Effect of acute hypoxia on muscle blood flow, VO2p, and [HHb] kinetics during leg extension exercise in older men

Abstract

The adjustment of pulmonary oxygen uptake (VO2p), heart rate (HR), limb blood flow (LBF), and muscle deoxygenation [HHb] was examined during the transition to moderate-intensity, knee-extension exercise in six older adults (70 ± 4 years) under two conditions: normoxia (FIO₂ = 20.9 %) and hypoxia (FIO₂ = 15 %). The subjects performed repeated step transitions from an active baseline (3 W) to an absolute work rate (21 W) in both conditions. Phase 2 VO₂p, HR, LBF, and [HHb] data were fit with an exponential model. Under hypoxic conditions, no change was observed in HR kinetics, on the other hand, LBF kinetics was faster (normoxia 34 ± 3 s; hypoxia 28 ± 2), whereas the overall [HHb] adjustment (τ' = TD + τ) was slower (normoxia 28 ± 2; hypoxia 33 ± 4 s). Phase 2 VO₂p kinetics were unchanged (p < 0.05). The faster LBF kinetics and slower [HHb] kinetics reflect an improved matching between O₂ delivery and O₂ utilization at the microvascular level, preventing the phase 2 VO₂p kinetics from become slower in hypoxia. Moreover, the absolute blood flow values were higher in hypoxia (1.17 ± 0.2 L min(-1)) compared to normoxia (0.96 ± 0.2 L min(-1)) during the steady-state exercise at 21 W. These findings support the idea that, for older adults exercising at a low work rate, an increase of limb blood flow offsets the drop in arterial oxygen content (CaO₂) caused by breathing an hypoxic mixture.

Repeated Ischemic Preconditioning Effects on Physiological Responses to Hypoxic Exercise

“…Owing to the environmental constraints imposed in hypoxic environments, which reduces alveolar O2 tension (PaO2) and perturbs O2 distribution, the overshoot is pronounced during exercise onsets in hypoxia. 39,46 This is indicative of an increased reliance on O2 extraction for a given V O2p, inferring a mismatch downstream from the conduit artery between the microvascular O2 distribution and O2 utilization. 32 The overshoot magnitude strongly correlates with V O2p when on-kinetics are > 20 s 'O2dependent' threshold, 32,39 indicating a delayed metabolic control during exercise.…”

“…This FiO2 was chosen as it ensured that simulated conditions were a similar severity to the most comparable repeated IPC study (0.12-0.13 FiO2), 10 and lower than the level shown to impair muscle and pulmonary kinetics (0.15 FiO2). 39,46 ***Insert fig 1 here*** Procedure A NIRS optode (Portamon, Artinis Medical Systems, Zetten, The Netherlands) was fitted to the medial gastrocnemius of the right leg prior to entering the chamber during the baseline and post testing visits and remained on for the duration of these visits. Specifically, the NIRS device was fitted 2/3 distance from the calcaneus and anterior fossa and secured with an elastic bandage (Tiger Tear, Hampshire, United Kingdom) to prevent movement and covered with an optically dense black material to minimize the intrusion of extraneous light.…”

“…2 Furthermore, the reduced pulmonary diffusion gradient also acutely disrupts the on-transient pulmonary V O2 and muscle microvascular response to moderate-intensity exercise, such that mismatches in O2 delivery and utilization have been reported. 39,46 The O2 delivery-V O2 matching is vital in setting blood-myocyte PO2, thus directly influencing oxidative metabolic regulation. 23 These factors limit exercise capacity at altitude but could feasibly be attenuated by targeted training interventions.…”

Quantification of energy expenditure of military loaded runs: what is the performance of laboratory-based equations when applied to the field environment?

“…Furthermore, the Bland-Altman analysis showed a non-significant bias (0.0004 kcal/kg/min) between estimates, a small imprecision (0.01 kcal/kg/min, equal to 5.9% of the signal). It is well known that the HR–VO 2 relationship during dynamic, aerobic exercise in a given individual is acutely affected by a number of factors such as exercise mode,23 priming,24 hypoxia,25 hydration,25 training,23 fatigue26 and increases in core temperature 27. Estimates of VO 2 that rely on HR measurements will be affected accordingly.…”

Increased testicular blood flow maintains oxygen delivery and avoids testicular hypoxia in response to reduced oxygen content in inspired air

“…In the human brain, hypoxia (14% for 18 min) increased oxygen metabolic rate and blood flow (compared to 21% oxygen 26 , consistent with the present study. In some organs, a reduced oxygen supply increased blood flow 27 , 28 due to release of vasodilators (decreased oxygen impairs re-phosphorylation of adenosine diphosphate (ADP), which is subsequently degraded to adenosine, a compound that causes vasodilation 15 .…”