Study objectives: Inspiratory capacity (IC) has been proposed as a simple method to assess acute changes in functional residual capacity (FRC) with bronchodilation, assuming that total lung capacity (TLC) is unchanged. This assumption is based on studies using body plethysmography, which may not accurately measure TLC in severely obstructed subjects. The aim of this study is to validate the use of IC measured by optoelectronic plethysmography (OEP) [ICoep], a noninvasive technique capable of computing changes in absolute lung volumes with great accuracy.
Methods and measurements: We studied 13 subjects with COPD in clinically stable condition at baseline and after 200 μg of inhaled albuterol. Changes in lung volumes were obtained from changes in chest wall volume (Vcw) measured by OEP and were compared with those measured by standard techniques.
Results: Albuterol treatment caused a small but significant increase in FEV1 and FVC, a significant decrease of Vcw at FRC (Vcwfrc), but no changes of Vcw at TLC (Vcwtlc) and breathing pattern variables. The reduction of Vcwfrc was not correlated with either spirometric or breathing-pattern variables. IC measured with a pneumotachograph was highly correlated with and not significantly different from ICoep (p < 0.001).
Conclusions: A single dose of inhaled albuterol does not significantly modify Vcwtlc in subjects with COPD, thus validating the use of IC to measure changes of FRC in the assessment of reversibility of airway obstruction.