Improving the Agreement Between the First Heart-Rate-Variability Threshold and the Gas-Exchange Threshold

Purpose: The first heart-rate (HR) -variability (HRV) -derived threshold based on detrended fluctuation analysis alpha 1 (DFA a1) has shown inconsistent agreement with the gas-exchange threshold (GET). This study examined whether a custom method of computing the first HRV threshold (HRVT₁) based on individual HRV characteristics would improve agreement. Methods: Fourteen participants underwent ramp incremental testing measuring gas-exchange variables and RR intervals. Comparisons were made between the oxygen consumption ðV̇O₂Þ=HR at the GET versus the V̇O₂=HR at the standard DFA a1 = 0.75 (HRVT_1s) or a custom value (HRVT_1c) based on the DFA a1 midway between the maximum seen during the early ramp incremental and 0.5. Results: Mean values for GET V̇O₂ versus HRVT_1s V̇O₂ and GET_HR versus HRVT_1sHR were statistically different (25.4 [3.3] vs 29.8 [6.8] mL·kg⁻¹·min⁻¹, P = .01, d = 0.80; 131 [11] vs 146 [22] beats·min⁻¹, P = .005, d = 0.91). There were no statistical differences when using the HRVT_1c (25.4 [3.3] vs 25.1 [5.7] mL·kg⁻¹·min⁻¹, P = .77, d = 0.08; 131 [11] vs 132 [17] beats·min⁻¹, P = .65, d = 0.12). Equivalence between GET and HRVT_1c V̇O₂=HR was also verified. Mean maximal DFA a1 during the early ramp incremental was 1.52 (0.22) with mean HRVT_1c of 1.01 (0.11). Pearson r correlation coefficients were between .67 and .70 for all GET to HRVT₁ comparisons. The second HRV threshold and respiratory compensation point parameters showed agreement and correlations in line with prior studies. Conclusions: The HRVT_1c showed stronger agreement to GET parameters than seen using the HRVT_1s. It is recommended that evaluations of the HRVT₁ consider this approach in determining the HR and V̇O₂ at this threshold.