British Journal of Anaesthesia, 119 (S1): i154–i166 (2017)

doi: 10.1093/bja/aex385
Respiration and the Airway

RESPIRATION AND THE AIRWAY

The evolution of airway management – new concepts

and conflicts with traditional practice
A. F. McNarry1,* and A. Patel2
1
NHS Lothian, Edinburgh, UK and 2Royal National Throat Nose and Ear Hospital and University College
London Hospital, London, UK
*Corresponding author. E-mail: althegasman@btinternet.com

Abstract
In the last 25 yr, there have been several advances in the safe management of the airway. Videolaryngoscopes and supra-
glottic airways, now in routine use by new trainees in anaesthesia, have had their genesis in the recent past. The 4th
National Audit Project of the Royal College of Anaesthetists and the Difficult Airway Society published in 2011 a seminal
report that has influenced airway management worldwide . Understanding how the report’s recommendations were con-
structed and how clinical guidelines compliment rather than contradict them is important in understanding the tenets of
safe airway management. Over the last 25 yr there has been an increasing understanding of the effects of human factors in
anaesthesiology: we may not perform in a predictable or optimal manner when faced with unusual and threatening chal-
lenges. The place of cricoid pressure in anaesthetic practice has also evolved. Current recommendations are that it be
applied, but it should be released rapidly should airway difficulty be encountered. The need to prevent hypoxaemia by
preoxygenation has long been recognized, but the role of high-flow nasal oxygen in anaesthesia is now being realized and
developed. Clinicians must decide how novel therapies and long-standing practices are adapted to best meet the needs of
our patients and prevent harm during airway management.

Key words: airway management; laryngeal mask airway; oxygen, Inhalation therapies; intubation, endotracheal

Airway management is the cornerstone of anaesthetic practice, first description of a supraglottic airway,2 but it was and still
and virtually every anaesthetic innovation in the past 25 yr has remains a revolution in safe airway management. In Verghese
had an impact on some aspect of airway care. Pulse oximetry, and Brimacombe’s 1993 study3 the cLMA was being used in
sevoflurane, remifentanil, disposable equipment, rocuronium almost one third of cases with a success rate of 99.8%. They
and sugammadex have all altered clinical practice. The chal- noted that fewer than 5% of patients had a laryngeal mask in
lenge when considering these innovations is knowing how they situ for procedures lasting more than two hours. By the time of
will effect clinical practice in the next 25 yr. the 4th National Audit Project (NAP4), supraglottic airway devi-
ces (SADs) were being used in 56.2% of general anaesthetics.4 In
2017, a case series described SAD use in patients for up to 11 h.5
Supraglottic airway devices Similar SADs were developed by other companies, and an
Brain’s description of the classic Laryngeal Mask Airway1 (cLMA, entirely new nomenclature based on the seal of the mask with
manufactured by Bivona and initially distributed by Colgate the oropharynx (oropharyngeal leak pressure) was created.2 6 7
Medical) in the British Journal of Anaesthesia in 1983 was not the Underlining its place in safe airway management, the term

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Laryngeal Mask Airway became a MeSH keyword in 1993. How then does the clinician proceed? For instance, is the LMA
Brimacombe reported there were 295 articles, abstracts or chap- Protector36 a better device than the Intubating Laryngeal Tube
ters featuring the cLMA in 1994 alone.2 with Drain Tube (iLTS-D; https://www.vbm-medical.com/products/
Supraglottic airway devices enable anaesthetists to be airway-management/intubating-laryngeal-tube-ilts-d/; accessed 7
hands-free during a procedure, but the cLMA’s success was as a October 2017)? Does the Baska Mask37 38 with its self-sealing cuff
result of more than its labour-saving properties. Brain stated it provide a better airway than any other? Which is the best SAD to
is likely to be ‘of particular value where difficulty is experienced use for airway rescue after failed tracheal intubation? Is one family
in maintaining the airway’. The increased interest in the poten- of devices as effective in adults and children?
tial of day surgery8 and the availability of propofol as an emul- Clinicians must prioritise three issues: 1. Effective oxygena-
sion in 1986 were also major contributors to the success of the tion and ventilation; 2. Minimizing aspiration risk; and 3. Ability
device (the original description recommended its use after thio- to insert the device effectively without resorting to complex
pental and alcuronium 0.2 mg kg1). By 1988 the benefit propofol methods or repeated attempts. Cost, educational opportunities
offered in terms of suppression of pharyngeal and laryngeal and the likelihood of airway trauma also inform any choice.
reactivity over thiopental was reported,8 and its use advocated.9 Regular rehearsal and clinical experience with any device will
Brain’s contribution to anaesthetic practice has already been improve its utility. Brimacombe found that as many as 750 LMA
celebrated as the cLMA reached its 30th birthday,10 11 and the insertions were required to overcome the long-term learning
impact of his innovation cannot be overstated. This article, curve of the cLMA.39
however, looks forward to the next 25 years.
Amidst the technological and clinical research that under-
pinned the development of SADs some simpler innovations have
also revolutionised anaesthetic practice. The Aintree Catheter
Videolaryngoscopy
facilitates tracheal intubation through a SAD.12–16 It was originally Many regard Jack Pacey, the vascular surgeon who invented the
described as a ‘disposable plastic tube’, although it was cleverly Glidescope40–42 in 2001, as the father of videolaryngoscopy (VL).
designed to be just shorter (by 3 cm) than the length of the cord However, optical devices designed to facilitate difficult tracheal
on a fibrescope allowing continued manoeuvrability of the fibre- intubation existed before this date. Katz and Berci43 coined the
scope tip.17 A guide to its use can be found at http://www.das.uk. phrase Optical Stylet in 1979. Regardless of their history, video-
com/files/AIC_abbreviated_Guide_Final_for_DAS.pdf (accessed 7 laryngoscopes are effective. A retrospective analysis by the
October 2017). Supraglottic airway devices can also be used to Multicenter Perioperative Outcomes Group44 reported 92% suc-
facilitate tracheal intubation directly18–20 and have an important cess using a videolaryngoscope as a rescue device after failed
role in rescuing failed intubation.21 22 Since the manufacture of intubation. A Cochrane Review45 comparing videolaryngoscopy
the LMA Proseal, various devices have also offered enhanced sep- with direct laryngoscopy stated ‘statistically significantly fewer
aration of the respiratory and gastrointestinal tracts. They have failed intubations were reported when a videolaryngoscope was
even been used as the primary airway for Caesarean section.23 used’, and ‘there were fewer failed intubations in those with an
Although blind intubation techniques are possible through devi- anticipated difficult airway when using a videolaryngoscope’.
ces such as the intubating laryngeal mask,24 reports of harm25 Reassuring as these statements appear, they were made based
and the wide availability of fibreoptic equipment in the UK,26 on 38 studies with 4127 participants and six studies with 830
have made such techniques redundant. participants, making the average number of participants per
Examples of SAD use in ‘extreme circumstances’ such as study 108 and 138, respectively.
a bridge to extubation in the ICU,27 managing the airway for Studies of videolaryngoscopy generate their own issues.
cardiac surgery,28 or for surgery in the prone position both Studies using tracheal intubation success as their primary out-
electively29 30 and with unexpected extubation31 are reported. come measure require many subjects (>1,000) in each arm to
Clinicians must decide when to choose a specific device, not effectively demonstrate superiority of one device over another,
just based on how it works, but on how likely it is to fail.32 if the VLs studied are 98% - 99% effective. This need for large
Individual anaesthetists must combine their knowledge of a numbers has led to several studies that looked at surrogates of
device’s performance alongside their ability to use that device intubation difficulty, such as time to intubation,46 47 or the suc-
effectively in each situation. cess rates of novices or medical students.48 Similarly, given the
Ramachandran’s study33 of 15,795 uses of the LMA Unique relatively low incidence of difficult intubation in the general
reported a failure rate of 1.1%, but if an anaesthetist does population, studies have chosen to use manikins,49 50 simulated
around 400 cases per yr and works for 30 yr as a consultant, it difficulty,51 52 or anticipated difficulty rather than actual
will take years for one individual to generate adequate data to difficulty. This myriad of inclusion criteria has led to some
prove the safety profile of a single device. Cook suggested a potentially conflicting results. For instance, a meta-analysis of
scoring system for choosing the best SAD34 based on seven the Pentax AWS53 vs Macintosh laryngoscope in 2014 suggested
factors from the presence of a sore throat to overall insertion that despite a superior laryngeal view, the Pentax Airway Scope
success. With the perpetual advent of new devices, findings provided little clinical benefit over a conventional laryngoscope.
rapidly become out of date but it is the methodology that must Cook’s suggestion54 that devices should be studied sequentially
be retained. from manikin to human subject has merit, although this is per-
The Difficult Airway Society’s ADEPT (Airway Device Evaluation haps not directly applicable to VLs. In a meta-analysis,55 only 13%
Project Team) process35 set out a framework whereby airway of ‘non-standard’ laryngoscopes had been tested on patients with
equipment should be evaluated using at least level 3 b (single case anticipated or known difficult airways. Mihai and colleagues55
control or historical control) evidence. This level of evidence could then suggested that multicentre collaborations are likely to be
then be used to inform purchasing decisions, based on properly needed, studying known difficult patients to fully understand
conducted trials rather than evaluations with small numbers. these devices. A taxonomy describing VLs has been developed56
Despite interest and ongoing work in this area, a UK-based study by Healy and colleagues (Fig. 1). While some parts of this are
specifically using the ADEPT methodology has yet to be published. already redundant (the Ctrach, a laryngeal mask with a camera

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 i156 | McNarry and Patel

Video laryngoscopes

Integrated channel Rigid blade

Video stylets
laryngoscopes laryngoscopes

Standard Angled
Airway Blade Rigid
blade blade

CTrach Pentax AWS Bonfils V-MAC GlideScope

Airtraq* C-MAC McGrath

Fig 1 A classification of videolaryngoscopic devices. CTrach image courtesy of LMA North America. Pentax AWS image courtesy of Ambu USA. Airtraq image cour-
tesy of Prodol Meditec S.A. Bonfils and C-MAC ÂV
C 2012 Photo Courtesy of KARL STORZ Endoscopy-America, Inc. GlideScope image courtesy of Verathon, USA. The

C 2012 Healy et al.; licensee BioMed

McGrath series 5 image courtesy of Aircraft Medical, UK. This figure is taken from Healy et al BMC Anesthesiol. 2012; 12: 32. V
Central Ltd. Reproduced under the terms of its Creative Commons Attribution License (2.0).

allowing visualisation of the glottis from the bowl of the device is The DAS 201521 guidelines adopted Zaouter’s stance68 recom-
no longer manufactured), its broad divisions into channelled, rigid mending ‘all anaesthetists should be trained to use, and have
video stylet, standard blade and angulated blade are useful. immediate access to, a videolaryngoscope’, but did not specify
Given the variety of devices available, it is not surprising that which laryngoscope, as the evidence did not exist to recommend
they have different features, different modes of failure and dif- one over another. Proficiency with any device requires training
ferent recommendations for successful use.57 58 A common early and regular practice.69 This is unlikely to be achieved if there are
statement about VLs was that they offered an excellent view of several devices across one hospital and neither trainees nor
the glottis but did not necessarily facilitate tracheal intubation.59 consultants are likely to become proficient in their use in such
However, this may relate more to our understanding of the proc- circumstances, particularly in potentially difficult airways.
ess of tracheal intubation than the properties of the device itself. After considering the evidence of efficacy, departments
The idea of ‘axes of alignment’ was introduced by Bannister and should choose a VL based on a variety of factors including cost
Macbeth in 1944,60 a year after Macintosh described his laryngo- of the initial device and any disposables, cleaning protocols and
scope.61 Greenland suggested considering this as two curves portability. Developing expertise will require frequent rather
(oro-pharyngeal and pharyngo-glotto-tracheal) may be more than exceptional use. This will enable understanding not just of
useful62 with VLs serving to move the eye along the primary how it works but recognition of those situations and airways
curve. This has been disputed as the complete theory of laryngo- where it may prove ineffective, as no VL is perfect.70–72 Practice
scopy and intubation,63 but it offers an effective way to consider to develop, retain and be able to pass on skills is essential.
videolaryngoscopy, and the shaping of a stylet or bougie should Looking to the next 25 yr, the anaesthetic community needs
tube insertion prove difficult. This is particularly the case with ongoing quality research into videolaryngoscopes, considering
hyper-angulated blades where a 60 curve is advocated.42 64 65 blade design and shape, and also how we view the role of the lar-
However useful, these adjuncts have been known to cause yngoscope, which is not actually to view the larynx, but to facili-
trauma themselves.66 67 With many devices now available, shap- tate tracheal intubation. Large multicentre studies will be required
ing the stylet to the shape of the blade in use may be the easiest to establish how effective these devices are in difficult airways
option. Similarly, operators must not become fixated on the and whether any one is superior to another. The ultimate chal-
screen but adopt a ‘patient-screen-patient’ approach to observe lenge is to be able to determine those patients where videolar-
the passage of the tracheal tube as it initially enters the oral cav- yngoscopy will be ineffective before the induction of anaesthesia.
ity. The design of certain VLs may mean that the camera does
not provide a view from the tip of the blade. This means that the
best glottic view may not correspond with the best chance of
Human factors and guidelines
passing the tracheal tube easily, and in these situations with- Alexander Pope wrote ‘to err is human’ in a poem in the 18th
drawing the blade slightly may be beneficial. Century. The notion that humans can and do make mistakes is

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 Evolution of airway management | i157

longer standing than the practice of modern anaesthesia. do not claim to be and are not perfect, and they are not designed
However, in a safety critical area such as anaesthesia strategies to dictate practice, coming with a disclaimer to this effect. They
to limit potentially catastrophic clinical errors should be distil the evidence from published literature, national and inter-
embraced. In the 21st Century, anaesthetists will often cite the national expert opinion, and the entire DAS membership to sup-
tragic case of Elaine Bromiley73 74 and the outstanding, unselfish port best practice in airway management, including a demand
work done by her widower Martin in raising awareness of the to wake the patient up if appropriate. The guidelines should at
role of human factors in anaesthetic practice. The Clinical least serve to stimulate an anaesthetist into developing more
Human Factors Group which he founded in 2007 (www.chfg.org; than one airway management plan for their patient, which they
accessed 7 October 2017) has a vision that extends beyond safe then must communicate with their team.
anaesthesia, to ‘a healthcare system that places an understand- The guidelines were specifically written to support UK and
ing of human factors at the heart of improving clinical, manage- Ireland based practice and should be read in that way, although
rial and organisational practice leading to significant they are used throughout the world. Other countries have
improvements in safety and efficiency’. The NAP4 report already published their own guidelines. Figure 2 shows an air-
included a chapter on human factors and stated that human way trolley designed to comply with the unanticipated difficult
factors contributed to airway issues, relating to either the indi- intubation guidelines, also serving as a cognitive aid to the
vidual or the team, in 75 (40%) of cases. A follow up study where guidelines. Figure 3 shows some of the many devices currently
12 contributing anaesthetists were interviewed, identified available on these trolleys.
human factors as causative in all cases, with a median of 4.5
factors per case.75 The RCoA’s role in acknowledging and under-
standing the challenges of human factors predates NAP4. The NAP4
first President of the new College of Anaesthetists, AA Spence,
Any consideration of airway management in the past 25 yr must
wrote an editorial in 1997 entitled ‘The expanding role of simu-
include the detailed findings of the NAP4 report.4 88 It is the larg-
lators in risk management’.76 He hoped that simulators would
est audit of serious airway complications in the world literature.
help anaesthetists in their formative years learn tricks of the
It was a prospective study of all the major airway events occur-
mind, and operate in a more disciplined manner.
ring in operating theatres, ICU or the emergency department that
Anaesthetists are unlikely to ever train as frequently as
resulted in serious harm occurring across the UK in a 12 month
Formula 1 pit crews, but anaesthesia is improving. The Cockpit
period, beginning September 2008. For inclusion, complications
Resource Management programmes in the aviation industry
of airway management had to have led to death, brain damage,
became Anaesthetic Crisis Resource Management courses, first
the need for an emergency surgical airway (front of neck access),
established by Gaba in the USA.77 Anaesthetists are recognizing
unanticipated ICU admission or prolonged ICU stay. Each
that poor non-technical skills are contributory to adverse
included case was then reviewed by an expert panel. One hun-
events.78 Unfortunately, despite increased awareness, evidence
dred and eighty-four cases were reported, including 38 deaths.
suggests that Can’t-Intubate, Can’t-Oxygenate situations have
The report generated 167 recommendations, divisible into rec-
been poorly managed.79 The use of a graded assertiveness com-
ommendations for the institution, for the department, and for
munication tool such as PACE (Probe, Alert, Challenge,
the individual practitioner. The recommendations complement
Emergency), adapted from the aviation industry,80 may be of
the guidelines even though they were created as recommenda-
benefit in averting an airway catastrophe. For effective use,
tions from ‘non-ideal practice’ rather than literature review.
everyone involved must understand how it works, and have the
NAP4 identified several recurring themes including poor airway
knowledge to know when to deploy it.81 By 2016, Merry noted
assessment, poor planning in the face of potential difficulty, a
events leading to harm during anaesthesia often result from
failure to plan for failure, and the inappropriate use or lack of use
omission of key planning steps (such as failure to anticipate and
of various pieces of airway equipment. The obese and those with
plan for a difficult airway) or other forms of basic oversight.82
head and neck pathology featured too commonly, and poor judg-
ment and a lack of education and training were contributory.
In recent years, anaesthetic departments have been inun-
Difficult airway society guidelines dated with guidelines and recommendations on airway man-
agement. Every anaesthetist must individually develop a plan
The original Difficult Airway Society (DAS) guidelines were not
that will effectively oxygenate their patient. Departments must
the first to suggest a method of dealing with a failed intubation.
ensure that the equipment and training is provided not just for
Tunstall described a Failed Intubation Drill for obstetrics in
anaesthetists but for the entire operating theatre team to facili-
1976.83 Many of his themes still resonate in current guidelines
tate effective airway management. The RCoA and DAS must
such as the decision to abandon repeat attempts at intubation
strive towards better airway management at local and national
being made promptly, asking the surgeon and the theatre sister
levels with audit and research into best practice.
to unscrub and help, and releasing cricoid pressure. The first set
of DAS Guidelines (for the management of the unanticipated
difficult airway)84 were begun in 1999 and set out to provide a
step-wise series of plans to remedy the situation when the pri-
Cricoid pressure
mary intubation plan failed. Although they set out to be simple, The application of cricoid pressure to reduce gastric insufflation
clear and definitive, when the time came to update them, one and regurgitation and to prevent pulmonary aspiration in those
ambition was to simplify them further and make them even at risk is an integral part of anaesthetic practice. It was origi-
more didactic. nally described in 1961 by Sellick (The Middlesex Hospital,
To date DAS has produced guidelines on unanticipated London) as a ‘simple manoeuvre’ to cause ‘occlusion of the
difficulty with intubation (200484 and 201521), extubation,85 upper oesophagus by backwards pressure on the cricoid ring
paediatrics,86 and obstetrics,87 with guidelines for the manage- against the bodies of the cervical vertebrae’.89 In his original
ment of the difficult airway in critical care in preparation. They paper, Sellick reported instillation of water at pressures of up to

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Fig 2 An airway trolley drawer-front combining airway equipment to manage an unanticipated difficult tracheal intubation with a cognitive aid to the DAS 2015
Guidelines.

100 cm H2O into the stomach of cadavers in a steep anaesthetic community with minimal further research, becom-
Trendelenburg position and found that cricoid pressure pre- ing an integral part of the rapid sequence induction/intubation
vented water from regurgitating into the pharynx. In a para- technique for almost 50 yr.90 Over the last 25 yr we have gained
lysed, anaesthetised patient with a latex tube filled with a better understanding of the factors that influence the efficacy
contrast placed in the oesophagus, he found the application of of cricoid pressure as described by Sellick. These include the
cricoid pressure led to a loss of contrast at the level of the risk of pulmonary aspiration, whether a force applied to
applied pressure. He then went on to study cricoid pressure in the cricoid reduced these risks, the reliability of oesophageal
patients considered at high risk for aspiration. In a series of 26 occlusion, optimal manual delivery of cricoid pressure, and a
patients no regurgitation occurred in 23 before or after the appli- recognition that application of cricoid pressure itself can inter-
cation of cricoid pressure. In three patients he witnessed regur- fere with all aspects of airway management.
gitation after cricoid pressure was released after tracheal
intubation.89
Anatomically the formation of a barrier appeared to provide Anatomy of hypopharyngeal/oesophageal compression
a good reason for using cricoid pressure in high-risk patients, The position of the oesophagus relative to the cricoid ring in the
and the practice was quickly and widely adopted across the axial plane was assumed by Sellick to be immediately posterior

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 Evolution of airway management | i159

cervical vertebra to occlude the hypopharyngeal lumen,

(iv) paralaryngeal pressure may achieve the desired compres-
sion at the oesophageal level, and (v) inappropriate cricoid
pressure (paralaryngeal) may achieve effective oesophageal
compression.95

Does cricoid pressure reduce the risk of regurgitation

and aspiration?
In Mendelson’s 1946 paper describing obstetric patients under-
going face mask ventilation with ether and nitrous oxide, the
incidence of aspiration was 1:667, with a mortality rate of
1:22,000.96 Subsequent studies during general anaesthesia sug-
gest an incidence of pulmonary aspiration that ranged between
1:200097 and 1:300098 increasing to 1:900 during emergency sur-
gery.99 The mortality associated with this incidence of aspira-
tion was between 1:45,000 and 1:70,000.98–100 More recent
studies show an incidence of aspiration of 1 in 7000 for elective
patients and a mortality at 1:1,000,000.101 The 4th National Audit
Project,88 described in total 42 cases of aspiration in 2,872,600
general anaesthetics of which there were 36 cases of aspiration
Fig 3 Airway devices described in the past 25 yr. From left to right (top of gastric contents. Twenty-nine of these 36 patients required
row): Ambu AuraGain, LMA Supreme, Proseal LMA mounted on introducer ICU admission and half required a prolonged stay.88 There were
tool, Airtraq, and McGrath MAC with X-blade. An Aintree (intubation) eight deaths as a result of aspiration of gastric contents
catheter is placed beneath them.
accounting for the single most common cause of death in
anaesthesia events.88
NAP4 highlighted that protection from regurgitation and
aspiration of gastric contents was still an important issue for all
to the cricoid ring, providing an anatomical explanation for the anaesthetists, with the majority of aspiration events occurring
occlusion he saw in his cadaveric experiments and in his series during maintenance (inappropriate supraglottic airway device
of 26 patients. However, a retrospective review of CT scans of use) or extubation compared with before or during induction.
healthy patients showed this not to be true; in 50% of subjects However, NAP4 still identified several cases where there was
the oesophagus sits posterolateral to the cricoid ring, mainly on omission of a rapid sequence induction with cricoid pressure
the left side.91 Oesophageal position was also investigated using despite strong indications for its use, followed by patient harm
MRI imaging with and without cricoid pressure in 22 awake or death from aspiration. There were no cases where cricoid
patients. It showed similar findings of posterolateral positioning pressure was reported to lead to major complications. NAP4
of the oesophagus, mainly on the left side with an increase in concluded rapid sequence induction with cricoid force did not
the lateral displacement of the oesophagus from 53% to 91% provide 100% protection against regurgitation and aspiration of
with cricoid pressure.92 Ultrasound examination to assess the gastric contents, but remained the standard for those patients
degree to which cricoid pressure compressed the oesophagus at risk.88
showed no reduction in the anteroposterior diameter of the
oesophagus. However, a novel pressure site (paralaryngeal) did
Is cricoid pressure performed optimally and is it
achieve the desired compression of the oesophagus.93 Our tradi-
tional understanding of the anatomical position of the oesopha-
harmless?
gus may be incomplete and ‘occlusion of the upper oesophagus Although Sellick originally described ‘cricoid pressure’ with firm
by backwards pressure on the cricoid ring against the bodies of pressure being applied once consciousness was lost, our current
the cervical vertebrae’90 may be suboptimal or mistaken. understanding of a force that is applied may mean that cricoid
Further MRI studies on awake volunteers found the hypo- force is a more accurate term. Most problems with cricoid force
pharynx not the oesophagus lay behind the cricoid ring and it occur when too much force is applied,102 but knowledge around
was the hypopharynx not the oesophagus that was compressed how much force to apply, when to apply it, and its application is
by cricoid pressure.94 With cricoid pressure, the mean anterio- inconsistent.103–106
posterior diameter of the hypopharynx was reduced by 35% Cricoid pressure is uncomfortable in the awake patient,103
(3.2 mm) which was thought to represent complete occlusion of particularly when the force is greater than 20 N (2 kg) causing
the lumen.94 This compression was maintained even when the retching,107 and leading to pulmonary aspiration108 or oesopha-
cricoid ring was lateral to the vertebral body whilst the distal geal rupture.109 110 Current guidelines recommend a 10 N (1 kg)
hypopharynx, the portion of the alimentary canal at the cricoid force initially to the awake patient increasing to 30 N (3 kg) after
level, was fixed with respect to the cricoid ring by a complex loss of consciousness.21 87 101 109 The application of cricoid pres-
network of muscular and ligamentous structures minimizing its sure may also influence the lower oesophageal sphincter caus-
mobility.94 ing relaxation and an increased potential for vomiting,
The significance of these findings is: (i) compression regurgitation and aspiration.110–113
and obliteration of the lumen may be at the level of the hypo- If active vomiting occurs when cricoid pressure is applied,
pharynx and not at the level of the oesophagus, (ii) appropriate cricoid pressure should be removed immediately. The very
cricoid pressure compresses the post-cricoid hypopharynx, problem we set out to prevent may actually be triggered by our
(iii) it may not be necessary to fix the cricoid ring against a actions.95

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Cricoid pressure can affect: Prolongation of apnoea time after induction of

anaesthesia
i. Facemask ventilation - increasing inspiratory pressures,
reducing tidal volumes, and may even cause complete air- Methods to prolong the apnoea time after the induction of
way obstruction.114–119 anaesthesia and administration of neuromuscular blocking
ii. Direct laryngoscopy - causing distortion of laryngeal struc- agents include; (i) pharyngeal insufflation of oxygen (3–10 litre
tures.120 Failed intubation is almost eight times as frequent min1),147 148 (ii) nasal oxygen delivered at flow rates of < 15 litre
in patients having a rapid sequence induction.88 min1 (described as nasal oxygenation during efforts to secure a
iii. Laryngeal mask airway placement - impeding successful tube or NO DESAT),144 149 150 and more recently (iii) high flow
placement and ventilation.121 122 humidified nasal oxygen at 30–70 litre min1,146 151–158 for which
the acronym THRIVE has been used (transnasal humidified
During the last 25 yr the practical application of cricoid pres-
rapid insufflation ventilatory exchange).
sure/force has been discussed123–128 and challenged.129–132 In
Unlike regular breathing where oxygen removal from and
the UK it remains a standard component of a rapid sequence
carbon dioxide (CO2) delivery to the alveoli are predictable using
induction and intubation. Cricoid pressure may reduce gastric
the respiratory quotient, during apnoeic oxygenation signifi-
distension during mask ventilation (the purpose for which it
cantly more CO2 (90%) is buffered in the bloodstream and tis-
was originally described),89 increasing the time to desaturation
sues, meaning only approximately 20 ml min1 is delivered to
in those at risk with poor respiratory reserve, sepsis, or high
the alveoli during apnoea.144 159 160 The subsequent reduction in
metabolic requirements and providing an early indication of the
lung volume generates a pressure gradient between the upper
ease of ventilation.21 The force used can be reduced to 20 N
airway and the alveoli, which results in mass movement of oxy-
(2 kg) if the patient is in a head-up position.125
gen down the trachea into the alveoli replenishing oxygen
The role of cricoid pressure has evolved from a procedure that
stores and increasing the safe apnoea time. The safe time is lim-
was never or rarely released when difficulties were encountered
ited by a significant increase in the alveolar CO2 concentration
to a pragmatic approach recognizing cricoid pressure can affect
leading to progressive respiratory acidosis.
all aspects of airway management. It should be released immedi-
1. Pharyngeal insufflation . Pharyngeal insufflation of oxygen
ately when airway difficulty (laryngoscopy, face mask ventilation
involves the administration of oxygen at 3–10 litres min1 via a
or laryngeal mask airway placement) is encountered.21 87 95 If cri-
nasal catheter placed through the nose into the oropharynx or
coid pressure is removed, this should be done under direct vision
an oral tube placed through the mouth directly into the oro-
with suction available and the assistant ready to reapply cricoid
pharynx. Oxygen insufflated at 3 litre min1 through an 8 Fr
pressure in the event of regurgitation or aspiration.
catheter inserted through the nose and emerging in the
oropharynx of apnoeic patients undergoing elective surgery,
sustained peripheral oxygen saturations >97% throughout a
Preoxygenation and increasing safe apnoea
10 min study period, whilst those who did not receive pharyng-
time
eal oxygenation desaturated.147 The authors concluded phar-
From the 1940’s onwards preoxygenation has been recognised yngeal oxygenation provided 10 min of ‘safe’ apnoea time in
as an important technique before the induction of general ASA I or II patients with unobstructed airways.147
anaesthesia to delay the onset of hypoxia, allowing more time 2. Nasal oxygenation during intubation (<15 litre min1). This sim-
for laryngoscopy, tracheal intubation and airway rescue.132–137 ple technique, first described by Levitan as NO DESAT in 2010,149
In the 1960 s preoxygenation was emphasised with the intro- involves the application of simple nasal cannulae on the patient
duction of cricoid pressure and in the 1970 s became essential under a face mask during preoxygenation. After induction of
as part of the rapid sequence induction and intubation techni- anaesthesia the nasal cannula flow rate is increased to a maxi-
que.90 As the development of a ‘can’t intubate can’t oxygenate’ mum of 15 litre min1, whilst airway patency is maintained by
situation is unpredictable, national guidelines have recom- either a jaw thrust or direct laryngoscopy. At this flow rate nearly
mended preoxygenation as desirable in all patients.138 The DAS 100% FiO2 is achieved and the apnoea time increased. The sim-
2015 guidelines for management of unanticipated difficult plicity of the technique, the universal availability of equipment
intubation in adults recommend ‘All patients should be pre- and minimal change to existing practice suggest it should be used
oxygenated before the induction of general anaesthesia’.21 87 more widely to increase the apnoea time in those at risk.144 149
3. Humidified high flow nasal oxygen (30–70 litre min1). The
term THRIVE was coined in 2015 by Patel and Nouraei to
Optimising the oxygen reservoir before induction of
describe the application of high-flow (30–70 litre min1) warmed
anaesthesia humidified oxygen (Fig. 4) in adult patients with difficult air-
Over the last 25 yr the optimum technique for facemask preoxy- ways.146 The high flow rates associated with this technique are
genation has been investigated. These studies describe techni- only tolerated because the oxygen administered is warmed and
ques of administering oxygen to optimise the oxygen reservoir humidified. In contrast, cold dry oxygen at high flow rates leads
before the induction of anaesthesia and include; (i) tidal volume to mucosal drying, pain, discomfort, sinus headaches and bleed-
breathing, (ii) single tidal capacity breathing, (iii) one vital ing from the nasal mucosa limiting cold dry oxygen to 10–15 litre
capacity breath followed by tidal volume breathing, (iv) four min1 in the awake patient. ‘THRIVE’ significantly extends the
deep (inspiratory capacity) breaths, (v) eight deep (inspiratory apnoea time but unlike traditional apnoeic oxygenation techni-
capacity) breaths, and (vi) extended deep breathing (12–16 ques it also improves the clearance of CO2, preventing the
inspiratory capacity breaths).139 Other techniques to increase increase seen in traditional apnoeic oxygenation techniques.
the oxygen reservoir before the induction of anaesthesia include: The clearance of CO2146 has been replicated in clinical trials by
(i) continuous positive airway pressure (CPAP),140–143 (ii) nonin- Gustafsson158 and To.156 The mechanism of CO2 clearance dur-
vasive bilevel positive airway pressure143 and (iii) head up posi- ing high oxygen flow rates has recently been investigated and is
tioning.21 144–146 thought to be mediated by the interaction between supraglottic

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 Evolution of airway management | i161

hypoxia during apnoea, prolonging the safe apnoea time in infants

and children.153 The accompanying editorial concluded that as a
new technique THRIVE had not been fully characterised in chil-
dren, but held great promise and may represent a transformative
technique for the safe management of difficult airways.154
Over the last 25 yr improvements to preoxygenation techni-
ques in different clinical situations have identified head up
positioning, continuous positive airway pressure, positive
end-expiratory pressure, bilevel positive airway pressure and
apnoeic oxygenation techniques to optimise the oxygen reser-
voir before induction of anaesthesia and prolong the apnoea
time after the induction of anaesthesia.

Conclusions
The NAP4 reported that anaesthetists are almost defined by
their ability to manage the airway. However, this is not a simple
task answered by a simple clinical trial, as it involves the inter-
action between the patient and their (patho-)physiology, any
airway management equipment used and its efficacy, and the
skill of the operator. As we seek to make airway management
safer in the next 25 yr, looking at these factors in isolation may
prove overly simplistic. An ability to effectively stratify patients
between low and high risk and then an ability to deal with those
patients identified as difficult, presumes that any such stratifi-
cation or device or innovation to manage difficulty is one hun-
dred percent effective, which it cannot be. Regardless of the
clinical and scientific discoveries of the present146 and the
future,161 162 anaesthetists must continue to make an airway
management plan for every patient that includes a second plan
should their primary plan fail. Together with their team they
must be trained in its expert delivery.

Authors’ contributions
Manuscript preparation: all authors
Fig 4 A humidified high flow nasal oxygen circuit by kind permission of Writing paper: all authors
Fisher and Paykel. Revising paper: all authors

Declaration of interest
flow vortices generated by the high nasal flow and cardiopneu- A.F.M. has been loaned a variety of equipment by Amby and
matic movements. Aircraft (Medtronic) for evaluation purposes in the past 5 years.
Patel and Nouraei146 concluded that warmed humidified high A.F.M. has also been loaned equipment by Accutronic, Aircraft
flow oxygen has the potential to transform the practice of anaes- Medical, AMBU, Cook, Fannin, Freelance, Medtronic, Storz, and
thesia by maintaining oxygen saturations after commencement Teleflex Medical for use in Airway Workshops. He has been
of apnoea to levels that change the nature of securing a definitive loaned equipment for clinical use by Fisher and Paykel, who
airway in emergency and difficult intubations from a hurried have also funded travel costs for a THRIVE development day in
stop-start, potentially traumatic undertaking, to a smooth event 2015. In 2017 A.F.M. acted as an expert advisor to the MHRA and
undertaken within an extended safe apnoeic window. received conference funding from MSD. A.F.M. is the RCoA and
When investigated in a randomized controlled trial of rapid DAS Airway Leads Advisor. A.P. has received travel, accommo-
sequence induction of anaesthesia during emergency surgery, dation and consultancy support from F&P Healthcare and is the
high flow nasal oxygen at 30–70 litre min1 was shown to be a current president of the Difficult Airway Society.
feasible and safe method for oxygenating patients. The high
flow group had a significantly longer apnoea time when com-
pared with face mask preoxygenation, whilst an equivalent
Funding
blood gas profile was maintained between the groups.48 155 This None declared
is a powerful demonstration of the benefits of high flow oxygen-
ation, but the increased length of time to tracheal intubation
must be considered with care in the ‘rapid-sequence’ setting.
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