Management of the Difficult Airway
This important topic can be divided into anticipated and unanticipated difficult airway management. Although both situations may require similar techniques, the approach, urgency and risk of adverse outcomes differ considerably between the two settings.
Difficulties arise most commonly at the start or end of anaesthesia, with the former the more common. Difficulty at the end of anaesthesia involves either airway obstruction or aspiration; when the problem is airway obstruction, difficulty may be categorized in the same manner as difficulty after induction. Difficult airway management may involve any of the four main categories of airway management:
Airway management is usually routine and straightforward, but each of the above techniques may fail. Anaesthetists are used to high levels of success at what they do and routine airway management does not usually fail. The frequency of failure of routine airway management is as follows:
Failure rates probably vary depending on definitions used, operator experience and the group of patients examined. Novices may have failure rates more than 10-fold higher. Difficulty may arise at least 10 times more often than failure. For example, difficult laryngoscopy (and hence difficult intubation) occurs in about 6% of intubations in unselected patients but in selected groups, e.g. those presenting for cervical spine surgery, this may be as high as 20%. The urgency of a procedure also contributes to ease and success. During emergency intubation (with rapid sequence induction), intubation fails approximately eight times as often as during elective intubation.
About a quarter of major airway emergencies occur in the Intensive Care Unit (ICU) or the emergency department and it is necessary to ensure that the same response to airway difficulty can be provided in these locations as in the operating theatre. In the emergency department, tracheal intubation may be difficult in 1 in 12, fail as often as 1 in 50, and an emergency surgical airway has been reported to be needed as often as 1 in 200 intubation attempts.
Guidelines: Organizational preparedness requires that those events which might reasonably be anticipated to occur can be managed appropriately in the organization. This in turn requires guidelines (or policies) and equipment. As a minimum, the guidelines should cover the following:
Guidelines will also ideally include unexpected failed mask ventilation and unexpected failed insertion of a SAD. Guidelines might also address the indications for fibreoptic intubation and management of extubation of the difficult airway. These guidelines need not be created by every institution and there is much to be said for nationally accepted or published guidelines being adopted as local policy (e.g. the Difficult Airway Society [DAS] guidelines in the UK). There are several advantages to widespread adoption of this approach; for example, practice becomes based on available evidence and employees who move between hospitals will be immediately familiar with emergency protocols.
Equipment: Logic dictates that the equipment needed to satisfy institutional preparedness is that which is needed for all the guidelines to be carried out in their entirety. This equipment should be procured, stored, maintained and checked appropriately to ensure that it is readily available whenever and wherever it is required. Difficult airway equipment (perhaps better described as advanced airway equipment) is usually maintained in an airway trolley (Fig. 22.1). It is advisable for all airway trolleys in an organization to have the same content and layout; this includes areas such as ICU and the emergency department. Organizing the airway trolley so that the layout of the equipment matches the flow of the airway guideline may improve compliance with the guideline and patient care; an example based on the DAS guideline is shown in Figure 22.2.
Communication and Training: The final aspect of institutional preparedness is communication and training. Guidelines are of limited value if they are not understood, accepted and practised by the relevant staff. Many hospitals have access to training in advanced airway management. Guidelines should be distributed widely. Training should involve the use of local guidelines and locally available equipment to ensure relevance. Where possible, those individuals who work together in teams should be trained together so that the chances of the team working well in an emergency are enhanced. The ‘team’ need not be limited to the anaesthetist and anaesthetic assistant, and some training (rather like a trauma team) allocates specific roles to surgeons, scrub nurses and other anaesthetists who attend to help in a crisis. While there is no evidence that such an approach improves outcome in real airway emergencies, it probably enables an organized, systematic approach and has the value of enabling a ‘team leader’ to oversee the management of the crisis, perhaps avoiding ‘task fixation’ and promoting ‘situation awareness’.
Institutional preparedness is a process whereby the organization facilitates good difficult airway management by individuals. Such preparation usually requires that the organization has a nominated airway lead for this role, usually a consultant anaesthetist.
Individual anaesthetists have a clear responsibility to be prepared to manage the difficult airway. At different stages of training and expertise the responsibilities will differ. The elements of individual preparedness are:
Through appropriate education, individual anaesthetists should ensure that they have the appropriate knowledge and skills to deal with anticipated and unanticipated airway difficulties and emergencies which they may reasonably expect to encounter. It is also important that the less experienced know the limitations of their expertise and therefore when to call for assistance.
Individual training requires that each anaesthetist is familiar with local guidelines and is trained to find and use the locally available equipment according to local guidelines. Training should also include an understanding of how the less experienced anaesthetist seeks senior assistance. To the trained anaesthetist, management of the difficult airway should become a part of routine practice.
Assessment and Planning a Strategy: While it is accepted that not all cases of airway difficulty can be anticipated (perhaps 50% are unanticipated), many can be. Airway assessment is discussed in detail in Chapter 21. Only principles are discussed here.
Assessment should include an assessment of the likely ease or difficulty of performing the planned primary airway technique and potential rescue techniques. Assessment should also specifically assess the risk of aspiration. When difficulty with one technique is identified, particular attention is required in assessment of other techniques, first because that rescue technique is more likely to be required and second because in patients in whom one technique fails there is an increased likelihood that other techniques will also fail. Multiple airway problems tend to co-exist in the same patient.
The importance of assessment and planning is underlined by the fact that several large studies examining major airway complications have identified failure to assess, failure to alter technique in the light of findings, and failure to have back-up plans as causes of poor outcomes.
A strategy is a logical sequential series of plans which aim to achieve oxygenation, ventilation and avoidance of aspiration and which are appropriate to the patient’s specific features and condition. Airway management should not rely on the success of plan A and should be based on a clear strategy that is communicated to all. An aphorism that may encapsulate this is ‘in order to succeed, it is necessary to plan for failure’. Guidelines are, in essence, a strategy for unexpected difficulty. The strategy should usefully identify a ‘place of safety’; this is a pre-planned rescue plan when problems arise; for instance, if insertion of a laryngeal mask is known to be successful this may be the place of safety, but alternatively the safest option may be to allow the patient to wake.
Many ‘airway disasters’ are associated with poor planning, use of techniques that are unfamiliar to the user (sometimes used incorrectly or sub-optimally), poor team-work and poor communication. Human factors associated with airway management complications include:
Task fixation is the tendency, having started a task (e.g. tracheal intubation), to persevere with attempts to complete that task, even when it is not in the patient’s interest (e.g. repeated attempts risking airway trauma and progression to CICV). Situation awareness should enable the anaesthetist (or someone else in the team) to realise that the task is failing and that another technique or approach is necessary, or perhaps that priorities have changed (e.g. from intubation to oxygenation and waking the patient). When a team works well together and hierarchical boundaries are broken down, communication within the team is enhanced. A level hierarchy should not be confused with lack of leadership; the ability of one person to step aside and observe the team behaviour may have value. Conversely, multiple individuals all pursing their own approach to managing the crisis is unlikely to be constructive or successful.
Several questions can usefully be considered before approaching any anticipated difficult airway. These questions are based on the preamble to the American Society of Anesthesiologists’ (ASA) Difficult Airway algorithm.
When airway difficulty is anticipated, the point at which the anaesthetist risks loss of control and ‘burning bridges’ is likely when general anaesthesia is induced. At this point, respiratory drive is diminished or obliterated, airway reflexes are largely ablated and the loss of muscle tone of the airway means that the risk of airway obstruction increases dramatically. If general anaesthesia is induced, the time from cessation of administration to patient waking is often up to 10 min, which is more than sufficient to cause profound hypoxaemia, hypoxic tissue injury or even death.
Securing the airway awake should be considered actively whenever significant difficulty in intubation is predicted. The argument for this increases when mask ventilation is predicted to be difficult, when rescue techniques such as direct tracheal access are predicted to be difficult and when there is a high risk of aspiration.
While not all anaesthetists currently have the skills and experience to perform AFOI, such skills must be available in every anaesthetic department at all times and should be deployed whenever indicated.
When general anaesthesia is administered, neuromuscular blocking drugs can be friend or foe during difficult airway management. It is notable that neuromuscular blockade is not usually necessary for mask ventilation or SAD insertion, but does facilitate tracheal intubation and probably insertion of an emergency surgical airway.
When mask ventilation is problematic, administration of a neuromuscular blocking agent usually makes ventilation easier. This has led some to promote the use of neuromuscular blockade when mask ventilation is difficult in patients with a difficult airway. However, it is important to note that the evidence of easier ventilation is derived from patients who are not particularly difficult in the first place. In contrast, there is no robust evidence that neuromuscular blockade makes ventilation reliably easy when dealing with patients with abnormal anatomy or an anticipated difficult airway. There is also no robust evidence that neuromuscular blockade reliably converts CICV to a situation in which ventilation is possible. The disadvantage of inducing neuromuscular blockade in this situation is that it commits the anaesthetist to securing the airway promptly and eliminates the option of waking the patient. Patients who are difficult or impossible to ventilate using a face mask are also at increased risk of failed intubation. Thus administration of a neuromuscular blocker may be reasonable when difficult mask ventilation occurs and waking the patient is not desirable but the anaesthetist must be prepared to manage a critical airway if it does not improve the situation.
In contrast, when both mask ventilation and intubation have failed, if waking the patient is not possible, there is little to be lost and much to be gained by administering a neuromuscular blocker. If successful, it will avoid an unnecessary emergency surgical airway and may save the patient’s life.
even if it was not part of the initial airway management strategy, if CICV occurs and waking the patient is not an option, a muscle relaxant should be given before determining the need to proceed to a surgical airway.
A couple of decades ago, the use of tracheal tubes as large as 10 mm internal diameter was routine. While smaller tracheal tubes are used by most anaesthetists, some persist in using tracheal tubes as large as 9 mm internal diameter. In the circumstance of difficult airway management, there is a strong argument for using small tracheal tubes. A smaller tracheal tube:
Spontaneous ventilation is rarely required for prolonged periods after tracheal intubation and all but the most impaired patient can breathe with ease through a tracheal tube of 6 mm internal diameter. Selection of a tracheal tube of 6.0–6.5 mm internal diameter may make airway management easier and use of tubes larger than 7.0 mm is rarely indicated.
Because problems relating to the management of a difficult airway are the leading cause of death related to anaesthesia, the need for guidelines to formalize and help in the management of the difficult airway has been recognized in many countries. Major complications arising from difficult airway management are relatively rare but they are frequent enough that almost all anaesthetists will encounter them in their careers.
The first guidelines were published in 1993 by the ASA. The introduction of guidelines in the USA has been demonstrated to have led to a reduction in death and brain damage claims (and therefore probably critical incidents) related to airway management, most notably at the time of induction of anaesthesia. Many European countries developed their own guidelines in subsequent years. While all these claim to be evidence-based, the paucity of robust evidence means that most guidelines differ significantly from each other, often reflecting local preferences. The UK guidelines were published by the Difficult Airway Society in 2004. The major differences between the guidelines published by the ASA and DAS are summarized in Table 22.1. Both emphasize the most important principles in airway management:
|ASA Guidelines 2012||DAS Guidelines 2004|
|Breadth of ‘difficult airway’ management covered||A clinical situation in which a conventionally trained anesthesiologist experiences difficulty with face mask ventilation of the upper airway, difficulty with tracheal intubation or both||A Cormack and Lehane grade 3 or 4 view despite optimal direct laryngoscopy, using an alternative laryngoscope and external laryngeal manipulation|
|Evaluation of the airway||11 non-reassuring findings||Not covered|
|Number of attempts at laryngoscopy allowed before moving to a different technique||> 3, multiple attempts||Up to 4 during routine intubation and 3 during RSI. A further single attempt if a more experienced anaesthetist arrives.|
|Techniques recommended for difficult intubation||Multiple including AFOI, blind, retrograde, LMA used as conduit and invasive airway access||Optimal laryngoscopy with gum elastic bougie (Plan A), then LMA/ILMA as conduit using fibreoptic control (Plan B)* then invasive airway access (Plan D)|
|Order of techniques||None given||Clear flow chart|
|Recommendations for extubation||Yes||Published separately (2012)|
|Recommendations on training||None given||Should form part of all anaesthetist training|
The ASA guidelines cover airway assessment and a number of different difficult airway situations. They offer the user a wide choice of options at each point of airway difficulty. They recommend multiple different techniques and it is likely that not all will be within the competence of all anaesthetists. They offer ‘choices’. In contrast, the DAS guidelines are didactic and present a single recommended pathway arranged in plans A to D and differentiating between the patient undergoing routine intubation or rapid sequence induction. They do not prescribe any advanced techniques but recommend simple procedures using equipment that should be familiar to all anaesthetists in training. They also strongly emphasize the need for regular practice of the recommended techniques using simulators and manikins where appropriate. The DAS guidelines are shown in Figures 22.3–22.5.
FIGURE 22.3 Difficult Airway Society guideline for unanticipated difficult tracheal intubation during routine induction of anaesthesia in an adult patient. (Adapted from Henderson JJ, Popat MT, Latto IP, Pearce AC. Difficult Airway Society guidelines for management of the unanticipated difficult intubation. Anaesthesia 2004; 59:675–964, with permission from Blackwell Publishing Ltd. LMA, laryngeal mask airway; ILMA, intubating laryngeal mask airway.)
FIGURE 22.4 Difficult Airway Society guideline for unanticipated difficult tracheal intubation during rapid sequence induction of anaesthesia in a non-obstetric adult patient. (Adapted from Henderson JJ, Popat MT, Latto IP, Pearce AC. Difficult Airway Society guidelines for management of the unanticipated difficult intubation. Anaesthesia 2004; 59:675–694, with permission from Blackwell Publishing Ltd.)
FIGURE 22.5 Difficult Airway Society guideline for management of failed intubation, increasing hypoxaemia and difficult ventilation in the paralysed, anaesthetized patient, with rescue techniques for the ‘can’t intubate, can’t ventilate’ situation. (Adapted from Henderson JJ, Popat MT, Latto IP, Pearce AC. Difficult Airway Society guidelines for management of the unanticipated difficult intubation. Anaesthesia 2004; 59:675–694, with permission from Blackwell Publishing Ltd.)
The first problem encountered in any difficult airway situation is often difficulty with face-mask ventilation. This is a vital step because it represents the basic and least invasive way of ensuring oxygenation of the patient. For mask ventilation to occur, a clear, sealed and patent airway from face mask to the lower airway is required. Difficulty can be diagnosed when there is inadequate chest movement despite high airway pressures (in the case of obstruction) or very low airway pressures (due to a leak during inspiration). Capnography and spirometry, both of which are available on most modern anaesthetic machines, can also identify poor ventilation before hypoxaemia occurs (Fig. 22.6).
Face-mask ventilation requires the combination of: establishing a seal between the mask and the face; maintaining a clear upper airway; and ventilation of the lungs. Flexion of the lower cervical spine, extension of the upper cervical spine and mandibular protrusion are required, ideally with good quality facial soft tissues to enable an adequate seal with the mask. Using a ‘C-grip’, the thumb and first finger are used to hold the mask pushing downwards while the remaining three fingers pull the chin, jaw and soft tissues into the mask while also maintaining head and neck positions (Fig 22.7). Manual ventilation is performed with the anaesthetist’s other hand. Problems with the upper airway can often be predicted by prior airway assessment. Patients for whom obtaining an adequate mask seal is often problematic include the edentulous elderly, bearded patients and those who require high ventilation pressures, such as the morbidly obese.
In a two-person (four-handed) technique, the senior anaesthetist holds the mask with two hands, one on either side of the mask, maintaining the airway seal and head and neck position, while a second person performs manual ventilation. In the three-person (six-handed) technique, the third person acts solely to improve airway positioning and seal with jaw thrust. In an alternative technique, one or two people maintain the airway and the reservoir bag is squeezed by a foot, or mechanical ventilation is employed. If ventilation is still not possible and the depth of anaesthesia is adequate an appropriate SAD should be inserted. Ensure adequate depth of anaesthesia and muscle relaxation where appropriate.
Every anaesthetist should have a strategy prepared for dealing with problems with intubation, including plans for the more serious situation of CICV. The DAS algorithm is a suitable approach and is shown in Figures 22.3–22.5. This, or another equally valid strategy, should be familiar to all anaesthetists who are practising without direct supervision.
This involves the first and best attempts at intubation. It requires good pre-oxygenation, optimal positioning, optimal anaesthesia, appropriate neuromuscular blockade and an appropriate laryngoscope blade. Use of external laryngeal manipulation and a (high quality) bougie are appropriate. If there is difficulty, the anaesthetist should call for help, but not send away their primary assistant. Attempts at intubation must be limited to no more than four (preferably fewer) as multiple attempts at laryngoscopy are associated with increased risk of airway trauma, failed rescue techniques, CICV, morbidity and mortality. The major concern is of converting a patient who cannot be intubated but can be ventilated into one who cannot be intubated or ventilated. A final attempt is deemed appropriate if a more experienced anaesthetist arrives and adequate face mask ventilation can be maintained.
There is no necessity to use the same laryngoscope blade for all attempts at intubation. Alternative blades include a long (size 4) Macintosh blade, the McCoy (levering laryngoscope) blade and a number of straight-bladed laryngoscopes (e.g. Miller, Henderson). Arguably, at least one attempt should be with a McCoy blade (Fig. 22.10) because it is recognized to move the fulcrum of the force applied to the airway distally and to improve the view when laryngoscopy is awkward. Use of a straight blade may offer the benefit of using an alternative approach to the laryngoscopy, such as a retromolar approach (Fig. 22.11), and this may be helpful, particularly if there is a small mandibular space.
This is appropriate if adequate ventilation is possible and anaesthesia can be maintained. A classic (cLMA) or intubating (ILMA) laryngeal mask airway is inserted to enable ventilation and is then used as a conduit for intubation. Fibreoptic guidance of intubation is recommended.