Interventional Bronchoscopy: Airway Stents

General description of procedure, equipment, technique

Airway Stents:

The definition of interventional pulmonology, originally published in European Respiratory Society (ERS) and American Thoracic Society (ATS) guidelines is “the art and science of medicine as related to the performance of diagnostic and invasive therapeutic procedures that require additional training and expertise beyond that required in a standard pulmonary medicine training program.” Clinical entities encompassed in this discipline include complex airway management, benign and malignant central airway obstruction, and pleural disease.

Diagnostic and therapeutic procedures in interventional pulmonology include rigid bronchoscopy, transbronchial needle aspiration, autofluorescence bronchoscopy, endobronchial ultrasound, transthoracic needle aspiration and biopsy, laser bronchoscopy, endobronchial electrosurgery, argon plasma coagulation, cryotherapy, airway stent insertion, balloon bronchoplasty and dilatation techniques, endobronchial radiation (brachytherapy), photodynamic therapy, percutaneous dilatational tracheotomy, transtracheal oxygen catheter insertion, medical thoracoscopy, and image-guided thoracic interventions. The following section will discuss airway stent insertion; additional procedures are discussed elsewhere.

The “stent” is named after the nineteenth-century British dentist, Charles R. Stent, who developed a dental impression material that was later used as a template to support healing skin grafts. Today, the term connotes any artificial support that maintains patency of a hollow tubular structure. When a large airway is compressed by an extraluminal lesion, airway stents play an important therapeutic role. Although there is no ideal stent, important attributes of the device include ease of insertion and removal, the absence of a proclivity to migrate in the airway, the capacity to withstand compression from external forces, minimal tendency to elicit reaction in the surrounding tissue, and the capacity to permit mobilization of respiratory secretions. Airway stents are primarily placed in the trachea, right and left main stem bronchi, and the right bronchus intermedius. Data are emerging regarding the placement of stents in lobar and segmental bronchi, referred to as “lobar salvage”.

Types of Stents:

Silicone stents: Silicone, a synthetic substance made of silicone elastomers, is firm and stable at high temperatures. Several tracheobronchial silicone stents are commercially available including the Dumon, Polyflex, Noppen, and Hood stents. The benefits of silicone stents over metallic stents include relative easy of removal and replacement, lower cost, and no tumor invagination. Disadvantages of silicone stents include their tendency to migrate and induce granuloma formation, insufficient flexibility to conform to irregular airways, higher rate of mucostasis, lower inner to outer diameter ratio, and the need for rigid bronchoscopy for placement.

Metallic stents:

The first generation airway metallic stents (Gianturco, Strecker, and Palmaz) were uncovered, rigid, non-conforming, stainless steel stents associated with an increased incidence of airway and vascular erosion. The second generation Wallstent and Ultraflex stents are available in both covered, partially and uncovered varieties while the Aero stent is a nitinol stent fully covered in polyurethane. All metallic stents are radiopaque, and exhibit varying degrees of dynamic expandability. When uncovered, metallic stents maintain ventilation and some ability to evacuate secretions if placed over lobar orifices.

Metallic stents include fixed-diameter stents, which require balloon dilatation, and self-expandable stents, which “spring” to a predetermined diameter once released. The stents are made of bare metal or have a thin coating of silicone, nylon, or polyurethane.

Metallic stents have gained popularity because of their ease of insertion. In addition, they may be placed in an outpatient setting via flexible bronchoscopy under local anesthesia. Other advantages of metallic stents over silicone stents in management of tracheobronchial obstruction include thinner walls, decreased migration and their ability to conform to tortuous airways. Similar to the silicone stents, however, metallic stents may induce granulation tissue formation. Other disadvantages are difficulty in removal, potential for wire fracture, increased expense, tumor invagination if not coated, and increased risk of perforation and erosion.

Choice of Stent:

Proper selection of stent type and size is critical to avoiding stent-related complications, including migration, granulation tissue formation, and airway perforation from excessive radial force of the stent.

Proper choice of a stent should not be based on ease of placement but rather the best stent for a given condition. For benign strictures, only silicone stents should be used, as they are easy to remove and replace.

Immediate and long-term complications should be considered before placing a stent. A useful “check list” of questions when considering stent placement includes :

Is a stent required?
Will the patient benefit from stent placement in terms of quality of life or prognosis?
Is there expandable lung distal to the obstruction?
Will the stent interfere with or prohibit a curative surgical procedure in the future?
Does the operator have the expertise, equipment, and team to place the stent?
What is the underlying airway pathology and which stent is ideal for that pathology?
Is it safe to place a stent in the anatomic site?
What are the required stent dimensions (length and diameter)?
Does the operator have the optimal stent, or should a more appropriate one be obtained?

Indications and patient selection:

The primary indications for airway stent placement include reestablishing patency of compressed or stenosed central airways, management of mixed stenoses if endoluminal resection insufficient to open airway, supporting weakened cartilages, and sealing fistulas and dehiscences to esophagus, mediastinum or pleural cavity. Stents may be used alone or in conjunction with other techniques for palliation of dyspnea, cough, or respiratory insufficiency that is due to central airway obstruction. An airway stent supports the airway wall against collapse or external compression, and it may impede extension of tumor into the airway lumen. Stents may be safely used in patients who are undergoing external-beam radiation therapy or brachytherapy.

Indications for stent placement for benign airway stenosis include complex tracheal stenosis as a bridge to surgery or for patients that are not surgical candidates, tracheal or bronchial stenosis from inflammatory or infectious process while waiting for response to systemic therapy or surgical resection, lung transplant anastomotic stenoses, and tracheobronchomalacia.

Contraindications:

Contraindications to airway stent placement include any contraindication to flexible or rigid bronchoscopy, absence of expandable lung distal to the obstruction, and lack of adequate operator expertise, equipment, and team to place the stent.

An FDA “Black Box” warning has been issued cautioning against the use of metallic stents in patients with benign airway disorders.

Details of how the procedure is performed:

Bronchoscopy should be performed in patients with symptoms or radiographic findings suggestive of central airway obstruction. When a stenosis is discovered the length and diameter of the lesion should be documented. A spiral CT scan with three-dimensional reconstruction is extremely helpful in the sizing of the stent needed. If a lesion is not amenable to endoscopic removal, bronchial dilatation should be done to enable insertion of a stent of the appropriate diameter.

Silicone stents are placed through a rigid bronchoscope using a specially designed stent deployer. Metallic stents can be placed with a flexible or rigid bronchoscope, using either a guidewire, direct visualization or an over the scope system, depending on the particular type of stent.

Interpretation of results:

Not applicable.

Performance characteristics of the procedure (applies only to diagnostic procedures):

Not applicable.

Outcomes (applies only to therapeutic procedures):

Studies have shown no significant survival difference between those patients with malignant airway obstruction who received palliative chemotherapy (median survival, 8.4 months) and those who received treatment with laser, stent placement, or both, followed by chemotherapy (median survival, 8.2 months).

In a multicenter trial of 1058 patients in whom 1574 stents were placed (of which 698 were for malignant airway obstruction), stent migration occurred in 9.5 percent, granuloma formation was observed in 8.0 percent, and stent obstruction by mucus occurred in 4.0 percent. In another study of 125 silicone stents placed in sixty patients with malignant disease and thirty patients with benign tracheobronchial disease, migration was observed in 13 percent and granuloma formation in 6 percent.

The Wallstent is a self-expanding device made of cobalt alloy braided filaments that also has special features of outward radial force provided by its integral design and an optional cover that prevents growth of tumor and granulation tissue through the stent’s thin wire mesh. In a report describing use of the Wallstent, stent-related obstructive granulation tissue was found in 11 percent of patients, and stent migration was not observed.

Ultraflex stents are flexible, self-expanding stents made of nitinol, an alloy with shape memory that deforms at low temperatures and regains its original shape at higher temperatures. They are available in covered and uncovered versions. In a study of 62 covered and uncovered Ultraflex stents placed in 60 patients with malignant airway stenoses, successful opening of the stenosis and relief of symptoms were achieved in all patients. Complications occurred in 23% of patients and included mucus plugging (8%), granulation tissue (5%), tumor ingrowth (5%), and migration (5%).

In a study of long-term outcomes in patients with malignant or benign airway strictures who were treated with Wallstents and Ultraflex stents, a complication rate of 0.06 complications per patient-month was reported. Median follow-up was 42 days for patients with lung cancer, 329 days for lung transplant recipients, and 336 days for other benign conditions. The most common complications were infectious tracheobronchitis and obstructing granulomas that required interventions to restore airway patency. Tumor in-growth and stent fractures were also seen.

Bioabsorbable airway stents are currently an area of intense research. The hope is that these stents will minimize foreign body reaction and the need for stent extraction, and allow for normal airway function after stent resorption. Various bioabsorbable materials have been utilized including vicryl filament, poly-L lactic acid, and polydioxanone (PDS). Drug-eluting airway stents using mitomycin C, paclitaxel, and doxycycline are also under investigation.

Alternative and/or additional procedures to consider:

There are no additional or alternative procedures to consider.

Complications and their management:

The most common complications are tumor in-growth, stent fracture, infectious tracheobronchitis, mucostasis, and obstructing granuloma. For obstruction by granuloma and tumor in-growth, endoscopic removal by laser, cryotherapy, or argon plasma coagulation may be required. Respiratory infections have been shown to increase the risk of granulation tissue formation following airway stenting in patients with malignant airway obstruction. Stents are also associated with increased risk of respiratory infections in patients undergoing airway interventions for malignant airway disease.

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