A two-bag system for continuous measurement of oxygen uptake

Collection of mixed expired gas in a bag has been a classic method for the estimation of VO2 during the steady state but has not been employed during unsteady state exercise in part because there is a need for suspending the acquisition of data during the period of gas analysis unless many bags are used. In this study a two-bag system is described in which one bag fills while the other is analyzed. Bag volume is under the control of the operator, and we employed volumes of 30 to 50 L. Thirty-one subjects were studied with this circuit in a progressive treadmill test. Although VO2 could be falsely elevated during periods of overbreathing, this source of error could be identified and its effect reduced if VO2 was plotted against both ventilation and power requirement. Plateau values of VO2 were identified only in six subjects and the ventilatory threshold in 16.