Introduction
An airway stent is a hollow prosthesis that maintains airway patency and provides structural support. Stent deployment is an integral skill for an interventional pulmonologist. The indications and selection of airway stents will typically dictate the deployment technique used. Therefore the interventional pulmonologist must be familiar with a variety of techniques depending on the pathology that warrants stenting. In this chapter we will discuss stent types, indication, and placement.
History
The word “stent” is named after Charles Stent, a British dentist who created dental splints in the 19th century. The first stents were surgically placed by Trendelenburg and Bond to treat airway strictures and endoscopic placement was first performed by Brunings and Albrecht in 1915. In 1965, a silicone stent with a tracheal stomal limb was invented by Montgomery, called the T-tube, for treatment of subglottic stenosis. The first strictly endoluminal airway silicone stent was developed and described by Jean-François Dumon. The silicone stent is what gave interventional pulmonology momentum, as this allowed central airway obstruction to be managed by pulmonologists with training in rigid bronchoscopy. Since then there has also been the emergence of metallic stents.
Types of Stents
An ideal stent would (a) be easy to place and remove, (b) be resistant to migration, (c) not form granulation tissue, (d) not become obstructed with secretions, (e) be able to conform well to the patient’s airway, (f) be customizable, (g) have sufficient radial force to maintain airway patency, and (h) be inexpensive. Unfortunately, this ideal stent does not (yet) exist.
Two main categories of stents exist: metallic and silicone. Both types of stents are used to manage central airway obstruction. A variety of stents are available for each main category ( Fig. 10.1 ) and each stent has small nuances that will help you choose one over the other. A basic overview of the comparison of metallic versus silicone stents is shown in Table 10.1 . One hybrid stent of special note is the dynamic stent, which is a bifurcated silicone stent that is constructed with anterior metal struts in the tracheal limb and only silicone at the posterior to simulate the tracheal rings and membranous trachea ( Fig. 10.1H ). This stent is placed with direct laryngoscopy and a pair of specialized rigid forceps. Finally, there are also hourglass stents that are made to resist migration when used to treat a stenotic airway and stent migration is a concern ( Fig. 10.1G ).
| Metal (Covered or Uncovered) | Silicone | |
|---|---|---|
| Insertion with flexible scope | Yes | No |
| Insertion with rigid scope | Yes | Yes |
| Granulation tissue formation | Yes | Yes (ends) |
| Tumor ingrowth | Yes (uncovered) | No |
| Migration | Rarely | Yes |
| Fracture | Yes | Very rarely |
| Infection | Yes | Rarely |
| Airway perforation | Rarely | Very rarely |
| Mucus plugging | Rarely | Yes |
| Modifiable | No | Yes |
| Able to reposition |
| Yes |
| Ease of insertion | Easy | Requires rigid bronchoscopy expertise |
| Conforms to tortuous airway | Yes | No |
| Internal-to-external diameter ratio | High | Low |
| Radial strength/force | Medium | High |
| Mucociliary clearance |
| No |
| Cost | More | Less |
Indications for Airway Stenting
Airway stents are placed when a patient develops respiratory symptoms and has imaging findings consistent with focal airway obstruction ( Table 10.2 ). The amount of obstruction should be able to explain the patient’s symptoms. Typically a 50% reduction in airway diameter is required for symptoms to arise. Generally a tracheal diameter of 8 mm or less is required before the patient will experience dyspnea on exertion, and a tracheal diameter of less than 5 mm will result in shortness of breath at rest. Importantly, airways distal to the area of stent placement need to be patent for stent placement to be effective; this can sometimes be determined with imaging but often must be assessed during bronchoscopy. Airway stents can be particularly useful in patients with lung cancer, of which 30% present with airway obstruction and 35% will die from asphyxia, hemoptysis, or postobstructive pneumonia. In a large, multicenter registry of 1115 procedures on 947 patients undergoing therapeutic bronchoscopy for central airway obstruction, one-third of patients underwent airway stenting that was associated with improvement in likelihood of achieving airway patency. Interestingly, patients with higher baseline dyspnea (Borg score) and nonlobar obstruction experienced greater improvements in dyspnea and health-related quality of life. Additionally, patients with higher American Society of Anesthesiology (ASA) score and lower functional status also had greater improvements in health-related quality of life. Of note, the United States Food and Drug Administration of the United States issued a black box warning on the use of uncovered metallic stents in the trachea to treat benign tracheal obstruction in 2005. The current standard of care is to treat benign tracheal obstruction with silicone stents or fully covered metallic stents. Fortin and colleagues reviewed the records of 30 patients who had third-generation fully covered metallic stents placed in the central airway for benign airway stenosis and found that 50% had to be removed for complications at a mean of 77 ± 96.6 days, the rest were removed at 122 ± 113.2 days without complication. The clinical success rate of stent treatment was 40.7% and no stent-related mortalities were reported.
| Indication | Metallic | Silicone |
|---|---|---|
| Tracheal tumor | Yes | Yes |
| Bronchial tumor in central airways | Yes | Yes |
| Small airway with tumor | Yes | No |
| Malacia/EDAC | Sometimes | Yes |
| Tracheoesophageal fistula | No | Yes |
| Anastomotic dehiscence | Yes | Rarely |
| Extrinsic compression | Yes | Yes |
| Benign tracheal stenosis a | No | Yes |
a This can include idiopathic, inhalational injury, posttracheostomy, postintubation, and autoimmune conditions (ex sarcoid, granulomatosis with polyangiitis, or systemic lupus erythematosus).
Stenting of more distal airways can be considered on a case-by-case basis to drain retained secretions, improve atelectasis, and assist with dyspnea, though this is technically more challenging and outcomes are less consistent, due for the most part to mucus clearance issues.
Preprocedural Planning
Patients’ histories, physical examination, and computed tomography (CT) imaging should be carefully reviewed to confirm the need for airway stenting and to plan the procedure. Patients best suited for airway stenting are usually symptomatic with possible impending respiratory compromise, and so a team approach should be designed. Patients are best served in centers wherein there can be collaboration between multidisciplinary team members, including interventional pulmonologists, anesthesiologists, critical care physicians, and thoracic surgeons or otolaryngologists. Although some patients may be unstable for transport to other medical centers, it is essential to recognize that there can be risks of deploying airway stents without the proper equipment, team members, and facilities to care for the patient.
It is important to recognize and plan for extrinsic, intrinsic, and complex stenoses. Tissue debulking may be necessary before stents can be deployed for intrinsic and complex stenoses, and so the proper equipment and expertise must be made available prior to the procedure, for example, rigid bronchoscopy, cryotherapy, argon plasma coagulation, electrocautery, or laser therapy.
The lung parenchyma distal to the stenosis should be carefully assessed. Airway stenting is most effective when the lung distal to the stenosis is viable. Stents placed in airways proximal to solid tissue masses may alleviate the stenosis and open the airway but not facilitate ventilation and therefore provide little clinical benefit. Likewise, CT imaging can help assess the vasculature in relation to airway stenosis, and if there is significant obstruction of distal lung perfusion, then stenting to improve ventilation may not yield clinical benefit. Therefore CT chest imaging with IV contrast can be extremely helpful for selection of patients most likely to benefit from stenting.
We recommend general anesthesia for stent deployment in order to facilitate airway management and adequate oxygenation and ventilation during the procedure. For distal tracheal or bronchial stenosis, the placement of an endotracheal tube may be routine, but for more proximal tracheal stenosis, endotracheal tube placement may be more difficult or impossible. Rigid bronchoscopy may therefore be necessary immediately upon the induction of anesthesia in order to secure the proximal trachea and follow with ablation or dilation of the stenosis before stenting. Rigid bronchoscopes also provide the interventional pulmonologist with more options for deployment of either silicone or metallic stents. Often the flexible scope is used through the rigid bronchoscope to assist with debulking or stent deployment. If a routine endotracheal tube is used, however, we recommend using at least an 8.0-size tube to facilitate ventilation around a therapeutic flexible bronchoscope.
Deploying Metallic Stents
Metallic stents can be uncovered, partially covered, or completely covered by silicone or polyurethane. Although they used to be made of stainless steel, now these stents are made of nitinol, a nickel-titanium alloy that is elastic and has shape memory so will return to original form after being folded and loaded onto a catheter for deployment. No one stent is perfectly suited to all airway pathologies, and interventional pulmonologists should learn the benefits and limitations of each type. In general, metallic stents should be considered for malignant airways disease, when the provider wants to maximize internal-to-external diameter, if the airway is irregular, and when an uncovered portion may be desired to maintain patency of an adjacent airway. Uncovered metallic stents are used on occasion for benign disease, specifically for anastomotic dehiscence following lung transplantation. All metallic stents share their ability to be easily folded into a low-profile deployment system. This means that they can be passed through a rigid bronchoscope or an endotracheal tube, and some can even pass through the working channel of a flexible bronchoscope, thus can be deployed without the use of rigid bronchoscopy. The most commonly used metallic stents today are the Ultraflex (Boston Scientific, Marlborough, MA, USA) ( Fig. 10.2 ), AERO and AEROmini (Merit Endotek, South Jordan, UT, USA) ( Fig. 10.3 ), and the Bonastent (Thoracent, Huntington, NY, USA) ( Fig. 10.4 ). Ultraflex stents are either uncovered or partially covered and are held onto the delivery catheter using a silk thread that you unwind by pulling on the string and releasing a series of crochet knots ( Fig. 10.2 ). They have both proximal and distal release options. The AERO, AEROmini, and Bonastent are all metallic, fully covered with silicone or polyurethane, and use an external clear deployment catheter to compress the stent that is subsequently unsheathed for deployment ( Figs. 10.3 and 10.4 ).
