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.

Arthur Guedel and the oropharyngeal airway

One-piece Guedel

Any definitive history of the development of the oropharyngeal airway (OPA) is likely to include reference to Arthur Guedel and the OPA he described in a short article entitled, ‘A nontraumatic pharyngeal airway’ published in the Journal of the American Medical Association (JAMA) in 1933. In little more than a dozen lines he described a device which today is almost synonymous with the term oropharyngeal airway.

Although Joseph Thomas Clover (1825-1882) is credited with first use of an artificial airway, as this was a nasopharnygeal device, it is Sir Frederic William Hewitt (1857-1916) who is usually acknowledged as being first to describe the use of an oropharyngeal airway, in an article entitled ‘An artificial air-way for use during anaesthetisation’, published in The Lancet in 1908.

Hewitt was appointed as anaesthetist to King Edward VII in 1901. He was also a founding member of the Society of Anaesthetists in London and was made a member of the Royal Victorian Order (4th class) in 1902 for personal service to the King. The Frederick Hewitt Lecture was inaugurated by the Royal College of Surgeons (now the Royal College of Anaesthetists), in 1950. Fittingly, the lecture is now given biennially with the Joseph Clover Lecture – two pioneers of airway management appropriately acknowledged for their contribution to anaesthesia and airway management. The names of those anaesthetists who have delivered a Hewitt or Clover lecture reads like a roll call of anaesthetic icons, and includes Sir Ivan Magill, Sir Robert Macintosh, Gordon Jackson Rees and Brian Sellick.

Hewitt and the airway he designed are discussed in a historical note published in Anaesthesia by RP Haridas entitled, ‘The Hewitt airway – the first known artificial oral ‘air-way’ 101 years since its description’. The original airway incorporated a straight rubber tube, but a curved version was later developed. Brimacombe described the Hewitt airway as the forerunner to many modern oropharyngeal airways (Laryngeal Mask Anesthesia. Principles & Practice. Elsevier Ltd. 2nd edition. 2005).

Hewitt was undoubtedly an anaesthetic giant of the late nineteenth and early twentieth century, yet his contribution to anaesthesia and airway management may be less immediately well known than that of Arthur Guedel.

Maltby confirms in ‘Notable Names in Anaesthesia’ that Arthur Guedel was born in Cambridge City, Indiana in 1883. Despite losing three fingers when he was a teenager, Guedel still managed to become an accomplished pianist and composer! His medical career started at the Medical College of Indiana in 1903. After graduating in 1908, he was interned at the City Hospital in Indianapolis where he was required to administer ether and chloroform. He eventually served as an anaesthetist in the American Expeditionary Forces in France in World War One, where the challenges faced by inexperienced personnel from the army medical corps provided the impetus to develop a classification of the stages of anaesthesia.

Guedel also created the first inflatable cuffs for ET tubes, experimenting with location, above, below or at the vocal cords, cuff pressures and possible inflation techniques. Around this time, Guedel would often use his own pet dog, appropriately named ‘Airway’, as part of his lecture demonstrations. Maltby confirms that Airway survived to enjoy ‘an honourable retirement with the Waters family in Madison, Wisconsin.’ A glance at the content of Guedel’s lecture demonstrations from the 1920s suggests that Airway’s survival to retirement was by no means a foregone conclusion and can itself be considered an achievement.

The recipient of the dog, Ralph Waters, wrote a personal tribute to his friend Arthur Guedel in the BJA in 1953 as part of the ‘Eminent Anaesthetists’ series. He confirmed that the saying, ‘If a man loves dogs he will love mankind’ was true of Guedel. He discusses their correspondence, at its most prolific between 1925 to 1945, his athleticism, and his motto for many years, ‘Maintain Flying Speed’, taken from the pilot of the time whose altitude began to fail as his forward progress diminished. Interestingly, he makes no mention of the oropharyngeal airway for which Guedel is perhaps most often remembered today.

Guedel originally described his oropharyngeal airway in JAMA as follows: ‘The airway herewith depicted is made of rubber and is sufficiently soft and flexible not to traumatize yet amply rigid to maintain an open oropharyngeal air passage under all conditions.’ He also confirmed that ‘the metal insert extends into the airway for about 2cm from the oral opening and prevents collapse of the rubber between the teeth’.

Thomas Baskett, in his 2004 article on Guedel published in Resuscitation, quoted Guedel’s own 1937 publication, ‘Inhalational Anesthesia: A Fundamental Guide’, describing use of the oropharyngeal airway during anaesthesia when ‘there is sufficient muscular relaxation to permit the lower jaw to fall backward allowing the base of the tongue to lie against the posterior wall of the pharynx. Depending upon the anatomical structure of the pharynx, this may partially or completely obstruct inspiration. It is usually remedied at once by the insertion of a pharyngeal airway which will hold the tongue forward from the pharyngeal wall.’

Dorsch and Dorsch in ‘Understanding Anesthesia Equipment (5th edition)’, describe an oropharyngeal airway as follows ‘…..may be made of elastomeric material or plastic. It has a flange at the buccal end to prevent it from moving deeper into the mouth. The flange may also serve as a means to fix the airway in place. The bite portion is straight and fits between the teeth or gums. It must be firm enough that the patient cannot close the lumen by biting. The curved portion extends backward to correspond to the shape of the tongue and palate.’

The International Standard ISO 5364: 2008 ‘Anaesthetic and respiratory equipment – oropharyngeal airways’ describes an oropharyngeal airway as a ‘device intended to maintain a gas pathway through the oral cavity and pharynx’. It confirms the size should be designated by the nominal length expressed in centimetres and provides a table to show how the length should be calculated. A table is provided confirming designated size (nominal length) , as well as tolerances and minimum inside dimensions. The latter is relevant to the ability to pass other devices, such as a suction catheter, through the airway.

The European Resuscitation Council (ERC) Guidelines for Resuscitation 2010 confirm that ‘An estimate of the size required is obtained by selecting an airway with a length corresponding to the vertical distance between the patient’s incisors and the angle of the jaw.’

The original Guedel airway was made of rubber with a metal insert. Most modern Guedel airways are made of plastic. Dorsch and Dorsch confirm that modifications to aid flexible fibreoptic intubation have been described and Guedel airways with the bite block incorporated into one moulding, thereby eliminating the danger of loose or detached bite blocks, are also available.

The Guedel airway has endured the test of time and remains one of most widely known and used airway adjuncts eighty years after it was first described. It is of simple design, but many of the best inventions often are.

 

The Macintosh laryngoscope – is there a pretender to the crown?

Over 70 years after Sir R R Macintosh first described it in his landmark study published in The Lancet in 1943, the Macintosh laryngoscope remains universally popular.

Intersurgical plastic single use Macintosh laryngoscope blade, InterForm stylet and InTube ET tube

The dominant position of the Macintosh cannot be attributed to a lack of alternatives. Dorsch and Dorsch, in their huge 1,000+ page book, Understanding anesthesia equipment, list over 45 types of laryngoscope blade. Although some of these are described as modifications of the Macintosh, including the Oxiport Macintosh, Polio, Fink, Tull Macintosh, Bizarri-Giuffrida, Upsher Low Profile, Upsher ULX Macintosh, Improved vision Macintosh and the Left handed Macintosh, the list of alternatives also includes a significant number of other designs, including the Wisconsin, Schapira, Soper, Guedel, Bennett, Seward, Phillips, Alberts, Robertshaw and the Bainton. This list is not exhaustive. If you don’t have easy access to a copy of Dorsch & Dorsch, an extract is included in Annex D of the international standard, ISO 7376:2009: Anaesthetic and respiratory equipment – Laryngoscopes for tracheal intubation.

 In current practice, the two major types of Macintosh blade are generally considered to be the American, also called the ‘standard’, and the English, sometimes known as the ‘classic’ or ‘e-type’. The latter description is a particular favourite, conjuring up the image of the classic E-Type Jaguar, once described by Enzo Ferrari as, ‘The most beautiful car ever made’. Such a comparison may seem a little tenuous, but surely they can both be considered design classics?

The primary differences between the English or German and the American Macintosh are the shape, height and length of the proximal flange and the distance from the light to blade tip. The performance of both types was compared in a 2003 study by Asai et al, published in the British Journal of Anaesthesia, entitled ‘Comparison of two Macintosh laryngoscope blades in 300 patients.’ There was a difference in the view of the glottis in 80 patients. Among these patients, the view was better for the English blade for 63 patients and the standard blade was better for 17 patients. The authors concluded, ‘In patients in whom laryngoscopy was unexpectedly difficult, the English blade provided a better glottic view significantly more frequently than the standard blade.’

Probably the closest rival to the Macintosh in terms of popularity is the Miller blade. Described two years earlier than the Macintosh, the 1941 paper by Miller in Anesthesiology and simply entitled, ‘A new laryngoscope’, described a straight blade which when compared to an ‘old style medium sized blade’, was ’rounded on the bottom, smaller at the tip, and has an extra curve beginning about two inches from the end. The internal diameter of the base is shallow, but adequate to permit the passage of a 38 catheter.’ This landmark study is free to access on the Anesthesiology web-site. Miller also described a modification of his adult laryngoscope for children in 1946, ‘A new laryngoscope for intubation of infants’, Anesthesiology. 7(2):205, March 1946. This paper is also free to access.

The Miller blade remains popular for children, with straight blades in general having been described as ‘superior in elevating the tongue, removing it from the field of view to facilitate a better visualization of the infant larynx than the curved blade laryngoscope’. Doherty JS et al, 2009. Pediatric Anesthesia, 19: 30–37.

 In 2009, The UK NHS Purchasing & Supply Agency’s Centre for Evidence-based Purchasing (CEP) produced a  Buyer’s Guide for Laryngoscopes (CEP08048). The scope of this guide only extended to Macintosh blades sizes 3 and 4. No Miller or other alternative blades were included, perhaps reflecting the fact that ‘The Macintosh is the most popular [blade] for use with adults in the United Kingdom…’

 In the UK 4th National Audit Project (NAP4) – Major complications of airway management in the UK, it is confirmed in relation to tracheal intubation that, ‘Direct laryngoscopy with a Macintosh blade remains the technique of first choice if not actively contraindicated when difficulty is not anticipated.’

 Intersurgical single use plastic Macintosh laryngoscope blade

Given the huge variety of options available, why is the popularity of the Macintosh so enduring?

 Scott and Baker provide some answers in their 2009 review article, How did the Macintosh laryngoscope become so popular? In a very informative and entertaining article, the authors, New Zealanders like Macintosh, suggest that poor straight blade laryngoscopy technique prior to the widespread use of muscle relaxants, commercial availability, Macintosh’s connections in the industrial sector and unprecedented influence on the development of anaesthesia, as key factors in the success of the Macintosh blade that can be traced back to ‘prevailing circumstances’ in the 1940s. They conclude that, ‘Despite being able to achieve superior laryngoscopy with paraglossal straight blade technique and the multiple alternatives available, the Macintosh laryngoscope remains ubiquitous and is regarded as the gold standard of direct laryngoscopy’.

 In 1984, over 40 years after publication of the original description of the Macintosh laryngoscope and 25 years before the review article published by Scott and Baker, Jephcott published  A historical note on its clinical and commercial development. It was estimated by Jephcott that well over 1 million Macintosh blades had been made and sold in the previous 40 years. With regard to the origin of the design, Jephcott confirmed Macintosh’s own account from a letter he received from him:

 “I had a bit of luck and the nous to take advantage of it. On opening a patient’s mouth with a Boyle-Davis gag I found the cords perfectly displayed. Richard Salt (a really excellent chap) was in the theatre with me: before the morning had finished he had gone out and soldered a Davis blade on to a laryngoscope handle and this functioned quite adequately as a laryngoscope. The important point being that the tip finishes up proximal to the epiglottis.” Interestingly, he continued by noting that, ‘The curve, although convenient when intubating with naturally curved tubes, is not of primary importance as I emphasised subsequently.’

 Jephcott confirms the Macintosh laryngoscope was originally produced by Medical and Industrial Equipment Ltd (MIE), quickly followed by The Longworth Scientific Instrument Company Ltd. In the USA, Foregger of New York started to make the device in 1943. Jephcott concluded his article by noting that, ‘Today the Macintosh laryngoscope is known throughout the world and is made by many firms in many countries. The technique discovered by Macintosh and the instrument he designed for its achievement has survived translation into plastic and the adoption of the fibre-light. No doubt they will endure other developments in years to come.’

 Jephcott’s prediction was correct. Since his article in 1984, the Macintosh blade has also survived translation in to single use metal blades and is incorporated in to the design of a number of video laryngoscopes. The Macintosh blade remains the dominant blade for direct laryngoscopy in the 21st century, with no obvious pretender to the crown.

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?

Airway management in a combat zone

Most discussions regarding airway management probably relate to use of airway devices during anaesthesia, in-hospital and out-of-hospital cardiac arrest (OHCA) and Emergency Medicine (EM). Each of these disciplines presents its own challenges, but perhaps the most hostile and challenging environment in which to manage the airway is during combat by those involved in Special Operations Medicine (SOM).

A report published in the Spring 2013 edition of the Journal of Special Operations, entitled, Proposed Change: Supraglottic Airways Committee on Tactical Combat Casualty Care (CoTCCC) Recommendation Paper, included a review of the use of Endotracheal Intubation (ETI) v Supraglottic Airways (SGA) and Surgical Airways v SGAs.

Many of the discussion points regarding use of ETI v SGAs will be familiar to most civilian practitioners of airway management and much of the evidence presented in the paper relates to non-military, non-combat situations. However, the paper does note that ETI ‘has been reported to be accomplished by military physicians on the battlefield with a success rate comparable to that achieved in the civilian setting.’ It also makes the point that intubating a trauma patient without sedation and paralysis will be difficult and that training courses that emphasise use of ETI and SGAs, often don’t discuss how to intubate or insert an SGA in a non-comatose patient.

Referencing a 2011 study by Timmerman, the paper also highlights that ‘SGAs may not be well tolerated by casualties who are not unconscious and may not be effective in casualties with direct airway trauma. The ideal casualty for SGA use is one who is unconscious and has no direct maxillofacial or neck trauma.’

The section discussing Surgical Airways v SGA is particularly interesting. Clearly, airway management in combat is not the same as management of a cardiac arrest patient in a civilian setting. The most common cause of airway death in combat casualties is maxillofacial trauma, with the result that management of the airway may be ‘complicated by blood, distorted anatomy, and a struggling patient.’

Referencing the manual by Butler et al, entitled, Prehopsital Trauma Life Support Manual (7th edition) – Military Version, published in 2010, the paper states that , ‘Many casualties with isolated maxillofacial injury can protect their own airways by simply sitting up, leaning forward, spitting out the blood in their airway and continuing to breathe in that position. Surgical airways should be reserved for those casualties in whom this strategy is not successful at maintaining an adequate airway. Maxillofacial injury should not trigger a knee-jerk reflex for the medic to attempt a surgical airway.’ Nonetheless, the TCCC noted that a surgical airway was the airway procedure of choice in a combat casualty when the airway is compromised by direct maxillofacial trauma.

There is an interesting discussion regarding pre-hospital cricothyroidotomy and the records from the Joint Theater Trauma Registry, including the differences in failure rates for cricothyroidotomy between combat medics, physicians and physicians’ assistants and the caution that should be exhibited when interpreting this data. There is also a review of some of the published evidence regarding different types of SGA, including the Laryngeal Mask Airway, LMA ProSeal® , Combitube®, King LT and the i-gel. Inevitably, since this report was completed last year, additional evidence has been published regarding the civilian use of SGAs and ETI during cardiac arrest, which the committee may have wanted to include had it been available to them.

The conclusions of the report include the following:

  • The recommendation for supraglottic airway use in Tactical Evacuation Care should be expanded to include SGAs other than the Laryngeal Mask Airway and Combitube®.
  • Use of SGAs is recommended as both a combat medic and a combat paramedic skill.
  • Casualties who are unconscious from haemorrhagic shock or severe Traumatic Brain Injury (TBI) but who have not suffered direct airway trauma are likely to be the best candidates for SGAs among the combat wounded.
  • Casualties who are able to tolerate either ETI or SGA without sedation and paralysis are likely to have a poor prognosis.

Sadly, the latter refers to the point that the among the combat wounded, the most common causes of unconsciousness are haemorrhagic shock and severe TBI. Both associated with high mortality.

Clearly, there are many combat situations where management of the airway is best achieved by techniques other than insertion of an SGA. However, when use of an SGA is indicated or appropriate, it would seem sensible that the device should be one which can be reliably inserted quickly and easily, incorporates a gastric channel to reduce the risk of aspiration (compared to an SGA without a gastric channel) and has a high seal pressure. A 2nd generation SGA would therefore probably seem a more logical choice than a 1st generation SGA.

Whilst this paper concluded that the recommendation for supraglottic airway use in Tactical Evacuation Care should be expanded to include SGAs other than just the Laryngeal Mask Airway and Combitube®, it did not make any specific recommendation regarding use of 2nd generation devices. This may be an interesting area for the committee to consider in any future review.

The Journal of Special Operations Medicine web-site can be accessed at the following web address http://www.jsomonline.org

NAP4 – two years on

NAP4, 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’, was published in March 2011.

Two years on, the key findings of the report continue to resonate. These include:

  • A high failure rate of emergency cannula cricothyroidotomy
  • Failure to correctly interpret a capnograph trace leading to several oesophageal intubations going unrecognised in anaesthesia.
  • Numerous cases where awake fibreoptic intubation (AFOI) was indicated but was not used.
  • Problems arising when difficult intubation was managed by multiple repeat attempts at intubation.

Poor airway assessment, a failure to plan for failure and poor judgement were also identified as key clinical themes in a number of the cases reported. Such is the breadth of NAP4, that the above represent little more than a highly selective short-list.

Interestingly, when the report was launched, one of the authors highlighted in a presentation on ‘Aspiration of gastric contents and of blood‘, which can be seen as a podcast on the RCoA web-site, that ‘there’s nothing new in NAP4’, referring to the fact that one of the major findings of the report (perhaps the major finding?), that aspiration of gastric contents was the single commonest cause of death in anaesthesia events, was also the finding of a report published in Anaesthesia way back in 1956 entitled, ‘Deaths associated with anaesthesia: A report on 1,000 cases‘. So, despite all the advances in airway management and anaesthesia over the last 50 years, aspiration remains a major concern.

For many, I am sure NAP4 did highlight a lot that was new, or at the very least, NAP4 probably provided evidence to support what logic and personal experience had suggested might be true. The ultimate judgement on the success of the report may only become evident in the years ahead, when it can be assessed what practical changes have been made in light of the many recommendations of the report.

A number of posters and abstracts at national and international conferences have already assessed or reported on changes implemented in their departments in the light of NAP4. At the annual DAS meeting in 2011, these included the following:

Audit comparing supraglottic airway (SAD) device use at a DGH to NAP4 guidelines. Thomas S – This poster reported on SAD use in obese patients, use in procedures with risk of reflux or previous difficult intubation and supraglottic training and awareness of NAP4.

Post NAP4 – Implications for intensive care nursing. Lamb RG et al – A report on a project looking at basic awareness of ICU nursing staff regarding Rapid Sequence Induction (RSI) and their familiarity with difficult airway equipment. The results were used to assess the need for further education of nursing staff who may be expected to assist with RSI.

Capnography use in ICU. Measuring up to NAP4. Cole S et al – A poster reporting on the results of a survey to measure capnography use in ICUs across Scotland and to describe factors influencing use.

Capnography use outside of theatres in the Northern Deanery before and after publication of NAP4. Metcalfe SE et al – An audit on use of capnography in the UK for patients undergoing anaesthesia and being intubated irrespective of location.

Some of the recommendations of NAP4, such as each department of anaesthesia having a ‘Departmental Airway Lead’, have long been advocated by the UK Difficult Airway Society. Many hospitals already have an airway lead, but following discussions between DAS and the RCoA, the college council has endorsed a strong recommendation that all anaesthesia departments should conform with this NAP4 recommendation. The responsibilities of the position should include, the overseeing of local airway training, ensuring local policies exist and are disseminated for predictable airway emergencies, liasing specifically with ICU and emergency departments to ensure consistency, and ensuring that difficult airway equipment is appropriate to the local guidelines and standardised within the organisation.

Further potential responsibilities have also been outlined. The RCoA intends to maintain a database of departmental airway leads. DAS also plans two follow-up surveys to study the impact of NAP4. A National survey of institutional responses to NAP4 and a national ‘sprint audit’ to collect national data on practice and activity over a short period. We await the results with interest. See the DAS Newsletter – Projects Edition December 2012 pp6-7 for further details

NAP5 – Accidental Awareness during General Anaesthesia in the United Kingdom, has just been published, but two years on from publication, its predecessor remains essential reading.

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.