Airway resistance (RAW) and specific airway conductance (sGAW) are measures that reflect airway function. However, little is known of the variability of these measures between different lung diseases. In a recent study published in the journal Respiratory Research, a team of researchers from the Department of Clinical and Experimental Medicine, University of Leuven in Belgium found that RAW and sGAW are significant factors that contribute to the diagnosis and differentiation of obstructive airway diseases.
A technique called “whole-body plethysmography” is an increasingly used and important component of comprehensive pulmonary function testing in respiratory medicine, and the two most commonly used resistive parameters are airway resistance (RAW) and specific airway conductance (sGAW).
RAW reflects changes in alveolar pressure over changes in airflow representing true resistance of the airways. Airway resistance is determined by airway narrowing and is therefore considered a good parameter for the diagnosis of airflow obstruction. As the technique to determine airway resistance is dependent on measuring thoracic gas volume (with larger volumes resulting in opened airways), specific airway resistance (sRAW) is divided by thoracic gas volume to determine airway resistance.
To examine the contribution of RAW and sGAW to a diagnosis of obstructive airways disease and their role in differentiating asthma from COPD, in the study titled “Airways resistance and specific conductance for the diagnosis of obstructive airways diseases,” Marko Topalovic and colleagues conducted a prospective cohort investigation, which included new test subjects admitted for the first time to a pulmonary practice between 2011 and 2012 across 33 hospitals in Belgium.
A total of 976 subjects were included in the analysis. The assessments were based on complete pulmonary function tests and supported by investigations of physicians’ discretion. A total of 651 subjects had a final diagnosis of obstructive diseases, 168 had another respiratory disease and 157 subjects had no respiratory disease (healthy controls).
The results revealed that RAW and sGAW were different between obstructive and other groups. Abnormal RAW and sGAW were found in 39 % and 18 % of the population, respectively, in which 81 % and 90 % had diagnosed airway obstruction.
Results from multiple regression analysis showed that sGAW was a predictor of an obstructive disorder. To differentiate asthma from COPD, RAW was found to be more relevant and statistically significant. In patients with asthma with normal FEV1/FVC ratio, both RAW and sGAW were more specific than sensitive diagnostic tests in differentiating asthma from healthy subjects.
Based on the study results the researchers concluded that airway resistance and specific conductance can contribute to the diagnosis of obstructive diseases and further disease differentiation.
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