Penetration of inhaled He and SF6 into alveolar space at low tidal volumes

Abstract

To study mixing of inspired gas with lung gas, penetration of simultaneously inspired helium (He) and sulfur hexafluoride (SF6) into alveolar space was determined in normal subjects at low tidal volumes (from 50 to 500 ml) and at varied lung volumes and speeds of inspiration/expiration. The volume of inspired gas reaching the alveolar space, termed alveolar-tidal volume, VTA, was calculated from preinspiratory lung volume, inspired volume, and inspired and expired alveolar test gas concentrations. The difference between the VTA values calculated for He and SF6, VTA(He) - VTA(SF6), was influenced by tidal volume, lung volume, and the speed of inspiration/expiration, but it was always positive. The results are qualitatively explainable on the basis of easier diffusive mixing of He in lung airways compared with SF6. Since Taylor dispersion would produce deeper penetration, and therefore higher VTA, for a less diffusible gas the results provide no evidence for its implication in pulmonary gas exchange.

Gas and aerosol mixing in the acinus

“…An approximate solution to this was given by Taylor, 1953, who found that radial diffusion coupled with a parabolic profile led to a net axial flux, in coordinates moving with the mean fluid velocity, which was proportional to the mean concentration gradient. This is precisely the form of Fick's law of diffusion, but the prefactor which would otherwise have been the molecular diffusivity is replaced by a new prefactor, ever since known as the effective diffusivity D eff , which was quadratic in the mean velocity but, counterintuitively, inverse with the molecular diffusivity D. To what extent such Taylor-type diffusion (including its extension to turbulent flow in the upper airways (Taylor, 1954), and oscillatory flow (Watson, 1983)) is important in pulmonary gas exchange remains an open question, it has been argued that it is not likely to be highly significant (Worth, 1977), although it may play a role in intermediate level bronchi (Wilson and Lin, 1970). By contrast, there is evidence (Kvale, 1975) that CO uptake is significantly enhanced in the presence of SF 6 as a carrier gas, suggesting that at least in some circumstances, Taylor-type diffusion may play a large role.…”

Ventilation: Total, Alveolar, and Dead Space

“…In experiments where inspiratoryand expiratory-flow rates were independently controlled, the volume of dead space increased with inspiratory-flow rate (148). Single-breath dead space is smaller during forced expirations from total lung capacity than it is during slow expirations; this was ascribed to different emptying sequences (7), but no change in dead space with expiratory-flow rate was observed when expiration was initiated from lessextreme lung volumes (148).…”

Pulmonary Parenchymal Tissue Volume and Pulmonary Capillary Blood Flow in Normal Subjects

“…Overland et al [15] mention Qc increases when the alveo lar volume increases. Although there exist different interpretations of the influence of the inspired volume on the intrapulmonary mixing of gases [ 16,24], we get the impression that part of the differences in the results, dependent on the inspired vol ume and the gas used, may be explained in terms of the dynamics of the intra-alveolar gaseous mixing which depends on the diffusivity characteristics of each gas. DME, with the highest molecular weight, may achieve a better intra-alveolar mixing when the inspired volume increases, while at the same time C2H2, which is more dif fusible, may have mixing dynamics more independent of volume changes.…”

“…Using these results as a basis, we carried out 10 mea surements with each gas at extreme re breathing frequencies (24-78 breaths/ min), observing a high correlation (r = 0.912) between the rebreathing frequency and the Vt-C2FU/Vt-DME relationship. These findings suggest that part of the dif ferences could be attributed to technical problems derived either from the relation between the time of response of the ana lyzer for each gas, the uneven distribution of the gases in the alveolar space, depend ing on their diffusivity [6,19,24], or the greater slope of the absorption curve of DME [23].…”