to ensure the device reported on is the device currently produced. There are currently too few comparative trials between devices to determine the clinical role (if any) of some devices. The ProSeal LMA (PLMA), i-gel and laryngeal tube (LT) are the most extensively investigated of the newer SGAs. The LMA Supreme and LMA Protector are also supported by an increasing body of evidence.

The majority of SGAs are designed to lie with their tip at the origin of the oesophagus, effectively ‘plugging’ the oesophagus. A seal around the larynx is then achieved by a more proximal cuff, which acts to elevate the base of the tongue, lift the epiglottis and seal the oropharynx. Some cuffs encircle the laryngeal inlet while others simply lie above it. All conventional SGAs are inserted beyond the epiglottis and may lead to downfolding of the epiglottis and airway obstruction. One of the design challenges of these devices is to enable placement of the distal portion of the device behind the cricoid cartilage without displacing the epiglottis during insertion.

Efficacy, Safety and New SGAs

As described above, SGAs may have a number of roles, and different designs and performance characteristics may be required for each role. In anaesthesia the cLMA was originally used almost exclusively for brief, peripheral operations, performed in slim patients, usually breathing spontaneously.

SGAs may be used for most operations. Contraindications include:

Major oral or pharyngolaryngeal pathology
Increased risk of full stomach and pulmonary aspiration
Reduced lung compliance such that airway pressures above the pharyngeal seal pressure are required
SGA would interfere with access to the surgical site
Surgery prevents access to the SGA during surgery

All are relative contraindications and require judgement and interpretation. Thresholds for use will therefore vary with device, clinician experience and individual opinion.

SGAs have become increasingly used for longer, more complex operations and in obese patients, during laparoscopic (and even open) abdominal surgery, during controlled ventilation and even in the prone position by some practitioners.

All these expansions of use offer potential benefit to patients but also raise questions of efficacy and safety. In low-risk patients the cLMA provides a clear airway and enables hands-free anaesthesia in > 95% of uses. The efficacy of the cLMA and many other SGAs for controlled ventilation diminishes rapidly as ventilatory compliance decreases (e.g. obesity, laparoscopy etc.) and both hypoventilation and gastric distension increase. This raises concerns over the safety of its use in these situations.

In the last two decades more than 40 new SGAs have been introduced. The majority are simply attempts to copy and compete with the cLMA: of interest to purchasers and managers. Of more interest to the anaesthetist are those designed to improve performance (efficacy and safety) and thereby increase the clinical utility of SGAs.

Efficacy depends on a number of factors including ease of insertion, manipulations required to maintain a clear airway throughout anaesthesia and tolerance during emergence. Efficacy during controlled ventilation requires the ventilation orifice of the SGA to sit over the larynx and that the SGA seals well with the laryngopharynx (airway seal).

Safety reflects the risk of complications occurring at all stages of anaesthesia and afterwards. Prevention of aspiration requires a good quality seal with the pharynx and oesophagus (oesophageal seal) to (i) prevent gas leaking into the oesophagus and stomach and (ii) prevent regurgitant matter passing from the oesophagus into the airway. A functioning drain tube will enable regurgitant matter to bypass the larynx and be vented outside: both protecting the airway and giving an early indication of regurgitation.

There are now numerous SGAs that are effective. The wise anaesthetist will choose the SGA they consider most safe and this becomes evermore important as the boundaries of clinical use of SGAs expand. Randomised controlled studies (RCTs) comparing different SGAs are generally inadequately powered to determine even efficacy, usually being powered against airway seal rather than clinical utility. RCTs cannot address the issue of safety directly, as the study size would need to be impractically large, and this needs a different approach. Another problem with SGA RCTs is that most are performed in low-risk patients (elective, slim, ASA 1–2) undergoing low-risk surgery (brief, supine and non-laparoscopic) and exclusively managed by experienced staff. These studies are useful in informing us of the basic efficacy of a device but translating the results of these studies (even in meta-analysis) to modern advanced use (either efficacy or safety) is not possible. Databases of many thousands of ‘real-life’ uses and registries of complications may provide more useful information on safety.

Classification of SGAs

There are several classifications of SGAs with most based on device anatomy and positioning. These are useful for designers but add little to the practical understanding of SGA use.

The author has divided SGAs in to first and second generation SGAs, and this is the most widely used classification. First generation SGAs (e.g. cLMA) are simple airway tubes. Second generation SGAs in contrast have specific design features to reduce the risk of aspiration (and are often more effective for controlled ventilation). It is important to note there is no ‘third generation SGA’ in this classification and when an SGA is described as third generation it is simply a marketing tool.

The main SGAs of interest are listed in Table 13.1, though the list is not exhaustive, and illustrated in Figure 13.1.

Table 13.1 Types of supraglottic airway – classified as first or second generation

Device	Comments
Second generation SGAs
ProSeal LMA	Described in the text. Silicone construction, reusable. High airway and oesophageal seal. Large drain tube. Extensive evidence base.
i-gel	Described in the text. First cuffless second generation SGA. Single use. Very easy insertion and low rate of sore throat. Moderate airway seal and low oesophageal seal. Effective conduit for intubation. Widespread use for CPR and out of hospital.
LMA Supreme	Described in the text. Single-use PVC device. Rigid curved stem, with central drain tube surrounded by two small airway lumens. Moderate airway seal. Not suitable for FOB-guided intubation.
LMA Protector	Described in the text. Single-use silicone device. Semi-rigid curved stem. High airway seal. Large airway tube facilitates use as conduit for intubation. Large drain tube.
Ambu AuraGain	Plastic SGA, with some features of ProSeal and of ILMA. Semi-rigid curved stem. Limited evidence base supports efficacy.
SLIPA	Described in the text. Atypical second generation SGA. No drain tube. Relatively large internal volume designed to act as ‘sump’ in event of regurgitation. Relatively high airway seal. Complex sizing process and concerns over trauma limit penetration of market.
Guardian CPV	A single-use, silicone SGA of very similar design to the PLMA and the first to use the Cuff Pilot. Minimal evidence base.
Baska mask	Cuffless, silicone construction. Three lumens: one airway tube opening to anterior of thin soft mask portion; two opening to distal posterior portion of mask, one as drain tube and one designed to have suction continuously applied. Changes in design and negligible current evidence base limit conclusions about efficacy or safety.
Eclipse	Cuffless design, not dissimilar to i-gel. Single use. Two material design aims to create soft mask portion with high seal and semi-rigid stem facilitating insertion. New entrant to market, currently without evidence base.
Combitube and Easytube	Forerunners of modern second generation SGAs. The Combitube comprises two tubes akin to two tracheal tubes of different lengths joined together, with distal and proximal cuffs. If the longer (tracheo-oesophageal) tube enters the glottis, the distal tube is inflated and the device is used as a tracheal tube. If the distal tube enters the oesophagus (it does 98% of time) the upper cuff is inflated in the pharynx and ventilation occurs through holes at the end of the shorter (pharyngeal) tube. Previously popular in out-of-hospital use, especially in North America. Now largely superseded. Cost, high rates of trauma and possibility of ‘failing dangerous’ generally preclude from use in anaesthetic practice.
Combitube and Easytube	The Easytube is a very similar, but more recently introduced device.
Laryngeal tube suction II/LTS-D	Described in the text. PVC construction. A development of the laryngeal tube with a second (posterior) tube running parallel to the airway tube to act as a drain tube. Reusable and single-use versions.
Gastro-LT and LMA Gasto	Relatively new versions of the LTS-D and LMA Protector, respectively, with dramatically expanded drain tube and reduced lumen airway, designed to accommodate an endoscope and so facilitate upper gastrointestinal endoscopic procedures.

First generation SGAs
Classic (cLMA)	Described in text in detail. The template for modern SGAs. Very extensive evidence base.
LMA Unique (uLMA)	Single-use version of the cLMA, made of siliconised PVC. Similar performance to the cLMA, but modest increase in failure rates and trauma.
Flexible LMA	Described in the text. Mask portion as per cLMA. Wire-reinforced stem makes it flexible and enables use in shared airway surgery without displacement. Extensive evidence base.
All LMs	The term ‘laryngeal mask’ (LM) refers to devices based on the ‘laryngeal mask airway’ made by a different manufacturer. Numerous versions, including flexible LMs. All differ somewhat from the original LMAs. Many are poorly or not evaluated in clinical practice.
AirQ blocker	This is a version of the ‘masked laryngeal airway’ (a high-performing cuffless LM) that incorporates a short posterior channel to facilitate passage of an orogastric tube (OGT) or an ‘oesophageal blocker’ (a short cuffed catheter). The channel does not extend as far as the mask portion of the device, so serves no function other than passage of the OGT and provides no tangible benefit if no OGT is passed. As such it is a first generation device.
Laryngeal tube	Described in the text. A narrow airway tube with proximal and distal cuffs (designed to sit in the oropharynx and oesophagus) and airway orifices lying between these through which ventilation takes place. A family of devices exist, with the range matching that of the LMA family. Uncertain evidence base.
CobraPLA and Tulip	The Cobra perilaryngeal airway (CobraPLA) is a single-use soft plastic SGA designed for use during anaesthesia. It has a wide tubed design with a proximal (pharyngeal) balloon and distally a soft plastic head (shaped a little like a cobra head) designed to seat in and seal the hypopharynx. The head’s anterior surface should abut the larynx and has a grille of soft bars, which are easily displaced for airway instrumentation. The Tulip airway is a single-use SGA comprising a tube which lies in the upper airway and a circumferential cuff that seals within the oropharynx. The airway does not extend into the hypopharynx or oesophagus. In this regard it strongly resembles the COPA (cuffed oropharyngeal airway) which was available in the early 1980s but superseded by the cLMA. It is marketed for airway management during resuscitation, including by bystanders, rather than anaesthesia. A concern with both the CobraPLA and Tulip airway is that they seal the oro/ hypopharynx but do not seal below the larynx. There is therefore a concern that, especially during controlled ventilation, their use may lead to gastric inflation. If regurgitation occurs there is no route of egress and the designs might be judged to increase the risk of pulmonary aspiration (0 generation SGA).

SGAs designed for intubation
Intubating LMA	Described in the text. Single-use and reusable versions. Includes specific tracheal tube and stabilising device to assist removal of SGA after removal.
LMA classic excel	Based on the cLMA but with a wider stem, easily detachable connector and with the grilles replaced by an epiglottic elevator. Available in North America only.
Ambu Aura-i	A first generation LM designed for routine use and for tracheal intubation.
Ambu AuraGain	A second generation LM designed for routine use and for tracheal intubation.
iLTS-D	An LT designed specifically for intubation. The only second generation intubating SGA.

FOB, flexible optical bronchoscope; ILMA, intubating LMA; SLIPA, streamlined liner of the pharynx airway.

Figure 13.1 (a) A number of SGAs are illustrated here. From left to right: Baska mask, LMA protector, Ambu Aura-i, intubating LMA, Gastro-LT, laryngeal tube suction II, laryngeal tube, classic LMA, ProSeal LMA, LMA Supreme, Guardian CPV, Cobra perilaryngeal airway, i-gel, Streamlined liner of the pharynx airway (SLIPA). (b) The Eclipse.

The cLMA is considered first as it is the SGA template, then the second generation devices as they are now of greater importance than first generation devices.

Classic LMA (cLMA)

The cLMA was designed by Dr Archie Brain in the United Kingdom in the 1980s and marketed in 1988. It has been used in well in excess of 200 million anaesthetics globally and there are several thousand studies published on the device. Consequently, the cLMA must be considered the SGA against which all others are judged. It is one of several members of the ‘LMA family’ (Figure 13.2).

Figure 13.2 LMA family. From left to right: classic LMA, flexible LMA, ProSeal LMA with introducer attached, ILMA tracheal tube, ILMA and ILMA stabilising rod.

The cLMA consists of a transparent silicone tube with an oval-shaped silicone mask at the distal end. The mask has an anterior cuff, with pilot balloon and its posterior surface is reinforced to prevent folding. Across the distal end of the airway tube are two soft silicone bars that prevent the tongue impeding insertion and the epiglottis causing obstruction after placement. Correctly placed the mask lies with the airway orifice facing anteriorly, the tip at the origin of the oesophagus and the cuff encircling the laryngeal inlet. The lateral cuff lies against the pyriform fossa and the proximal cuff the base of the tongue. The mask is held in a stable position by the hypopharyngeal constrictor muscles laterally and cricopharyngeus inferiorly. Inflation of the mask cuff produces a low-pressure seal around the larynx.

The cLMA is designed for use up to 40 times. Standard masks are not MRI compatible due to metal in the pilot tube valve. MRI compatible masks are available with a metal-free valve and are colour coded with yellow pilot balloons.

Practicalities and Routine Use

There are eight sizes of cLMA available (1, 1½, 2, 2½, 3, 4, 5, 6) for use from neonates to large adults. Features of each size mask are described in Table 13.2. Size selection is according to patient weight in children (up to size 3) and in adults, as a ‘rule of thumb’ in Western patients a size 4 is used for adult women and a size 5 for adult men. In smaller Asian patients one size smaller may be appropriate. Using a larger size cLMA for a given patient is likely to result in an improved airway seal, but may increase minor airway trauma.

Table 13.2 Characteristics of cLMAs

Size	Patient group	Weight range (kg)	Maximum cuff volume (mL)	Length of airway tube (mm)	Maximum size of tracheal tube that will pass
1	Neonate	< 5	4	108	3.5
1.5	Infant	5–10	7	135	4.5
2	Child	10–20	10	140	5.0
2.5	Child	20–30	14	170	6.0
3	Child/small adult	30–50	20	200	6.5
4	Adult	50–70	30	205	6.5
5	Adult	70–100	40	230	7.0
6	Large adult	> 100	50	–	–

Length of airway tube: from connector to grille.

Maximum size of tracheal tube: based on an uncuffed, well lubricated, Portex blue line tracheal tube, without forcing.

The cLMA is designed to be used during spontaneous or controlled ventilation. It is an alternative both to anaesthesia with face mask or tracheal tube. Its introduction transformed the routine practice of anaesthesia and face mask anaesthesia is now rarely practised. Good case selection is the first key to successful, safe use, with good insertion technique being the second.

Importantly, like all SGAs, the cLMA does not reliably protect against aspiration of regurgitated gastric contents and is contraindicated for patients who are not starved or who may have a full stomach. The cLMA seals with the pharynx to a pressure of 18–22 cmH₂O and the oesophagus to a pressure of 30–50 cmH₂O but has no drain tube. The cLMA does provide good protection from pharyngeal secretions above the cuff entering the larynx.

The depth of anaesthesia required for cLMA insertion is greater than that needed for insertion of a Guedel airway, but less than for several other SGAs or for tracheal intubation. Prior to insertion there should be no eyelash reflex and no response to purposeful jaw thrust. Propofol is the ideal anaesthetic induction agent for insertion as it profoundly reduces airway reflexes (in contrast to thiopentone). Addition of a rapidly acting opioid, intravenous lidocaine (up to 1.5 mg kg⁻¹) and nitrous oxide each improve insertion conditions. Neuromuscular blocking drugs are not needed. The cLMA may also be inserted with topical anaesthesia of the airway or bilateral supraglottic nerve blocks. Insertion causes minimal haemodynamic response.

Insertion is best performed with the patient in the ‘sniffing position’ (flextension). The cLMA should be visually inspected, free from foreign bodies and the mask completely deflated. Failure of a cLMA to maintain complete deflation indicates a cuff leak.

The smaller the leading edge of the device is, the less likely it is to catch on the tongue or epiglottis during insertion, both of which may impair placement. Full deflation enables the cLMA to slide behind the cricoid cartilage. The posterior of the mask should be well lubricated. During insertion the non-intubating hand holds the head to prevent flexion of the head. The airway is held like a pen, with the index finger placed at the anterior junction of the airway tube and mask. The mouth is opened, the mask inserted and its posterior aspect pressed onto the hard palate. The index finger is then advanced towards the occiput, which causes the cLMA to pass along the roof of the mouth and then the posterior pharynx (Figure 13.3). The device is advanced in a single smooth movement until it is felt to stop, on reaching the cricopharyngeus muscle. Chin lift or jaw thrust applied by an assistant aids insertion. Once fully inserted the tube should be held while the intubating finger is withdrawn.