Do Agreements between HRV and Gas Exchange Thresholds Still Hold under Hypoxic Conditions?

Area of Science:

• Exercise Physiology
• Cardiovascular Physiology

Background:

• Heart rate variability thresholds (HRVT), derived from detrended fluctuation analysis alpha 1 (DFA a1), generally align with gas exchange-based thresholds under normoxic conditions.
• The influence of acute hypoxia on the agreement between HRVTs and gas exchange equivalents remains less understood.

Purpose of the Study:

• To investigate whether acute hypoxia affects the agreement between HRVTs and gas exchange-derived thresholds during incremental cycling.
• To compare oxygen consumption (V̇O2) and heart rate (HR) at the gas exchange threshold (GET) and respiratory compensation point (RCP) with HRVTs under normoxic and hypoxic conditions.

Main Methods:

• Twelve participants completed incremental ramp tests in normobaric hypoxia (FIO2 ≈ 13.5%) and normoxia.
• Gas exchange, ventilatory responses, and high-sampling rate electrocardiogram for DFA a1 were used to determine thresholds.
• Comparisons were made between GET:HRVT1 and RCP:HRVT2 using Pearson's r correlation and Bland-Altman analysis.

Main Results:

• No statistically significant differences in V̇O2 and HR were observed between GET:HRVT1 and RCP:HRVT2 in either condition.
• Equivalence testing passed for all normoxic comparisons and for GET:HRVT1 in hypoxia.
• Pearson's r correlations were weaker in hypoxia (0.58–0.79) compared to normoxia (0.86–0.96), with increased bias and limits of agreement during hypoxic testing.

Conclusions:

• HRVTs maintain alignment with GET and RCP in both normoxia and hypoxia, but the strength of this relationship is diminished in hypoxia.
• While HRVT alone may offer some guidance for training in hypoxia, its reliability is reduced.
• Further research incorporating complementary surrogate markers is recommended for more robust training boundary guidance during hypoxic exercise.