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The ability of different pulmonary function tests to detect dose-response effects depends on both the variability and sensitivity of the methods used. In clinical trials bronchodilatation is usually assessed by spirometry. Body plethysmography and impulse oscillation (IOS) are alternative techniques. The advantage of IOS is that it is simpler to perform and requires minimal effort from the subject. It can also measure different components of respiratory impedance, including both central and peripheral airways resistance.
The authors of this paper compared the sensitivity and variability of the above pulmonary function methods to measure the dose-response effects of salbutamol in both healthy subjects and asthmatics. The study involved 12 healthy subjects, 12 with mild asthma (FEV1 >80% predicted), and 12 with moderate asthma (FEV1 <80% predicted). Pulmonary function was measured by all three methods on day 1 and repeated after 30 minutes to assess variability. One week later the measurements were repeated at the same time of day, after which increasing doses of nebulised salbutamol were administered and pulmonary function was recorded after each dose. The most sensitive measurements were spirometry in healthy individuals and plethysmography in those with mild asthma. All three pulmonary function methods showed similar sensitivity in individuals with moderate asthma. However, spirometry showed the least variability of the tests. The sensitivities of the tests also varied with the degree of airflow obstruction.
This study suggests that the airways of healthy subjects respond differently from those of asthmatic subjects. In clinical trials the results of dose-response curves in healthy individuals are not necessarily applicable to those with asthma. This needs to be taken into account when choosing the most appropriate test to measure pulmonary function.