Fig. 13.1
Classification of central airway obstruction based on qualitative and quantitative criteria. Dynamic features refer to the phase of respiration during which there is flow limitation. In a fixed obstruction, there is limitation to flow both during inspiration and expiration, while in dynamic obstruction, only during a respiratory phase, as is the case with tracheomalacia. The quantitative criteria could be objectively assessed. For instance, based on physiologic data, for tracheal stenosis, the severity of airway narrowing can be quantified as mild (<50% narrowing), moderate (50–70%), and severe (>70%); the extent is the vertical length of the stenosis and, based on outcomes from bronchoscopic and open surgical interventions, can be quantified as mild (<1 cm), moderate (1–4 cm), and severe (>4 cm). Functional impairment can be objectively assessed using a variety of validated tools such as MRC dyspnea scale or WHO functional class
Historical Perspective
Since the beginning of documented airway stent insertion at the end of nineteenth century, tracheobronchial prostheses have been generally made of two types of materials: metal or rubber. As the understanding of airway physiology and its interaction with the prosthetic materials has advanced, the manufacturers take into consideration the biomechanical and biocompatibility characteristics, even though this information is not always available to the practicing bronchoscopist. Clinically used airway stents are currently made of polymers, alloy metallic mesh, or a combination of the two (aka hybrid stents). In general, the pure metallic stents have been abandoned because of severe complications.
The future may see the incorporation of treatment agents such as chemotherapeutic (i.e., mitomycin C, paclitaxel), radioactive agents or bioabsorbable stents [1]. In theory, stents made of bioabsorbable polymers may be ideal, especially in pediatric population, as they can support the airway wall and dissolve after the remodeling process is completed, thus providing temporary airway stiffness, sometimes necessary in infants with tracheobronchomalacia. Such stents have the advantage of potentially avoiding the need for repeated interventions under general anesthesia for removal or revision [2–4]. Only pilot human studies of bioabsorbable stents have been published to date [5, 6]. Bioabsorbable drug-eluting stents have the potential advantage of reducing the risk of stent-related complications, but they have only been studied in animal models of benign tracheal stenosis [1]. In animal models, novel bioabsorbable stents (made of polycaprolactone) with cisplatin elution have been developed to overcome some of the problems associated with chronic indwelling stents (tumor ingrowth, fracture, migration) [7]. The mechanical strength of these stents was shown to be comparable to the strength of Ultraflex SEMS and provided a steady release of cisplatin for >4 weeks in vitro. The in vivo study showed sustained cisplatin levels in rabbit trachea for >5 weeks with a minimum drug level in blood. Histologic examination showed an intact ciliated epithelium and marked leukocyte infiltration in the submucosa of the stented area, findings suggesting potential use in malignant CAO. In a recent human study, six biodegradable polydioxanone tracheal stents were safely implanted in four patients with benign inoperable tracheal stenosis. The authors report that all patients had “some” benefit from treatment and suggested that further research is needed to fully assess the outcomes of this therapy [8]. Whether these stents will be incorporated into clinical practice remains to be determined.
As of this writing, the originally described problems of migration, granulation, mucus plugging, infection, and even airway perforation and fatal hemoptysis are still present after stent insertion [9]. Therefore, operators have to carefully review the indications and expected results before inserting airway stents.
Indications
Airway stents are generally used for symptomatic extrinsic airway compression with or without associated airway mucosal infiltration. Stents can also be used if there is still significant (generally considered more than 50%) narrowing after the endoluminal component of a purely exophytic or mixed type of obstruction has been treated using one or more bronchoscopic techniques2 [10]. Various stents have been used as well for sealing malignant esophagorespiratory and bronchial stump fistulas. Stents are occasionally used to improve symptoms of severe tracheobronchomalacia or excessive dynamic airway collapse, in patients who are refractory to more conservative measures (i.e., continuous positive airway pressure) and are not candidates for an open surgical procedure (i.e., tracheobronchoplasty for diffuse disease or sleeve resection for focal disease) [11, 12]. Studies performed within the last 20 years have shown that airway stents improve lung function in patients with central airway obstruction. In this section, we will describe the indications of stent insertion based on the mechanism of obstruction.
Extrinsic Compression
Extrinsic compression from benign or malignant thyroid disease, primary lung tumors (Fig. 11.2), mediastinal masses, or massive intrathoracic lymphadenopathy is the most common indication for airway stent insertion. Rarely, vascular abnormalities such as aortic aneurysm, vascular sling, and double aortic arch may cause symptomatic airway obstruction and may require stent insertion for patients who do not undergo corrective surgery.
Intraluminal Obstruction
Stent insertion may be useful in selected cases of endoluminal exophytic benign central airway obstruction (CAO); this is the case of refractory endobronchial recurrent respiratory papillomatosis (RRP) when medical and other endobronchial therapies fail to restore airway patency. Case reports show that papilloma debulking and silicone stents can offer adequate control of symptoms [13]. However, histologically benign intraluminal obstruction necessitating stent insertion is mostly caused by strictures, either idiopathic or related to other disorders. The most common cause of benign strictures is post-intubation and post-tracheostomy stenosis (Fig. 13.2), but it is important to note that a variety of other conditions associated with strictures should be ruled out before making the diagnosis of idiopathic stenosis. This is relevant as the management strategies need to be individualized. Examples include granulomatosis with polyangiitis (GPA; formerly Wegener granulomatosis), amyloidosis, sarcoidosis, ulcerative colitis, post-tuberculosis, or Klebsiella rhinoscleromatis infection. For example, 12–23% of patients with GPA develop tracheobronchial stenosis. A recent multicenter retrospective study of 47 patients with GPA-associated tracheobronchial stenosis found that these patients benefit from a delay in any interventional procedures following the diagnosis, allowing for a “cooling off” period from the associated inflammation. It is also advisable to have patients on an increased dose of corticosteroids to >30 mg/day during the periprocedural period [14].
Fig. 13.2
Indications for airway stent insertion. Severe extrinsic compression of the right mainstem bronchus due to primary lung cancer, before (a) and after (b) silicone stent insertion. Severe, complex post-tracheostomy, triangular or A-shaped stenosis with malacia in a nonsurgical candidate before (c) and after (d) a 16 × 40 mm straight silicone stent was inserted. Follow-up bronchoscopy triggered by excessive coughing and inability to raise secretions demonstrated restored tracheal patency, but the stent migrated down to the main carina (e) and required removal. Benign gastro-tracheal fistula (f) after esophagectomy and gastric pull-up procedure. As repeat surgery was unsuccessful at closing the fistula, a fully covered SEMS was used (g). Four weeks later the stent was removed and, fortunately, the airway wall completely healed (h) without recurrence of the fistula during the follow-up
The remainder of this section will focus on the role of stent insertion for benign stenoses associated with intubation (PITS) and tracheostomy (PTTS). The incidence rate of benign tracheal stenosis following intubation has historically ranged from 0.6 to 19% and following tracheostomy from 6 to 65%. Fortuitously the advent of low-pressure cuffs has substantially decreased these rates (by up to tenfold), yet still 1–5% of patients suffer from traumatic symptomatic PITS or PTTS, typically occurring 2–3 months following the event [15]. It remains to be determined whether the introduction of new mechanical ventilators with continuous endotracheal cuff pressure monitoring could further reduce the incidence of PITS. For post-intubation or post-tracheostomy strictures, stent placement should be considered only in inoperable patients; in addition, patients need to be symptomatic and the lumen of the airway below half of its normal after other interventional endoscopic techniques have been applied.
Benign airway obstruction can be classified in a variety of ways, and management techniques and success rates vary based on the type of stenosis. For example, a simple web-like stricture (extent less than 1 cm), which is dilated and does not recur, will not require a stent [16, 17]; a complex stricture, however, often has associated chondritis, and dilation alone (with or without laser assistance) is not usually successful, and a stent would be required to maintain airway patency [18]. Another way of classifying strictures uses the terms “structural” and “dynamic”: a structural stenosis is a result of scarring and fixed constriction of the airway—this is the most common form. A dynamic stenosis is a form of focal, localized malacia with variability of obstruction dependent on the variability of transthoracic pressures during respiration. Another classification has been proposed to exist: that of a dynamic A-shaped tracheal stenosis (DATS) which is an amalgamated variation that combines both a structural stenosis from a fractured anterior cartilage ring with a dynamic stenosis from posterior malacia (Fig. 13.2). This results in a triangular “A-shaped” trachea on imaging. This is an important finding as the structural component is not the result of scaring/shrinkage of the trachea, and as such the management of DATS differs significantly from that of other structural forms of benign airway strictures. Specifically, patients with DATS do not benefit from dilation alone. At the same time, due to the dynamic component to the stenosis, patient experiences higher rates of stent migration than typical structural stenosis patients (Fig. 13.2) [15].
Silicone stent insertion performed using rigid bronchoscopy under general anesthesia is considered an acceptable alternative to surgery for inoperable patients with complex tracheal strictures. A 2016 retrospective study of 90 patients undergoing stenting for histologically benign airway obstruction showed that in patients with simple stenosis undergoing stenting, there was a 100% success rate with a single stent placed and mean stent duration of 5.6 months. On the other hand, patients with complex stenoses did not fare as well: 45% required multiple re-stenting procedures, 60% required stent repositioning, the stents remained in place for 12 months, and despite this the success rate was 70% at 1 year [17]. In an older study of 42 patients with complex stenoses, only 9 were surgical candidates, and 33 were treated with silicone stent insertion, with a success rate of 69% [19]. The success rate of bronchoscopic treatment once stents are removed (usually after at least 6 months) in cases of complex stenosis is reportedly low (17.6%) suggesting the need for long-term indwelling airway stent. A higher rate of airway stability after stent removal (46.8%, in 22 out of 47 patients) was described after stents remained in place for a longer period of time (mean of 11.6 months) [20], with almost 50% of patients (12/22) having their stents for more than 12 months. Predictors of success of bronchoscopic treatments are stenoses less than 1 cm in vertical extent and without associated malacia (i.e., chondritis). Lesion extent (i.e., height) and intubation-to-treatment latency have also been reported to independently predict the success of bronchoscopic intervention. In one study, 96% of patients with lesions <3 cm in height were successfully treated bronchoscopically, but the success rate decreased to 20% for lesions longer than 3 cm. Patients with stenosis present for more than 6 months since the original injury were also less likely to be successfully treated bronchoscopically [21], suggesting that the established fibrotic tissue counteracts the expansile force of the remaining cartilage [22]. In fact, knowing the integrity of the cartilage in post-intubation or post-tracheostomy stenoses is important in the treatment decision-making process. In complex post-intubation/tracheostomy stenosis, cartilage integrity or lack thereof is not always easily assessed on white-light bronchoscopy, mainly because of the overlying stenotic hypertrophic tissues [23] (Fig. 13.3). To assess the integrity of the cartilage, one may use high-frequency endobronchial ultrasound (20 MHz balloon-based radial probe) during the bronchoscopic intervention. The EBUS image using this system has a high resolution and allows visualization of the stenotic tissue and the cartilaginous structures and may be a surrogate of gross histology for tracheal stenosis; for instance, in idiopathic tracheal stenosis, the cartilage is known to be normal, but there is clear hypertrophy of the mucosa and submucosa as visualized by EBUS as well. On the other hand, in complex stenoses, there is partial or total destruction of cartilage histologically which can be identified by EBUS [23] (Fig. 13.3).
Fig. 13.3
Rigid bronchoscopic and sonographic view of laryngotracheal stenosis. In the upper panel, the circumferential post-intubation tracheal stenosis is noted, but on white-light imaging, the cartilage cannot be assessed. High-frequency endobronchial ultrasound (20 MHz probe) can identify the cartilage and its disruption. The knowledge that the cartilage is affected could impact management since simple laser-assisted mechanical dilation without stent insertion is unlikely to maintain airway patency in the long term. In the lower panel, idiopathic subglottic stenosis at the level of the cricoid is seen on white-light imaging, but the intact cricoid cartilage itself is only identified on high-frequency endobronchial ultrasound
When used for benign stenosis, silicone stents are preferable and can be helpful for splinting post-intubation/tracheostomy stenoses and are considered appropriate to palliate airway narrowing in nonsurgical candidates3 [18, 24, 25]. Stent-related complications, however, are not uncommon in this disease and include migration, obstruction from secretions, infection, and significant granulation tissue formation at the proximal or distal extremities of the stent [9, 26].
Silicone T-tubes (Montgomery T-tubes) or tracheostomy tubes are sometimes used for benign tracheal strictures; they should be inserted through the area of stenosis, if possible, to conserve airway not involved by the stenosis lesion. For most patients who do not require mechanical ventilatory support, a silicone T-tube could provide symptomatic improvement [27]. These therapies are warranted in the few patients with critical stenoses who are neither candidates for surgery or indwelling airway stent insertion or who develop recurrence after such interventions [18]. T-tubes can also be used when tracheal resection and reconstruction or dilation techniques are either not available or have failed or as a solution for patients who had silicone stent placement complicated by frequent migrations [26]. In a large case series including 53 patients with complex tracheal stenoses (24 post-tracheostomy), silicone T-tube insertion was effective in 70% of patients with limited complications [28]. The sharper edge of the proximal aspect of the T-tube, in cases when it has to be cut, suboptimal tracheostomy tract (i.e., non-midline stoma), as well as its placement within 0.5 cm from the vocal cords are known risk factors for granulation tissue development4 [28]. In addition, airway secretions may become dry and cause obstruction. Patients, families, and referring physicians probably benefit from instruction on how to care for and monitor T-tubes. Frequent bronchoscopies may be necessary to remove mucus plugs, with some investigators performing a 3–4 biweekly bronchoscopies, followed by once every 4 weeks once stent patency has been documented [28].
Self-expandable metallic stents (SEMS) have been associated with significant complications and are to be avoided, if possible, in benign disorders. Immediate symptomatic improvement is reported and expected, but the long-term complications are common and may be life threatening [29].
Self-expandable silicone stents, contrary to metal stents, have the advantage of being easily removable. They are, however, placed under rigid bronchoscopy or suspension laryngoscopy. Some of these silicone stents have been studied in benign airway obstruction including tracheal stenosis and malacia [30]. While immediate symptom palliation was established in most cases, the incidence of complications was high (75%) with stent migration occurring in 69% of cases [30, 31].
Postoperative Tracheobronchial Stenosis
A variant of histologically benign tracheal stenosis, postoperative tracheal stenosis (POTS) is a challenging problem following tracheal resection. Despite improved recognition and surgical techniques, the rate of POTS is 2–9% following tracheal resection. The majority patients with POTS are not candidates for further surgical management due to a combination high general surgical risk, poor lung function, and technical difficulties associated with previously resected tracheal segments. As such, bronchoscopic intervention is considered a therapeutic option. In a single-center retrospective review, 30 patients with POTS managed by bronchoscopic intervention were studied. Interventions included dilations (balloon or bouginage), YAG laser, and stenting (63% underwent silicone stents, no metallic stents were used). The majority (97%) achieved improvement in dyspnea within 24-h post-procedure. Stents were successfully removed in 37% of patients. Average stent duration in those amenable to removal was 7 months; 16% of those with stents removed developed tracheobronchomalacia [32].
Mixed Obstruction: Malignant Central Airway Obstruction
Malignant central airway obstruction (CAO) is a frequent complication of primary lung cancer and other cancers, which metastasize to the chest (especially breast, colon, melanoma, and renal cell cancers). Malignant CAO can be intrinsic (endobronchial/intraluminal), extrinsic, or a mixed obstruction, which has features of both intrinsic and extrinsic obstructive patterns. The most common form of malignant CAO is a mixed obstruction [33]. In a series of 172 patients who underwent stent insertion for malignant CAO at a tertiary cancer institution, 62.5% of the stents were placed for mixed disease, while only 16.4% and 14.8% were placed for extrinsic compression and intraluminal obstruction, respectively [9]. In general, the management principles for malignant intraluminal obstruction are the same as those for benign disease: if there is still obstruction after recanalization with various ablative techniques, if extrinsic compromise is present, or if there is a loss of airway structure (i.e., severe malacia due to cartilage invasion and destruction by tumor), a stent is placed to maintain airway patency.
Management of malignant CAO often requires a combination of multiple different management modalities. The choice of technique and method is operator dependent and is contingent not only on the etiology of the obstruction but also operator familiarity and preference. To study the impact of procedural volume and choice of technique in bronchoscopic management of malignant CAO, a large multicenter retrospective review of bronchoscopic management of patients with malignant CAO was undertaken from the American College of Chest Physicians (CHEST) Quality Improvement Registry, Evaluation, and Education (AQuIRE) registry. Overall the study found that despite significant inter-institutional differences in procedural preferences and volumes, there was no impactful difference in technical success and that one specific therapeutic modality could not be recommended over another [33].
Interventional treatment of malignant CAO is considered to be primarily palliative as once cancer progresses to the point of CAO, it is almost invariably incurable. As such endoscopic interventions focus predominantly on attempting to improve quality of remaining life. Relieving the CAO due to malignant disease has been proposed to prevent post-obstructive pneumonia, sepsis, and septic shock, allow extubation, change in level of care, permit initiation of systemic therapy, and potentially improve survival. There is evidence that bronchoscopic therapies often provide acute relief of the obstruction, improve quality of life, and serve as a therapeutic bridge until systemic treatments become effective [34–36]. Prospective studies show that bronchoscopic intervention for malignant CAO is associated with improvement in the six-minute walk test (6MWT), spirometry, and dyspnea [37]. In addition, studies show that airway stent insertion resulted in significant palliation of symptoms in patients with malignant CAO as evaluated by Medical Research Council (MRC) dyspnea scale and performance status [38].
In the AQuIRE registry mentioned above, bronchoscopic interventions were associated with a significant decrease in dyspnea (decrease in Borg score by 0.9 ± 2.2). Specifically, 48% reported clinically significant improvement in dyspnea, 43% reported no change, and 9% had worsened dyspnea. Of particular relevance, dyspnea improved proportionally to the pre-procedure severity of dyspnea: the more dyspneic prior to procedure, the more improvement in dyspnea after the intervention. Another notable finding was that those with lobar (as opposed to more central) obstruction were less likely to have much improvement in dyspnea. Bronchoscopic interventions were also associated with a significant increase in health-related quality of life (HRQOL) . Overall 42% had a significant improvement of HRQOL, 33% remained unchanged, and 25% reported worsened HRQOL. Again, as with the predictors of dyspnea relief, a higher baseline Borg (i.e., worse baseline dyspnea) predicted a more pronounced improvement in HRQOL, while those with lobar obstruction were found to have less improvement in HRQOL [33]. While airway patency was improved in >90% of patients, less than half improved their HRQOL scores. These findings suggest that we need better prediction models for who will improve dyspnea and HRQOL after such interventions. Despite the focus on palliation and improved quality of life with these procedures, a significant post-procedural survival advantage was also apparent in those without severe performance limitations prior to their procedures when compared with historical controls [38].
The presence of stridor (reflecting critical CAO) prior to intervention was found to be a poor prognostic indicator for survival in patients undergoing bronchoscopic intervention for malignant CAO: those without stridor had a 1-year and 2-year survival of 35.5% and 31%, respectively, while those with stridor had a 1-year and 2-year survival of 12.5% and 0%, respectively. Patients requiring stent placement for malignant CAO as opposed to dilation ± other non-stenting interventions had significantly lower 1- and 2-year survivals [39]. It is not clear whether lower survival rates are because of the stenting or just because patients requiring stents had more severe/extensive airway obstruction.
Subsequent chemotherapy and/or radiotherapy has been shown to increase disease-free survival during the first year after restoration of airway patency [34, 40]. A retrospective single-center study of 48 patients with malignant CAO who underwent bronchoscopic intervention reviewed the effects of chemotherapy following bronchoscopic interventions. The patients who received post-procedural palliative chemotherapy had a median survival of 6 months with a 1-year and 2-year survival of 35% and 31%, respectively. Those patients who received no post-procedural chemotherapy had a median survival of 2.5 months with a 1-year and 2-year survival of 18% on 14%, respectively [39]. In addition, it appears that airway stent insertion followed by adjuvant therapy may improve survival of treatment-naive patients with severe symptomatic airway obstruction caused by advanced lung cancer. In one study, while the performance status and dyspnea scales improved in both treatment-naive and terminal-stage lung cancer, the median survival time and 1-year survival rate after stent insertion were 1.6 months and 5.1%, respectively, in the terminal-stage group and 5.6 months and 25.0%, respectively, in the treatment-naive group [41].
Lung cancer patients who develop respiratory failure due to CAO have particularly poor prognoses: only 25% are successfully liberated from the ventilator, and 40–70% die in the hospital. In addition to the quality of life issues, ventilated patients are often not considered candidates for additional oncologic treatment. Furthermore, patients with malignant CAO are given low priority for ICU level admission in the Society of Critical Care Medicine ICU admission recommendations [42], as they are considered to have low probability of reversibility and survival. A small single-center retrospective study addressed this assumption of lack of reversibility. Twelve patients with non-small cell lung cancer with associated CAO resulting in respiratory failure requiring mechanical ventilation who were not candidates for surgical procedures were managed with bronchoscopic intervention and various combinations of mechanical debulking, laser resection, and airway stenting: 66% underwent stenting. The majority (83%) was successfully liberated from mechanical ventilation, and the post-procedural median survival was 313 days. As such bronchoscopic intervention should be considered for lung cancer patients with respiratory failure due to CAO [43].
Stump Fistulas
A less common indication for stent insertion is to cover large stump fistulas after lobectomy or more commonly, after pneumonectomy [44]. In general, management strategies for bronchopleural fistula (BPF) depend on the underlying histology (malignant versus benign), size, time to fistula formation postsurgery, and health status of the patient. Surgery is the treatment of choice of this condition, but bronchoscopic techniques have been advocated as an option when surgery is not possible or has to be postponed [45]. Surgical repair is not a good option for patients requiring mechanical ventilatory support because postoperative mechanical ventilation is associated with a high failure rate due to persistent barotrauma on the repaired stump [45]. As a general rule, when stents are used for this indication, a large stent must be used to seal the stump fistula as tight as possible in order to prevent aspiration pneumonia and empyema and allow satisfactory single-lung ventilation when the patient requires mechanical ventilation. Stent selection would depend on the size and location of the fistula, as well as on the physical properties of the stent and the operator’s ability to manage potential stent-related complications. Several case reports and case series of endobronchial stent insertion for isolated fistulas have been published [46]. The effect of case selection is difficult to assess from the limited literature on this topic.
Esophagorespiratory Fistulas
Tracheoesophageal or broncho-esophageal fistulas can be covered by airway stents. While these fistulas can be congenital, the majority are acquired either after esophagectomy, after intubation, or in the setting of malignancy. Benign esophagorespiratory fistulas (ERFs) are not expected to improve after stent insertion, and, in fact, it should only be considered as a palliative intervention if there are no operative modalities (Fig. 13.2) [47].
Malignant ERF is common in esophageal cancer, having a 5–15% occurrence, and occurs rarely in bronchogenic carcinoma (~1%). Once developed, the prognosis is poor, with a poor QOL and 3–4 month survival. Although surgical resection and reconstruction has the greatest potential benefit, it comes at a high cost of complications and prolonged hospitalized recovery. Alternatively, gastro/jejunostomy tube feeding is a strategy utilized to minimize effect of malignant ERF, but this may not be accepted by patients and has the potential to further reduce quality of remaining life [48]. Palliation for malignant ERF is usually achieved with endoscopic placement of esophageal, airway, or parallel (dual) stent insertion (in the esophagus and airway). Dual stent insertion appears to work better than a single prosthesis. Particular attention should be paid to airway compression or erosion caused by placement of esophageal stents; if there is concern for significant tracheobronchial obstruction, operators should consider placement of an airway stent prior to the esophageal one (Fig. 13.4).
Fig. 13.4
Airway stents in obstruction caused by esophageal tumors. In the upper panel, chest computed tomography (CT) shows severe tracheal narrowing from a mediastinal mass, known to be esophageal carcinoma. Bronchoscopy confirmed the CT findings, and a partially covered metallic stent was placed to palliate the airway obstruction prior to esophageal stent insertion for dysphagia. In the lower panel, severe tracheal and right mainstem obstruction occurred after the insertion of an esophageal stent and resulted in respiratory failure in this patient with poor lung function from his previous pneumonectomy. A partially covered metallic stent was inserted from the lower trachea to the mainstem bronchus, palliating the obstruction and allowing liberation from mechanical ventilation
The choice of tracheal stent used for ERF closure should take into consideration the size and location of the fistula. The Freitag classification system [49] was developed to systematically define the location and severity of central airway stenosis, but this system can be used to define the location of an ERF: Location I, upper third of trachea; II, middle third or trachea; III, lower third of trachea; IV, carina; V, right mainstem; VI, bronchus intermedius; VII, left mainstem; and VIII, left distal bronchus. Using this system and by defining a small fistula as one that is <1 cm in size, a single center developed an algorithm for stent choice in ERF stenting: an I-shaped stent for small fistulas in Locations I, II, and VIII; an L-shaped stent for small fistula in Locations V, VI, VII; and a Y-stent for any fistulas in Locations III or IV or large fistulas in Locations II, V, and VII. This approach resulted in complete fistula closure in 72% of patients and clinically beneficial partial closure in the remaining patients [48].
A dedicated fistula stent, the DJ cufflink-shaped prosthesis, was designed exclusively for closure of malignant ERF secondary to esophageal or lung cancer. It can be sized to the fistula diameter to occlude the abnormal communication [50, 51]. Insertion of silicone Y-stents was shown to improve symptoms, reduce infections, and improve the quality of life in patients with malignant ERF. Mean survival of these patients, however, remains dismal and is in the range of 2 months [52]. A conservative palliative approach including only symptomatic control but no palliative interventions (i.e., stent insertion) is not unreasonable especially since interventions in this frail population could be harmful. Without treatment, however, survival may be limited to only a few days [53]. On the other hand, a recent prospective study of 112 patients with malignant ERF, airway stents were inserted in 65 (58%) patients, esophageal stents in 37 (33%) patients, and both airway and esophageal stents in 10 (9%) patients. Contrary to previous data, the authors found an overall mean survival of 236.6 days (airway stent 219.1 days, esophageal stent 262.8 days, and combined airway-esophageal stent 252.9 days). Since a few patients are operable, currently airway and/or esophageal stent insertion is mainly used with a palliative intent to improve the quality of life (QOL) in patients with malignant ERF [54].
Expiratory Central Airway Collapse
Airway stent insertion has been used to improve cough, secretions, and QOL in patients with expiratory central airway collapse (ECAC) [11, 12]. There are, however, different morphologic types of ECAC, for some of which stent insertion is not physiologically justifiable. Excessive dynamic airway collapse (EDAC) is due to bulging of the posterior membrane within the airway lumen during exhalation that narrows the lumen by 50% or more, and the cartilage is intact in this process. Tracheobronchomalacia (TBM) , on the other hand, refers to softening of the airway cartilaginous structures [55]. The decision to insert an airway stent in these processes is complicated by at least two factors: (1) the lack of standardized definitions and cutoff values to define abnormal airway narrowing and (2) the lack of clear understanding if these entities are truly responsible for airflow limitation. In fact, the limit between normal and abnormal narrowing of the central airways has not been physiologically established, and different investigators propose different cutoff values. In addition, there is no standardized way to measure the narrowing in terms of location or respiratory maneuver (Table 13.1) [55]. To illustrate this lack of consensus, a study found that almost 80% of normal individuals met the currently accepted 50% narrowing during forced exhalation criterion [56]. In an attempt to provide a common language for these patients with ECAC, a classification system was proposed based on objective quantifiable criteria, which can be applied before and after stent insertion (Table 13.1) [55].
Table 13.1
Summary of classification systems for expiratory airway collapse
First author/year | Parameters | Comments |
|---|---|---|
Rayl/1965 | Extent: proximal, mediastinal, and intrapulmonary airways | Collapse during cough on cine-bronchography |
Johnson/1973 | Severity: four degrees and focal type | TM: more than 50% collapse during coughing on fluoroscopy |
Feist/1975 | Etiology: congenital and acquired | TM: more than 50% collapse during coughing on fluoroscopy |
Jokinen/1977 | Severity: mild, moderate, severe | First classification based on bronchoscopic findings |
Extent: TM, TBM, BM | ||
Mair/1992 | Etiology: congenital, extrinsic compression, acquired | Described for pediatric TBM |
Severity: mild, moderate, severe | Empirical severity score | |
Masaoka/1996 | Etiology and extent criteria | TBM: >80% collapse during expiration |
Pediatric, adult, and secondary | ||
Murgu/2007 | Functional class | Stratification criteria (Functional class, extent and severity are objectively assessed) |
Extent | Morphology includes EDAC and three forms of TBMa | |
Morphology | Origin: idiopathic or secondary | |
Origin (Etiology) | ||
Severity |
Studies show that in the short term (up to 10–14 days), airway stabilization with silicone stents in patients with expiratory central airway collapse (malacia and EDAC) improves symptoms, quality of life, and functional status [11, 12]. QOL and functional status scores improved in 70% of patients, and dyspnea scores improved in 91% of patients after stent insertion [12]. Stent-related complications in this case series included obstruction from mucus plugging and migration, and almost 10% of patients (5/52 patients) had complications related to the bronchoscopic procedure itself. Because the dynamic features of expiratory central airway collapse continuously alter the shape of the central airways as well as the surface contact between a stent and the airway wall, stent-related complications may occur more frequently in dynamic forms of airway obstruction than in fixed benign obstruction. Although not life threatening, these stent-related adverse events required multiple repeat bronchoscopies [11]. In another series of patients with mostly TBM, adverse effects from silicone stent insertion were very common, however, with a total of 26 stent-related adverse events noted in 10 of 12 patients (83%), a median of 29 days after intervention [11]. TBM due to relapsing polychondritis (RP) is one disease for which stent insertion is often necessary due to a diffuse lack of airway cartilaginous support. Both self-expandable metallic stents and silicone stents have been used in patients with malacia from RP [57, 58]. Sometimes, more than one stent may be required if symptoms persist after stent insertion, presumably because of distally migrated choke points [58]. Because airway stents are not the best solution for this disease, a more conservative approach such as continuous positive airway pressure (CPAP) may be safer. CPAP may indeed be considered a “pneumatic stent.” The excessive airway narrowing in ECAC and the resulting turbulent flow result in increased airway resistance. This requires greater trans-pulmonary pressures to maintain expiratory airflow and will increase the work of breathing and result in dyspnea. Thus, noninvasive positive-pressure ventilation such as CPAP decreases pulmonary resistance and can be used to maintain airway patency, facilitate secretion drainage, and improve expiratory flow. Small studies showed that nasal CPAP improves spirometry values, sputum production, atelectasis, and exercise tolerance, but its long-term efficiency has not been clearly demonstrated [59]. As of this writing, however, the limited published evidence suggests that QOL and functional status are improved in patients with ECAC undergoing stent insertion, but the lung function as measured by FEV1 has not been consistently reported to improve after stent insertion or other forms of central airway stabilization (i.e., membranous tracheoplasty) [12]. These facts raise questions about the physiologic basis for stent insertion for both fixed and dynamic forms of CAO.
Physiologic Rationale for Airway Stent Insertion
In general, for symptomatic patients with fixed tracheal obstruction, a stent is inserted to improve the lumen to less than 50% obstruction; for symptomatic patients with dynamic obstruction, stents are meant to stabilize the airway at the collapsible segment responsible for flow limitation (aka choke point).
For tracheal stenosis , symptoms depend on the amount of pressure drop along the stenosis; this depends on the degree of the obstruction but also on the flow velocity through the airway narrowing. This flow dependence of symptoms explains why different patients with similar degree of airway narrowing have different clinical presentation, depending on their level of activity. These facts highlight the need to individualize treatment based not just on degree of narrowing as assessed by radiographic or bronchoscopic imaging but also on the stenosis impact on functional status. In fact, functional status and dyspnea scales may be more relevant than static lung function measurements, which were shown to weakly correlate with the MRC dyspnea scales in laryngotracheal stenosis [60]. In addition to functional status, a classification system for tracheal stenosis should include the extent, morphology, and severity of airway narrowing, factors that impact the decision to insert an airway stent. To quantify the severity of airway narrowing, the cutoff values used in the available systems are 50% and 70% to define moderate and severe stenosis, respectively [61]. These values seem to be justified by physiologic studies in which the investigators found that the effect of the glottis narrowing was noted to be of the same order as that of the 50% stenosis; these data suggests that a 50% or less narrowing may not even be clinically detected or require treatment; however, a significant pressure drop is seen at 75, 85, and 90% stenosis, pressure drop which correlates with significant work of breathing [62]. Based on these physiologic data, therefore, one could classify stenosis as mild, when less the 50% narrowing; moderate, from 50 to 70%; and severely narrowed when more than 70% of the lumen is occluded, justifying the practice of improving the airway lumen to less than 50% narrowing, with stent insertion, if necessary.
For expiratory central airway collapse , it is still not clear what degree of airway collapse is physiologically significant; furthermore, as of this writing, there are no accepted noninvasive physiologic tests to predict response to stent insertion. However, when patients have clear inability to raise secretions and recurrent pneumonia or even respiratory failure, then a stent is inserted regardless of the cause of collapse. From flow dynamics standpoint, the clinically relevant question in this process is whether stent insertion improves the expiratory flow. Physiologists proposed a theory to explain expiratory flow limitation, theory which is useful to understand the role of stent insertion in patients with dynamic CAO such as malacia or EDAC. Physiologic studies showed that once expiratory flow becomes limited at a given lung volume, there would be a region within the intrathoracic airway where intrabronchial and extra-bronchial pressures become equal (equal pressure point, EPP) (Fig. 13.5) [63]. At a given lung volume, driving pressure upstream (alveolarward) from the EPP would be equal to lung elastic recoil, because pleural pressure (Ppl) equals the intraluminal pressure (PL); downstream from the EPP (mouthward), airways would be compressed during expiration. This region of compression of intraluminal caliber is referred to as a flow-limiting segment (FLS) or “choke point.” As lung volume decreases and pleural pressure (Ppl) increases during forced expiration, the EPP migrates upstream, resulting in a lengthening of the increasingly narrow downstream segment. This increases airway resistance and prevents further increases in expiratory airflow, causing the EPP to become fixed when airflow becomes constant. EPP and therefore the FLS have tracheal location at high lung volumes (TLC), but as lung volume decreases during exhalation, the FLS moves peripherally, but they still stay in the central airways, in the lobar/segmental, the farthest in subsegmental bronchi [64]. Therefore, if the choke points in humans are often located in the lobar bronchi, a mainstem bronchial or tracheal collapsibility in the form of EDAC, often seen on CT or bronchoscopy, should not result in any pressure drop between the mouth and the choke point and should not affect flow. In fact, physiologists suggest bronchoscopic or radiologic detection of expiratory tracheal or mainstem bronchial compression (EDAC) should trigger a search for causes of airflow obstruction within the lung, not the central airways [65]. Loss of pressure in the abnormally narrowed peripheral airways in patients with asthma, COPD, or bronchiolitis will lead to decreased intraluminal pressure by the time that airflow reaches central airways, so that these airways (trachea and mainstem bronchi) will collapse at the weakest point, which is the posterior membrane. Thus, EDAC is most often a reflection of peripheral airway disease, but it can also be seen with morbid obesity due to increased pleural pressure and possible flow limitation at rest. A study of patients with obesity and COPD and normal volunteer controls found that EDAC was significantly associated with BMI (69% tracheal collapse among morbidly obese patients with BMI ≥ 35 compared to 57% in others, p = 0.002) [66]. EDAC has been documented in 22% of patients with COPD assessed by dynamic chest CT and in morbidly obese patients under general anesthesia likely due to positive pressures throughout the chest [67]. This does not mean that EDAC is responsible for flow limitation. In fact, even when defined as forced expiratory collapse of >80%, according to some reports, EDAC is not flow limiting as there is no significant correlation between end-expiratory or dynamic expiratory collapse and percent predicted FEV1 [68].
