Chest
Translating Basic Research into Clinical PracticeSmall Airway Disease in Asthma and COPD: Clinical Implications
Section snippets
Assessment of Small Airway Disease
The assessment and monitoring of small airway involvement in asthma and COPD is a challenging matter because the region is relatively inaccessible for functional measurements. Until now there has been no test with accepted cutoff values to measure the presence and severity of small airway involvement.
Functional Parameters
Pulmonary small airway function tests have been developed with respect to the physiology of small airways.1 Although the cross-sectional area increases toward the periphery of the lung, gas velocity decreases and airflow changes from turbulent to laminar. Obstruction of the small airways affects the distribution of ventilation and may lead to small airway closure accompanied by air trapping. Pulmonary function tests that are used to assess small airway pathology can be subdivided in tests
Noninvasive Markers of Inflammation
Peripheral airway inflammation can be examined by measuring nitric oxide (NO) concentrations in single breath exhaled air during different flow rates.15 Using a mathematical model, it is possible to discriminate between the bronchial and alveolar contribution. Alveolar NO measurements have shown good reproducibility and responsiveness to small particle inhaled corticosteroids (ICS) (Table 1, Table 2).16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39
Imaging Techniques
Imaging techniques, such as high-resolution CT (HRCT) scanning and MRI with inhaled hyperpolarized gases, can be used to evaluate indirect signs of small airway obstruction, such as inhomogeneity of ventilation and air trapping, which are associated with remodeling.40, 41, 42, 43, 44, 45 All above-mentioned parameters have been used in research and contributed to the progress made to understand involvement of small airways in asthma and COPD. Each test has its own features making it more or
Pathology of Small Airways in Asthma
Despite interest in small airway disease in asthma, the number of articles about the underlying pathology is surprisingly small, probably due to the relative inaccessibility of the small airways. Several studies collected tissue by using lung material from autopsied patients with fatal asthma, or from patients with asthma needing lung resection because of malignancy. More recent studies in subjects with nocturnal asthma used transbronchial biopsies, an invasive technique that limits its use for
Pathology of Small Airways in COPD
COPD is defined as an abnormal inflammatory response of the lung to noxious particles and gases, mainly cigarette smoke.66 The smaller airways (<2 mm in internal diameter) offer little resistance in normal lungs but are the major site of obstruction in COPD. To assess the role of small airways in COPD, we raised the following questions:
Asthma
With the introduction of the solution hydrofluoroalkane (HFA) technology, pressurized metered dose inhalers (pMDIs) have become available, generating small particles with an average size of approximately 1 μm. Examples of currently available small-particle pMDIs are HFA-beclomethasone, HFA-beclomethasone/formoterol, HFA-flunisolide, and ciclesonide. The most important advantage of small-particle pMDIs is their higher lung deposition than conventional inhalers (50% to 60% vs 10% to 20%) and
Conclusion
There is now a considerable amount of evidence that small-airway inflammation contributes importantly to the clinical expression of asthma and COPD. This is important, since new devices have become available that allow drugs to better target the small airways. Several studies in asthma have shown a larger improvement in small airway function and inflammation with small-particle aerosols than with larger-particle aerosols. This may be clinically relevant, especially for patients with
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