Chapter 7 – Difficult Airway Prediction




Chapter 7 Difficult Airway Prediction


Fauzia Mir and A. Agatha Crerar-Gilbert



Key Points





  • Airway assessment preoperatively is essential for patient management and can reduce significant morbidity and mortality.



  • A comprehensive airway assessment can determine whether a patient is safe to be anaesthetised or an awake technique for securing the airway is essential. Thus it establishes a clear airway management plan for the patient.



  • An airway assessment is aimed to determine whether the patient has any features suggestive of difficult bag mask ventilation or difficult laryngoscopy, both of which determine the anaesthetic approach for airway management.



  • There are several investigations which can help with airway assessment preoperatively. The merits and limitations of these should be kept in mind when evaluating the airway.



  • Extubation of a patient with difficult airways poses significant challenge. Difficult Airway Society (DAS) extubation algorithms are helpful to formulate appropriate extubation plan.


Airway complications are a significant source of perioperative morbidity and mortality, contributing to perioperative death in 1:48,000, and a principal cause of perioperative death in 1:176,000 cases (Auroy et al., 2009).


While airway assessment is essential and mandatory in surgical patients and is a core skill for all anaesthetists, suboptimal airway evaluation continues to be the main reason for unanticipated difficult intubation (L’Hermite et al., 2009). The aim of airway assessment is to identify a difficult airway and plan a perioperative strategy anticipating and reducing the incidence of unexpected difficult airways.


Planning for the management of a difficult airway includes identifying the suitable airway expertise, choosing appropriate anaesthetic techniques and planning ahead for a primary and alternative airway management strategy. The important distinction to be made is whether the patient’s airway can be secured under anaesthetic or an awake anaesthetic technique is necessary. Furthermore, in anticipation of a ‘can’t intubate, can’t ventilate’ scenario, the anterior neck access should be evaluated preoperatively. Broadly speaking, airway difficulties may be related to the patient’s anatomical features or the presence of a pathologically challenging airway.


Screening for difficult airways in a pre-assessment clinic aims to identify potential difficulties in three areas; whether the patient will be difficult to mask ventilate (MV), difficult to intubate using conventional laryngoscopy or problematic to extubate at the end of the procedure (Patel, 2014). Airway management may pose a dilemma at any of these three stages (Broomhead, 2010).


In this chapter, we use the term ‘laryngoscopy’, referring to a conventional laryngoscopy with the use of a standard Macintosh blade. Although numerous ‘low-skill’ fibre-optic techniques are available and can be employed in anticipated challenging airways, this depends on individual expertise and experience. The role of an anaesthetist in pre-assessment is to identify potential difficulties in standard MV and conventional laryngoscopy, while alternative techniques of airway management will be a matter of personal choice.



History


A history of previous airway management problems reported by the patient or documented in the notes is important. Previous head and neck pathology and surgery especially malignancy, radical neck dissection, radiotherapy to the head and neck and previous tracheostomy in either elective or emergency situations can all be significant in planning airway management.


History and examination may reveal dyspnoea related to compression of the upper airway, dysphonia and dysphagia. Although symptoms of upper airway compression are very suggestive of potentially difficult airways, a vast majority of patients with significant airway pathology are completely asymptomatic.


Broadly speaking, airway difficulties may be related to the patient’s anatomical features or the presence of a pathologically challenging airway.


In what follows, we describe the clinical assessment of patients aimed at predicting difficult MV and difficult laryngoscopy.



Mask Ventilation


The incidence of difficult MV has been reported as high as 5 per cent (Langeron et al., 2000), while near impossible MV was found in 0.15 per cent of cases (Kheterpal et al., 2006). While the incidence of impossible MV is very low, the consequences are potentially catastrophic. Therefore the potential for impossible MV must be recognised and a clear contingency plan formulated.


Four grades of difficulty in MV are described (Kheterpal et al., 2006). Grade 1 represents easy MV, Grade 2 describes ventilation by mask with oral airway/adjuvant with or without muscle relaxant, Grade 3 denotes difficult ventilation (inadequate, unstable or requiring two providers) with or without muscle relaxant and Grade 4 represents inability to MV. Potential for Grade 3 and Grade 4 MV can be identified by the presence of a number of clinical factors.


Features specific to predict Grade 3 MV are: body mass index (BMI) >30 kgm2, severely limited jaw protrusion, snoring, having a beard, Mallampati classification 3 or 4 and age more than 57 years. Potential for difficult MV relies on the presence of two or more of these factors.


Factors indicative of impossible MV (Grade 4) are: neck radiation changes, male sex, obstructive sleep apnoea, Mallampati classification 3 or 4 and the presence of a beard (De Hert et al., 2011; Kheterpal et al., 2009).


In clinical practice, predictors for difficult MV can be remembered by using the mnemonic MOANS.




  • Mask seal (receding chin, a beard)



  • Obesity or obstruction (history of snoring)



  • Age >55 years



  • No teeth (abnormal or protruding teeth)



  • Stiff lungs (such as asthma, pulmonary fibrosis)/stiff chest (such as barrel chest)


The presence of two or more of these features predicts difficult MV with sensitivity of 72 per cent and specificity of 73 per cent and has been shown to be indicative of the need for adjuncts and manoeuvers for successful mask seal (for example, use of oral-nasal airway, or better positioning).


Components of the mnemonic MOANS are based on the appreciation of the physiological mechanisms maintaining the patency of the upper airways. Firstly, the facial anatomical features such as receding chin and a beard may affect the mask seal (M in MOANS) and result in difficulty in MV. In elderly edentulous patients (A and N in MOANS) loose skin and lack of teeth result in the loss of facial structural support for the adequate seal between the facemask and facial soft tissues and difficult MV.


Secondly, obese patients (O in MOANS), especially those with a history of obstructive sleep apnoea (OSA), have significantly reduced patency of the nasopharyngeal space, which is further reduced post induction of anaesthesia, causing loss of the upper airway muscle tone and upper airway collapse. This reduction or loss of the nasopharyngeal airspace contributes to the difficulty in MV.


Lung pathology (S in MOANS) such as COPD, smoking-related excessive secretions and hyper-reactive airways predispose to laryngospasm and bronchospasm on induction. This can increase the difficulty in MV. Previous radiotherapy may result in tissue rigidity/stiffness of the lungs and thorax and compromise in MV. The head and neck tissues may be affected, limiting their movement and causing difficulties in achieving a good seal with the facemask. This may result in inadequate patency of the airway and in difficulty in MV.


One must be aware that difficult MV may also result from repeated attempts at intubation due to airway trauma, bleeding and oedema. This may convert a ‘can’t intubate, can ventilate’ into a ‘can’t intubate, can’t ventilate’ situation.



Difficult Laryngoscopy


Certain groups of patients are known to be associated with an increased risk of difficult laryngoscopy. These include pregnant patients, obese patients with OSA, patients with a neck circumference of more than 45 cm and patients with acromegaly (Calder, 2001). Also otolaryngology patients, patients with previous airway trauma, diabetics with ‘stiff joint syndrome’, patients with cervical rheumatic disease limiting neck movements and patients with congenital syndromes with facial and neck malformations warrant considering at risk for difficult laryngoscopy.


There are a number of bedside tests in use for predicting difficult laryngoscopy. Some of these are described in Table 7.1.




Table 7.1 Bedside tests for airways examination.

























































Test Performance of the test Test result
Mallampati classification (modified) (Samsoon & Young, 1987) the patient is examined in sitting position, the head held neutral with the mouth fully open and the tongue protruded and without phonation Class I: Soft palate, fauces, uvula and pillars visible
Class II: Soft palate, fauces, uvula visible
in obese and diabetic patients this test is performed with the neck extended (Mashour et al., 2008) Class III: Soft palate, base of uvula visible
Class IV: Soft palate not visible at all
Thyromental distance is measured along the straight line from the thyroid notch to the lower border of the mandibular mentum with the head fully extended and the mouth closed Normal value is 6.5cm
Thyrosternal distance is measured along the straight line from the thyroid notch to the upper border of the sternal manubrium with the head fully extended and the mouth closed Normal value is 12.5cm
Interincisor gap is measured with the mouth fully open in the midline; in edentulous patient inter-gingival distance is measured Normal value is 4–4.5cm
Mandibular protrusion examines the patient’s ability to bring the lower incisors in front of the upper incisors Grade A: the patient is able to bring lower incisors in front of the upper incisors
Grade B: the lower incisors and upper incisors are tip-to-tip
Grade C: the patient is unable to perform A and B
Neck extension an angle is measured between patient’s head and neck in full flexion and full extension; the bridge of the nose being a suitable reference point Flexion and extension is graded into three levels: >100°
between 100° and 80° and <80°.
Upper lip bite test (Khan, Kashfi, & Ebrahimkhani, 2003) examines the patient’s ability to bite the upper lip with the lower incisors Class I: the lower incisors can bite the upper lip, making the mucosa of the upper lip totally invisible;
Class II: the same biting manoeuver reveals a partially visible upper lip mucosa;
Class III: the lower incisors fail to bite the upper lip.


Legend: cm=centimetre, ° = degree


Mallampati classification is the most commonly used airway evaluation test. This test assesses the size of the tongue, the oropharyngeal space and their relationship to each other, indicating the potential difficulty in MV and laryngoscopy. Mallampati Classes 1 and 4 have been shown to be more reliable in predicting easy or difficult laryngoscopic views, respectively, compared with Classes 2 and 3, which lack consistent laryngoscopic correlation (Mallampati et al., 1985).


The predictive value of Mallampati classification is enhanced by combining it with other bedside tests such as mouth opening (Eberhart et al., 2010), thyromental distance or thyrosternal distance. Use of these indices has been shown to increase the positive predictive value of Mallampati class to 100 per cent with the negative predictive value of 93 per cent (Iohom, Ronayne and Cunningham, 2003).


Adequate mouth opening as measured by the inter-incisor gap is another important bedside test as a 3 cm inter-incisor gap is required for the laryngoscope blade insertion while a minimum of 2 cm gap is required for the insertion of a Supraglottic Airway Device.


Limited neck mobility may prevent adequate extension of the neck during laryngoscopy. Decreased neck mobility is seen in patients with arthritis, previous radiotherapy and radical neck surgery. Radiotherapy may render the skin, subcutaneous tissues and muscles stiff, preventing adequate extension of the neck. Radiotherapy-related stiffness of the soft tissues of the neck also indicates the potential for difficulty for the tongue to be displaced to the submandibular space during laryngoscopy, thus worsening the glottic view and making intubation difficult.


The upper lip bite test (ULBT) is also considered a useful indicator of difficult laryngoscopy. In the initial studies a ULBT of class III was reported to be a better predictor of difficult intubation compared with the Mallampati classification (Khan et al., 2003; Tremblay, 2008).


Difficult laryngoscopy is a multifactorial problem, and while it is clear that no single test can be used and relied on totally, a combination of factors allows the prediction of difficult airways with high accuracy. Hence multifactorial scoring systems and numerical predictors have been developed. However, the complexity of such systems limits their use in clinical practice; examples of such scoring systems are Wilson and El-Ganzouri scores.


To evaluate airways in everyday practice the mnemonic LEMON is commonly used.




  • Look (facial features such as protruding teeth, receding chin, loose teeth and other anatomical features suggestive of difficult laryngoscopy such as reduced mouth opening)



  • Evaluate (the distance between patient’s incisors and the distance between the thyroid notch and the floor of the mouth. These parameters and normal values are described in Table 7.1



  • Mallampati score



  • Obstruction may be indicated by the presence of signs and symptoms of upper airway obstruction; eliciting the extent and nature of the obstruction may require a CT/MRI or a nasal endoscopy (please refer to accessory investigations section)



  • Neck mobility (refer to Table 7.1 and the preceding section)



Accessory Investigations


Although bedside airway examination is sufficient in the majority of cases, positive predictive value of this assessment for detecting difficult airways is low (Nørskov et al., 2015). In some instances when more accurate assessment of airway pathology is required, use of preoperative accessory tools such as nasendoscopy, virtual endoscopy or ultrasound may be indicated. Flexible nasendoscopy can be performed by the anaesthetist in the clinic or preoperatively on the ward. This is useful for identifying upper airway pathology such as oedema and distorted anatomy, especially in peri-glottic lesions causing total airway obstruction under anaesthetic. This assessment is helpful in formulating a plan if a fibre-optic intubation is expected to secure the airway indicating whether this will be feasible or not. However it must be appreciated that while the pharyngeal tissue tone ensures a patent airway in an awake patient, this is lost under anaesthetic when the pharyngeal tone is reduced.


Further investigations such as CT scans and MRI have little proven value in airway assessment in the general population. Since these scans are performed in awake patients, results depend on the respiratory phase during which the scan is taken. This may not reveal vital information applicable to the patient under general anaesthetic, which changes the dynamic picture of the patient’s airway.


Although of limited value in the general population, these modalities may be particularly useful in patients with known airway abnormalities such as previous head and neck surgery, tumours or other growths, radiation, trauma or congenital syndromes. One needs to be aware that airways in such patients may have changed significantly since previous intubation as a result of tumour growth or surgery, making intubation more difficult than in the past. An anaesthetist should therefore look for previous difficulties encountered as documented in anaesthetic charts, but not be falsely reassured by an uneventful airway management. A full and detailed airways examination and vigilance in anticipation of difficult airways is essential.



Extubation


Extubation of a patient with difficult airways is an elective process which should be well planned and executed. Recent UK data from a number of studies have suggested that respiratory complications are frequently reported during extubation and in the post-operative recovery period.


One in six complications reported in NAP4 were from the emergence phase and a further one in six came from the recovery area. This number emphasises that vigilance is essential in the immediate and early post-extubation period. Increased risk of complications after extubation is associated with the type of surgery (for instance laryngeal or head and neck free flap surgery), duration of surgery, position of the patient intraoperatively (for instance Trendelenberg or prone position) and surgery in obese patients. Elective tracheostomy may be warranted in some patients to prevent post-extubation problems.


Difficult Airway Society guidelines (2011) for extubation give a framework for planning the extubation phase in patients with potentially difficult airways (Figures 7.17.3).





Figure 7.1 Extubation guidelines: basic algorithm. Difficult Airway Society Extubation Algorithm 2011.





Figure 7.2 Extubation guidelines: low risk algorithm. Difficult Airway Society Extubation Algorithm 2011.





Figure 7.3 Extubation guidelines: ‘at risk’ algorithm. Difficult Airway Society Extubation Algorithm 2011.



Summary


Although numerous predictors of difficult airways exist, none of them on its own is sufficient to predict difficult airways. A combined use of these parameters is advocated with clinical picture and patient history in mind to give more accurate risk assessment. The airway management plan should include predicting difficult MV and difficult laryngoscopy, as well as an extubation strategy with a plan made for anterior neck access if required. A strategic plan should be made preoperatively whether the patient can be safely anaesthetised or the airway should be secured in an awake state.


In summary, the preoperative airway assessment is not necessarily to give an exact risk of a difficult airway, but to identify this risk in order to transform an unanticipated (potentially repeated laryngoscopic attempts, an increased incidence of airway and haemodynamic complications) into an anticipated difficult airway with planned and coordinated airway management plans in place before induction of anaesthesia.

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Sep 15, 2020 | Posted by in ANESTHESIA | Comments Off on Chapter 7 – Difficult Airway Prediction
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