Thoracic gas volume measurement. Increased variability in patients with obstructive ventilatory defects

We measured the thoracic gas volume (FRC) in normal subjects and patients by body plethysmography to determine the within-subject variability and to identify factors which might influence the variability. The coefficient of variation (CV), a measure of variability, was significantly greater in patients with obstructive ventilatory defects than in patients with restrictive ventilatory defects and normal subjects, and there was a direct correlation between the CV and the FRC and an inverse correlation between the CV, the FEV₁/FVC, and the FEV₁. These results suggest that the disease state and its severity influence the variability in FRC measurements and that both factors should be considered in the interpretation of serial pulmonary function studies measuring lung volumes.