Diffusivity, respiratory rate and tidal volume influence inert gas expirograms

Abstract

We modified, and developed software for, a computer-controlled quadrupole mass spectrometer to measure complete breath-by-breath expirograms of helium (He) and sulfur hexafluoride (SF 6) exhaled during the infusion of saline saturated with the inert gases. He and SF 6 have similar blood solubilities but very different gas phase diffusivities allowing examination of the influence of gas phase diffusivity on steady state inert gas expirograms. We studied six normal human volunteers in nine separate studies and examined the influence of tidal volume (V t) and breathing frequency (f) on the airways dead space (V d aw) and alveolar plateau slope (phase III) for the inert gases and CO 2. The experimental data showed a reduction in V d aw with rapid shallow breathing, while phase III slope increased by a factor of two to three. We critically evaluated the data and methodology of these and previously reported studies of continuous and single breath washout of He and SF 6. In general the 15 to 20 ml differences in V d aw between He and SF 6 were in keeping with previous studies by others. The ratio of phase III slopes of SF 6 to He reported by us previously (Scherer et al., J. Appl. Physiol. 64: 1022–1029, 1988) was 3.13. In the current study, which includes the analysis of more than 400 He and SF 6 breaths, the ratio of SF 6 to He slope was 1.85. The difference between the two studies was largely related to the improved methodology of the current study, particularly for the measurement of He. The results support the conclusion that diffusivity is an important component of both phase II and phase III of the expirogram. However, the difference in phase III between He and SF 6 is somewhat less than previously reported.