Assessment of Ventilatory Thresholds from Heart Rate Variability in Well-Trained Subjects during Cycling

Abstract

The purpose of this study was to implement a new method for assessing the ventilatory thresholds from heart rate variability (HRV) analysis. ECG, V·O₂, V·CO₂, and V·_E were collected from eleven well-trained subjects during an incremental exhaustive test performed on a cycle ergometer. The “Short-Term Fourier Transform” analysis was applied to RR time series to compute the high frequency HRV energy (HF, frequency range: 0.15 - 2 Hz) and HF frequency peak (f _HF) vs. power stages. For all subjects, visual examination of ventilatory equivalents, f _HF, and instantaneous HF energy multiplied by f _HF (HF · f _HF) showed two nonlinear increases. The first nonlinear increase corresponded to the first ventilatory threshold (VT₁) and was associated with the first HF threshold (T_RSA1 from f _HF and HFT₁ from HF · f _HF detection). The second nonlinear increase represented the second ventilatory threshold (VT₂) and was associated with the second HF threshold (T_RSA2 from f _HF and HFT₂ from HF · f _HF detection). HFT₁ , T_RSA1, HFT₂, and T_RSA2 were, respectively, not significantly different from VT₁ (VT₁ = 219 ± 45 vs. HFT₁ = 220 ± 48 W, p = 0.975; VT₁ vs. T_RSA1 = 213 ± 56 W, p = 0.662) and VT₂ (VT₂ = 293 ± 45 vs. HFT₂ = 294 ± - 48 W, p = 0.956; vs. T_RSA2 = 300 ± 58 W, p = 0.445). In addition, when expressed as a function of power, HFT₁, T_RSA1, HFT₂, and T_RSA2 were respectively correlated with VT₁ (with HFT₁ r² = 0.94, p < 0.001; with T_RSA1 r² = 0.48, p < 0.05) and VT₂ (with HFT₂ r² = 0.97, p < 0.001; with T_RSA2 r² = 0.79, p < 0.001). This study confirms that ventilatory thresholds can be determined from RR time series using HRV time-frequency analysis in healthy well-trained subjects. In addition it shows that HF · f _HF provides a more reliable and accurate index than f _HF alone for this assessment.

Key words

Exercise - respiratory components - time frequency analysis - short-term fourier transform

References

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