Oxygenated mixed venous PCO2 in healthy subjects.

Despite the large number of blood gas measurements made over the years, precise information is lacking on intrasubject and intersubject variability of carbon dioxide tension. Accordingly, the oxygenated mixed venous CO2 tension was measured in 202 healthy volunteers and the influence of potentially disturbing factors such as intake of coffee and food, exercise and menstruation on the stability of this tension was studied in selected subjects. In the 102 men the mean value was 5.1 mm Hg (SD, 2.9 mm Hg), significantly higher (P less than 0.001) than that of the 100 women, 46.03 mm Hg (SD, 2.48 mm Hg). Among the women the value was significantly lower (P less than 0.01) in those taking oral contraceptives (mean, 44.9 mm Hg; SD, 2.3 mm Hg) than in those not taking these agents (mean, 46.5 mm Hg; SD, 2.4 mm Hg). Intrasubject variability of values was greatest in women taking oral contraceptives, mean day-to-day variation over two mentrual cycles being +/- 3.26 mm Hg. After moderate exercise the tension usually returned to +/- 1 mm Hg of resting values within 5 minutes. Intake of food and coffee had negligible effects on the tension.