The measurement of lung volumes using body plethysmography: a comparison of methodologies.

BACKGROUND The statement of the American Thoracic Society and European Respiratory Society on the measurement of static lung volumes (SLV) suggests a preferred and alternate method for measuring and calculating SLV. OBJECTIVE To determine if differences in functional residual capacity (FRC), vital capacity (VC), residual volume (RV), and total lung capacity (TLC), obtained using preferred and alternate measurement and calculation methodologies, exist in a clinical setting. METHODS Patients attending for SLV at a hospital-based laboratory were recruited. Following spirometry, SLV was measured via body plethysmography, using the preferred and alternate methods in random order. Volumes were calculated using the preferred and alternate calculation methods. Subjects were classified according to standard ventilatory function interpretative strategies. Differences of the means between the measurement methods, and calculation methods were assessed. RESULTS One hundred eight data sets were analyzed. Significant, but small differences (< 150 mL) in the means for VC and TLC, and RV and TLC were found in the normal and restricted groups, respectively. No significant differences in SLV parameters were found in subjects with air-flow obstruction. Twelve of the 108 changed ventilatory function classification between methods, with the alternate method delivering a lower inspiratory capacity and TLC without a change in RV in 66% of this subgroup. Identical results were obtained when data were analyzed using both calculation methods. CONCLUSIONS Differences in FRC, VC, RV, and TLC obtained using the preferred and alternate measurement methodologies exist in the clinical setting in select classification groups and individuals. Differing calculation methods dependent on measurement method used may be unnecessary.