Alveolar and dead space volume measured by oscillations of inspired oxygen in awake adults

Forced sinusoidal oscillations in the inspired concentration of a low- solubility inert gas can be used to measure airways dead space and alveolar volume. When inspired oxygen is oscillated about its mean value in the same way, the ratio between the amplitudes of the resulting end-expired and inspired oxygen oscillations is the same as that of an inert gas such as argon. Thus, oxygen forcing oscillations can be used to measure lung volume. In nine healthy spontaneously breathing adults, the FI(O2) (mean F(O2) = 0.26, mean minute volume = 8.5 L/min) was forced to sinusoidally oscillate with an amplitude of ± 0.04. The mean airways dead space measured using FI(O2) oscillations with a forcing period of 3 min was 0.17 ± 0.04 L, and the airways dead space measured by a single-breath CO2 technique was no different at 0.19 ± 0.03 L. An oxygen oscillation of the same period measured the mean end-expired alveolar volume at 3.1 ± 0.7 L. Adding together the airways dead space and end-expired alveolar volume, obtained by the oxygen oscillation technique, provided a measure of FRC that at 3.3 ± 0.7 L matched the FRC of 3.3 ± 0.8 L measured by whole-body plethysmography. A third measure of FRC using a multiple-breath nitrogen washout technique gave a smaller volume of 3.00 ± 0.85 L. The advantage of using FI(O2) oscillations is that accurate FRC measurements can be made continuously, without interfering with the subject's natural breathing rhythm.