Effect of Acute Hypoxia on Post-Exercise Parasympathetic Reactivation in Healthy Men

Hani Al Haddad,Martin Buchheit,Pitre C Bourdon,Alberto Mendez-Villanueva

doi:10.3389/fphys.2012.00289

Abstract

In this study we assessed the effect of acute hypoxia on post-exercise parasympathetic reactivation inferred from heart rate (HR) recovery (HRR) and HR variability (HRV) indices. Ten healthy males participated in this study. Following 10 min of seated rest, participants performed 5 min of submaximal running at the speed associated with the first ventilatory threshold (Sub) followed by a 20-s all-out supramaximal sprint (Supra). Both Sub and Supra runs were immediately followed by 15 min of seated passive recovery. The resting and exercise sequence were performed in both normoxia (N) and normobaric hypoxia (H; FiO2 = 15.4%). HRR indices (e.g., heart beats recovered in the first minute after exercise cessation, HRR60s) and vagal-related HRV indices [i.e., natural logarithm of the square root of the mean of the sum of the squared differences between adjacent normal R–R intervals (Ln rMSSD)] were calculated for both conditions. Difference in the changes between N and H for all HR-derived indices were also calculated for both Sub and Supra. HRR60s was greater in N compared with H following Sub only (60 ± 14 vs. 52 ± 19 beats min−1, P = 0.016). Ln rMSSD was greater in N compared with H (post Sub: 3.60 ± 0.45 vs. 3.28 ± 0.44 ms in N and H, respectively, and post Supra: 2.66 ± 0.54 vs. 2.65 ± 0.63 ms, main condition effect P = 0.02). When comparing the difference in the changes, hypoxia decreased HRR60s (−14.3% ± 17.2 vs. 5.2% ± 19.3; following Sub and Supra, respectively; P = 0.03) and Ln rMSSD (−8.6% ± 7.0 vs. 2.0% ± 13.3, following Sub and Supra, respectively; P = 0.08, Cohen’s effect size = 0.62) more following Sub than Supra. While hypoxia may delay parasympathetic reactivation following submaximal exercise, its effect is not apparent following supramaximal exercise. This may suggest that the effect of blood O2 partial pressure on parasympathetic reactivation is limited under heightened sympathetic activation.

Highlights

Exercising in altitude has grown in popularity in recent years
To better understand the respective impact that hypoxia and exercise intensity have on post-exercise autonomic activity, we examined the time course of parasympathetic reactivation following submaximal and supramaximal exercise performed both in normoxic and hypoxic conditions
These results are in line with the belief that altitude should generally exceed 3500 m to substantially alter cardiac parasympathetic activity at rest (Yamamoto et al, 1996; Buchheit et al, 2004), and suggest that the hypoxia-induced changes in post-exercise parasympathetic reactivation observed in the present study were unlikely affected by changes in resting autonomic activity

Summary

Introduction

Exercising in altitude has grown in popularity in recent years. For example, the number of registered participants in the international Ultra-Trail of the Mont Blanc has increased from 700 in 2003 up to 10,000 in 2012 (http://www.ultratrailmb.com/documents/ Newsletters/Letters/nl2_fevrier2012_EN.html). Since an increased sympathetic activity and/or a delayed parasympathetic reactivation following exercise has been associated with an increased risk of sudden cardiac death (Billman, 2002), exercising in a hypoxic environment is likely to increase the prognosis of developing cardiac arrhythmia (Clark et al, 1963). While the acute effects of hypoxia on cardiac autonomic nervous system function have been well investigated at rest and during exercise (Somers et al, 1989a; Yamamoto et al, 1996, Perini and Veicsteinas, 2003; Buchheit et al, 2004, Iwasaki et al, 2006), the effect of hypoxia on parasympathetic reactivation following exercise has, to our knowledge, never been documented

Methods

Results

Discussion

Conclusion