Sources of Error in Flow-Volume Curves: Effect of Expired Volume Measured at the Mouth vs That Measured in a Body Plethysmograph

The popularity of the maximum expiratory flow-volume curve (FVC) is in part due to the effort independence of expiratory flow. Of interest are expiratory flow rates at specific lung volumes, usually 50 and 25 percent of vital capacity (VC); V ˙ max50 and V ˙ max25, which make accurate assessment of lung volumes essential. Changes in lung volume during the test are due to both the volume of gas expired and the volume change due to gas compression (Vcomp). In normal subjects, Vcomp is small but may be considerable in those with airflow obstruction. When the FVC is measured in a plethysmograph (FVCp), both expired volume and Vcomp are measured. When the volume of the FVC is derived from gas expired at the mouth (FVCm), Vcomp is not considered and differences in V ˙ max25 or V ˙ max50 may occur. The magnitude of these errors was assessed in 30 children and young adults: nine normal subjects, ten with cystic fibrosis (CF) and 11 with asthma. For V ˙ max50, use of FVCm instead of FVCp resulted in an error of 8 ± 7 percent (mean ± 1 SD) in the normal subjects compared to 32 ± 23 in those with CF (p 1 or RV/TLC but were related to a combination of expiratory effort, the shape of the FVCp, and the absolute volume of gas that was being compressed (p