Treatment of severe tracheobronchomalacia: Ten-year experience

Surgical treatment for severe pediatric tracheobronchomalacia: the 20-year experience of a single center

OBJECTIVE

In children with tracheobronchomalacia, surgical management should be reserved for the most severe cases and be specific to the type and location of tracheobronchomalacia. The goal of this study is to describe the presentation and outcomes of children with severe tracheobronchomalacia undergoing surgery.

METHODS

Retrospective case series of 20 children operated for severe tracheobronchomalacia at a tertiary hospital from 2003 to 2023. Data were collected on symptoms age at diagnosis, associated comorbidities, previous surgery, age at surgery, operative approach, time of follow-up, and outcome. Surgical success was defined as symptom improvement.

RESULTS

The most frequent symptoms of severe tracheobronchomalacia were stridor (50 %), cyanosis (50 %), and recurrent respiratory infections (45 %). All patients had one or more underlying conditions, most commonly esophageal atresia (40 %) and prematurity (35 %). Bronchoscopy were performed in all patients. Based on etiology, patients underwent the following procedures: anterior aortopexy (n = 15/75 %), posterior tracheopexy (n = 4/20 %), and/or posterior descending aortopexy (n = 4/20 %). Three patients underwent anterior aortopexy and posterior tracheopexy procedures. After a median follow-up of 12 months, 16 patients (80 %) had improvement in respiratory symptoms. Decannulation was achieved in three (37.5 %) out of eight patients with previous tracheotomy. The presence of dying spells at diagnosis was associated with surgical failure.

CONCLUSIONS

Isolated or combined surgical procedures improved respiratory symptoms in 80 % of children with severe tracheobronchomalacia. The choice of procedure should be individualized and guided by etiology: anterior aortopexy for anterior compression, posterior tracheopexy for membranous intrusion, and posterior descending aortopexy for left bronchus obstruction.

Single-center review on safety of biodegradable airway stenting in pediatric population

BACKGROUND

Tracheobronchomalacia (TBM) and airway stenosis are recognized etiologies of airway obstruction among children. Their management is often challenging, requiring multiple interventions and prolonged respiratory support with associated long-term morbidity. Metallic or silicone stents have been used with mixed success and high complication rates. More recently biodegradable Ella stents (BES) provided an attractive interventional option.

OBJECTIVES

We report our experience in the treatment of TBM and vascular airway compression using BES. We deliberately downsized them to minimize intraluminal granulation tissue formation.

MATERIALS AND METHODS

Retrospective study over an 8-year period between November 2012 and December 2020 of pediatric patients with severe airway obstruction requiring airway stenting for extubation failure, malacic death spells, recurrent chest infections, or lung collapse.

RESULTS

Thirty-three patients (5 tracheal and 28 bronchial diseases) required 55 BES during the study period. The smallest patient weighed 1.8 kg. Median age of patient at first stent implantation was 13.1 months (IQR 4.9-58.3). The majority of the bronchial stents were in the left main bronchus (93%), of which 57% for vascular compression. Repeat stents were used in 19 patients (57.7%), with a range of two to four times. We did not experience erosion, infection, or obstructive granuloma needing removal by forceps or lasering. Three stent grid occluded with secretions needing bronchoscopic lavage. Stent migration occurred in three patients.

CONCLUSIONS

BES holds promise as a treatment option with low rate of adverse effects for a specific subset of pediatric patients with airway malacia or vascular compression. Further studies are warranted.

Bronchial Remodeling Following Airway Stenting in Pediatric Patients With Tracheobronchial and Congenital Heart Disease

Background

Treatment of tracheobronchial disease in medically complex infants with congenital heart disease (CHD) is often challenging. When conservative management or surgery fails or is contraindicated, airway stenting can allow for advancement of care or weaning of respiratory support.

Methods

We identified 8 cases of airway stenting with balloon-expandable coronary bare-metal stents performed at our institution between February 2019 and September 2022 to relieve conservative treatment-refractory tracheobronchial disease in pediatric patients with CHD. All patients underwent rigid microlaryngoscopy, bronchoscopy, and flexible bronchoscopy as well as computed tomography angiography.

Results

Eight patients underwent technically uncomplicated placement of balloon-expandable coronary bare-metal stents in the trachea or bronchus. Immediate improvement in respiratory parameters was noted following stent placement. Six patients were able to wean mechanical ventilation following stent placement, with a median of 2.5 days of mechanical ventilation following the procedure (range, 0-219). All stents were subsequently endoscopically removed at a median of 6.8 months (range, 0.4-16.3 months). In 6 patients, bronchoscopy after stent removal demonstrated a rounder configuration of the airway consistent with bronchial remodeling.

Conclusions

In pediatric patients with tracheobronchial and CHD, airway stenting with balloon-expandable bare-metal coronary stents relieved respiratory symptoms with minimal complications and resulted in bronchial remodeling after stent removal.

The clinical application of flexible bronchoscopy in a neonatal intensive care unit

OBJECTIVE

Flexible bronchoscopy is widely used in infants and it plays a crucial role. The aim of this study was to investigate the value and clinical safety of flexible bronchoscopy in a neonatal intensive care unit.

METHODS

A retrospective analysis was performed on the clinical data of 116 neonates who underwent flexible bronchoscopy and the outcomes of 147 procedures. A correlation analysis was performed on the relationship between flexible bronchoscopy findings, microscopic indications, and clinical disease.

RESULTS

The 147 procedures performed were due to the following reasons: problems related to artificial airways, 58 cases (39.45%); upper respiratory problems, 60 cases (40.81%) (recurrent dyspnea, 23 cases; upper airway obstruction, 17 cases; recurrent stridor, 14 cases; and hoarseness, six cases), lower respiratory problems, 51 cases (34.69%) (persistent pneumonia, 21 cases; suspicious airway anatomical disease, 21 cases; recurrent atelectasis, eight cases; and pneumorrhagia, one case), feeding difficulty three cases (2.04%). The 147 endoscopic examinations were performed for the following reasons: pathological changes, 141 cases (95.92%); laryngomalacia, 78 cases (53.06%); mucosal inflammation/secretions, 64 cases (43.54%); vocal cord paralysis, 29 cases (19.72%); trachea/bronchus stenosis, 17 cases (11.56%) [five cases of congenital annular constriction of the trachea, seven cases of left main tracheal stenosis, one case of the right middle bronchial stenosis, two cases of tracheal compression, and two cases of congenital tracheal stenosis]; subglottic lesions, 15 cases (10.20%) [eight cases of subglottic granulation tissue, six cases of subglottic stenosis, one cases of subglottic hemangioma]; tracheomalacia, 14 cases (9.52%); laryngeal edema, five cases (3.40%); tracheoesophageal fistula, four cases (2.72%); rhinostenosis, three cases (2.04%); tracheal bronchus, three cases (2.04%); glossoptosis, two cases (1.36%); laryngeal cyst, two cases (1.36%); laryngeal cleft, two cases (1.36%); tongue base cysts, one case (0.68%); and pneumorrhagia, one case (0.68%). Complications were rare and mild.

CONCLUSION

Flexible bronchoscopy is safe and effective for diagnosing and differentiating neonatal respiratory disorders in neonatal intensive care units.

A Delayed Anatomic Diagnosis and Management Challenge in an Initially Asymptomatic Infant With Type II Pulmonary Artery Sling: A Case Report

Pulmonary artery sling (PAS) is a rare but fatal malformation. Patients with PAS tend to develop obstructive symptoms in few weeks of life. Conversely, some patients may be otherwise mild or asymptomatic in their early life. Currently, no consensus on the intervention timing and treatment strategy for asymptomatic and mild cases has been reached. Moreover, the extent of tracheal stenosis is another determining factor for the choice of intervention timing since clinical symptoms might not correspond well with the degree of stenosis. Lack of comprehensive assessment of entire airways confer underestimation of disease severity and in turn improper choice of treatment regimens and poor outcomes. Herein, we described an infantile case of PAS, who was scheduled initially for periodic outpatient follow-up on account of the absence of symptoms and inadequate imaging assessment at diagnosis. The patient developed recurrent wheezing and progressive respiratory distress at 7 months of age. After left pulmonary artery (LPA) reimplantation without tracheal intervention, bronchoscopy was performed due to failure to wean from mechanical ventilation, which demonstrated complete tracheal cartilage rings, a long segment tracheal stenosis, a low tracheal bifurcation at T6, and the absence of a separate right middle lobe bronchus. The patient was finally diagnosed with type IIb PAS and extubated successfully following conservative treatment. Miserably, neurological sequelae were devastating, leading to poor outcomes. Comprehensive airway evaluation using bronchoscopy is substantial to early identification of all components responsible for airway compromise in PAS anatomic subtypes. Considering severe concomitant maldevelopment of the bronchial tree in children with type IIb PAS, early and complete correction by surgery might decrease perioperative morbidities and mortalities of these patients.

Stent intervention for children with CHD and tracheal stenosis

CHD is closely related to respiratory system diseases (Mok Q, Front Pediatr 2017; 5: 2296-2360). Flexible fibreoptic bronchoscopy will diagnose anatomical lesions of the trachea and perform interventions at the same time for children with indications. We report a case of pulmonary artery sling with severe tracheostenosis in a 11-month-old boy. Tracheal stents were placed with good prognosis.

Thoracoscopic aortopexy for symptomatic tracheobronchomalacia

AIM

Symptomatic tracheobronchomalacia can be fatal. Successful treatment includes aortopexy. We report outcomes of the thoracoscopic approach in a single centre.

METHODS

All patients undergoing thoracoscopic aortopexies from 2009 to 2018 were retrospectively reviewed. Data was reported as median (interquartile range). Risk factors for subsequent tracheostomy were analyzed with logistics regression model, p < 0.05 as significant.

RESULTS

Twenty-one patients with mid to distal tracheomalacia (n = 17) and bronchial involvement (n = 4) were determined on bronchoscopy, tracheobronchogram, or CT thorax. Preoperative patient demographics and comorbidities, e.g., gastro-oesophageal reflux disease, prematurity, and cardiac anomalies were recorded. Indications for thoracoscopic aortopexy were apparent life-threatening event(s) (n = 14), recurrent chest infections (n = 5), and failure to wean invasive ventilation (n = 2). Thoracoscopic aortopexies (n = 20) with conversion to open (n = 1) were performed. Intraoperative bleeding (n = 2) occurred, and chest tube (n = 1) was inserted for monitoring. Intraoperative bronchoscopy (n = 17) confirmed improvement of tracheomalacia. Anesthetic time was 140 (90-160) minutes. Postoperatively, 2 patients had dehiscence of the aorta from the sternum. They underwent redo open aortopexy with posterior tracheopexy, and 1 required subsequent tracheostomy. Another 2 patients required tracheostomies. Potential risk factors for subsequent tracheostomy were investigated, and only the association of tracheobronchomalacia was close to significance (OR 16 (95% CI 0.95-267.03), p = 0.05). Follow up duration was 365 (72-854) days. Symptoms resolution occurred in n = 17 (81%) of patients.

CONCLUSION

Different modalities were used to delineate the site of tracheobronchomalacia and its etiology. Tracheomalacia with bronchial involvement may be a risk factor for subsequent tracheostomy.

LEVEL OF EVIDENCE

Level 3 (Case Series).

ERS statement on tracheomalacia and bronchomalacia in children

Tracheomalacia and tracheobronchomalacia may be primary abnormalities of the large airways or associated with a wide variety of congenital and acquired conditions. The evidence on diagnosis, classification and management is scant. There is no universally accepted classification of severity. Clinical presentation includes early-onset stridor or fixed wheeze, recurrent infections, brassy cough and even near-death attacks, depending on the site and severity of the lesion. Diagnosis is usually made by flexible bronchoscopy in a free-breathing child but may also be shown by other dynamic imaging techniques such as low-contrast volume bronchography, computed tomography or magnetic resonance imaging. Lung function testing can provide supportive evidence but is not diagnostic. Management may be medical or surgical, depending on the nature and severity of the lesions, but the evidence base for any therapy is limited. While medical options that include bronchodilators, anti-muscarinic agents, mucolytics and antibiotics (as well as treatment of comorbidities and associated conditions) are used, there is currently little evidence for benefit. Chest physiotherapy is commonly prescribed, but the evidence base is poor. When symptoms are severe, surgical options include aortopexy or posterior tracheopexy, tracheal resection of short affected segments, internal stents and external airway splinting. If respiratory support is needed, continuous positive airway pressure is the most commonly used modality either via a face mask or tracheostomy. Parents of children with tracheobronchomalacia report diagnostic delays and anxieties about how to manage their child's condition, and want more information. There is a need for more research to establish an evidence base for malacia. This European Respiratory Society statement provides a review of the current literature to inform future study.

Innovative management of severe tracheobronchomalacia using anterior and posterior tracheobronchopexy

OBJECTIVES/HYPOTHESIS

Combined anterior and posterior tracheobronchopexy is a novel surgical approach for the management of severe tracheobronchomalacia (TBM). We present our institutional experience with this procedure. Our objective was to determine the utility and safety of anterior and posterior tracheopexy in the treatment of severe TBM.

STUDY DESIGN

Retrospective chart review.

METHODS

All patients who underwent anterior and posterior tracheopexy from January 2013 to July 2017 were retrospectively reviewed. Charts were reviewed for indications, preoperative work-up, tracheobronchomalacia classification and severity, procedure, associated syndromes, synchronous upper aerodigestive tract lesions, and aberrant thoracic vessels. Main outcomes measured included improvement in respiratory symptoms, successful extubation and/or decannulation, vocal fold immobility, and new tracheotomy placement.

RESULTS

Twenty-five patients underwent anterior and posterior tracheopexy at a mean age of 15.8 months (range, 2-209 months; mean, 31 months if 2 outliers of 206 and 209 months included). Mean length of follow-up was 26.8 months (range, 13-52 months). Indications for surgery included apneic events, ventilator dependence, need for positive pressure ventilation, tracheotomy dependence secondary to TBM, recurrent pneumonia, and exercise intolerance. Many patients had other underlying syndromes and synchronous upper aerodigestive tract lesions (8 VACTERL, 2 CHARGE, 1 trisomy 21, 1 Feingold syndrome, 17 esophageal atresia/tracheoesophageal fistula, 20 cardiac/great vessel anomalies, 1 subglottic stenosis, 1 laryngomalacia, 7 laryngeal cleft). At preoperative bronchoscopy, 21 of 25 patients had >90% collapse of at least one segment of their trachea, and the remaining four had 70% to 90% collapse. Following anterior and posterior tracheopexy, one patient developed new bilateral vocal-fold immobility; one patient with a preoperative left cord paralysis had a new right vocal-fold immobility. Postoperatively, most patients had significant improvement in their respiratory symptoms (21 of 25, 84%) at most recent follow-up. Three patients with preexisting tracheotomy were decannulated; two patients still had a tracheotomy at last follow-up. Two patients required new tracheotomy for bilateral vocal-fold immobility.

CONCLUSIONS

Combined anterior and posterior tracheopexy is a promising new technique for the surgical management of severe TBM. Further experience and longer follow-up are needed to validate this contemporary approach and to minimize the risk of recurrent laryngeal nerve injury.

LEVEL OF EVIDENCE

4 Laryngoscope, 130:E65-E74, 2020.

Outcome of long-term complications after permanent metallic left bronchial stenting in children

OBJECTIVES

We describe the way we treated 7 children with critical long-term complications after metallic balloon-expandable stenting in the left mainstem bronchus.

METHODS

Endoscopic follow-up included a first bronchoscopy 3 weeks after stenting, then monthly for 3 months, every 4-6 months up to 1 year and at scheduled times to calibrate stent diameter up to final calibration. When major complications occurred, patients underwent chest computed tomographic angiography.

RESULTS

In 1 of the 7 children (median age 2.8 years), metallic left bronchial stenting served as a bridge to surgery. After a median 4-year follow-up, all 7 children experienced recurrent stent ovalizations with stent breakage in 3 and erosion in 1. In 4 children, computed tomographic angiography showed abundant peribronchial fibrous tissue, in 2 left mediastinal rotation and in 1 displacement along the left bronchus after pulmonary re-expansion as the cause of stent-related complication. Of the 7 children, 6 underwent surgery (5 posterior aortopexy and 1 section of the ligamentum arteriosus) and 3 required nitinol stents placement within the metallic ones. One patient completed the follow-up, and 1 patient was lost to follow-up. All 5 remaining children still have permanent bronchial stents in place, patent and re-epithelialized after a median 10.5-year follow-up. There were no deaths.

CONCLUSIONS

Satisfactory anatomical relationships when children have stents placed in the left mainstem bronchus alone do not guarantee the final success. Several mechanisms intervene to cause critical stent-related complications in children during growth. Permanent metallic stents should be used carefully, and only in selected patients.

Technique, pitfalls, quality, radiation dose and findings of dynamic 4-dimensional computed tomography for airway imaging in infants and children

This retrospective review of 33 children's dynamic 4-dimensional (4-D) computed tomography (CT) images of the airways, performed using volume scanning on a 320-detector array without anaesthesia (free-breathing) and 1.4-s continuous scanning, was undertaken to report technique, pitfalls, quality, radiation doses and findings. Tracheobronchomalacia (airway diameter collapse >28%) was recorded. Age-matched routine chest CT scans and bronchograms acted as benchmarks for comparing effective dose. Pitfalls included failure to administer intravenous contrast, pull back endotracheal tubes and/or remove nasogastric tubes. Twenty-two studies (67%) were diagnostic. Motion artefact was present in 16 (48%). Mean effective dose: dynamic 4-D CT 1.0 mSv; routine CT chest, 1.0 mSv, and bronchograms, 1.4 mSv. Dynamic 4-D CT showed tracheobronchomalacia in 20 patients (61%) and cardiovascular abnormalities in 12 (36%). Fourteen children (70%) with tracheobronchomalacia were managed successfully by optimising conservative management, 5 (25%) underwent surgical interventions and 1 (5%) died from the presenting disorder.

Indirect management of full-thickness tracheal erosion in a complex pediatric patient

Prolonged tracheostomy dependence in pediatric patients can be associated with significant complications, including damage to the tracheal wall requiring reconstruction. We present a case of an 8 year-old female with full-thickness tracheal erosion secondary to the presence of a tracheostomy tube combined with a narrow thoracic inlet. A direct tracheal reconstruction was considered but eliminated due to the poor tissue quality of the trachea. Instead, a multi-disciplinary surgical team conceived of a novel indirect approach to manage the patient's tracheal defect. To our knowledge the use of indirect repair of a full-thickness tracheal defect has not been reported in the literature.

Losartan in combination with propranolol slows the aortic root dilatation in neonatal Marfan syndrome

Neonatal Marfan syndrome, in contrast to classical Marfan syndrome, is characterized by rapidly progressive multi-valvular cardiac disease and death from congestive heart failure, typically within the first year of life. Due to the rarity of this condition, treatment for neonatal Marfan syndrome has not been well studied. In this report, a combination of losartan and propranolol reduced the aortic root dilatation rate after three months of losartan therapy. Genetic analysis in this patient revealed a mutation in exon 25 of the FBN1 gene, which typically results in a shorter life expectancy. However, the patient's heart failure was controlled by losartan, propranolol and other anti-congestive medications, which may have prolonged his survival.

Trans-nasal flexible bronchoscopy in wheezing children: Diagnostic yield, impact on therapy, and prevalence of laryngeal cleft

AIM

Persistent or recurrent wheezing is a common indication for flexible bronchoscopy, as anatomic and infectious or inflammatory changes are highly prevalent. We sought to evaluate the prevalence of anatomic, infectious, and inflammatory disease in a cohort of children undergoing flexible bronchoscopy for wheezing or poorly controlled asthma.

METHODS

We retrospectively reviewed all children <18 years old who underwent flexible bronchoscopy at our center from October 29, 2012-December 31, 2016 for the primary or secondary indication of wheezing (persistent, frequently recurring, or atypical) or poorly controlled asthma.

RESULTS

A total of 101 procedures were identified in 94 patients, aged 3 months to 18 years. Potential anatomic causes for wheezing identified in 45.7% of patients and inflammatory changes in 49.5% of procedures. This included the identification of a laryngeal cleft in 17% for which half required medical or surgical management. Tracheobronchomalacia was the most commonly identified anatomic lesion. Thirty children from this cohort had poorly controlled asthma. Among this subgroup, 54% had increased neutrophils on BAL and 30% had an anatomic contributor to wheezing, including one with a laryngeal cleft. Based on findings from flexible bronchoscopy, management changes made in 63.8% of patients. This included medication changes in 54 and surgical intervention in 9.

DISCUSSION

We conclude that transnasal flexible bronchoscopy has high yield in children with recurrent, persistent, or atypical wheezing and those with poorly controlled asthma. Laryngeal cleft has a reasonably high prevalence that warrants specific evaluation in this population.

Airway stenting in a child with spondyloepiphyseal dysplasia congenita: 13-Year survival

We describe the case of a boy with spondyloepiphyseal dysplasia congenita. At birth, he experienced severe respiratory distress necessitating tracheotomy. Endoscopy done because mechanical ventilation failed to resolve desaturations disclosed severe tracheo-bronchomalacia. A Polyflex silicone stent was placed in the trachea (replaced by Y-Dumon stent) and 2 Palmaz metallic stents in the mainstem bronchi (overlapped with 2 Jomed stents 5 years later). Airway stenting guaranteed a suitable respiratory status and allowed a child who was expected to die at birth, to reach 13.5 years old in good conditions.

Tracheobronchomalacia Is Associated with Increased Morbidity in Bronchopulmonary Dysplasia

RATIONALE

Tracheobronchomalacia is a common comorbidity in neonates with bronchopulmonary dysplasia. However, the effect of tracheobronchomalacia on the clinical course of bronchopulmonary dysplasia is not well-understood.

OBJECTIVE

We sought to assess the impact of tracheobronchomalacia on outcomes in neonates with bronchopulmonary dysplasia in a large, multi-center cohort.

METHODS

We preformed a cohort study of 974 neonates with bronchopulmonary dysplasia admitted to 27 neonatal intensive care units participating in the Children's Hospital Neonatal Database who had undergone bronchoscopy. In hospital morbidity for neonates with bronchopulmonary dysplasia and tracheobronchomalacia (N=353, 36.2%) was compared to those without tracheobronchomalacia (N=621, 63.8%) using mixed-effects multivariate regression.

RESULTS

Neonates with tracheobronchomalacia and bronchopulmonary dysplasia had more comorbidities, such as gastroesophageal reflux (OR=1.65, 95%CI 1.23- 2.29, P=0.001) and pneumonia (OR=1.68, 95%CI 1.21-2.33, P=0.002) and more commonly required surgeries such as tracheostomy (OR=1.55, 95%CI 1.15-2.11, P=0.005) and gastrostomy (OR=1.38, 95%CI 1.03-1.85, P=0.03) compared with those without tracheobronchomalacia. Neonates with tracheobronchomalacia were hospitalitized (118 ± 93 vs 105 ± 83 days, P=0.02) and ventilated (83.1 ± 91.1 vs 67.2 ± 71.9 days, P=0.003) longer than those without tracheobronchomalacia. Upon discharge, neonates with tracheobronchomalacia and BPD were more likely to be mechanically ventilated (OR=1.37, 95CI 1.01-1.87 P=0.045) and possibly less likely to receive oral nutrition (OR=0.69, 95%CI 0.47-1.01, P=0.058).

CONCLUSIONS

Tracheobronchomalacia is common in neonates with bronchopulmonary dysplasia who undergo bronchoscopy and is associated with longer and more complicated hospitalizations.

Computational modeling of airway instability and collapse in tracheomalacia

Background

Tracheomalacia (TM) is a condition of excessive tracheal collapse during exhalation. Both acquired and congenital forms of TM are believed to result from morphological changes in cartilaginous, fibrous and/or smooth muscle tissues reducing airway mechanical properties to a degree that precipitates collapse. However, neither the specific amount of mechanical property reduction nor the malacic segment lengths leading to life threatening airway collapse in TM are known. Furthermore, the specific mechanism of collapse is still debated.

Methods

Computational nonlinear finite element models were developed to determine the effect of malacic segment length, tracheal diameter, and reduction in tissue nonlinear elastic properties on the risk for and mechanism of airway collapse. Cartilage, fibrous tissue, and smooth muscle nonlinear elastic properties were fit to experimental data from preterm lambs from the literature. These elastic properties were systematically reduced in the model to simulate TM.

Results

An intriguing finding was that sudden mechanical instability leading to complete airway collapse occurred in airways when even a 1 cm segment of cartilage and fibrous tissue properties had a critical reduction in material properties. In general, increased tracheal diameter, increased malacic segment length coupled with decreased nonlinear anterior cartilage/fibrous tissue nonlinear mechanical properties increased the risk of sudden airway collapse from snap through instability.

Conclusion

Modeling results support snap through instability as the mechanism for life threatening tracheomalacia specifically when cartilage ring nonlinear properties are reduced to a range between fibrous tissue nonlinear elastic properties (for larger diameter airways > 10 mm) to mucosa properties (for smaller diameter airways < 6 mm). Although reducing posterior tracheal smooth muscle properties to mucosa properties decreased exhalation area, no sudden instability leading to collapse was seen in these models.