Validity and between-days reliability of two different metabolic systems for measuring gas exchange during walking

Portable metabolic systems for assessing gas exchange outside the laboratory and clinical settings are increasing their popularity. We aimed to determine the validity and the reliability of the K5 portable system (Cosmed, Rome, Italy) and the Omnical (Maastricht Instruments, Maastricht, The Netherlands) metabolic cart.

Gas exchange was measured, using the K5 and the Omnical in a randomised and counterbalanced order, in 35 participants (19 women; age: 26 ± 3 yrs.) using a walking protocol on 2 non-consecutive (48 h apart) days. The protocol consisted of a 5 min walk and 2.5 min rest cycle starting at 3.5 km/h and increasing in 1 km/h to 6.5 km/h. The protocol was repeated twice after a 10-15 min resting. To determine validity and reliability, we conducted two-factor repeated measures analysis of variance (ANOVA), intraclass correlation coefficients (ICC), coefficients of variation (CV) and Bland-Altman analyses.

Repeated measures ANOVA showed that mean oxygen consumption, carbon dioxide production and energy expenditure were higher for the K5 compared to the Omnical on both visits (all P < 0.001). Respiratory exchange ratio (RER) was similar among systems at faster (≥4.5 km/h) walking speeds. ICC, CV and Bland-Altman showed a better reliability of the Omnical (ICCs ranged from 0.71 to 0.86, and CVs from 3.1 % to 7.1 %) compared to the K5 (ICCs ranged from 0.40 to 0.98, and CVs from 2.6 % to 6.3 %).

The K5 portable system provides similar RER values to the Omnical metabolic cart, although the Omnical reliability was better for most outcomes.