A model of pulmonary airway exchange of soluble gases

The conducting airways of the lungs play an important role in heating and humidification of inspired air and the exchange of highly soluble gases. The authors have developed a mathematical model that describes airway heat, water, and soluble gas exchange which has been used to explain the impact that breathing technique and inspired air conditions have on breath alcohol [blood:gas partition coefficient (λ)=1760] concentration. The model predicts that alcohol exchanges entirely with the conducting airways and not within the alveoli. On inspiration, the air reaches equilibrium with the partial pressure of alcohol in the bronchial circulation prior to entering the respiratory bronchioles. On expiration, approximately 20-50% of the alcohol absorbed from the airways during inspiration is redeposited to the airways, depending on the breathing manoeuvre. The degree of exchange of gas delivered via the bronchial circulation depends strongly on λ with the most soluble gases equilibrating most efficiently. An in situ isolated dog trachea preparation was used to quantitate the degree of diffusion- and perfusion-limitation to airway gas exchange for incorporation into the model. Although the level of excretion of the six inert gases differed depending on λ, the responses of each gas to changes in blood flow were similar, regardless of λ. The results show that tracheal gas exchange is limited equally by perfusion and diffusion