Validity and reliability of the Vyntus CPX metabolic gas analyzer compared with the Douglas Bag method

To validate the Vyntus CPX system against Douglas Bag method.

Twelve cycle-trained men and women completed an incremental cycling protocol starting at 50 W, increasing by 50 W every fifth minute up to 350 W or to near exhaustion. After a 5 min break a test of peak oxygen uptake (V̇O₂peak) was carried out. Gas exchange parameters were continuously measured using the Vyntus CPX system in mixing chamber mode, while expired air was simultaneously collected in Douglas Bags during the final 60 s of each stage.

Mean systematic differences were observed for V̇O₂ (+ 0.108 L/min, 4.6%) and respiratory exchange ratio (RER, -0.032, -3.2%). The Vyntus CPX measured proportionally higher V̇O₂ (+ 0.032 L/min) and lower V̇CO₂ (-0.032 L/min) per L/min increase in V̇O₂ and V̇CO₂, respectively, measured by the Douglas Bag. Consequently, the Vyntus CPX proportionally underestimated RER by 0.017 per 0.1-unit increase. From paired measurements on two days, coefficient of variation (CV) for the gas exchange variables were 3.4%-5.7%. After adjusting for biological variability, systematic test-retest differences were observed for V̇CO₂ (+ 0.066 L/min, 2.0%) and RER (+ 0.021, 2.3%). Proportional bias was evident across all gas exchange variables.

The Vyntus CPX demonstrated agreement comparable with other studies on similar commercial instrument but showed reduced accuracy at higher levels of V̇O₂ and V̇CO₂. The variability in technological error was somewhat larger than what is generally considered acceptable for gas exchange measurements. The Vyntus CPX showed systematic test-retest differences for V̇CO₂ and RER, with proportional bias evident for all measured gas exchange variables.