Removal of observer variability from the determination of the volume of isoflow.

The initial attractiveness of the volume of isoflow (VisoV) as an index of early small airways dysfunction has faded, perhaps because of VisoV's high variability, a significant part of which is contributed by the observer. We suggest here that the observer variability can be removed by filtering the flow data in the reciprocal volume domain, by using a modified foreign gas mixture in which some of the helium is replaced by neon, and by reading VisoV from a plot of density dependence vs. lung inflation. Support for these suggestions is drawn from both model simulations and experiments. Model simulations suggested that VisoV would be increased by the substitution of neon for some of the helium in the usual helium-oxygen (Helox) mixture. This was confirmed in tests on matched groups of normal and asymptomatic asthmatic children. The inclusion of 20% neon (Heneox20) was found to reduce the coefficient of variation in the mean value of VisoV in both groups of subjects. An interesting result of this test was that VisoV using Heneox20 was significantly higher in the asthmatic group than in the normal group, whereas VisoV using Helox was not. The results for density dependence (DD) between the 2 groups were not significantly different using Heneox20 but were using Helox. Plots of DD (derived from heavily filtered flow curves) vs. lung inflation showed an unambiguous value for VisoV.