Adaptation of the inert gas FRC technique for use in heavy exercise

We automated the inert gas rebreathe technique for measurement of end-expiratory lung volume (EELV) during heavy exercise. We also assessed the use of two gas tracers (He and N₂) vs. a single gas tracer (He) for measurement of this lung volume and compared the two-tracer EELV to changes in the inspiratory capacity (defined with transpulmonary pressure) and shifts in the end-expiratory pressure from rest through heavy exercise. A computer program switched a pneumatic valve when flow crossed zero at end expiration and defined points in the He and N₂ traces for calculation of EELV. An inherent delay of the rebreathing valve (50 ms) caused virtually no error at rest and during light exercise and an error of 74 ± 9 ml in the EELV at peak inspiratory flow rates of 4 l/s. The measurement of EELV by the two gas tracers was closely correlated to the single-gas tracer measurement (r = 0.97) but was consistently higher (120 ± 10 ml) than when He was used alone. This difference was accentuated with increased work rates (2-5% error in the EELV, rest to heavy exercise) and as rebreathe time increased (2-7% error in the EELV with rebreathe times of 5-20 s for all work loads combined). The double-gas tracer measurement of EELV agreed quite well with the thoracic gas volume at rest (P > 0.05). Repeat measurements performed during the same exercise load were in close agreement (r = 0.97, coefficient of variation ± 3%), and changes in the EELV from rest through heavy exercise agreed with changes in the inspiratory capacity (r = 0.86) and with positive shifts in the end-expiratory esophageal pressure (r = -0.66). The automated technique, with two gas tracers, appears to give sensitive, reproducible, and accurate measurements of EELV during heavy exercise.