There is currently an absence of strong evidence for any of the proposed therapies for adult and pediatric tracheobronchomalacia (TBM).1,2 Invasive treatments for TBM mostly comprise airway stent insertion and membranous tracheobronchoplasty.3–6 These interventions may improve patients’ symptoms and quality of life. A significant improvement in physiology parameters, however, is not a consistent finding.3,5,6 Complications from both procedures increase morbidity and health care costs.3–6 Conservative management for TBM includes treatment of the underlying causative diseases (eg, relapsing polychondritis, COPD), pulmonary hygiene, and intermittent application of noninvasive positive pressure ventilation (NIPPV) in the form of continuous positive airway pressure (CPAP) or bilevel positive pressure ventilation (BiPAP).2 When applied to patients suffering from TBM or the related disorder, excessive dynamic airway collapse (EDAC), NIPPV is sometimes referred to as “pneumatic stenting” and can be a sole treatment or as an adjunct in patients who remain symptomatic and have residual central airway collapse after invasive therapies.4,7–9
The excessive airway narrowing that occurs during expiration in patients with TBM results in an increased turbulent flow and airway resistance. Although it has not been clearly demonstrated by physiological studies in patients with TBM, it is possible that the reduced airway cross-sectional area may require greater than normal transpulmonary pressures to maintain an adequate expiratory airflow. This results in an increase in the work of breathing and dyspnea. NIPPV decreases respiratory system resistance and can be used to maintain airway patency, improve expiratory flow, and assist with secretion drainage. Small studies demonstrated that the application of CPAP for pediatric and adult TBM improves spirometry values, secretion management, atelectasis, and exercise tolerance.8,9 The equal pressure point and wave speed theories offer complementary explanations for expiratory airflow limitation.10,11 NIPPV affects the variables described in these theories including intraluminal airway pressure, airway compliance, and cross-sectional area at the flow-limiting segment. CPAP was shown to decrease the resistance of the collapsible airways and prevent critical airway closure.12–14 NIPPV increases airway lumen cross-sectional area (thus the name “pneumatic stenting”) and airway elastance, making the airway stiffer and increasing flow velocity. Furthermore, CPAP prevents the development of flow-limiting segments in the upper airways early during expiration, facilitating lung emptying.15
Application of NIPPV also improves lung volumes, and thus the elastic recoil, which is one of the determinants of flow limitation based on equal pressure point theory.10 This fact offers an alternative explanation for symptomatic improvement in patients suffering from TBM.16 The measured flow and the contour of the flow-volume loop depend upon the lung volume at which they are measured; the expiratory flow being greater at higher than at lower lung volumes. In a study of infants with TBM and controls, the application of CPAP significantly increased maximal expiratory flow at functional residual capacity because of an increase in lung volumes.16 The maximal expiratory flows measured at different levels of CPAP were not different when compared at the same lung volumes, suggesting that the optimal level of CPAP for these patients may be related to increasing lung volumes to a level at which patients are not flow-limited during tidal breathing. In the absence of high-quality published data regarding the appropriate pressure levels that improve symptoms and flow in TBM, physicians could prescribe empirically chosen NIPPV levels or base them on the results from NIPPV-assisted bronchoscopy.17
CPAP or BiPAP assistance during flexible bronchoscopy can be used to evaluate the effectiveness of intermittent NIPPV application to symptomatic patients with expiratory central airway collapse (TBM or EDAC). It is part of the dynamic bronchoscopy (Dynamic bronchoscopy allows observations of airway changes in response to various respiratory maneuvers while placing the patient in the upright, supine, and lateral decubitus positions. It is performed using flexible bronchoscopy with minimal or moderate sedation so that the patient can follow commands. The patient is asked to inhale and exhale deeply, cough, flex, and extend the neck.) performed to evaluate these patients.18 NIPPV-assisted bronchoscopy could also be valuable in the assessment of patients with small tracheoesophageal or bronchoesophageal fistulas, which become evident once positive pressure is applied, and to prevent respiratory distress and intubation in high-risk patients requiring flexible bronchoscopy.18,19 In the critical care setting, bronchoscopy can be performed with CPAP or BiPAP assistance, potentially avoiding the risks of refractory hypoxemia, intubation, and mechanical ventilation.20,21 NIPPV applied during bronchoscopy helps assure an uncomplicated bronchoscopy in hypercapnic COPD patients with pneumonia,22 compensates for the hypotonicity of the upper airway muscles in patients with obstructive sleep apnea,23 and improves volume and flow in spontaneously breathing children with collapsible upper airways.24
For TBM and EDAC, NIPPV-assisted bronchoscopy allows physicians to evaluate the changes and the degree of airway collapse and assess patient’s symptoms during positive pressure ventilation. This is relevant, as the exact NIPPV settings that maintain airway patency in expiratory central airway collapse of various degrees of severity have not been methodically studied. Therefore, the pressure settings are individualized and determined during NIPPV-assisted bronchoscopy, although there are preliminary reports on CPAP-assisted computed tomography scanning.25 The limited published literature suggests that CPAP of 7 to 10 cm H2O usually assures airway patency.9,26
The degree to which a flexible bronchoscope reduces the airway lumen depends upon individual patients tracheal cross-sectional area and bronchoscope’s outer diameter; in general, however, the flexible bronchoscope is expected to occupy at least 10% to 15% of the normal tracheal lumen and can therefore increase the resistance to expiratory flow and intraluminal pressure. Studies of pulmonary mechanics during transnasal flexible bronchoscopy have also demonstrated that functional residual capacity increases by 17%. There is also a significant decrease in vital capacity, forced inspiratory flow, and FEV1. Airway pressure changes are also affected by the extent and duration of suctioning. NIPPV application during bronchoscopy will affect these physiological parameters. Standardization of technique is warranted. Flow dynamics studies are necessary to accurately quantify the degree of pressure changes during NIPPV-assisted bronchoscopy and their clinical relevance. The airway pressures applied during bronchoscopy to maintain airway patency may be indeed elevated by the presence of the bronchoscope itself; it is expected that once the scope is removed from the airway, to maintain the same airway lumen patency, patients may need higher pressures than the ones applied during bronchoscopy.
Contraindications to NIPPV-assisted bronchoscopy includes respiratory failure requiring intubation, hemodynamic instability, encephalopathy, recent myocardial infarction, presence of facial deformities, or recent head and neck or gastric surgery that would prohibit facemask applications or increase the risk of gastric insufflation and aspiration.27 Caution must also be applied in patients with COPD. Results from one study showed that CPAP levels of 10 and 15 cm H2O increased the emphysematous zones in all sectors of the lungs in patients with COPD compared with causing little hyperaeration in healthy volunteers.28 This is potentially harmful, given the already hyperinflated lungs of patients with COPD. Thus, optimal CPAP levels should counteract the increased effort of breathing while improving expiratory flow to its maximum without causing further hyperinflation.29 This could be evaluated by HRCT scanning when radiologic density threshold is used for quantification of hyperinflated zones30 but from practical standpoint, until more evidence is available, it may be more feasible for physicians to titrate the pressure levels based on changes in airway patency noted bronchoscopically and based on patients’ symptoms. The technique involves securing a full facemask to patient’s face with elastic straps, then connect it to the ventilator using a dual axis swivel adapter. The bronchoscope is advanced to the nares through the swivel adapter and facemask (Fig. 1). The initial pressure is 0 cm H2O and then it is titrated upward in increments of 3 cm H2O until airway lumen during exhalation is at least 50% of that noted during inspiration. The airway dynamics are evaluated in the upright and supine positions, as well as on and off CPAP to evaluate the degree of airway narrowing (Fig. 1).
The use of intermittent NIPPV for patients suffering from TBM and/or EDAC can result in symptomatic improvement. These patients, however, often have underlying disorders including obesity, COPD, and asthma, so that the true impact of their narrowed central airways on symptoms is difficult to discern. The use of validated performance status and dyspnea scores are warranted for this treatment as they are for the other modalities proposed for these entities.3–6 Although it makes physiological sense, there may be a placebo effect with NIPPV application. A randomized sham-controlled trial needs to be performed to determine whether “pneumatic stenting” should be offered to patients suffering from TBM. Further, there are no studies comparing CPAP versus BIPAP for this disease. Until more evidence becomes available, CPAP may be preferred over BiPAP, as the inspiratory and expiratory trigger sensitivity of BIPAP mode ventilation may be inadequate in patients with a high respiratory rate or small tidal volume, which could result in patient-ventilator asynchrony. Future studies of these disorders and their treatments must use a physiological approach and clearly defined outcome measures.
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