Anaesthesia for airway stenting

Learning objectives.

By reading this article you should be able to:

• •List the indications for airway stenting and understand the rationale for stent insertion.
• •Describe the challenges and risks associated with airway stenting procedures.
• •Discuss the options for inducing and maintaining anaesthesia, airway management and ventilation during the procedure.
• •Identify the obstructed central airway and describe its emergency management.

Key points.

• •Airway stents are used in the management of central airway obstruction.
• •Airway stents can be inserted for benign or malignant disease and for pulmonary or extrapulmonary pathologies.
• •Airway stenting procedures pose challenges for the anaesthetist, including safely inducing and maintaining anaesthesia, providing adequate gas exchange and managing life-threatening postoperative complications.
• •In extreme circumstances, where the degree of airway obstruction is prohibitively severe, extracorporeal membrane oxygenation may be required.
• •Jet ventilation is often required during airway stenting procedures. The anaesthetist should be familiar with jet ventilation techniques.

Airway stents are tracheobronchial prostheses that are used to relieve airway obstruction and maintain airway patency. The Montgomery T-tube, developed in the 1960s, represents the earliest prototype of an airway stent and remains in use for the treatment of subglottic and proximal tracheal stenoses.¹ The first endoluminal stents were introduced by Dumon in 1987.²

Airway stents are primarily used for treating obstructions involving the central airways. Here, the term ‘central airways’ refers to the trachea and main bronchi. Obstructions involving the trachea or carina are more demanding for the anaesthetist and pose greater risk to the patient than those involving the bronchi. In this article we discuss the challenges that airway stenting procedures present for the anaesthetist. These challenges include managing patients with advanced thoracic disease, risk stratification, safely inducing and maintaining anaesthesia in the presence of airway obstruction, managing a shared airway and treating life-threatening complications.

Types of airway stents

A wide variety of airway stents are available for clinical use. The type of stent chosen depends on individual patient anatomy and the underlying disease process.

Airway stents are manufactured from metal, silicone or a combination of materials. Metallic stents may be fixed-diameter or self-expanding, and may be covered or uncovered. Fixed-diameter metallic stents require balloon dilatation during deployment. Self-expanding metallic stents are typically manufactured from nitinol, an alloy of nickel and titanium. Nitinol stents are highly elastic and have shape memory. Self-expanding stents have the advantage of being able to be inserted using flexible bronchoscopy as opposed to requiring rigid bronchoscopy. Covering materials for metallic stents include polyurethane, polytetrafluorethylene (Teflon), and silicone. Silicone stents have a fixed diameter and may be reinforced with polypropylene or carbon fibre. The advantages and disadvantages of the different types of stents are summarised in Table 1.

Table 1.

Advantages and disadvantages of different types of airway stents.

Type of stent	Advantages	Disadvantages
Silicone	•Repositioning or removal usually possible •Less tumour invasion and granulation tissue formation	•Rigid bronchoscopy required for deployment •Increased risk of migration. Some silicone stents, including the Dumon stent, have integrated studs on the external wall to reduce the risk of migration •Higher rates of infection than metallic stents
Metallic	•May be inserted using flexible bronchoscopy •Larger internal diameter/external diameter ratio than silicone equivalent •Self-expanding devices that exert a greater radial force on the airway mucosa	•Challenging to reposition or remove •increased risk of vascular perforation •Higher rates of tumour invasion and granulation tissue formation
Hybrid	•Composed of more than one type of material. Harnesses benefits and mitigates disadvantages of each	•Usually more costly than stents composed of a single material

Airway stents maintain their position by exerting radial pressure and frictional forces on the airway mucosa (Fig. 1).³ Metallic stents – particularly uncovered metallic stents – also become embedded into the airway mucosa, meaning they are less likely to migrate but are more difficult to reposition or remove. Consequently, silicone stents are preferred for patients with non-malignant disease, as subsequent removal may be required.

Fig 1.

Tracheal stenting with metallic stent. (A) Severe extrinsic compression of the trachea from advanced oesophageal cancer. (B) Appearances after placement of an uncovered metallic stent. Image reproduced from Nagano and colleagues³ (article distributed under the terms of the Creative Commons Attribution Licence).

More recently, attempts have been made to produce biodegradable stents. Biodegradable stents would be potentially useful in situations in which stent removal was anticipated – for instance in patients who develop tracheal stenosis after tracheostomy. Biodegradable stents are not currently available for clinical use and clinical trials have yielded mixed results.⁴

In addition to differing types of materials, stents also vary in shape and size. For example a Y-shaped stent may be required for a patient with carinal disease. Custom-made stents are now available that can be produced to suit an individual patient's anatomy.⁵

Indications for airway stenting

Patients typically undergo stenting procedures for symptomatic narrowing of the central airways caused by mechanical obstruction or tracheobronchomalacia. Occasionally, airway stenting is used for treating a fistulous connection involving the airways and surrounding structures.

Malignant disease

The primary indication for airway stenting is malignancy. Causes include cancers of the respiratory tract (e.g. primary lung cancer, adenoid cystic carcinoma, carcinoid disease); cancers involving the adjacent structures (e.g. sarcomas, lymphomas, and cancers involving the oesophagus and the thyroid gland); and metastatic disease. Overall, 20–30% of patients with primary lung cancer develop obstruction of the central airways.⁶ Tumours that metastasise to the airway include breast and colorectal cancers. For most patients with malignant disease, airway stenting is a palliative procedure, which, nevertheless, is effective in providing symptomatic relief and improving quality of life.⁷ Airway obstruction caused by malignant disease can be categorised as extraluminal, intraluminal or mixed. Airway stenting can be used for both types of obstruction.

Non-cancer and benign disease

Airway stenting may be used for treating obstruction from benign disease or causes unrelated to cancer. However, airway stenting in patients without cancer is problematic because of a high rate of late complications and difficulties related to repositioning or removal, particularly for metallic stents. The United States Food and Drug Administration has published an advisory cautioning against using metallic stents in patients with benign disease.⁸

Anastomotic strictures are a recognised complication of lung resection surgery and lung transplantation. Airway stenosis can occur after prolonged periods of tracheal intubation or after tracheostomy. Although airway stenting is an option in these patients, definitive surgery, in the form of tracheal resection, is usually preferred.⁹

Tracheobronchomalacia may be congenital or acquired and is associated with a variety of multisystem diseases. However, in adults, the cause is frequently unknown. Occasionally, tracheobronchomalacia occurs as a late presentation of previously undiagnosed tracheobronchomegaly (Mounier–Kuhn syndrome). Acquired tracheobronchomalacia may be associated with chronic respiratory disease (e.g. asthma, chronic bronchitis, emphysema), and inflammatory diseases (e.g. relapsing polychondritis), toxin exposure and chronic airway compression (e.g. thyroid goitre, vascular abnormalities).¹⁰

Clinical presentation

Patients with central airways obstruction present with a variety of respiratory problems including cough, dyspnoea, haemoptysis and infection. Clinical signs associated with central airways obstruction include wheeze or crackles on auscultation. Stridor may be present if there is severe obstruction involving the trachea.¹¹ The degree of endoluminal obstruction required to cause symptomatic obstruction has not been clearly defined. Greater than 50% obstruction may be considered severe. A frequently cited myth is that exertional symptoms do not occur until the tracheal diameter is less than 8 mm and rest symptoms do not occur until the endoluminal diameter is less than 5 mm. Although the degree of obstruction is an important determinant of airflow limitation, multiple factors may contribute to a patient's perception of dyspnoea. Respiratory muscle strength, the presence of pulmonary parenchymal or cardiac disease, metabolic demand, along with pain and anxiety all contribute to the sensation of dyspnoea.¹²

Before surgery

Preoperative planning

Ideally, all patients presenting for airway stenting should be reviewed by a multidisciplinary team, including the operator (respiratory physician, thoracic surgeon), anaesthetist and radiologist. Depending on the circumstances, it may be appropriate to have input from a palliative care physician or an intensivist. Key issues to determine before surgery are the type of stent to be used, the location and severity of the obstruction, the operative technique, the need for general anaesthesia, rescue interventions, postoperative management and the ceiling of care. Patients should be appropriately counselled regarding the risks and benefits of the procedure and the goals of care.¹³

Multidetector computed tomography (MDCT) with three-dimensional (3D) reconstruction is the optimal imaging technique before airway stenting. This technique accurately characterises the location, severity and extent of the lesion, and therefore helps operative planning and selection of the appropriate stent. Multidetector computed tomography also allows dynamic assessment of the large airways if tracheobronchomalacia is suspected.¹⁴

The operative technique depends on the location of the obstruction and the type of stent deployed. In general, self-expanding metallic stents involving the trachea or bronchi can be deployed using flexible bronchoscopy. If the procedure can be done with flexible bronchoscopy, then conscious sedation is often appropriate. Insertion of fixed-diameter metallic stents and silicone stents usually requires rigid bronchoscopy. Rigid bronchoscopy involves placing a hollow, large-bore metal tube into the trachea via the mouth allowing visualisation of the larynx, trachea and bronchi. A deep plane of general anaesthesia is required as the rigid bronchoscope is highly stimulating. General anaesthesia and rigid bronchoscopy may also be required for high-grade obstructions involving the trachea or carina, irrespective of the type of stent used.

The use of any other bronchoscopic interventions must be clarified preoperatively. For example if laser ablation is required, techniques to prevent and manage airway fires are necessary.¹⁵ It is useful to consider the optimal location for inducing anaesthesia. If there is concern regarding maintaining airway patency, we favour inducing anaesthesia in the operating theatre rather than the anaesthetic room, as this approach allows rigid bronchoscopy to be carried out rapidly as a rescue technique.

Anaesthesia assessment and optimisation

Patients presenting for airway stenting typically have important comorbidities. In particular, patients with thoracic malignancies may have a significant smoking history and associated cardiovascular and pulmonary disease. Guidelines for the preoperative management of patients undergoing thoracic surgery tend to focus on lung resection surgery, and therefore are not fully applicable to airway stenting procedures.¹⁶ Co-existing diseases should be optimised as far as possible. Specifically, pulmonary infection, bronchospasm and unstable coronary artery disease should be treated and fluid and electrolyte disturbances corrected. However, it is important to recognise that airway stenting is frequently a palliative procedure that is performed for symptomatic relief of airway obstruction. It is often not possible to correct all abnormalities, particularly when patients present acutely. A list of appropriate preoperative investigations is shown in Table 2.

Table 2.

Preoperative investigations.

Investigation	Rationale	When to consider
Full blood count	Assess for anaemia. Important determinant of oxygen-carrying capacity.	All patients.
Biochemistry	Assessment of thyroid function and investigation for paraneoplastic syndromes may be important.	Thyroid function biochemistry important if underlying thyroid pathology is the indication for airway stenting. Useful to exclude or confirm a paraneoplastic syndrome in patients with underlying malignant disease.
ECG	Allows baseline comparison in the event of signs of myocardial ischaemia or infarction during the perioperative period.	All patients.
Chest X-ray	Allows assessment of any pulmonary collapse or pneumonia and any mediastinal shift.	All patients.
Transthoracic echocardiogram	Allows assessment of baseline cardiac function.	Patients with risk factors for or known cardiac disease
Chest CT with three-dimensional reconstruction	Allows detailed assessment of airway anatomy. Areas of stenosis may be identified. May assist in airway management planning.	All patients.
Pulmonary function tests	Permits assessment of the level of obstruction and whether the obstruction is fixed or dynamic. Also allows assessment of underlying pulmonary parenchymal disease.	All patients.

Extracorporeal support

In cases of very severe tracheal obstruction, where inducing anaesthesia and initiating positive pressure ventilation might precipitate total airway occlusion, venovenous extracorporeal membrane oxygenation (VV ECMO) may be considered. This device provides satisfactory gas exchange in the absence of pulmonary ventilation and has the advantage that it can be initiated with the patient awake, before induction of anaesthesia. Careful planning is required in terms of assembling the appropriate team and equipment, and discussing pertinent risks with the patient. Outcomes for patients are improved if VV ECMO is used electively, rather than as a rescue technique after failed management of the obstructed airway.¹⁷ Nevertheless, complications related to VV ECMO can occur including vascular injury during cannulation and complications related to the circuit and anticoagulation (bleeding and thrombosis).¹⁸

Management during the procedure

Airway stenting involves a shared airway. As such, clear and effective communication between the anaesthetist and the operator is paramount at all stages of the procedure.

Sedative premedication in an unmonitored environment is not advisable in patients presenting for airway stenting procedures because of the risk of compromising respiratory function further and the potential for complete airway obstruction.

Conscious sedation

As outlined above, general anaesthesia is usually required for procedures involving rigid bronchoscopy or high-grade obstructions involving the trachea. Flexible bronchoscopy can typically be undertaken with adequate local anaesthetic topicalisation of the airway. Conscious sedation may improve tolerance of the procedure. Judicious use of short-acting opioids as part of a sedative regimen is particularly useful for their antitussive effects. Transnasal humidified rapid-insufflation ventilatory exchange (THRIVE) is another useful adjunct that can minimise the risk of inadequate gas exchange during flexible bronchoscopy carried out under conscious sedation.

General anaesthesia

In addition to standard monitoring, placement of an intra-arterial catheter is appropriate for patients requiring general anaesthesia for airway stenting procedures. Depth of anaesthesia monitoring (e.g. bispectral index) is useful, as TIVA is typically used for procedures requiring rigid bronchoscopy. A dedicated, reliable i.v. catheter should be used for TIVA.¹⁹

Anaesthesia may be induced using either an inhalation or intravenous technique. There are no data recommending one technique over another, and the choice is determined by the patient's condition and the anaesthetist's preference and experience. General anaesthesia reduces the tone of the trachealis muscle resulting in the trachea becoming less rigid. Reduced parenchymal traction decreases the cross-sectional area of the large airways. These effects are exacerbated by neuromuscular blocking drugs. Coughing can precipitate acute airway obstruction. Thus, irrespective of the technique chosen, extreme caution should be exercised when inducing general anaesthesia in patients with a high-grade tracheal obstruction. In this setting, a reasonable approach is to induce anaesthesia but to maintain spontaneous ventilation and then assess the degree of dynamic airway obstruction with flexible bronchoscopy before giving neuromuscular blocking drugs. Topicalisation of the airway with a lidocaine spray and ensuring an adequate depth of anaesthesia help prevent coughing during this time. Rescue with rigid bronchoscopy (or emergency VV ECMO) should be immediately available when inducing anaesthesia in patients with severe tracheal obstruction.

The choice of airway device depends on factors related to the patient and to the type of procedure being carried out. For airway stents placed using flexible bronchoscopy, either a tracheal tube or supraglottic airway device may be used, with or without neuromuscular blocking drugs. Compared with a tracheal tube, a supraglottic device facilities access to the proximal trachea and may be associated with less coughing during emergence from anaesthesia, potentially reducing the risk of stent migration. The use of THRIVE has been described for tubeless airway surgery.²⁰ Transnasal humidified rapid-insufflation ventilatory exchange facilitates oxygenation and allows some carbon dioxide elimination in the absence of ventilation, and so is another option for selected patients undergoing general anaesthesia for procedures performed using flexible bronchoscopy.

Irrespective of the airway used, anaesthesia may be maintained with either a volatile anaesthetic agent or TIVA. However, for procedures performed using a rigid bronchoscope, TIVA offers several advantages over inhalation anaesthesia. To facilitate inhalation anaesthesia, the anaesthesia circuit may be connected to a side port of a rigid bronchoscope via a swivel connector. However, this approach requires intermittently occluding the working port of the bronchoscope in order to ventilate and deliver anaesthesia gases to the patient. In addition, it is not possible to accurately measure end-tidal gas concentrations. The leak of volatile anaesthetic agents from the airway can also result in significant pollution of the operating theatre environment, with high gas flows required in order to overcome the circuit leak. For these reasons, TIVA in combination with jet ventilation is typically preferred when rigid bronchoscopy is required.²¹ The addition of an infusion of remifentanil to target-controlled anaesthesia with propofol is useful, as the rigid bronchoscope is highly stimulating to the airway.

Long-acting opioids are best avoided or given at low dose. Patients undergoing airway stenting procedures do not experience significant postoperative pain, but using long-acting opioids risks causing postoperative respiratory depression and reduces the patient's ability to clear their respiratory secretions.

Jet ventilation

Jet ventilation involves the delivery of high-flow, compressed gas at high pressure (0.3–3 bars). Jet ventilation can be delivered by a variety of systems, which can broadly be divided into manual and electrically powered devices. Electrically powered jet ventilators are convenient as they free the anaesthetist from manually ventilating the patient. For both techniques, the fraction of inspired oxygen (Fio₂) delivered tends to be lower than anticipated, because of the entrainment of air.

Low-frequency jet ventilation refers to jet ventilation that provides breaths within a physiological range (8–30 min⁻¹). Low-frequency jet ventilation is typically used with manual devices. The majority of ventilation occurs as a result of convection (bulk flow). With high-frequency jet ventilation, breaths are delivered at rates greatly in excess of the physiological range – typically 120–600 impulses min⁻¹, which generates tidal volumes of 1–3 ml kg⁻¹. High-frequency jet ventilation is typically used with electrically powered ventilators. Ventilation is achieved by convection along with several other gas-stream mechanisms.²²

When using an electrically powered jet ventilator, alarms should be set to limit the insufflation pressures delivered to the patient's airway. Two important alarms are usually set: the pause pressure and the peak inspiratory pressure. The pause pressure reflects the mean airway pressure, and when a preset high value is reached, further inspiration is suspended in order to prevent gas trapping. When choosing the settings for an electrically powered jet ventilator, there are three major factors that affect the volume of gas delivered to the patient: (1) the frequency, expressed in counts per minute (CPM); (2) the inspiratory time, expressed as a percentage of the total cycle time that is spent in inspiration; and (3) the driving pressure, typically 0.3–3 bars. Reasonable initial settings are a fraction of inspired oxygen of 1.0, a CPM of 100, an inspiratory time of 50%, a driving pressure of 1.5 bars, a pause pressure of 25 mbar and a peak inspiratory pressure of 25 mbar.²³

Complications

The ACCS AQuIRE (American College of Chest Physicians Quality Improvement Registry, Evaluation, and Education) registry recently published the results from a large, prospective review of therapeutic bronchoscopy for central airways obstruction.²⁴ The overall rate of complications was relatively low, being 3.9%. However, when complications did occur, they tended to result in serious adverse events including permanent disability and death. The mortality rate attributed to procedural complications was 0.5%. However, the overall 30-day mortality rate was 14.8%, which may reflect disease progression in a population with underlying, end-stage disease. Patients undergoing bronchoscopy for airway stenting had an increased 30-day mortality rate compared with other types of therapeutic bronchoscopy (22.4% vs 9.7%). Again, this difference is likely to be explained by the fact that patients requiring stents have more severe underlying disease.

Early complications

Early complications are those related to the procedure itself. Rigid bronchoscopy may result in dental damage along with injuries to the mouth, oropharynx and airways. When combined with positive pressure ventilation, airway injury risks causing barotrauma. Even in the absence of an airway injury, jet ventilation can result in gas embolism, tension pneumothorax, tension pneumomediastinum and development of subcutaneous emphysema.²⁵ Tension pneumothorax or tension pneumomediastinum can result in severe hypotension or even cardiac arrest. As in any procedure involving a shared airway, inadequate oxygenation and ventilation can occur, with the potential for hypoxaemia, hypercarbia and respiratory acidosis. Hypoxaemia and hypercarbia are more likely to occur in patients with pre-existing pulmonary disease. Predictors of adverse cardiorespiratory events include ASA physical status 4 and a baseline oxygen saturation less than 96%.²⁶ If there is persistent hypoxaemia, the procedure should be paused, and the cause investigated. Tracheal intubation and conventional ventilation should be initiated, and pneumothorax excluded by clinical examination and chest radiography.

The most serious early complication of airway stenting procedures is acute airway obstruction. Acute airway obstruction can occur at the time of anaesthesia induction, during the procedure or in the recovery period. As discussed above, patients with high-grade tracheal obstruction are at risk of complete airway obstruction after induction of anaesthesia. An inability to lie flat, stridor at rest and marked hypoxia predict this problem. Immediate rigid bronchoscopy to identify and relieve the obstruction can be life-saving and should be immediately available. Prophylactic or emergency VV ECMO may be appropriate in some patients. Airway obstruction during or after the procedure can occur from stent dislodgement or bleeding into the airway.

Airway obstruction in the immediate postoperative period is easily misdiagnosed as the residual effects of anaesthesia, particularly in patients with severe underlying respiratory disease. The patient may emerge from anaesthesia normally but subsequently become obtunded because of progressive hypercarbia, with or without hypoxia. There may be evidence of abdominal respiratory effort in the absence of effective ventilation. Alternatively, the patient may not establish effective spontaneous ventilation at the end of the procedure, in which case the situation may resemble residual neuromuscular block. Unless treated rapidly, the patient may progress to cardiorespiratory arrest. Immediate rigid bronchoscopy is mandatory to diagnose the problem and relieve the obstruction. If airway obstruction is not identified, tracheal intubation and positive pressure ventilation should be initiated. An urgent chest radiograph should be obtained to exclude tension pneumothorax and tension pneumomediastinum.

Late complications

In the context of airway stenting, late complications are those occurring beyond the immediate postoperative period. Late complications are more likely in patients with benign disease, who have a longer life expectancy, than in patients with malignant disease. Late complications include stent migration, stent fracture, in-stent stenosis, erosion of the stent into surrounding structures and chronic bacterial colonisation of the stent.

Stent migration is more likely to occur in patients who have undergone isolated tracheal stenting and those with silicone stents placed compared with other patients. Granuloma formation within or adjacent to the stent can lead to airway stenosis. Bacterial colonisation may predispose granuloma formation and lead to increased mucus production. Even in the absence of bacterial colonisation, airway stents impair mucociliary clearance and can lead to mucous plugging, and atelectasis. Erosion of the stent into surrounding structures – particularly the oesophagus – can result in haemorrhage, fistula formation and infection.

Granulomas can be treated with cryotherapy or with heat ablative therapy, in the form of laser therapy or argon plasma coagulation. Removal or repositioning of silicone stents can usually be achieved with fibreoptic or rigid bronchoscopy. However, removal of metallic stents – particularly after a prolonged period in situ – is associated with a high-risk of airway or vascular injury. Therefore, the anaesthetist should be prepared for major haemorrhage or conversion to an open thoracic procedure.

Conclusions

Airway stenting is typically performed for central airways obstruction in patients with malignant disease. Challenges for the anaesthetist include safely inducing and maintaining anaesthesia, ensuring adequate gas exchange and managing immediately life-threatening postoperative complications. For procedures performed under general anaesthesia with a rigid bronchoscope, TIVA offers several advantages over inhalation anaesthesia. Jet ventilation is typically required. Extracorporeal support may be needed in cases of extreme airway obstruction.

Declaration of interests

The authors declare that they have no conflicts of interest.

MCQs

The associated MCQs (to support CME/CPD activity) will be accessible at www.bjaed.org/cme/home by subscribers to BJA Education.

Biographies

Niamh Barnwell FCAI has completed a specialist training fellowship in cardiothoracic anaesthesia at the Mater Misericordiae University Hospital, Dublin. Her interests include cardiothoracic anaesthesia, transoesophageal echocardiography and high-risk anaesthesia.

Martin Lenihan FCAI is lead cardiothoracic anaesthetist at the Mater Misericordiae University Hospital, Dublin. His interests include cardiothoracic anaesthesia, anaesthesia for heart and lung transplant and transoesophageal echocardiography. He has previously undertaken international fellowships in cardiac and thoracic anaesthesia.

Matrix codes: 1A01, 1C02, 2A01, 2A03, 3A01, 3G00