Determination of oxygenated, mixed venous blood CO₂ tension by a breath-holding method

¹ Department of Physiology, University of Rochester School of Medicine and Dentistry, Rochester, New York

At the end of a normal expiration the subject inhaled a given volume of gas mixtures containing different concentrations of CO₂ in O₂ from 5 to 17%. These were held in the lung for 3 and then again for 12 seconds and were then expired and analyzed. Analyses were made with an infrared analyzer and times were obtained from the graphical record. If the rate of change of CO₂ tension is plotted against the mean CO₂ tension a straight line results which passes through zero rate at the tension which equals the tension of CO₂ in the mixed oxygenated venous blood. From the slope of this straight line it is possible to calculate the cardiac output if the lung volume and slope of the CO₂ dissociation curve of the blood are known. Data are presented from 37 experiments on 10 subjects. The method is believed to be theoretically sound but has not been validated as a practical clinical method. Occasional erratic points were obtained, especially in untrained subjects. The standard error of the mean value for venous CO₂ tension was 1.9 mm Hg.