The technical development of airway devices has evolved over time and so might be viewed from an evolutionary perspective. The introduction of a simple solitary new tool initiates a totally novel category of equipment and in so doing offers yet unseen opportunities for further developments. Each significant innovation is usually followed by an avalanche of variants possessing new features and improved function. Sometimes fairly minor changes to the basic design are made primarily to capitalise on economic benefits; however, in general the remorseless laws of evolution with their basic mechanisms, mutation (variation, change) and selection, unshakeably result in the disappearance of unsuitable material and a tendency for the fittest, most successful devices to prevail. The application of a taxonomical approach when seeking to create order in the world of airway devices is not just an idle pastime. It might help to establish an insight into what is currently a very crowded field. However, taxonomy needs to be applied according to the established methods of classification developed by the famous Swedish botanist, physician and zoologist Carl Linnaeus (1707 to 1778) in the structured and logical manner seen in his monumental work of taxonomy of living creatures.1 He used taxonomic units, known as ‘taxa’ which he arranged in a hierarchical classification scheme. He developed an order of ranks to permit the categorisation of the constituent species, subspecies, families and super-families depending on the variation and distribution of common key features that he recognised as the denominators of relationship between the involved units. The resulting connection between the subjects can be seen as stemming from a common origin based on the growth of a single developmental tree with various branches of increasing complexity. Some branches effectively run directly from their early origins to recently existing forms, but many more appear to have stopped in a blind ending, thus marking the curtailment and extinction of less successful variants. The same principle can be applied to the evolution of airway management devices.
The first airway device with a proven potential for longevity was Dr Morton's ether inhaler which appeared in the middle part of the 19th century. The conduit representing the ‘interface’ between the ether-soaked sponge retained inside a glass bottle and the patient was a mouthpiece. This mouthpiece can be considered as the earliest and simplest airway device that fulfilled both basic requirements for anaesthesia: first, to connect the human airway to a gas flow (or a specific volume of air containing ether vapour as it was then); and, second, to offer at least some prospect of observing inhaled gas (or vapour) channelled into and out of the patient's airway. The simplest ‘interface’ used in current anaesthetic practice as a conduit between the patient's airway and the gas delivery system is the face mask. Hence, this can be regarded as the direct descendant of Morton's mouthpiece, and the genealogical source of the evolutionary tree of airway devices. Essentially, this is the origin from which all other airway devices have been derived.
When applied correctly, the underlying principle governing the use of a facemask is the ability to maintain a gas-tight seal. When achieved, it works well in the majority of cases. The natural limitations of the face mask (and of its precursors such as the ether masks of those times) prompted the development of a more invasive device that might reliably improve performance and patient safety. The result was the tracheal tube introduced independently and nearly simultaneously in different places by O’Dwyer and Kuhn.2,3 Initially, placement of the tracheal tube was performed in a blind fashion by direct manual insertion, but this proved to be very difficult and in times of uncontrolled reflex activity, dangerous as well! To overcome this, the first laryngoscopes comprising a handle, a blade and a light source were developed in1895 by Kirstein.4 By acknowledging the tracheal tube and the laryngoscope, we have identified the first essential instruments that have been so well conceived in their design and are of such time-honoured utility that they continue to underpin the principles governing techniques for securing the airway both in anaesthesia and elsewhere. These two instruments also represent both of the dual principles applied to airway devices; They can be classified as either ‘gas channelling’ devices to facilitate oxygen and anaesthetic gas delivery, such as the tracheal tube, or as ‘introducers’, devices that facilitate the introduction of an airway management device, normally under visual control, such as the laryngoscope. For this reason we can apply this distinction to all other airway devices and classify them either as a ‘channeller’ (by being a descendant of the tracheal tube) or an ‘introducer’ (by being a descendant of the direct laryngoscope). As usual in science, one can simplify the nomenclature of complex terms by using abbreviations and combining them to form acronyms. Likewise, this can be done in the taxonomy of airway devices by using abbreviations for typical basic features, thus facilitating the creation of certain taxonomic clades, species and their descendants and subclades. These can be organised in hierarchical order starting with domain, kingdom, phylum, class, order, family, genus and ending with distinct species. Within the species, there can be various subspecies and races.
With regard to the taxonomy of airway devices, one can allocate the first position (as first of four hierarchical levels) of the acronym a ‘C’ for all ‘channellers’ (such as tracheal tubes) or an ‘I’ for all ‘introducers’ (such as laryngoscopes). This level can be viewed as a ‘domain’, either I or C. The second level reflects the fundamental difference such as the entry place for channellers and the basic technique of introducers; this level can be called the ‘family’. The third level might represent the species and the fourth the subspecies, both based on elements that differentiate all the devices at that specific level. In contrast to the living world, here we cannot invoke the rule that the absolute requirement to produce fertile descendants defines subsequent development of the species.
Close review of the channellers identifies immediate derivatives of the original tracheal tubes through observation of their main distinguishing ‘family’ criterion: the point of introduction of the tube into the airway. Here we have three variants: the oral route (O), the nasal route (N) and the trans-tracheal route (T). This permits us to apply the respective abbreviations as CO for the orally inserted tube, CN for the nasal and CT for the tracheostomy tube. The denomination is more specific when we arrive at the ‘species’ level when we add the position of the distal opening as related to the glottis: O for ‘oral’, S for supraglottic, I for infraglottic (but above the carina) and, finally, C for beyond the carina. A reference to the distinction between supraglottic and periglottic openings is ignored here for the sake of simplicity and because the difference represents a relatively minor technical detail. A secure airtight seal can be created by blocking any leak around the device by inflating a cuff. Tubing that does not possess a cuff creates an exception and, therefore, a difference in classification. To acknowledge this distinction at the subspecies level, one can add a B for a cuffed (blocked) device and a U for an uncuffed device. According to this, the regular (uncuffed) oropharyngeal airway becomes COOU,5 whereas the now ‘extinct’ cuffed version, which briefly appeared in the 1990s only to disappear soon after, would be classified as a COOB.6 Consequently, a list of the most widespread channelling instruments with their four character acronyms can be constructed and represented as follows:
- COOU: channelling, oral insertion, oral opening, uncuffed – for example, oropharyngeal airway (e.g. Guedel tube).
- COOB: channelling, oral insertion, oral opening, cuffed – for example, the meanwhile extinct C.O.P.A. (e.g. cuffed oropharyngeal airway).
- COSB: channelling, oral insertion, supraglottic opening, cuffed – for example, laryngeal mask airway or laryngeal tube.7,8
- COSU: channelling, oral insertion, supraglottic opening, uncuffed – for example, iGel or SLIPA.9,10
- COIB: channelling, oral insertion, infraglottic opening, cuffed – for example, conventional (oro)tracheal tube.
- COIU: channelling, oral insertion, infraglottic opening, uncuffed– for example, uncuffed (paediatric) orotracheal tube.
- COCB: channelling, oral insertion, subcarinal opening, cuffed– for example, double lumen tube.
- CNSU: channelling, nasal insertion, supraglottic opening, uncuffed– for example, nasopharyngeal airway (e.g. Wendl tube).
- CNIB: channelling, nasal insertion, infraglottic opening, cuffed– for example, nasotracheal tube.
- CTIB: channelling, transtracheal insertion, infraglottic opening, cuffed– for example, tracheostomy tube.
- CTIU: channelling, transtracheal insertion, infraglottic opening, uncuffed– for example, uncuffed tracheostomy cannula.
The ‘channelling’ element of the airway devices’ family tree is represented in Fig. 1.
When we switch to the family of introducers, an analogous system can be created to represent the variants within this heterogeneous group of devices. Here we have two equally populated subgroups (families) of introducer devices: laryngoscopes (L) and stylets (S). Laryngoscopes can be divided into two species: the direct vision type (labelled D – an example of this would be the conventional laryngoscope) and the indirect type, represented by devices with an optical system (older fashion), or more recently, with a video screen, labelled with V. Meanwhile, at the species level, we have a rather large group of video assisted laryngoscopes which differ among each other by having a tube guiding facility (G) or not having this (U), thus representing two subspecies. With regard to the stylet family, we can distinguish those which are used in a ‘blind’ fashion (N for nonvisualising), including gum elastic bougies, soft tip-guidewires, tube exchangers and countless other locally used stylets. Conversely, visualising stylets (labelled V) can be either rigid (R) such as the Bonfils intubation stylet or flexible (F) such as the flexible intubation fibreoptic endoscope. A third variant of visualising stylets is the hybrid or composite type (C) in which rigid and flexible elements are integrated in one instrument, such as the SensaScope.11 As demonstrated with the channellers, we can categorise all the well known introducers in a similar manner with their respective acronyms:
- ILDU: introducer, laryngoscope, direct viewing, without tube guiding facility – for example, the classical direct laryngoscopes such as the Macintosh and Miller.12,13
- ILVG: introducer, laryngoscope, video assisted, with tube guiding facility – for example, Airtraq and Pentax AWS.
- ILVU: introducer, laryngoscope, video-assisted, without tube guiding facility – for example, Glidescope.14
- ISNR: introducer, stylet, nonvisual, rigid – for example, rigid guidewire to stiffen tracheal tubes.
- ISNF: introducer, stylet, nonvisual, flexible – for example, gum elastic bougie and COOK tube exchanger.15
- ISVR: introducer, stylet, visual, rigid – for example, Bonfils intubation stylet.
- ISVF: introducer, stylet, visual, flexible – for example, flexible fibreoptic bronchoscope.
- ISVC: introducer, stylet, visual, composite – for example, SensaScope.11
An overview of the ‘introducers’ family tree is represented in Fig. 2.
Both introducers and channellers can be combined in a single taxonomic diagram to illustrate the whole taxonomy. However, such a representation does become somewhat crowded even though the diagram does not include every variant of every device previously used for airway management. An overview of the interrelationship among airway management devices is represented in Fig. 3.
Previously both, Brimacombe and Miller, have proposed detailed classifications of supraglottic airway devices organised according to their respective features and their chronological introduction.16,17 These summaries, although detailed, only concentrate their attention on specific classes of airway devices. The taxonomic system I present here attempts to provide a comprehensive overview embracing the entire range of airway management devices, although such a broad remit is inevitably at the expense of incorporation of every detail. The creation of such a classification structure does facilitate the incorporation of new instruments with features common to some of the existing devices and also provides the opportunity to incorporate future devices which are developed to include novel features that are as yet unrecognised. The acronyms used in this classification have been allocated according to a logical systematic appraisal of known airway management devices. It should be acknowledged that although the acronyms may not necessarily be easily memorised nor widely recognised, the system does provides a useful overview of all airway device categories and can be adapted to accommodate future developments.
Assistance with the Editorial: this Editorial is based on a refresher course held at the Euroanaesthesia meeting 2012 in Paris.
Financial support and sponsorship: the author of this Editorial is involved in the development and design of airway management devices produced by the Swiss company Acutronic Medical Systems (CH-8816 Hirzel) such as the SensaScope mentioned in this article (ISVC). This involvement does not entail financial benefits, but occasional support for demonstration and presentation of this equipment at airway-related meetings and workshops.
Conflicts of interest: none declared.
Comment from the Editor: this Editorial was checked and accepted by the Editors, but was not sent for external peer-review.
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