ENT anaesthesia — shared airway, laser precautions, microlaryngoscopy, tracheostomy

Overview and Framework

The shared airway describes any clinical situation in which both the anaesthetist and the operating surgeon or proceduralist require simultaneous or sequential access to the same anatomical field, the mouth, pharynx, larynx, trachea, or proximal bronchi. This creates a fundamental conflict: the anaesthetist's primary responsibility is to maintain a patent, protected airway and ensure adequate gas exchange, while the surgeon's primary goal is unobstructed operative access to the same structures. Neither goal is inherently subordinate to the other, and managing this tension safely requires meticulous pre-operative planning, explicit communication, and a shared mental model of the procedure.

Shared-airway cases encompass an enormous range of complexity, from straightforward dental extractions under general anaesthesia to microlaryngoscopy under total intravenous anaesthesia (TIVA) with jet ventilation, rigid bronchoscopy, laser airway surgery, or endoscopic subglottic procedures. What unites them is the requirement for each team to cede some degree of their preferred working conditions. The anaesthetist may accept a narrower or unconventional airway device, a different depth of anaesthesia, or an interrupted ventilation strategy; the surgeon accepts an airfield partly occupied by tubes, catheters, or fibreoptic equipment.

From a consultant perspective, the key skill is identifying the specific constraints of each case and constructing an anaesthetic plan that satisfies both parties' minimum requirements without compromising patient safety. This demands familiarity with the full range of airway techniques, ventilation strategies, and the pharmacological flexibility to achieve the required degree of relaxation or spontaneous breathing. It also demands the confidence to set explicit limits, the ability to state clearly when a proposed surgical technique poses unacceptable risk to the patient, and to negotiate alternatives before the patient is anaesthetised.

Common Clinical Scenarios

Pre-Operative Planning and Communication

Surgeon-Anaesthetist Briefing

Pre-operative discussion between the anaesthetist and surgeon is not optional in shared-airway cases, it is a safety-critical step. Key points to establish include:

• Nature of the lesion: Is it supraglottic, glottic, subglottic, tracheal? Is it obstructing? Is it vascular, fragile, or at risk of bleeding into the airway?
• Planned surgical technique: Cold-steel dissection, microlaryngoscopy forceps, CO₂ laser, KTP laser, coblation, balloon dilation?
• Ventilation strategy: What tube size or technique will provide adequate surgical exposure?
• Duration: Brief procedures may tolerate apnoeic techniques; longer procedures require sustained ventilation
• Position: Supine with extended neck (typical for laryngoscopy) versus sitting versus lateral
• Contingency: What is the rescue plan if the airway is lost intraoperatively?

Key principle: If the briefing reveals irreconcilable requirements, for example, the surgeon needs an entirely clear larynx but the patient has severe OSA and cannot tolerate apnoea, this must be resolved before anaesthesia commences, not after.

Preoperative Airway Assessment

Standard assessment applies (Mallampati, mouth opening, thyromental distance, neck movement, Cormack-Lehane prediction) but must be supplemented by reviewing any available endoscopic or imaging findings. A patient with a posterior commissure carcinoma may have a Mallampati I oropharynx yet a grade 3-4 laryngoscopic view because the tumour displaces the posterior larynx. CT and flexible nasendoscopy findings should be reviewed by the anaesthetist, not just the surgeon.

• Consider whether awake fibreoptic intubation is preferable to gaseous induction
• Assess whether the lesion may bleed on instrumentation
• Plan the tube type, size, and route (oral vs nasal) before entering the operating room

Ventilation Strategies in the Shared Airway

The central technical challenge is maintaining adequate oxygenation and CO₂ elimination when surgical access conflicts with standard endotracheal ventilation. Several strategies exist, each with distinct advantages and limitations.

Small-Bore Endotracheal Tube

The simplest compromise, a cuffed microlaryngoscopy tube (typically 5.0-6.0 mm ID in adults, with a long cuff designed for subglottic placement) occupies the posterior commissure, leaving the anterior two-thirds of the glottis and supraglottis accessible. This is adequate for most diagnostic and minor laryngeal procedures.

• Provides full airway protection and controlled ventilation
• Increased airway resistance requires pressure-controlled ventilation; peak airway pressures rise considerably with tube ID < 6.0 mm
• Limits access to the posterior commissure and interarytenoid region

Supraglottic Airway Devices

An LMA or SLIPA may be appropriate when surgery is entirely above the glottis (oropharynx, tonsil, palate) and the larynx itself does not require access. The device sits below the surgical field.

• Permits unobstructed surgical access to oropharynx
• Less suitable if surgical bleeding risk is high (no cuff to protect the larynx)
• Reinforced ("flexible" or "armoured") LMAs are less likely to kink under surgical retractors

Tubeless Techniques: Apnoeic Oxygenation

During brief procedures (typically < 3-5 minutes per apnoeic interval), the trachea is not instrumented and ventilation is paused to allow the surgeon complete laryngeal access. The patient is pre-oxygenated, deeply anaesthetised (usually TIVA), and an adequate period of SpO₂ > 95% maintained during the apnoeic interval.

• Transnasal Humidified Rapid-Insufflation Ventilatory Exchange (THRIVE) via high-flow nasal oxygen (30-70 L/min) prolongs safe apnoea time significantly by providing apnoeic oxygenation and some CO₂ washout, extending the safe apnoeic window from ~3-4 minutes to 10-15 minutes or more in healthy patients
• PaCO₂ still rises at approximately 3 mmHg/min during apnoea; THRIVE blunts but does not eliminate this rise
• Not suitable for patients with severe obesity, significant respiratory disease, or high oxygen requirements

Jet Ventilation

Jet ventilation delivers high-velocity pulses of oxygen-enriched gas into the airway, entraining air by the Venturi effect and achieving tidal volume without an endotracheal tube occupying the glottis. It may be delivered supraglottically (supra-glottic jet ventilation, SGJV) via a rigid laryngoscope port or subglottically (subglottic jet ventilation, or transtracheal jet ventilation via percutaneous catheter).

Key parameters (manual or automated jet ventilator):

• Driving pressure: typically 0.5-3.5 bar (higher pressures for transtracheal or subglottic delivery; lower for supraglottic)
• Frequency: 6-20 breaths/min (conventional jet) or 60-600 breaths/min (high-frequency jet ventilation, HFJV)
• FiO₂: effectively 1.0 with supraglottic jet but entrainment dilutes delivered oxygen

Hazards of jet ventilation:

• Barotrauma, pneumothorax, pneumomediastinum, surgical emphysema, particularly if there is any outflow obstruction (tumour, oedema, subglottic stenosis). The jet must have a clear exit path
• Gas trapping, insufficient expiratory time causes dynamic hyperinflation; chest must be seen to fall before the next jet
• Mucosal drying, humidification preferred for prolonged use
• Awareness, no volatile agent delivery; TIVA is mandatory
• CO₂ monitoring, ETCO₂ is unreliable; arterial blood gas sampling is required for procedures lasting more than 15-20 minutes

Examination tip: Jet ventilation is absolutely contraindicated when the airway cannot reliably be assessed for obstruction, an anxious patient moving, an uncooperative larynx, or a lesion likely to completely obstruct expiratory flow.

Rigid Bronchoscopy Ventilation

During rigid bronchoscopy, ventilation is delivered through the side-arm of the bronchoscope (Sanders injector or ventilating bronchoscope). Principles are identical to jet ventilation. The proximal end of the bronchoscope is often open or intermittently occluded with a glass cap, making full ventilation/ETCO₂ monitoring impossible. This reinforces the requirement for TIVA and periodic arterial blood gas monitoring.

Laser Airway Surgery: Specific Hazards

Laser surgery of the larynx and trachea carries unique hazards that directly involve the anaesthetist.

Airway Fire

An airway fire is among the most catastrophic intraoperative complications. The surgical laser provides ignition; oxygen and nitrous oxide are oxidisers; the endotracheal tube and airway secretions are fuel.

Risk-reduction strategy (the fire triad):

• Laser-safe tubes have double cuffs inflated with saline (coloured with methylene blue to indicate cuff breach)
• Jet ventilation eliminates the tube-as-fuel risk but does not eliminate mucosal fire risk
• Airway fire drill must be rehearsed: immediately stop laser, flood field with saline, remove ETT, ventilate with 100% O₂ via facemask, reintubate, assess burn extent, arrange bronchoscopy and ICU

Other Laser Hazards

• Eye injury: All staff and the patient require appropriate wavelength-specific protective eyewear (CO₂ laser: plain glass; KTP/Nd:YAG: specific optical density eyewear)
• Smoke plume: Contains carcinogens and viable viral particles; smoke evacuator mandatory

Dental and Oral Surgery

Dental and maxillofacial cases require the oral cavity to be accessible while protecting the lower airway from blood, bone fragments, dental debris, and irrigation fluid. This makes throat pack insertion routine practice.

Throat Pack

• Purpose: Provides a seal around the tube or LMA in the posterior oropharynx, preventing soiling of the larynx and tracheobronchial tree
• Pack must be documented in the anaesthetic chart and a visible external marker attached (e.g. tape to the patient's cheek or pack tied to the ETT)
• Retained throat pack is a never event, a WHO-style check must occur before extubation
• Packs must be removed before extubation, with the patient still anaesthetised or in a sufficiently deep plane to suppress laryngeal reflexes while allowing safe retrieval

Nasal Intubation

Most intraoral surgical procedures require nasal intubation (nasal RAE tube or reinforced nasal tube) to relocate the airway circuit out of the surgical field.

• Contraindicated with basal skull fracture, significant coagulopathy, nasal obstruction, or mid-face trauma
• Nasopharyngeal structures may bleed significantly on passage, vasoconstrictor (0.05% xylometazoline or cocaine 4-5%) applied topically pre-insertion
• Preformed nasal RAE tubes have a fixed curvature, tube tip position must be confirmed as a correctly positioned tracheal tube, not a bronchial intubation, and re-confirmed if head position changes

Tracheostomy Under General Anaesthesia

Elective surgical tracheostomy while the patient is orally intubated requires coordinated withdrawal of the ETT as the tracheal window is created.

• The anaesthetist advances the ETT cuff distal to the planned incision site initially, then withdraws the tube progressively as the trachea is entered
• Communication is essential: the surgeon must alert the anaesthetist before entering the trachea; the anaesthetist must confirm SpO₂ and ETCO₂ are acceptable before tube withdrawal
• The ETT is never fully removed until the tracheostomy tube is confirmed in position and CO₂ is detected
• Difficult airway trolley must be immediately available; the original intubating route (oral) must be maintainable throughout

Maintaining Adequate Anaesthesia Without Inhalational Agents

Jet ventilation, rigid bronchoscopy, and laser techniques are all TIVA-mandatory because volatile agents cannot be delivered reliably through open systems and because circuit leakage poses occupational exposure risk.

• Propofol target-controlled infusion (TCI) using the Schnider or Marsh model is the standard backbone (effect-site target typically 3-5 mcg/mL for surgical anaesthesia)
• Remifentanil TCI (effect-site target 3-8 ng/mL) provides excellent obtundation of airway reflexes and allows rapid titration
• Neuromuscular blockade (suxamethonium 1-1.5 mg/kg for intubation; rocuronium 0.6-1.2 mg/kg for maintenance) is used when immobility and vocal cord abduction are required; sugammadex 200 mg (2 mg/kg) for moderate block reversal or 400-600 mg for deep block reversal must be immediately available
• Depth of anaesthesia monitoring (BIS or entropy) is strongly recommended given the reliance on TIVA with no ETCO₂ during open techniques

Emergence and Extubation in Shared-Airway Cases

Post-operative airway oedema, haematoma, surgical debris, and altered anatomy make extubation in shared-airway cases higher risk than average. Key considerations:

• Awake extubation is the default for any patient with a difficult airway, significant intraoperative surgical trauma, or uncertainty about airway patency post-operatively
• Inspect the throat pack site and oropharynx by direct laryngoscopy before extubation
• Dexamethasone 8 mg IV given intraoperatively reduces airway oedema for laryngeal and subglottic procedures
• If significant oedema is anticipated, consider elective postoperative intubation and delayed extubation in ICU, or surgical tracheostomy at the time of the primary procedure
• Cuff-leak test (deflate ETT cuff and assess for airflow around tube before extubation) provides some reassurance but has limited sensitivity for excluding clinically significant oedema in isolation

Summary and Examination Strategy

Shared-airway cases represent some of the highest-stakes situations in anaesthetic practice because airway loss and inadequate surgical access carry simultaneous, competing risks. Examination questions in this domain typically probe:

1. Recognition of the specific constraints in a given scenario (what the surgeon needs vs what the anaesthetist requires)
2. Technique selection, being able to justify a choice among small tube, LMA, apnoeic oxygenation, THRIVE, and jet ventilation with reference to patient factors, procedure type, and duration
3. Laser fire, the fire triad, prevention, and emergency management
4. TIVA pharmacology, propofol/remifentanil dosing, NMB and reversal in open airway scenarios
5. Communication, the consultant-level expectation that the plan is agreed and verbalised before induction, that contingencies are explicit, and that the team is briefed

The unifying principle across all shared-airway scenarios is that the anaesthetist retains ultimate responsibility for the airway throughout the procedure, surgical convenience never supersedes patient safety, and that every technical concession made to the surgeon must be accompanied by a clear backup plan if that concession proves untenable.

Sources

• ANZCA Professional Documents (PS series)
• ANZCA clinical guidelines