Impact of k4b2 calibration drift on respiratory measurements during walking and running

The K4b² (COSMED s.r.l, Rome, Italy) is a portable metabolic analyser designed to measure breath-by-breath respiratory variables in the field. While the majority of published evidence suggests that the K4b² is valid and reliable during rest and exercise, it has been previously demonstrated that the accuracy of the %O₂ and %CO₂ measurements can drift over time (McLaughlin et al. 2001. Int J Sports Med 22(4): 280-284). However, the effect of K4b² calibration drift on respiratory variables during prolonged exercise tests is presently unclear.

PURPOSE: To examine the accuracy of K4b² measurements of VO₂, VCO₂ and V_E following a one hour delay between calibration and measurement.

METHODS: Ten male participants (age: 31 ± 11 years, height: 181 ± 3 cm and body mass: 84.1 ± 10.0 kg) completed three maximal discontinuous incremental exercise tests on a motorized treadmill at speeds corresponding to 4, 5, 6, 8, 10, 12, 14 and 16 km·hr^-1, or until volitional exhaustion. Participants completed 3 min of exercise at each speed, followed by 3 min active recovery for all speeds above 10 km·hr^-1. Respiratory variables were measured continuously using either a laboratory based metabolic cart (Oxycon Pro, Carefusion, Germany) (OP), a K4b² calibrated immediately before use (K4b²) or a K4b² calibrated one hour prior to the test (K4b²DEL).

RESULTS: VCO₂ and V_E were not different when measured by K4b² (P=0.27, P=0.53 respectively) or K4b²DEL (P=0.14, P=0.39 respectively) compared to the OP. VO₂ was not different between OP and K4b² (P=0.19) but K4b²DEL overestimated VO₂ compared to the OP at faster running speeds (P=0.05). Bland and Altman analysis indicated good agreement in the measurement of VO₂ between OP and K4b² with a mean difference of 38 ml·min^-1 and limits of agreement between -285 and 208 ml·min^-1. The mean difference between K4b²DEL and OP was 84 ml·min^-1 with limits of agreement between -469 and 302 ml·min^-1.

CONCLUSIONS: The K4b² overestimated VO₂ during faster running speeds following a one hour delay between calibration and measurement. Whilst care should be taken when using this device for longer duration exercise tests, the extent of the differences were small and newer models of the K4b² are fitted with a periodical recalibration mechanism which may eliminate this issue.