The rate of disappearance of acetylene or nitrous oxide from the alveolar gas during rebreathing was studied in dogs and in man. The subjects, hyperventilating in steady state, were connected at end-expiration to a re breathing bag for 30 sec. After the initial 2-4 breaths the alveolar inert gas concentration followed a single exponential course until the beginning of recirculation. From the slope and intercept of this exponential at zero time the capillary blood flow and the tissue volume of the lung were calculated. Simultaneous determinations of the cardiac output were made with direct Fick method for oxygen in dogs and indirect Fick method in man. A good correlation was found between both methods. A theory is represented which takes into consideration the effect of ventilation limitation upon inert gas transfer within the rebreathing system

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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.

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