A proposal for updating the classification of SADs and a new scoring system

The scoring and classification of supraglottic airways (SADs) is an interesting topic and currently the subject of much debate.

In 2011, a paper by Cook and Howes entitled, Recent developments in efficacy and safety of supraglottic airway devices, published in Continuing Education in Anaesthesia, Critical Care and Pain, described a classification of SADs into 1st and 2nd generation devices. The simplicity of this classification had immediate appeal and it quickly became established as the most widely used method for classifying SADs.

In A critique of elective pediatric supraglottic airway devices by White, Cook and Stoddart, a 1st generation device was described as asimple airway tube’ and 2nd generation as a device that ‘incorporates specific design features to improve safety by protecting against regurgitation and aspiration’.

Numerous publications, presentations and reviews subsequently utilised this classification, including the seminal  NAP4 report, the 4th National Audit Project of the Royal College of Anaesthetists (RCoA) and the Difficult Airway Society (DAS) on ‘Major Complications of Airway Management in the United Kingdom’, which made a number of recommendations regarding the use of SADs, and in particular the use of 2nd generation devices.

Its use has not been confined to the UK. The term is widely understood, accepted and used internationally. However, the classification of SADs as either 1st or 2nd generation was not the first classification.

In 2004, A Proposed Classification and Scoring System for Supraglottic Sealing Airways: A Brief Review by Miller was published in Anesthesia & Analgesia. This categorised SADs by the sealing mechanism. Three primary groups were identified:

  • Cuffed perilaryngeal sealers – such as the laryngeal mask airway
  • Cuffed pharyngeal sealers – such as the Cuffed Oropharyngeal Airway (COPA™)
  • Uncuffed anatomically preshaped sealers – such as i-gel®

This classification was further subdivided, so cuffless perilaryngeal sealers could be either ‘directional’ or ‘non-directional’, and cuffed pharyngeal sealers could be designated as ‘with’ or ‘without’ oesophageal sealing. The sealing mechanisms were described in detail, in conjunction with force vectors, frictional force and whether a device was reusable or single-use; or incorporated a mechanism to provide additional protection against aspiration.

In 2009, five years after Miller’s paper, the International standard, ISO 11712:2009(E) Anaesthetic and respiratory equipment – Supralaryngeal airways and connectors was published. This standard included five classifications of supralaryngeal designs. Further details can be obtained from my 2012 blog post on the classification of SADs.

Whatever their merits or limitations, neither the classification in the international standard or Miller’s classification from 2004 ever enjoyed the same measure of popularity or widespread use currently evident for the categorisation in to 1st and 2nd generation. It is therefore particularly interesting to note a proposal by the originator of the 1st/2nd generation classification, Professor Cook, for an update in correspondence to the editor of the British Journal of Anaesthesia (BJA).

The proposal is to add the suffix ‘i’ to 1st or 2nd generation to indicate those devices which enable intubation (eg with success >50%) and then include ‘d’ for direct intubation and ‘g’ for guided intubation. The correspondence provides three examples of SADs classified in this manner, as follows:

  • cLMA – 1st generation ‘ig’
  • Intubating LMA – 1st generation ‘id, ig’
  • i-gel® – 2nd generation ‘ig’

Further discussion regarding an updated classification can be seen on the BJA Out of the blue E-letters archive. Alternative classifications have been proposed, including one by Michalek and Miller in, ‘Airway Management Evolution – In a search for an ideal extraglottic airway device.

The scoring, as opposed to the classification of SADs, also has an interesting history. Miller proposed a ‘provisional scoring of airways’ in his 2003 paper already discussed above. This identified desirable features of airways for routine use and then for each variable (easy insertion, seal for IPPV etc) assigned a score to each device. An updated version, also by Miller, along similar lines, appeared in the second chapter of the book, The i-gel supraglottic airway, edited by Michalek and Donaldson.

The most recent scoring of SADs appeared in a particularly interesting editorial entitled, ‘Time to abandon the ‘vintage’ laryngeal mask airway and adopt second-generation supraglottic devices as first choice’ by Cook and Kelly.

This editorial notes that SADs now have important roles beyond airway maintenance during routine low-risk surgery, including airway maintenance in obese and higher risk patients and airway management outside the operating theatre by experts and novices, most especially during cardiac arrest. Other examples are also provided.

As a result, the authors state it is worth considering ‘whether one device can be the best device for all such functions and perhaps considering whether some devices might no longer be needed. This discussion raises the question as to whether the cLMA (and equivalent SADs) have any role in modern airway practice or whether it is time to move on.’

The editorial discusses a number of interesting and important issues related to the use of SADs, including the question of safety and efficacy, what sort of evidence should be sought when deciding which SAD to select – particularly if safety is the major concern – and the value and limitations of randomised controlled trials in answering safety related questions. Other issues of importance are also discussed, so it is critical the editorial is read in its entirety to fully appreciate the context in which the scoring system included in the paper is provided.

The scoring system itself lists the desirable features of a SAD (airway seal, overall insertion success, aspiration protection, avoiding sore throat etc) for a specific application (routine use during elective anaesthesia, use by a novice at a cardiac arrest etc), provides a maximal score for each parameter according to its importance and then allocates a score for each parameter for each device.

The authors confirm the ranking and allocated scores are based on their judgement, clinical experience and knowledge of the literature and also acknowledge that others may allocate maximal and relative scores differently.

I will not spoil your enjoyment of this editorial by revealing the results here, except to say the authors comment that, in the tables provided, it is notable that different circumstances lead to different SADs ranking highest and that ‘the cLMA rarely ranks highly in such analyses.’

The four scoring tables provided are as follows:

  • Table One: Choice of airway for routine use during elective anaesthesia
  • Table Two: Choice of airway for use by a novice at a cardiac arrest
  • Table Three: Choice of airway for expert rescue after failed intubation during rapid sequence induction
  • Table Four: Choice of airway for rescue after failed intubation followed by intubation through the SAD

In summary, a number of methods for classifying and scoring SADs have been proposed over the years. The most popular and widely used classification remains the categorisation of SADs as either 1st or 2nd generation. An update to this classification has been proposed to indicate those devices which enable intubation and a new scoring system has been published as part of an editorial in the BJA.

i-gel is a registered trademark of Intersurgical Ltd. cLMA is an abbreviation for the LMA Classic. LMA and LMA Classic are registered trademarks of Teleflex Incorporated or its affiliates. COPA is a trademark of Mallinckrodt Medical, Inc.

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The bougie – is it immortal?

Despite the technique of using an introducer to facilitate intubation being described over 65 years ago, the bougie, or tracheal tube introducer, remains a popular airway adjunct. With the development of more hi-tech aids to manage the airway, such as video laryngoscopes, is the demise of the bougie imminent or is it destined for immortality?

As Dr J J Henderson confirmed in correspondence entitled, ‘Development of the gum-elastic bougie’ – published in Anaesthesia in 2003, although Robert Reynolds Macintosh, who designed the Macintosh Laryngoscope – described by Sir Anthony Jephcott  as, ‘the most numerously and widely made durable item in the history of anaesthesia’ – is usually given credit for the first use of introducers to facilitate tracheal intubation, the technique was described a year earlier by Minnitt & Gillies in their ‘Textbook of Anaesthetics’, published by E & S Livingstone Ltd in 1948.

Dr Henderson confirms that in relation to passage of a tracheal tube with the Macintosh laryngoscope, the Minnitt & Gillies publication suggested, ‘This is an easy matter when a semi-rigid gum elastic catheter is passed’. 

 Sir Robert Macintosh (1897 – 1989), knighted in 1955, described the technique in his landmark illustrated 1949 article entitled, ‘An aid to oral intubation’ as follows:

 ‘One of the difficulties in passing tubes beyond a certain size is that the body of the tube obscures the view of the cords through which the tip must be directed. In order to overcome this I thread the tube over a long gum-elastic catheter, the tip of which is then passed through the cords under direct vision. Using the catheter as a guide, the tube is gently pushed down into position and the guide is then withdrawn.’

A few years later in 1952, responding to correspondence from a Dr Rook, Barnard (Anaesthesia 1952;7:119) described a technique he found of practical value when for any reason intubation proved difficult:

‘A small gum-elastic bougie is pushed through the Magill’s tube until about two inches extend beyond the distal end. The bougie is then bent forwards at an angle of 45 degrees or less. A Macintosh’s laryngoscope is passed and the bougie is passed through the larynx. The Magill’s tube is then passed well into the trachea and the bougie is removed’.

Use was not widespread until after the introduction of the Endotracheal Tube Introducer by Eschmann Bros & Walsh Ltd in the 1970s. This device incorporated a coudé tip, one of a number of differences to the device originally described by Macintosh in 1949. Dr Venn, who designed the device whilst working as an anaesthetic advisor to Eschmann, has described the development of the bougie in correspondence published in Anaesthesia in 1993 and the story was expanded further by Dr Henderson, from additional information provided by Dr Venn, in an article entitled, ‘Development of the gum-elastic bougie’ published in the same journal ten years later.

One irony is, as El-Orbany et al noted in Anesthesiology in 2004, the Eschmann Tracheal Tube Introducer is not gum, elastic or a bougie. The gum elastic bougie was originally a urinary catheter designed for dilation of urethral strictures. The material of the Eschmann device was different in that it had two layers: a core of tube woven from polyester threads and an outer resin layer. Other differences were the length, longer at 60cm, to allow the railroading of an endotracheal tube and the ‘presence of a 35 degree curved tip, permitting it to be steered around obstacles’.

Since its introduction, the bougie, or tracheal tube introducer, has grown in popularity, and whilst an equipment list for management of the difficult airway might include a number of different types of devices and airway adjuncts, such as alternative styles of rigid laryngoscope blades, supraglottic airways, video laryngoscopes and flexible fibreoptic intubation equipment, it is also likely to include some form of tracheal tube introducer or guide – single use or reusable.

The Difficult Airway Society (DAS) 2005 list of recommended equipment for routine airway management includes a ‘Tracheal tube introducer (gum-elastic-bougie)’ and an ‘Introducer (bougie)’ is included as part of ‘Plan A: Initial tracheal intubation plan’ in the DAS algorithm for ‘Unanticipated difficult tracheal intubation – during routine induction of anaesthesia in an adult patient’. Intubating bougies are also mentioned in the section on Recommendations for Extubation in the ‘Practice Guidelines for Management of the Difficult Airway’ , an updated report by the American Society of Anesthesiologists (ASA) Task Force on Management of the Difficult Airway, published in 2013.

Single use alternatives from a variety of manufacturers have been available for a number of years, and the Association of Anaesthetists of Great Britain and Ireland (AAGBI) Safety Guideline document, Infection Control in Anaesthesia, published in 2008, states:

Bougies: ‘Re-use of these items has been associated with cross-infection. Manufacturers recommend that a gum elastic bougie may be disinfected up to five times between patients and stored in a sealed packet. It is preferable that alternative single-use intubation aids are employed where possible.’

Even a cursory search of the published literature relating to tracheal tube introducers produces literally hundreds of studies, case reports and correspondence, comparing different types of introducers and a variety of potential extended applications, as well as the effects of sterilisation on multi-use devices, (Anaesthesia, 2011, 66, pages 1134 – 1139), and the forces required to remove bougies from tracheal tubes (Anaesthesia, 2009, 64, pages 320 – 322). There is even a report, published in Anaesthesia in 2007, regarding a home-made bougie. A quite alarming story in this modern age of device regulation, the author describes fashioning a bougie in Indonesia from a wire coat hanger and an ordinary giving set whilst waiting for a bougie to arrive from England! The author used the device on more than 40 occasions, commenting that, ‘In four patients I do not think I would have been able to intubate the trachea without it.’

It seems the bougie continues to be perceived as a useful airway adjunct for the persistent epiglottis-only view, but as Dr Richard Levitan has described in his overview of the Bougie (Tube Introducer), ‘The bougie is not a heat seeking missile, i.e., it does not ‘find’ the trachea automatically; laryngeal landmarks, i.e. the epiglottis at a minimum, or preferably the posterior cartilages must be sighted to place the bougie in the trachea.’

Given that many airway conferences now often include a debate comparing direct laryngoscopy to video laryngoscopy and provocatively ask whether the days of the standard laryngoscope are numbered, it is interesting that even those who feel the value of a bougie may sometimes be overstated, do not seem to suggest the bougie is in imminent danger of being consigned to the history books as a relic of anaesthesia practice from days gone by. Perhaps the bougie is immortal?

Facial Analysis to Classify Difficult Intubation

For any practitioner of airway management, predicting a difficult intubation in advance, reliably and with confidence, has obvious benefits. Know it in advance and you can plan for it. You can work out your plan A, as well as your plan B, plan C and plan D, refresh your memory of the appropriate algorithms, check the appropriate equipment is available and consult with colleagues. All in advance. All relatively stress free.

There are many predictive tests for difficult intubation, including thyromental distance, the mallampati classification and sternomental distance, as well as mnemonic difficult airway identifiers, such as LEMON (Look externally, Evaluate, Mallampati, Obstruction, Neck mobility) for difficult direct laryngoscopy, and MOANS (Mask seal, Obstruction, Age, No teeth, Stiff lungs) for difficult mask ventilation. For anticipation of difficult supraglottic/extraglottic device use, there is RODS (Restricted mouth opening, Obstruction of the upper airway, Distorted airway, Stiff lungs) and for difficult surgical cricothyrotomy, there is SHORT (previous neck Surgery, Haematoma, Obese, previous Radiation therapy, Tumour).

For tests such as thyromental distance, sternomental distance and the mallampati classification, the sensitivity and specificity vary, but range from poor to fair. Combinations of these, such as thyromental distance and mallampati, can be a useful predictor of difficult intubation, but still have their limitations, particularly since there can be variation in measurement conditions. These issues are explored in the meta-analysis Predicting Difficult Intubation in Apparently Normal Patients by Shiga et al, published in Anesthesiology. The full text is free to download.

Last month, an exciting project to develop a computer algorithm to accurately predict how difficult (or easy) it would be to intubate a patient using digital images was revealed by Tufts Medical Center in Boston, USA. This project follows on from a previous study at Tufts, which used computerised facial structure analysis, combined with thyromental distance, to produce a model for predicting difficult intubation. Their model accurately classified 70/80 airways, compared with 47/80 for mallampati + thyromental distance. The computing power needed might currently exceed the capability of today’s mobile phones, but the objective is to produce a computer algorithm that could utilise high speed computers, perhaps over a network, and the digital camera in a mobile phone for the image/photographic input. The dream scenario though has to be – data input and computation of the data in a totally self contained handheld device. Of course, even if this becomes possible, the algorithm will need to demonstrate a clear superiority to current techniques to be clinically useful. All very exciting!

You can keep up to date with progress of this study, ‘Facial Analysis to Classify Difficult Intubation’ at ClinicalTrials.gov.