Impulse oscillometry in COPD: identification of measurements related to airway obstruction, airway conductance and lung volumes.
BACKGROUND
Impulse oscillometry system (IOS) assesses pulmonary resistance and reactance. We set out to investigate which IOS measurements are related to airflow obstruction, airway conductance and lung volumes in chronic obstructive pulmonary disease (COPD).


METHODS
Ninety-four COPD patients were recruited and 58 agreed to follow up after 1 year. IOS measurements (R5, R20, X5 & Fres), body plethysmography (sGaw, FRC, TLC, RV & IC) and spirometry (FEV(1)) were performed. Pearson or Spearman correlation determined the relationships between IOS and other measurements.


RESULTS
R5, X5 and Fres were all significantly associated (p<0.05) with FEV(1), sGaw, TLC, RV and IC. However, R20 was not related to any of these measurements except for RV. The strongest associations were observed between FEV(1) and the reactance measurements X5 (r=0.48) and Fres (r=-0.44), and sGaw with X5 (r=0.47) and Fres (r=0.51). The r values for the associations with TLC and IC were all <0.25. There was no statistically significant change in the FEV(1), R5, X5 or Fres after 1 year, but R20 significantly increased over the year. The changes in R5 and R20 did not significantly correlate with the changes in FEV(1). In contrast, X5 changes were significantly related to FEV(1) changes over 1 year (r=-0.27, p=0.05), while for Fres changes there was a trend to statistical significance (p=0.08).


CONCLUSIONS
IOS reactance measurements are more closely related than resistance measurements to other pulmonary function measurements in COPD patients. The IOS reactance measurements appear to be indicative of changes in pulmonary compliance caused by airflow obstruction.
Kolsum
Umme
U
North West Lung Research Centre, University of Manchester, South Manchester University Hospitals Trust, Southmoor Road, Wythenshawe, Manchester M23 9LT, UK. ukolsum@meu.org.uk
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