Dynamic vs. fixed bag filling: Impact on cardiac output rebreathing protocol

A main purpose of this study was to compare the repeatability (2.77 multiplied by the within-subject SD) between two different rebreathing protocols on cardiac output (Q˙), pulmonary diffusing capacity for carbon monoxide (DLCO) and nitric oxide (DLNO), and pulmonary capillary blood volume (Vc). This study compared two bag volume conditions [Fixed Bag Volume (FBV) = BV fixed at 60% of forced vital capacity; Dynamic Bag Volume (DBV) = BV matched to tidal volume at each stage of exercise]. Ten females (age = 27 ± 8 years; V˙O2,peak=2.5±0.6 L/min) had measurements at rest (12%), 50%, 88%, and 100% of V˙O2,peak on two study days in a balanced, crossover design. Neither the slope nor intercept of Q˙ vs. V˙O2 were different between FBV or DBV. The slope of DLCO vs. Q˙ was the same but intercept was higher for FBV. The BV affected the slope and the intercept between DLNO vs. Q˙ (p < 0.05). The repeatability was similar between both protocols for Q˙ (2.8–3.4 l/min) and DLNO (33–42 ml/min/mmHg) regardless of exercise intensity. However, the measurement error was lower for Vc when using the FBV protocol (p < 0.05). This study shows that the pattern of BV used during rebreathing maneuvers affects DLNO vs. Q˙ relation more than it affects DLCO and Vc vs. Q˙. In addition FBV protocol provides more repeatable Vc measurements at any exercise stage compared to the DBV protocol.