Comparison of oxygen uptake on-kinetic calculations in heart failure

Purpose: The analysis of oxygen (O₂) uptake on-kinetics during steady-rate is gaining interest in the heart failure (HF) population. The rate change in O₂ at the initiation of exercise can be assessed via nonlinear regression time constant (TC) or an algebraic equation (mean response time [MRT]). These calculations are presumed to be interchangeable, but research supporting this claim is limited. This investigation compares and contrasts two of the more commonly used O₂ uptake on-kinetic calculations. Method: Twenty-eight subjects diagnosed with compensated HF and 19 age, sex, and activity-matched controls underwent a symptom-limited exercise test and a steady-rate exercise session (6 min). Peak O₂ uptake, O₂ uptake at ventilatory threshold, the O₂ uptake TC (TC), and the O₂ uptake mean response time (MRT) were calculated for each subject. Results: O₂ uptake on-kinetics was significantly faster for the control group (P < 0.05) regardless of calculation method. There was a significant difference between the O₂ uptake TC and MRT for the HF group. All O₂ uptake on-kinetic calculations were significantly correlated with aerobic capacity. Conclusions: O₂ uptake TC and MRT values may not be interchangeable in the HF population. All O₂ uptake on-kinetic calculations did produce a significant difference between experimental and control groups and correlated with indicators of aerobic capacity. The 10-s O₂ uptake on-kinetic calculations may be preferable secondary to expired gas fluctuations associated with breath-by-breath measures. Further work is, however, needed to determine which averaged O₂ uptake on-kinetic expression is optimal given the significant difference between TC and MRT. A mechanism for this difference may be the oscillatory ventilatory expired gas pattern demonstrated by some patients with HF.