Airways impedance during single breaths of foreign gases

The changes in airways resistance (Raw) and inertance (Iaw) during single inspirations of pure methane, helium, neon, and ethane at a flow of 0.1 l/s were measured in six healthy subjects by use of a forced-oscillation technique. Raw and Iaw were computed from respiratory transfer impedance obtained at a frequency of 20 Hz by applying pressure oscillations at the chest and measuring flow at the mouth with a bag-in-box system. Compared with the air data, the changes of Iaw after inhalation of 500 ml of gas averaged -41.1% with methane, -82.8% with helium, -25.8% with neon, and +4.8% with ethane. These changes were slightly less than the changes in gas density (-45%, -86%, -31%, and +5%, respectively). The inhaled volumes at which 50% of the changes had occurred (V50) did not differ significantly among gases and were approximately 100 ml. For Raw the data were more noisy than for Iaw; they were discarded in two subjects because of a strong and irreproducible volume dependence in air. Consistent differences were seen between the remaining subjects, one of whom exhibited a predominant viscosity dependence of Raw, one a predominant density dependence, and two an intermediate pattern. V50s were larger for Raw than for Iaw, indicating a more peripheral distribution of Raw. For Raw, V50s were lower with helium than with methane, in agreement with the notion that density-dependent resistance is located mainly in the large airways. The results suggest that some information on the serial distribution of Raw and Iaw may be derived from impedance measurements with foreign gases.