Delay time adjustments to minimize errors in breath-by-breath measurement of V̇O₂ during exercise

If the delay time between gas concentration and flow signals is not adequately corrected during breath-by-breath analysis of expired gas, an error in calculation of oxygen consumption (V̇O₂) will result. To examine the frequency and delay time dependences of errors in V̇O₂ measurement, six healthy men exercised at 100, 200, and 250 W on a cycle ergometer while breath-by-breath assessment of V̇O₂ was made simultaneously with collection of expired air. Subjects breathed first at normal rates (15-30 breaths/min) and then at 70 breaths/min. Each subject performed each level of exercise twice by using erroneous values for the delay time between gas concentration and flow signals. At normal breathing frequencies, errors in V̇O₂ measurement were ± 10% over the full range of delay times used, and the errors were not tightly correlated with variations in delay times from optimum. However, at 70 breaths/min, errors approached ±30% as the variations in delay times deviated ±0.1 s from the optimal, and the errors were highly correlated with the variations in delay times. We conclude that there is greater potential for errors in V̇O₂ measurement with incorrect delay time at higher breathing frequencies. These findings suggest that the optimal delay time for breath-by-breath systems should be adjusted by using high breathing frequencies.