Convective and diffusive gas mixing in human lungs: experiments and model analysis

Area of Science:

• Pulmonary physiology
• Respiratory mechanics
• Gas exchange

Background:

• Alveolar gas mixing is crucial for efficient respiration.
• Understanding gas equilibration dynamics during breath-holding is essential for assessing lung function.

Purpose of the Study:

• To investigate the equilibration of inspired gases with residual lung gas.
• To quantify the effects of breath-holding time and gas properties on lung mixing.
• To evaluate the roles of diffusion and convection in alveolar gas exchange.

Main Methods:

• Employed a single-breath technique with varying breath-holding times.
• Utilized helium (He), argon (Ar), and sulfur hexafluoride (SF6) as test gases.
• Analyzed end-expired and mean lung gas concentrations.
• Applied a serial three-compartment lung model for quantitative analysis.

Main Results:

• Gas mixing in the lung was consistently imperfect (end-expired/mean lung concentration ratio < 1).
• The ratio of mean lung to end-expired concentration increased with breath-holding time (tB) for all gases.
• Sulfur hexafluoride exhibited the poorest mixing, while helium showed the best.
• Bohr dead space (VD) was greatest for SF6 and smallest for He, decreasing towards a common asymptote with increasing tB.

Conclusions:

• Both diffusion and convection are effective mechanisms for gas equilibration in the lung during breath-holding.
• Stratified inhomogeneities significantly impact gas exchange without convection.
• Convective gas mixing in the alveolar space mitigates the restrictive effects of stratification on resting gas exchange.