Bronchial airways, diameters

The physiological effects of the image and mirror image of the leading bronchodilator albuterol are dramatically different. The R image increases the bronchial airway diameter, whereas the S mirror image cancels this effect and is a suspected inflammatory agent. 

Sparrow, M.P., Willet, K.E. and MitcheU, H.W. (1990). Airway diameter determines flow-resistance and sensitivity to contractile mediators in perfused bronchial segments. Agents Actions 31, 63-66. 

Figure 9. Diameters and surface areas of bronchial airways as a function of age, according to the UCI lung model.

In the present study the bronchial morphometry of Yeh and Schum (1980) is utilized instead of the Weibel model. This is a more accurate description of the bronchial airway lengths, diameters and branching pattern. It does not assume dichotomous branching and therefore does not suffer from the problem of the artificially high surface area leading to low alpha dose in the more distal airways common to other models of the human airways. 

Endocrine-like cells with biochemical characteristics of Amine Precursor Uptake and Decaboxylase (APUD) were quantified in the tracheal, bronchial, and bronchiolar epithelia of the guinea pig by four histochemical stains Grimelius, en-block silver, lead-haematoxylin-en block silver and periodic acid-Schiff-lead haematoxylin (Marchevsky et al. 1983). There were significant differences between the number of epithelial cells in the various locales of the respiratory tree 318 5.63 epithelial cells/ mm in the trachea, 263 17.11 epithelial cells/mm in the bronchi and 193 4.21 epithelial cells/mm in the bronchioles. Boers et al. (1996) did not find any difference in the number of neuroendocrine cells between large (airway diameter >4.5 mm) and small (airway diameter <1.2 mm) conducting airways in 9 human subjects without pulmonary disease out of 250 autopsy cases. 

The aerodynamic particle diameter determines the fate of particles in the respiratory system. Coarse particles are deposited in the nose and nasopharynx. Smaller particles that pass the upper airway can be deposited in the bronchial region and lower airway. A size-selective deposition model and sampling of particles has been standardized both in Europe and internationally. The... 

It is seen that the diameters of bronchioles (averaged over generations 11 - 15) vary little with age. The increase in bronchial size is greater, but still less than might be expected if airways are simply scaled for overall body dimensions (illustrated by the dashed curves in Figure 9, which are functions of body weight W). Since bronchiolar diameter does not change much with age it is likely that the thickness of bronchiolar epithelium is also relatively constant. However, in the case of the bronchi, it is reasonable to assume that epithelial thickness is proportional to bronchial diameter. Thus, it is necessary to use age dependent conversion factors between the surface density of alpha-decays and dose to cells. 

It is well known that enhanced deposition in the first few airways occurs due to the turbulence produced. Turbulent diffusion is accounted for by using factors (ratio of observed deposition to calculated diffusion deposition) to correct the diffusion deposition. These had formerly been measured by Martin and Jacobi (1972) in a dichotomous plastic model of the upper airways. The data used here are from measurements performed by Cohen (1986) using hollow casts of the upper bronchial tree which included a larynx. This cast was tested using cyclic flow with deposition measured for 0.03, 0.15 and 0.20 urn diameter particles. Her turbulent diffusion factors are used in the calculation here (14 for generation 0, and 2 for generations 1 to 6). 

One of the most striking pathological features of chronic asthma is an increase in the smooth muscle mass of the airways (DunniU etal., 1969 Heard and Hossain, 1973 Hossain, 1973 Ebina etal., 1990). Hyperplasia (increase in cell number) rather than hypertrophy (increase in cell size) predominantly produces this increase in mass, and the associated reduction in airway luminal diameter. This fixed increase in muscle mass contributes to the component of airways obstruction observed in chronic asthmatics which is irreversible with bronchodilators. It also leads to an exaggerated response to bronchoconstrictor stimuli, equivalent to the characteristic bronchial hyperresponsiveness of asthmatics (James et al., 1989 Pare et al., 1991 Pare, 1993). 

Rohrer [82] measured the diameter and length of the elements of the bronchial tree and constructed a dimensional model on which he based his reasoning on flow resistance in the human airways. 

It has recently been recognized that a fraction of particles deposited on surfaces of ciliated thoracic airways is not removed from these airways within 40 h but retained for longer periods (4). This long-term retention phenomenon was observed for particles smaller than 6 pm in diameter. Although particle removal from thoracic airways is therefore overestimated by the radiotracer technique, it affects bronchial deposition by less than 10%. 

The most important morphological examination prior to insertion of an airway stent is a spiral computed tomography (CT) of the major airways. A CT enables a delineation of the airway obstruction. It further demonstrates the length of stenosis, grade of obstruction, and allows calibrated measurements of the smallest diameter of the obstruction. CT can also differentiate a mucosal obstruction from a submucosal cause of obstruction, and further extra-tracheal or exo-bronchial disease responsible for airway comprise. Another advantage of CT is the possibility to assess the relation of the underlying cause of obstruction to other crucial mediastinal and/or pulmonary structures (e.g., major vessels, esophagus, heart, lymph nodes). 

Appropriate assessment of the location of the tracheal or bronchial obstruction is mandatory to choose the correct stent and to allow proper placement. In the case of a proximal tracheal lesion the distance to the vocal cord has to be assessed and measured. In the case of a distal tracheal lesion the relation to the Carina has to be defined in order to assess whether a single tracheal stent or an additional unilateral or bilateral bronchial stent is required to achieve sufficient luminal diameter in the central airways. In the case of bronchial obstruction, the relation to lobar bronchial orifices has to be analyzed. 

Tracheo-bronchial (TB) region. This begins at the larynx and comprises the trachea, bronchi and bronchioles. The branching airways are smaller and more numerous at each division, ending with the terminal bronchioles which are about 0.5 mm in diameter. 

At any point on the bronchial tree, the transport surface is determined by the inside diameter and the number of airways at this level. Moving from the center toward the periphery, diameters decrease, but the number of airways increases exponentially so that the transport surface decreases proportionally. As a result, the transporting surface of the airways decreases from the peripheral to the central airways. Accumulation of mucus in the central airways is normally countered by the higher mucus transport rate centrally than peripherally, and possibly by a greater reabsorption of watery constituents centrally. 

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