Rebreathing Method for Measurement of Mixed Venous

so the CO2 tension can be measured in a gas rather than in a liquid (Campbell and Howell, 1960a). The problem is to achieve this equilibrium before the prevention of its excretion causes the pressure of the CO2 in the blood to rise excessively. A rapid equilibrium will be achieved if a gas mixture is rebreathed which contains enough CO2 to enrich the alveolar gas to approximately the same tension as the mixed venous blood. If this mixture contains too much Co0, equilibrium will not be reached until the blood has carried away the excess; if it contains too little, CO2 will be evolved from the mixed venous blood. In either The periods of time required for these two stages obviously depend on the volume of the bag, the volume of the lungs, the volume of the breaths, the blood PcoW, and the volume of pulmonary blood flow providing CO. In practice the most important factor is the ratio of the tidal volume of breathing to the volume of the bag. Obviously, if the bag is too large too much time will be required to mix its contents with the lungs and raise the Pco2. On the other hand, if the bag is too small the subject will have insufficient "room" in which to breathe. The aim should be to have a bag volume abeut twice the tidal volume, but the ratio is not critical. Fig. 1 shows the Pco2 at the mouth of a subject breathing in and out of a bag starting with 100% 02 in the bag. The records were obtained from a normal subject with a resting tidal volume of 0.7 litre and a mixed venous Pco2 of 50 mm. Hg. He performed the two-stage procedure three times with different initial bag volumes (a, 1 1.; b, 2 1.; c, 4 1.). The three records have been superimposed to emphasize the influence of the tidal-volume: bag-volume ratio. In a this ratio was more than 1: 2; in b it was about 1: 3; and in c it was less than 1: 5. Comparison of the three records during the first stage shows that the cyclical variation in Pco2 had almost disappeared, indicating fairly complete mixing between bag and lungs, after 60 seconds in a, 90 seconds in b, but not within 90 seconds in c. Furthermore, the Pco2 in the bag remained above mixed venous after 45 seconds in a and 75 seconds in b, but not in c. Hence, an initial bag volume varying from 1 to 2 1. and a tidal volume ratio varying from 1:1.5 to 1 :3 permitted the " preparation " of a suitable Pco2 in