Determination of pulmonary blood flow by rebreathing of soluble inert gases

The rate of disappearance of acetylene or nitrous oxide from the alveolar gas during rebreathing was studied in dogs and man. The subjects, hyperventilating in steady state, were connected at end expiration to a rebreathing 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 tissue volume of the lung were calculated. Simultaneous determinations of the cardiac output were made with the direct Fick method for oxygen in dogs and the indirect Fick method in man. A good correlation was found between both methods. A theory is presented which considers the effect of limitation of ventilation upon inert gas transfer within the rebreathing system.