Effects of Acute Normobaric Hypoxia on Non-linear Dynamics of Cardiac Autonomic Activity During Constant Workload Cycling Exercise

Abstract

Aim: Measurements of Non-linear dynamics of heart rate variability (HRV) provide new possibilities to monitor cardiac autonomic activity during exercise under different environmental conditions. Using detrended fluctuation analysis (DFA) technique to assess correlation properties of heart rate (HR) dynamics, the present study examines the influence of normobaric hypoxic conditions (HC) in comparison to normoxic conditions (NC) during a constant workload exercise.

Materials and methods: Nine well trained cyclists performed a continuous workload exercise on a cycle ergometer with an intensity corresponding to the individual anaerobic threshold until voluntary exhaustion under both NC and HC (15% O₂). The individual exercise duration was normalized to 10% sections (10-100%). During exercise HR and RR-intervals were continuously-recorded. Besides HRV time-domain measurements (meanRR, SDNN), fractal correlation properties using short-term scaling exponent alpha1 of DFA were calculated. Additionally, blood lactate (La), oxygen saturation of the blood (SpO₂), and rating of perceived exertion (RPE) were recorded in regular time intervals.

Results: We observed significant changes under NC and HC for all parameters from the beginning to the end of the exercise (10% vs. 100%) except for SpO₂ and SDNN during NC: increases for HR, La, and RPE in both conditions; decreases for SpO₂ and SDNN during HC, meanRR and DFA-alpha1 during both conditions. Under HC HR (40-70%), La (10-90%), and RPE (50-90%) were significantly-higher, SpO₂ (10-100%), meanRR (40-70%), and DFA-alpha1 (20-60%) were significantly-lower than under NC.

Conclusion: Under both conditions, prolonged exercise until voluntary exhaustion provokes a lower total variability combined with a reduction in the amplitude and correlation properties of RR fluctuations which may be attributed to increased organismic demands. Additionally, HC provoked higher demands and loss of correlation properties at an earlier stage during the exercise regime, implying an accelerated alteration of cardiac autonomic regulation.

Keywords: autonomic nervous system; detrended fluctuation analysis; endurance exercise; heart rate variability; hypoxia; voluntary exhaustion.

FIGURE 1

Flow chart of the constant workload until voluntary exhaustion during normoxic and normobaric hypoxic condition. IAT: individual anaerobic threshold; WU (1): Warm-Up at 100 W; WU (2): Warm-Up at 150 W; CD: Cool-Down at 100 W.

FIGURE 2

Heart rate (A), blood lactate concentration (B) during resting state and all cycling conditions during normoxia (NC, black color) and normobaric hypoxia (HC, gray color). WU (1): Warm-Up at 100W; WU (2): Warm-Up at 150W; 10–100%: Percentage of continuous workload; CD: Cool-Down at 100W. ^*Significant compared to preceding measurement; §Significant change WU (1) vs. CD at 100W; ¥Significant change 10% vs. 100%; #Significant change NC vs. HC (p ≤ 0.05).

FIGURE 3

Oxygen saturation of the blood (A) and rate of perceived exertion RPE, (B) during resting state and all cycling conditions during normoxia (NC, black color) and normobaric hypoxia (HC, gray color). WU (1): Warm-Up at 100 W; WU (2): Warm-Up at 150 W; 10–100%: Percentage of continuous workload; CD: Cool-Down at 100 W. ^*Significant compared to preceding measurement; §Significant change WU (1) vs. CD at 100 W; ¥Significant change 10% vs. 100%; #Significant change NC vs. HC (p ≤ 0.05).

FIGURE 4

Short-term scaling exponent (DFA-alpha1) during resting state and all cycling conditions during normoxia (NC, black color) and normobaric hypoxia (HC, gray color). WU (1): Warm-Up at 100 W; WU (2): Warm-Up at 150 W; 10–100%: Percentage of continuous workload; CD: Cool-Down at 100 W. ^*Significant compared to preceding measurement; §Significant change WU (1) vs. CD at 100 W; ¥Significant change 10% vs. 100%; #Significant change NC vs. HC (p ≤ 0.05).