Composite flow-volume curves matched at total lung capacity in the study of density dependence of maximal expiratory flows

Abstract

The helium to air difference in maximal expiratory flow at mid-vital capacity (ΔHe 50%) and the volume at which air and helium expired flow rates become identical (V_iso $\mathop V\limits^ \bullet$) are currently measured from maximal expiratory flow-volume (MEFV) curves with the largest forced vital capacities (FVC) superimposed at residual volume (RV). True FVC and maximal flows may be obtained by superimposing several MEFV tracings to construct a “composite” (envelope) curve. From both theoretical and practical points of view, total lung capacity (TLC) is a more logical and more reproducible reference for matching air and helium-oxygen MEFV curves. The use of composite MEFV matched at TLC was compared to the conventional procedure in 41 healthy subjects aged 18 to 54 years. No significant difference for δHe 50% and V_iso $\mathop V\limits^ \bullet$ resulted from the application of the modifications proposed but the number of subjects with absent V_iso $\mathop V\limits^ \bullet$ increased with superimposition at TLC. The intersubject variability of the ΔHe 50% was decreased and that of V_iso $\mathop V\limits^ \bullet$ was increased. The sensitivity of both variables to the relationship between air and He + O₂ FVC is stressed.