Heart-rate variability threshold as an alternative for spiro-ergometry testing: A validation study

Although spiro-ergometry is the established "gold standard" for determination of the second ventilatory threshold (VT2), it is a costly and rather time-consuming method. Previous studies suggest that assessing the second anaerobic threshold (AT2) on the basis of heart rate variability (HRV) during exercise may be a more cost-effective and noninvasive manner. However, appropriate validation studies, are still lacking. Eleven healthy, moderately trained subjects underwent 3 incremental exercise tests. Ventilation, oxygen uptake (Vo₂), CO₂ production (Vco₂), and HRV were measured continuously. Exercise testing was performed in 3 oxygen (FiO₂) conditions of inspired air (14, 21, and 35% of oxygen). Participants and assessors were blinded to the FiO₂ conditions. Two research teams assessed VT2s and HRVT2s independently from each other. Mean workloads corresponding to VT2 and HRVT2 in hypoxia were, respectively, 19 ± 17% (p = 0.01) and 15 ± 15% (p = 0.1) lower in comparison with hyperoxic conditions. Bland-Altman analysis showed low estimation bias (2.2%) and acceptably precise 95% limits of agreement for workload -15.7% to 20.1% for all FiO₂ conditions. Bias was the lowest under normoxic conditions (1.1%) in comparison with hypoxia (3.7%) and hyperoxia (4.7%). Heart rate variability-based AT2 assessment had a most acceptable agreement with VT2 under normoxic conditions. This simple HRVT2 assessment may have potential applications for exercise monitoring in commercial fitness settings.