Distal trachea

Finer particles ( < 3 pm), termed respirable particles, pass beyond the ex-trathoracic airways and enter the tracheobronchial tree. Impaction plays a significant role near the tracheal jet, but sedimentation predominates as the effects of rapid conduit expansion dampen in the distal trachea and beyond. Sedimentation occurs when gravitational forces exerted on a particle equal drag forces, i.e., when particle velocity falls to u . As mean inspiratory air-stream velocity gradually declines along the tracheobronchial tree, particle momentum diminishes and 0.5-3 pm MMAD particles settle out of the airflow and onto mucosal surfaces. 

Fig. 12.1. a A 61-year-old male patient who presented with dyspnea and hemoptysis. Chest X-ray demonstrates mediastinal lymph node enlargement and severe narrowing of the distal trachea, b Computed tomography of the thorax confirms the mediastinal lymph node enlargement and shows infiltration of the trachea by the lymph nodes. Bronchoscopy revealed a tumor of the distal trachea that was found to be an adenocarcinoma at histology. The tumor was bleeding spontaneously, c A spot 1 ... 

Fig. 12.5a, b. Three-dimensional reconstruction of the trachea in a 58-year-old male patient with lymph node metastases of a hypopharynx carcinoma and invasion of the distal trachea causing high-grade obstruction... 

Clearance in the upper, or ciliated, region is governed by the rate of mucus transport along the airways. These rates have been measured in the human nose and in dogs, rats, and other species. Asmundsson and Kilbum, Hilding, and Iravani established that mucociliary clearance rates increase from the distal bronchi toward the trachea. Because bronchial openings retard mucus flow, bifurcations receive an accumulation of mucus and associated particles. The rate of mucus production and mucus thickness and velocity vary from one person to another. Thickness increases and velocity decreases greatly when some toxic elements are present in the airway. 

The pharynx, larynx, trachea and bronchi are lined with pseudostratified, ciliated columnar epithelium that contain at least eight cell types, including mucous secretory goblet and Clara cells, which produce a protective mucus layer of 5-10 jum thickness (see Table 9.2). Subepithelial secretory glands, present in the bronchial submucosa, also contribute to the mucus blanket [9]. Through coordinated ciliary movement a propulsive wave is created, which continuously moves the mucus layer up towards the larynx. Consequently, the mucosal surface of trachea and bronchi is constantly swept to remove inhaled materials. As the bronchi divide into bronchioli, the ciliated columnar respiratory epithelium is much thinner and changes to a simpler non-ciliated cuboidal epithelium. The epithelium in the terminal and respiratory bronchioles consists of ciliated, cuboidal cells and a small number of Clara cells. However, Clara cells become the most predominant type in the most distal part of the respiratory bronchioles [10]. 

Griscom NT, Martin TR (1990) The trachea and esophagus after repair of esophageal atresia and distal fistula computed tomographic observations. Pediatr Radiol 20 447-450... 

The presence of cytochrome P 450 monooxygenases within Clara cells has been correlated with their susceptibility to metabohcally activated pulmonary cytotoxicants. Species-specific differences in susceptibihty to cytotoxicants exist between Clara cells located in the trachea and bronchi and those in distal bronchioles (Plopper et al. 1992). 

Modifications of this stent were carried out by Westaby and colleagues (Westaby et al. 1982). The caudal end was fashioned as a Y in order to allow stenting of the trachea and the main bronchi as well as to prevent distal migration. This stent design also did not use a side arm, which allowed placement by endoscopy. 

Recent developments of plastic stents have aimed to improve the resistance of plastic stents to external compression forces. Therefore, metal has been incorporated into the plastic material of the stent. One of the latest developments is the dynamic bifurcation stent made of silicone (Freitag et al. 1994). This Dynamic stent (Riisch, Kernen, Germany) is reinforced with horseshoe-shaped steel struts. A posteriorly located flexible membrane allows dynamic compression of the stent during coughing, whereas the steel struts prevent airway compression from external forces. Theoretically, this stent mimics the mechanical dynamics of the normal trachea. The distal end is a Y shape which rides on the carina to prevent distal migration. 

A flexible bronchoscope is introduced into the trachea to assess the proximal and distal margin of the tracheal and/or bronchial obstruction. Flexible bronchoscopy should further delineate the location of the stricture in relation to adjacent bronchial ostia and the carina. If a stricture can not be passed with a flexible bronchoscope, recanalization can be obtained using a regular (usually hydrophilic coated) guidewire and an angiographic catheter. Once the lesion has been traversed, the postobstructive bronchial system should be visualized by injection of a small amount of non-ionic iodinated contrast material (ideally a non-ionic dimer with a low osmolality should be used). 

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