BT_AM 1.8 - ANZCA Fellowship Study Notes
Overview and Clinical Importance
Preoperative airway assessment is a fundamental component of the pre-anaesthesia consultation and a core consultant anaesthetist competency. The primary goal is to identify patients in whom laryngoscopy, intubation, mask ventilation, supraglottic airway placement, or surgical airway access may be difficult or impossible, allowing appropriate planning, equipment preparation, and patient communication before anaesthesia is induced.
Failure to anticipate a difficult airway remains a leading cause of anaesthesia-related morbidity and mortality. A structured, systematic approach to airway assessment - integrated with history, examination, and relevant investigations - is essential perioperative practice.
Definitions
| Term | Definition |
|---|---|
| Difficult mask ventilation | Inability of an unassisted anaesthetist to maintain $SpO_2 > 90\%$ with $FiO_2 = 1.0$ using positive-pressure mask ventilation |
| Difficult laryngoscopy | Unable to visualise any part of the vocal cords with conventional laryngoscopy (Cormack-Lehane grade 3-4) |
| Difficult intubation | Tracheal intubation requiring more than three attempts or more than 10 minutes |
| Failed intubation | Inability to place tracheal tube after multiple attempts |
| Difficult surgical airway | Inability to perform emergency front-of-neck access |
History
A targeted history should precede physical examination. Key elements include:
Previous Anaesthetic History
- Prior difficult intubation or difficult mask ventilation (most predictive single factor)
- Documentation in anaesthetic records - check previous records, patient-held alert cards, anaesthetic database flags
- Prior head and neck surgery, radiotherapy, or surgery that may have altered anatomy
Symptoms Suggesting Airway Pathology
- Stridor at rest or on exertion - suggests significant airway compromise (>50% cross-sectional area reduction)
- Dysphagia, odynophagia, dysphonia - may indicate supraglottic, glottic, or pharyngeal pathology
- Positional dyspnoea - concerning for extrinsic airway compression (e.g. mediastinal mass, large goitre)
- Snoring, witnessed apnoeas, daytime somnolence - features of obstructive sleep apnoea (OSA), associated with difficult mask ventilation and potentially difficult intubation
Relevant Conditions
- OSA (obesity, male sex, age, smoking are risk factors)
- Rheumatoid arthritis - atlanto-axial instability, cricoarytenoid involvement, limited mouth opening
- Ankylosing spondylitis - fixed cervical flexion, limited neck extension
- Diabetes mellitus (stiff joint syndrome - "prayer sign")
- Acromegaly, hypothyroidism - macroglossia, goitre
- Tumours, abscesses, trauma, burns, angioedema affecting the airway
- History of ACE inhibitor use - risk of angioedema
Physical Examination - Components of the Airway Assessment
The ASA Task Force components of the airway examination provide a comprehensive framework:
Mouth Opening (Inter-incisor Distance)
| Finding | Threshold of Concern |
|---|---|
| Inter-incisor (or inter-gum if edentulous) distance | $< 3$ cm |
Normal adult mouth opening is approximately 4-6 cm. Reduced mouth opening limits laryngoscope blade insertion and manoeuvring.
Dentition
- Note length and prominence of upper incisors - long upper incisors reduce the effective space for laryngoscope manipulation
- Prominent overbite (maxillary incisors overhanging mandibular incisors at rest)
- Inability to prognath (advance mandibular incisors anterior to maxillary incisors) - concerning finding indicating limited temporomandibular joint mobility
Mallampati Classification
Performed with the patient seated, mouth fully open, tongue maximally protruded, and phonation avoided. Assesses pharyngeal space visibility.
| Class | Structures Visible |
|---|---|
| I | Soft palate, fauces, uvula, tonsillar pillars |
| II | Soft palate, fauces, uvula |
| III | Soft palate, base of uvula only |
| IV | Hard palate only - soft palate not visible |
- Mallampati class III or higher is concerning for difficult laryngoscopy
- A highly arched or very narrow palate is an additional concern
- Sensitivity and specificity are modest when used in isolation (~50%); predictive value improves when combined with other tests
Thyromental Distance
$$TMD = \text{distance from thyroid notch to bony chin (mentum) with neck fully extended}$$
| Finding | Threshold of Concern |
|---|---|
| Thyromental distance | $< 6$ cm |
A short thyromental distance suggests a relatively posterior larynx, reduced submandibular space compliance, or both - indicating that the larynx will be more anterior and difficult to visualise at laryngoscopy.
Mandibular Space Compliance
- The submandibular space must accommodate the tongue during direct laryngoscopy
- Concerning if the space feels stiff, indurated, occupied by mass, or non-resilient
- Clinical examples: Ludwig's angina, post-irradiation fibrosis, haematoma, tumour
Neck Length and Circumference
- A short, thick neck (increased neck circumference) is associated with difficult mask ventilation and difficult intubation, particularly in the context of OSA and obesity
- A large neck circumference (e.g. >40-43 cm) is an independent predictor of difficult intubation in OSA patients
Head and Neck Range of Motion
| Finding | Threshold of Concern |
|---|---|
| Unable to touch chin to chest (flexion) | Concerning |
| Unable to extend neck | Concerning |
- Normal neck extension is approximately 35°; limited extension reduces the ability to align the oral, pharyngeal, and laryngeal axes
- Cervical spine disease, rheumatoid arthritis, ankylosing spondylitis, and post-surgical/radiation fibrosis commonly limit range of motion
Facial Hair
- Heavy facial hair significantly impairs the ability to achieve an effective bag-mask ventilation seal
- Should be documented and considered when planning induction strategy
Multivariate Predictive Models
No single test reliably predicts all difficult airways. Combining multiple variables increases predictive accuracy.
Wilson Risk Score
Combines five factors: weight, head and neck movement, jaw movement, receding mandible, and buck teeth. Scores ≥2 suggest increased risk.
LEMON Score (Emergency Airway)
| Letter | Assessment |
|---|---|
| L | Look externally (trauma, obesity, beard, large tongue) |
| E | Evaluate 3-3-2 rule |
| M | Mallampati score |
| O | Obstruction (foreign body, epiglottitis, abscess) |
| N | Neck mobility |
The 3-3-2 Rule
| Measurement | Description | Threshold |
|---|---|---|
| Mouth opening | 3 finger-breadths between incisors | $< 3$ finger-breadths concerning |
| Mandible length | 3 finger-breadths from chin to hyoid | $< 3$ finger-breadths concerning |
| Hyoid to thyroid notch | 2 finger-breadths | $< 2$ finger-breadths concerning |
Predictors of Difficult Mask Ventilation
The mnemonic MOANS summarises key predictors:
| Letter | Factor |
|---|---|
| M | Mask seal (beard, abnormal facial anatomy, trauma) |
| O | Obese / Obstruction (BMI $> 26$ kg/m²) |
| A | Age $> 55$ years |
| N | No teeth (edentulous - loss of facial support) |
| S | Snores / Stiff lungs (OSA, COPD, asthma) |
Predictors of Difficult Supraglottic Airway Placement
The mnemonic RODS:
| Letter | Factor |
|---|---|
| R | Restricted mouth opening |
| O | Obstruction at or below glottis |
| D | Distorted airway or supraglottic anatomy |
| S | Stiff lungs / Spine (limited neck extension) |
Predictors of Difficult Front-of-Neck Access (FONA)
- Obesity with short neck (difficult landmark identification)
- Prior neck surgery or radiotherapy causing fibrosis and distorted anatomy
- Haematoma or infection over the anterior neck
- Fixed flexion deformity of the cervical spine
- Tracheal deviation (mass, thyroid enlargement)
- Identification of the cricothyroid membrane should be attempted preoperatively and documented
Special Investigations
Imaging
- Plain lateral neck X-ray: tracheal deviation, subglottic narrowing, pre-vertebral soft tissue swelling
- CT neck/thorax: defines extent of mass lesions, tracheal compression, mediastinal involvement - particularly important for anterior mediastinal masses and large goitres
- MRI: soft tissue characterisation, extent of supraglottic tumours
- Flow-volume loops: flattening of inspiratory or expiratory limb suggests fixed or variable extrathoracic/intrathoracic obstruction
Endoscopic Assessment
- Awake flexible nasendoscopy or fibreoptic bronchoscopy allows direct visualisation of supraglottic, glottic, and (with bronchoscope) subglottic/tracheal anatomy
- Essential in patients with stridor, voice change, suspected pharyngeal/laryngeal pathology, or tracheal/mediastinal compression
- Degree and length of tracheal obstruction has direct implications for airway management planning (e.g. ETT position relative to lesion)
Documentation
The ANZCA pre-anaesthesia consultation requires documented airway and dental condition assessment in the anaesthetic record. This must include: - Findings of the structured examination - Mallampati class - Relevant measurements (inter-incisor distance, thyromental distance) - Risk classification - Planned airway strategy and contingency planning
Anaesthetic Implications
Risk Stratification and Planning
Every patient should be stratified as: 1. Predicted normal airway - standard technique, maintain backup equipment ready 2. Predicted difficult airway - primary technique modified, all rescue equipment available, surgical team aware 3. Known difficult airway - awake technique (e.g. awake fibreoptic intubation) strongly favoured; human factors optimised
Decision Framework for Predicted Difficult Airway
| Factor | Consideration |
|---|---|
| Predicted difficult intubation AND easy mask ventilation | Video laryngoscopy as primary; modified RSI may be appropriate |
| Predicted difficult intubation AND difficult mask ventilation | Awake fibreoptic intubation preferred; avoid paralysis before airway secured |
| Cannot-intubate-cannot-oxygenate risk | Surgical airway prep, pre-mark CTM, FONA equipment immediately available |
| Tumour/extrinsic compression | Consider fibreoptic bronchoscopy under sedation to assess and simultaneously intubate |
| Significant tracheal/subglottic stenosis | Bronchoscopic assessment under sedation; ETT positioned above/across lesion as appropriate; tube changer at extubation |
Key Perioperative Principles
- Awake intubation preserves airway tone, respiratory drive, and allows continuous neurological assessment
- Maintain spontaneous ventilation until airway is secured in cases of severe obstruction or anticipated cannot-intubate-cannot-oxygenate scenario
- Preoxygenation is critical - target $SpO_2 \geq 98\%$; consider CPAP/HFNO in obese patients to prolong safe apnoea time
- Extubation planning is as important as intubation; consider airway exchange catheter placement before extubation in any difficult airway patient
- Communication: document difficult airway clearly in the anaesthetic record, inform the patient (consider issuing a Medic Alert card), and communicate to recovery staff and subsequent treating teams
OSA-Specific Considerations
- OSA patients have reduced upper airway muscle tone potentiated by anaesthetic agents and opioids, increasing post-induction and postoperative airway obstruction risk
- Associated with difficult mask ventilation (multiple MOANS criteria often present)
- Positioning (ramped position, 25-30° head-up) improves laryngoscopy conditions and extends safe apnoea duration in obese OSA patients
- Postoperative monitoring with continuous oximetry; consider HDU/ICU for severe OSA or high-risk surgery
Human Factors and Communication
- Ensure adequately skilled assistance is present
- Explicitly communicate the airway plan - primary strategy, first rescue (e.g. video laryngoscopy), second rescue (LMA/SAD), and final rescue (surgical airway)
- A structured team briefing before induction is best practice for any anticipated difficult airway
A thorough, systematic airway assessment - integrating history, structured physical examination, multivariate scoring, and targeted investigations - remains the cornerstone of safe anaesthetic planning and the prevention of critical airway events.