The forced oscillation technique is a non-invasive and effort-independent test used to characterize the mechanical impedance of the respiratory system. Total respiratory impedance was assessed from 4 to 52 Hz in 15 normal subjects breathing air and a helium-oxygen mixture. 2. Breathing helium-oxygen reduced respiratory resistance and its frequency dependence as well as respiratory reactance very significantly. Resonant frequency during He-O2 breathing was 1.88 times higher than during air breathing. 3. It is concluded that by impedance measurement of the respiratory system in normal subjects a density-dependent decrease of respiratory resistance due to decreased turbulence in the larger airways and a density-dependent decrease in the inductive reactance can be found during breathing of low density gas mixtures.
