An approach to evaluating supraglottic airways for routine and special-purpose use in airway management is needed. A new classification is proposed according to the sealing mechanism.
Supraglottic airways is a general term that includes airways with and without sealing characteristics. There are currently at least 17 sealing supraglottic airways that have been described. An orderly approach is needed when assessing the merits of each.
Three main sealing mechanisms are identified: cuffed perilaryngeal sealers, cuffed pharyngeal sealers, and cuffless anatomically preshaped sealers (Table 1). Features that could provide a way of further subdividing the three main groups of sealing mechanisms are single-use versus reusable and whether the device offers protection from aspiration of gastric contents.
Three figures illustrating the three sealing mechanisms are shown. In all three figures, the airway pressure in the pharynx proximal to the sealing site determines the force vector. The resultant force vector is always perpendicular to the mean plane of sealing and is always labeled E, because it is an expulsive force. The maximum seal pressure is determined by the opposing resultant frictional force, F, derived from the forces that can be exerted on the surrounding supraglottic tissues divided by the sealing area. These are usually made of forces against the tongue and posterior and lateral pharyngeal walls.
In each of the three figures, the force vectors allow us to compare the magnitude and direction of the expulsive force E. In the case of the laryngeal mask (LM) (Fig. 1A), because of the direction of E, the magnitude of the frictional force vector F is less than in the other two groups, with a consequent lower seal pressure. The ProSeal™ LMA (PLMA) (Fig. 1B) overcomes this limitation by means of its posterior pharyngeal cuff, which enlarges the force vector that pushes against the posterior pharyngeal wall, the P vector, and enlarges an opposing force vector derived from the base of the tongue, the T vector (similar to pharyngeal cuff sealers), with a resultant enlarged value for F. Increased frictional resistance (F) of the perilaryngeal cuff occurs as it pushes against the base of the tongue. In the case of the pharyngeal cuff sealers (Fig. 2), F is larger because it is derived from the force vectors of P and T that are shown in opposing directions, thus providing a mechanical advantage. The anatomically preshaped sealer (Fig. 3) provides for similar direction of forces as in Figure 2, except that, in addition, there is the force provided by the “heel” of the Streamlined Liner of the Pharynx Airway (SLIPA™) in the nasopharynx, the frictional force vector F1 that adds to F.
Cuffed Perilaryngeal Sealers
The classical example of this is the laryngeal mask airway (LMA), for which the sealing mechanism comprises a cuff that surrounds the entrance to the larynx.
Perilaryngeal Cuffs (Without Directional Sealing)
Reusable: LMA-Classic™ and Intubating Laryngeal Mask (ILMA; The Laryngeal Mask Company Ltd., Maidenhead, UK); Laryngeal Airway Device (Marshall Products Ltd., Bath, UK).
Single-use: LMA-Unique™ (Intavent Orthofix Ltd., Maidenhead, UK); SoftSeal™ LM (Portex, Hythe, UK) and many others pending.
Mechanism of Sealing.
This mechanism comprises a perilaryngeal cuff that relies on the simple apposition of the cuff that surrounds the larynx. The limitation in seal pressure is due to the direction of the resultant forces of the cuff that produce a somewhat insubstantial junction between cuff and mucosa. The ILMA may improve the seal by virtue of the stiff conduit. This may assist by increasing the mechanical force applied to the sealing cuff. In one sense, it is possible to improve the seal of the ILMA by means of a directional force via the stiff conduit. This application may involve levering off the posterior pharynx and may contribute to sore throats.
There is minimal risk of precipitating laryngospasm on insertion of the LM because of the backward-slanting aspect of the cuff in the collapsed state. This, in effect, allows the pointed aspect of the LM to slide off the glottis into the entrance of the esophagus. There is excellent tolerance at light planes of anesthesia, and there is the theoretical ability of the LM to seal the entrance to the esophagus to prevent airway gases from passing into the esophagus during positive pressure ventilation. LMA-Unique and SoftSeal LM are single use, with an associated zero risk of cross-infection.
Disadvantageous features include a low seal pressure that may be inadequate for positive pressure ventilation (1). There is very little storage of any liquid that might enter the bowl of the LM before aspiration is likely to occur (2), and regurgitation liquid is likely to pass anterior to the cuff into the bowl of the LM rather than posterior to it (3). This is due to the forces at the pointed aspect of the cuff in the esophagus. The natural radius of curvature of the tube of the LM exceeds the required bending radius once the LM is in position. This results in a greater force in the posterior as opposed to the anterior aspect of the entrance to the esophagus (Fig. 4). In Figure 1, the kink in the middle of the perilaryngeal cuff of the LM in the Ambu trainer provides a visual illustration of these forces. An increased risk of gastroesophageal insufflation has been noted (4) that is probably due to this mechanism. This somewhat nullifies the theoretical esophageal sealing advantage. In addition, the backward-slanting profile of the collapsed cuff increases the likely tendency, on insertion of the LM, for it to get caught at the back of the mouth. This may necessitate insertion by using a gloved hand to push the device against the hard palate to ease insertion. Alternatively, the technique of twisting the partially inflated device sideways is helpful.
Perilaryngeal Airways with Directional Sealing Cuffs
Reusable: PLMA (The Laryngeal Mask Company Ltd.).
Single-use: GO2 Airway (Augustine Medical, Inc.), formerly referred to as the Glottic Aperture Seal (5).
Mechanism of Sealing.
The sealing site is around the entrance or at the entrance to the larynx. The quality of the seal is enhanced by means of a directional sealing cuff that is designed to push directly against the glottis off the posterior pharyngeal wall. The result is a better seal. The perilaryngeal cuff of the PLMA provides a larger seal area than the rim or foam pad of the GO2 airway that seals the glottic opening (5). Generally, the smaller the seal area, the better the seal pressure achieved for any given cuff inflation pressure.
Higher seal pressures are achieved than with the LMA (5,6). There is less risk of regurgitation liquid getting into the airway channel, because of improved sealing pressures. In the case of the PLMA, the drainage tube is effective in preventing gastroesophageal insufflation and allows regurgitation liquid to escape via the drainage tube (7,8). However, occasionally gastroesophageal insufflation does occur (9), and if the rate of regurgitation is excessive, aspiration may occur (8).
The GO2 Airway has been withdrawn from the market because of difficulty in reliably locating the glottic aperture accurately; this resulted in a small probability of successful insertion. The more rounded profile of the PLMA, by virtue of the inflated posterior cuff combined with the absence of a preformed pharyngeal curve but instead flexible conduits, means that it may more easily rotate out of position than the LMA-Classic. Malpositioning the PLMA can result in occlusion of the distal orifice of the drainage tube secondary to folding of the tip of the cuff during insertion. When this happens, there is a distinct vulnerability to aspiration (10). Finally, the PLMA is not for single use.
Cuffed Pharyngeal Sealers
Mechanism of Sealing.
The mechanism of sealing comprises an airway with a pharyngeal cuff that seals at the base of the tongue; it consists of a simple cuff surrounding a tube. Here, the forces that achieve an effective seal are approximately perpendicular to the direction of the airway channel and the direction of potential airway expulsion. This results in an increased frictional resistance to oppose the force of expulsion as airway pressure increases.
Without Esophageal Sealing Cuffs
Single-use: Cuffed Oropharyngeal Airway (COPA®; Mallinckrodt Medical, Athlone, Ireland); PAxpress™ (PAX; Vital Signs Inc., NJ); CobraPLA™ (Engineered Medical Systems, Indianapolis, IN).
Single-use: The principle of sealing within a “bounded” tubelike space means that better sealing pressures (4,11,12) may be achieved than with perilaryngeal cuff sealers, because the pressure within the cuff is exerted perpendicular to the airway channel (an exception being the COPA airway) (13), thereby effecting a mechanical advantage not known in the simple perilaryngeal cuff sealers.
There is no sealing of the downward outlet into the esophagus with the theoretical possibility of pumping air into the esophagus during positive pressure ventilation. Reliance is solely placed on the tone of the gastroesophageal sphincter. There is no protection from aspiration should passive regurgitation occur (14). There may be a less specific location of the cuff, with a more frequent need to adjust the airway or a more frequent loss of airway after changes in cuff pressure (e.g., with nitrous oxide diffusion) or adjustments of position of the head and neck (15). In the case of the PAX airway, this is compensated for by the use of an esophageal locator (appearance of gills) to prevent this. The compensatory esophageal locating mechanism in the PAX airway would appear to be the design feature that contributes to mucosal trauma and sore throats (11,16).
With Esophageal Sealing Cuffs
Reusable: Laryngeal Tube® (VBM Medizintechnik GmbH, Sulz a.N, Germany) (17); Laryngeal Tube Suction (VBM Medizintechnik GmbH) (18); Airway Management Device (AMD™; Nagor Ltd., Isle of Man; Biosil Ltd., Cumbernauld, UK); Elisha Airway Device™ (EAD).
Single-use: Combitube® (Tyco-Healthcare/Sheridan, Argyl, NY); Easytube® (Willy Rüsch AG, Kernen, Germany).
Apart from the Laryngeal Tube, these airways provide a means for minimizing the aspiration risk by providing access to the esophagus. The access provided by the AMD is provided only if a gastric tube is inserted through the lower cuff and into the esophagus, which is a substantial limitation, particularly if there is difficulty in passing the tube (19). It may be advisable, therefore, to always insert a gastric tube before inserting the device (20). Both the Combitube and Easytube are single use, which obviates infection risk. The Elisha Airway Device is anatomically shaped to provide a very specific location, making for a likely stable single position with each insertion without requiring further manipulations once it is positioned.
The Combitube has a rather rigid pharyngeal balloon, and the tube is stiff, creating the potential for traumatic insertion (21–23). The Combitube has a cardiovascular response to insertion that exceeds that of inserting a tracheal tube and may be a serious hazard to patients with cardiovascular disease (24). Congestion of the tongue is a possibility with excessive increases in cuff pressure and the potentially associated lingual nerve damage [a problem especially noted in cuffed devices (25), particularly if they are permeable to nitrous oxide] (21,26–28). There may be variability of cuff location in some of these devices, especially if there is no obvious curve that fits the natural curvature of the pharynx. The result may be a more frequent need to adjust the airway or a more frequent loss of airway after changes in cuff pressure (e.g., with nitrous oxide diffusion) or adjustments of the position of the head and neck (6,15). The AMD has suffered from an unacceptable incidence of intraoperative loss of the airway (29,30). This may be minimized with inflation of the lower cuff first, before insertion, with better location of the device. The AMD has a long pharyngeal cuff with a persistent tendency to slip out of position and to lose the airway during surgery. Perhaps this finding will change with new insertion instructions (31).
Cuffless Anatomically Preshaped Sealers
Mechanism of Sealing.
This mechanism is an anatomically preshaped hollow airway that provides for sealing the outlet from the pharynx at the base of the tongue and the entrance to the esophagus by virtue of the resilience of the walls of the shaped airway. The pressure inside the airway assists in keeping the airway shape as the pressure increases. Two mechanisms and two forces (F and F1 in Fig. 3) resist the expulsive force as the airway pressure increases. Both forces are derived from the anatomical shape of the airway. This makes for a very stable airway once it is in position.
Single-use: SLIPA or standard SLIPA and SLIPA wedge.
Aspiration protection is provided by means of the hollow structure that allows for effective storage of regurgitation liquid within the device—a sumplike effect (8). It may be safe to suction within the device if regurgitated liquid is present with a negligible risk of precipitating laryngospasm. Being anatomically preshaped makes for very specific and obvious correct or incorrect placement and a stable airway once it is in position. Because there is no cuff, there is enhanced simplicity of use and no influence on sealing pressure by nitrous oxide. There is no risk of cross-infection because it is for a single use. Finally, it may be collapsed in the anterior-posterior plane so that it may be inserted through a narrow space between the teeth if there is a stiff jaw.
Skill and experience are required for more careful selection of size, because there is no expandable cuff. There is less flexibility in this design than with cuffed devices; thus, it is contraindicated if upper airway anatomy is clearly abnormal or distorted in the case of the standard SLIPA.
Provisional Scoring of Airways
The above approach to supraglottic airways provides a useful framework for creating scoring systems appropriate to desirable objectives. For example, a supraglottic airway that will have multiple applications in diverse circumstances, such as for application on a difficult airway trolley, will be different from scoring airways with the objective of routine use only. The core requirements relevant to routine airway management will of course be much simpler and may or may not be included in the requirements for difficult airways.
“Core” desirable features, which practitioners would consider appropriate to have in any supraglottic airway suitable for routine use, are listed below.
1. A noninvasive (supraglottic) airway conduit.
2. Easy insertion, even by a nonspecialist.
3. Good first-time insertion success rate.
4. Stable airway once positioned, i.e., a reliable hands-free airway.
5. Sufficient sealing quality to apply positive pressure ventilation.
6. Minimal associated risk of aspiration.
7. Minimal risk of cross-infection, i.e., a single-use device.
8. Minimal risk of serious side effects.
In the discussion surrounding the above classification, the assessment of airways from the point of view of being used as routine airways has already been discussed. The results can be seen in Table 2.
No attempt is made to use a similar approach to put a value on a suitable airway for special advanced airway requirements or multipurpose applications, because it is unlikely to succeed with the background complexity of the subject of difficult airway management. It may be more useful to view the additional features separately. These may include suitability of the airway for application in (a) the presence of upper airway abnormalities, (b) for introducing a fiberscope, (c) for intubating patients, and (d) for access to the esophagus. To generate a scoring system for these requirements is probably inappropriate because the related circumstances are complex and too individual to permit much usefulness.
Cuffed Perilaryngeal Sealers
The LMA has an excellent record for circumstances of failed intubations and for use in difficult airways (32). Under these circumstances, it is invaluable for the quick and simple introduction of the fiberscope into the trachea. However, the supplied tube may be too narrow for introducing an endotracheal tube (33). The ILMAFastrach was developed specifically for the purpose of intubating blindly through it. In routine cases, this is approximately 88% successful after two attempts and is particularly useful for difficult airway management, for which the overall success rate is reported as being approximately 98% (34). Unlike the LMA, it is as stimulatory on the cardiovascular system as conventional intubation (35). The rigid accentuated curve-shaped stem of the ILM provides a means of providing enhanced directional sealing against the glottis. The PLMA is not quite as good as the LMA for fiberscope applications. For the insertion of a nasogastric tube, it has been nearly 100% successful (36,37).
Cuffed Pharyngeal Sealers
In the presence of upper airway abnormality, the ability of the pharyngeal cuff to expand into any shape to effect a seal would suggest that this group of airways might be quite effective. Alternatively, the presence of a tumor of any origin may affect the reliable location of the pharyngeal cuff and make for an unreliable position and possibly a loss of airway during surgery. Varying degrees of success with passing a fiberscope have been noted with the many different airway types in this group (11,38–40). The new Cobra airway is designed with a large conduit so that an endotracheal tube may be inserted through it (15). The Combitube has been particularly noted for its useful application in the presence of a bleeding upper airway (41). It has been modified for the purpose of applying the fiberscope with a fair degree of success, but it does take longer to insert a tracheal tube than with the LMA or ILMA (38).
Cuffless Anatomically Preshaped Sealers
The SLIPA was designed for routine use. Another version, the SLIPA wedge, may have a role in difficult airway management, but there are no data as yet.
Three basic supraglottic airway sealing mechanisms have been described. The explanation for each provides reasons for the differences observed in sealing pressures for the different types. This approach may help us to understand the mechanisms of action and to better assess whether new airways that may be introduced are likely to achieve the stated design objectives. The introduction of a scoring system for airways may be considered premature, given the limited amount of data upon which a score can be given in the case of new airways, with the result that there are many gaps (Table 2). That will always be the situation with new devices. Nevertheless, the attempt fulfills the need for a more consistent approach in evaluating supraglottic airways. It is likely to be controversial and is not devoid of subjective interpretation. The large amount of accumulated data with regard to the LMA and other airways has provided useful information on which the newer devices could base their design features to take into account the limitations of the current supraglottic airways. The apparent advantages for the newer devices remain in the realm of speculation until sufficient data are present for substantiating the respective claims.
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