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The carbon dioxide tension of oxygenated mixed venous blood has long been used, in normal individuals and frequently in subjects with various forms of disease, in the determination of cardiac output according to the Fick principle. The method of rebreathing mixtures of CO2 and oxygen, for establishing equilibrium between the CO2 tensions of the lungs and the incoming venous blood, and at the same time oxygenating this blood, was first described by Christiansen, Douglas and Haldane (1); later by Henderson and Prince (2) and others. In 1922 Douglas and Haldane (3) showed that the same arteriovenous differences in CO2 content were obtained in a given subject, whether the rebreathing procedure equilibrated CO2 tensions only and oxygenated the blood in the lungs, or whether this procedure equilibrated simultaneously both CO2 and oxygen tensions of incoming venous blood with the rebreathed air. Field, Bock, Gildea, and Lathrop (4) showed that arterial blood drawn during the course of rebreathing a mixture of 6 per cent CO2 and 94 per cent oxygen was fully oxygenated, and that the CO2 tensions in arterial blood and in the rebreathed air were the same. Most workers have been able to demonstrate an equilibrium of CO2 tensions by similar rebreathing technique; Hamilton, Moore and Kinsman (5), however, were unable to establish equilibrium or " plateau " levels of CO2 during rebreathing in experiments with a small number of subjects. Richards and Strauss (6) in 1930 reviewed the numerous assumptions made in various rebreathing methods for estimating the values of the gases of the mixed venous blood. They showed experimentally that the same equilibria or " plateau " levels of CO2 tension of oxygenated mixed venous blood could be regularly obtained in a normal subject after 15 to 20 seconds of rebreathing, with the use of initial mixtures in the rebreathing bag which differed in CO2 tensions by 10 mm. or more. By constructing a nomogram of the subject's blood, and by plotting on this nomogram the CO2 tensions (as obtained by rebreathing) of (a) oxygenated and (b) true mixed venous blood, these authors found that the (a) and (b) CO2 tensions represented approximately the same CO2 content of the blood. It is of interest to inquire how far the method of rebreathing can be applied to the determination of CO2 tensions in oxygenated mixed venous blood in cases with various forms of pulmonary disease. This inquiry forms the subject of the present paper. Specifically, answers have been sought, in each patient studied, to the following questions. 1. During the course of the rebreathing procedure can a constant or nearly constant (" plateau ") level of CO2 tension be demonstrated, which persists over a five second interval, or longer (e.g., from 15 to 20 seconds after the start of rebreathing) ? 2. If this is the case, can the same plateau level of CO2 tension be arrived at, in successive experiments, when the initial CO2 tensions in the rebreathing bag differ by several millimeters? 3. When such a transient equilibrium of CO2 tension in the rebreathed air is established, is there at the same time a close agreement between the tensions of CO2 and oxygen in the air sample taken at the end of expiration (" alveolar" air), and the CO2 and oxygen tension in the blood leaving the lungs (arterial blood) ? That the establishment of these three equilibria indicates that the plateau level of CO2 tension is the same as that of the oxygenated mixed venous blood, may be argued as follows: The complete oxygenation of the arterial blood

APPLICABILITY OF REBREATHING METHOD FOR DETERMINING MIXED VENOUS CO2 IN CASES OF CHRONIC PULMONARY DISEASE