Cerebral oxygenation during the Richalet hypoxia sensitivity test and cycling time-trial performance in severe hypoxia

Load carriage physiology in normoxia and hypoxia

PURPOSE

To determine the effects of load carriage in normoxia and normobaric hypoxia on ventilatory responses, hemodynamics, tissue oxygenation, and metabolism.

METHODS

Healthy males (n = 12) completed 3 randomly ordered baseline graded exercise tests in the following conditions: (1) unloaded normoxic (U: FIO2 = 20.93%), (2) loaded (~ 30 kg) normoxic (LN), and (3) loaded hypoxic simulating ~ 3650 m (LH: FIO2 =  ~ 13%). Thereafter, experimental exercise trials were completed in quasi-randomized order (i.e., U completed first) consisting of 3 × 10 min of walking (separated by 5 min seated rest) with stages matched with the U condition (in ascending order) for relative intensity, absolute oxygen consumption ([VO2]; 1.7 L min-1), and walking speed (1.45 ± 0.15 m s-1).

RESULTS

Load carriage increased perceived exertion and reduced VO2max (LN: - 7%; LH: - 32%; p < 0.05). At matched VO2, stroke volume and tidal volume were reduced and maintained with LN and LH vs. U, respectively (p < 0.05). Increases in cardiac output and minute ventilation at matched VO2 (with LH) and speed (with LN and LH), were primarily accomplished via increases in heart rate and breathing frequency (p < 0.05). Cerebral oxygenated hemoglobin (O2HHb) was increased at all intensities with LN, but deoxygenated hemoglobin and total hemoglobin were increased with LH (p < 0.05). Muscle oxygen kinetics and substrate utilization were similar between LN and U, but LH increased CHO dependence and reduced muscle O2HHb at matched speed (p < 0.05).

CONCLUSION

Load carriage reduces cardiorespiratory efficiency and increases physiological strain, particularly in hypoxic environments. Potential load carriage-induced alterations in cerebral blood flow may increase the risk for altitude illnesses and requires further study.

Independent effects of acute normobaric hypoxia and hypobaric hypoxia on human physiology

The purpose of this study was to examine the effects of acute normobaric (NH, decreased FiO₂) and hypobaric (HH, 4200 m ascent) hypoxia exposures compared to sea level (normobaric normoxia, NN). Tissue oxygenation, cardiovascular, and body fluid variables measured during rest and a 3-min step-test following 90-min exposures (NH, HH, NN). Muscle oxygenated hemoglobin (O₂Hb) decreased, and muscle deoxygenated hemoglobin (HHb) increased environmentally independent from rest to exercise (p < 0.001). During exercise, brain O₂Hb was lower at HH compared to NN (p = 0.007), trending similarly with NH (p = 0.066), but no difference between NN and NH (p = 0.158). During exercise, HR at NH (141 ± 4 beats·min^-1) and HH (141 ± 3 beats·min^-1) were higher than NN (127 ± 44 beats·min^-1, p = 0.002), but not each other (p = 0.208). During exercise, stroke volume at HH (109.6 ± 4.1 mL·beat^-1) was higher than NH (97.8 ± 3.3 mL·beat^-1) and NN (99.8 ± 3.9 mL·beat^-1, p ≤ 0.010) with no difference between NH and NN (p = 0.481). During exercise, cardiac output at NH (13.8 ± 0.6 L) and HH (15.5 ± 0.7 L) were higher than NN (12.6 ± 0.5 L, p ≤ 0.006) with HH also higher than NH (p = 0.001). During acute hypoxic stimuli, skeletal muscle maintains oxygenation whereas the brain does not. These differences may be mediated by environmentally specific cardiovascular compensation. Thus, caution is advised when equating NH and HH.

Self-Paced Endurance Performance and Cerebral Hemodynamics of the Prefrontal Cortex: A Scoping Review of Methodology and Findings

Recent research has suggested that top-down executive function associated with the prefrontal cortex is key to the decision-making processes and pacing of endurance performance. A small but growing body of literature has investigated the neurological underpinnings of these processes by subjecting the prefrontal cortex to functional near-infrared spectroscopy (fNIRS) measurement during self-paced endurance task performance. Given that fNIRS measurement for these purposes is a relatively recent development, the principal aim of this review was to assess the methodological rigor and findings of this body of research. We performed a systematic literature search to collate research assessing prefrontal cortex oxygenation via fNIRS during self-paced endurance performance. A total of 17 studies met the criteria for inclusion. We then extracted information concerning the methodology and findings from the studies reviewed. Promisingly, most of the reviewed studies reported having adopted commonplace and feasible best practice guidelines. However, a lack of adherence to these guidelines was evident in some areas. For instance, there was little evidence of measures to tackle and remove artifacts from data. Lastly, the reviewed studies provide insight into the significance of cerebral oxygenation to endurance performance and the role of the prefrontal cortex in pacing behavior. Therefore, future research that better follows the guidelines presented will help advance our understanding of the role of the brain in endurance performance and aid in the development of techniques to improve or maintain prefrontal cortex (PFC) oxygenation to help bolster endurance performance.

Integrative physiological assessment of cerebral hemodynamics and metabolism in acute ischemic stroke

Restoring perfusion to ischemic tissue is the primary goal of acute ischemic stroke care, yet only a small portion of patients receive reperfusion treatment. Since blood pressure (BP) is an important determinant of cerebral perfusion, effective BP management could facilitate reperfusion. But how BP should be managed in very early phase of ischemic stroke remains a contentious issue, due to the lack of clear evidence. Given the complex relationship between BP and cerebral blood flow (CBF)-termed cerebral autoregulation (CA)-bedside monitoring of cerebral perfusion and oxygenation could help guide BP management, thereby improve stroke patient outcome. The aim of INFOMATAS is to 'identify novel therapeutic targets for treatment and management in acute ischemic stroke'. In this review, we identify novel physiological parameters which could be used to guide BP management in acute stroke, and explore methodologies for monitoring them at the bedside. We outline the challenges in translating these potential prognostic markers into clinical use.

Hypoxic Respiratory Chemoreflex Control in Young Trained Swimmers

During an apnea, changes in PaO₂ activate peripheral chemoreceptors to increase respiratory drive. Athletes with continuous apnea, such as breath-hold divers, have shown a decrease in hypoxic ventilatory response (HVR), which could explain the long apnea times; however, this has not been studied in swimmers. We hypothesize that the long periods of voluntary apnea in swimmers is related to a decreased HVR. Therefore, we sought to determine the HVR and cardiovascular adjustments during a maximum voluntary apnea in young-trained swimmers. In fifteen trained swimmers and twenty-seven controls we studied minute ventilation (V_E), arterial saturation (SpO₂), heart rate (HR), and autonomic response [through heart rate variability (HRV) analysis], during acute chemoreflex activation (five inhalations of pure N₂) and maximum voluntary apnea test. In apnea tests, the maximum voluntary apnea time and the end-apnea HR were higher in swimmers than in controls (p < 0.05), as well as a higher low frequency component of HRV (p < 0.05), than controls. Swimmers showed lower HVR than controls (p < 0.01) without differences in cardiac hypoxic response (CHR). We conclude that swimmers had a reduced HVR response and greater maximal voluntary apnea duration, probably due to decreased HVR.

Exercise Overrides Blunted Hypoxic Ventilatory Response in Prematurely Born Men

Purpose

Pre-term birth provokes life-long anatomical and functional respiratory system sequelae. Although blunted hypoxic ventilatory response (HVR) is consistently observed in pre-term infants, it remains unclear if it persists with aging and, moreover, if it influences hypoxic exercise capacity. In addition, it remains unresolved whether the previously observed prematurity-related alterations in redox balance could contribute to HVR modulation.

Methods

Twenty-one prematurely born adult males (gestational age = 29 ± 4 weeks], and 14 age matched controls born at full term (gestational age = 39 ± 2 weeks) underwent three tests in a randomized manner: (1) hypoxia chemo-sensitivity test to determine the resting and exercise poikilocapnic HVR and a graded exercise test to volitional exhaustion in (2) normoxia (F_iO₂ = 0.21), and (3) normobaric hypoxia (F_iO₂ = 0.13) to compare the hypoxia-related effects on maximal aerobic power (MAP). Selected prooxidant and antioxidant markers were analyzed from venous samples obtained before and after the HVR tests.

Results

Resting HVR was lower in the pre-term (0.21 ± 0.21 L ⋅ min^-1 ⋅ kg^-1) compared to full-term born individuals (0.47 ± 0.23 L ⋅ min^-1 ⋅ kg^-1; p < 0.05). No differences were noted in the exercise HVR or in any of the measured oxidative stress markers before or after the HVR test. Hypoxia-related reduction of MAP was comparable between the groups.

Conclusion

These findings indicate that blunted resting HVR in prematurely born men persists into adulthood. Also, active adults born prematurely seem to tolerate hypoxic exercise well and should, hence, not be discouraged to engage in physical activities in hypoxic environments. Nevertheless, the blunted resting HVR and greater desaturation observed in the pre-term born individuals warrant caution especially during prolonged hypoxic exposures.

Quantifying the effects of acute hypoxic exposure on exercise performance and capacity: A systematic review and meta-regression

OBJECTIVE

To quantify the effects of acute hypoxic exposure on exercise capacity and performance, which includes continuous and intermittent forms of exercise.

DESIGN

A systematic review was conducted with a three-level mixed effects meta-regression. The ratio of means method was used to evaluate main effects and moderators providing practical interpretations with percentage change.

DATA SOURCES

A systemic search was performed using three databases (Google scholar, PubMed and SPORTDiscus). Eligibility criteria for selecting studies: Inclusion was restricted to investigations that assessed exercise performance (time trials (TTs), sprint and intermittent exercise tests) and capacity (time to exhaustion test, TTE) with acute hypoxic (<24 h) exposure and a normoxic comparator.

RESULTS

Eighty-two outcomes from 53 studies (N = 798) were included in this review. The results show an overall reduction in exercise performance/capacity -17.8 ± 3.9% (95% CI -22.8% to -11.0%), which was significantly moderated by -6.5 ± 0.9% per 1000 m altitude elevation (95% CI -8.2% to -4.8%) and oxygen saturation (-2.0 ± 0.4%; 95% CI -2.9% to -1.2%). TT (-16.2 ± 4.3%; 95% CI -22.9% to -9%) and TTE (-44.5 ± 6.9%; 95% CI -51.3% to -36.7%) elicited a negative effect, whilst indicating a quadratic relationship between hypoxic magnitude and both TTE and TT performance. Furthermore, exercise less than 2 min exhibited no ergolytic effect from acute hypoxia. Summary/Conclusion: This review highlights the ergolytic effect of acute hypoxic exposure, which is curvilinear for TTE and TT performance with increasing hypoxic levels, but short duration intermittent and sprint exercise seem to be unaffected.

Effect of dead space on breathing stability at exercise in hypoxia

Recent studies have shown that normal subjects exhibit periodic breathing when submitted to concomitant environmental (hypoxia) and physiological (exercise) stresses. A mathematical model including mass balance equations confirmed the short period of ventilatory oscillations and pointed out an important role of dead space in the genesis of these phenomena. Ten healthy subjects performed mild exercise on a cycloergometer in different conditions: rest/exercise, normoxia/hypoxia and no added dead space/added dead space (aDS). Ventilatory oscillations (V˙E peak power) were augmented by exercise, hypoxia and aDS (P<0.001, P<0.001 and P<0.01, respectively) whereas V˙E period was only shortened by exercise (P<0.001), with an 11-s period. aDS also increased V˙E (P<0.001), tidal volume (VT, P<0.001), and slightly augmented PETCO₂ (P<0.05) and the respiratory frequency (P<0.05). These results confirmed our previous model, showing an exacerbation of breathing instability by increasing dead space. This underlines opposite effects observed in heart failure patients and normal subjects, in which added dead space drastically reduced periodic breathing and sleep apneas. It also points out that alveolar ventilation remains very close to metabolic needs and is not affected by an added dead space. Clinical Trial reg. n°: NCT02201875.

Cycling Time Trial Is More Altered in Hypobaric than Normobaric Hypoxia

PURPOSE

Slight physiological differences between acute exposure in normobaric hypoxia (NH) and hypobaric hypoxia (HH) have been reported. Taken together, these differences suggest different physiological responses to hypoxic exposure to a simulated altitude (NH) versus a terrestrial altitude (HH). For this purpose, in the present study, we aimed to directly compare the time-trial performance after acute hypoxia exposure (26 h, 3450 min) by the same subjects under three different conditions: NH, HH, and normobaric normoxia (NN). Based on all of the preceding studies examining the differences among these hypoxic conditions, we hypothesized greater performance impairment in HH than in NH.

METHODS

The experimental design consisted of three sessions: NN (Sion: FiO2, 20.93), NH (Sion, hypoxic room: FiO2, 13.6%; barometric pressure, 716 mm Hg), and HH (Jungfraujoch: FiO2, 20.93; barometric pressure, 481 mm Hg). The performance was evaluated at the end of each session with a cycle time trial of 250 kJ.

RESULTS

The mean time trial duration in NN was significantly shorter than under the two hypoxic conditions (P < 0.001). In addition, the mean duration in NH was significantly shorter than that in HH (P < 0.01). The mean pulse oxygen saturation during the time trial was significantly lower for HH than for NH (P < 0.05), and it was significantly higher in NN than for the two other sessions (P < 0.001).

CONCLUSION

As previously suggested, HH seems to be a more stressful stimulus, and NH and HH should not be used interchangeability when endurance performance is the main objective. The principal factor in this performance difference between hypoxic conditions seemed to be the lower peripheral oxygen saturation in HH at rest, as well as during exercise.

Effect of oral nitrate supplementation on pulmonary hemodynamics during exercise and time trial performance in normoxia and hypoxia: a randomized controlled trial

Background: Hypoxia-induced pulmonary vasoconstriction increases pulmonary arterial pressure (PAP) and may impede right heart function and exercise performance. This study examined the effects of oral nitrate supplementation on right heart function and performance during exercise in normoxia and hypoxia. We tested the hypothesis that nitrate supplementation would attenuate the increase in PAP at rest and during exercise in hypoxia, thereby improving exercise performance.

Methods: Twelve trained male cyclists [age: 31 ± 7 year (mean ± SD)] performed 15 km time-trial cycling (TT) and steady-state submaximal cycling (50, 100, and 150 W) in normoxia and hypoxia (11% inspired O₂) following 3-day oral supplementation with either placebo or sodium nitrate (0.1 mmol/kg/day). We measured TT time-to-completion, muscle tissue oxygenation during TT and systolic right ventricle to right atrium pressure gradient (RV-RA gradient: index of PAP) during steady state cycling.

Results: During steady state exercise, hypoxia elevated RV-RA gradient (p > 0.05), while oral nitrate supplementation did not alter RV-RA gradient (p > 0.05). During 15 km TT, hypoxia lowered muscle tissue oxygenation (p < 0.05). Nitrate supplementation further decreased muscle tissue oxygenation during 15 km TT in hypoxia (p < 0.05). Hypoxia impaired time-to-completion during TT (p < 0.05), while no improvements were observed with nitrate supplementation in normoxia or hypoxia (p > 0.05).

Conclusion: Our findings indicate that oral nitrate supplementation does not attenuate acute hypoxic pulmonary vasoconstriction nor improve performance during time trial cycling in normoxia and hypoxia.

Ventilatory oscillations at exercise: effects of hyperoxia, hypercapnia, and acetazolamide

Periodic breathing has been found in patients with heart failure and sleep apneas, and in healthy subjects in hypoxia, during sleep and wakefulness, at rest and, recently, at exercise. To unravel the cardiorespiratory parameters liable to modulate the amplitude and period of ventilatory oscillations, 26 healthy subjects were tested under physiological (exercise) and environmental (hypoxia, hyperoxia, hyperoxic hypercapnia) stresses, and under acetazolamide (ACZ) treatment. A fast Fourier transform spectral analysis of breath-by-breath ventilation evidenced an increase in peak power under hypercapnia (vs. normoxia and hyperoxia, P < 0.001) and a decrease under ACZ (vs. placebo, P < 0.001), whereas it was not modified in hyperoxia. period was shortened by exercise in all conditions (vs. rest, P < 0.01) and by hypercapnia (vs. normoxia, P < 0.05) but remained unchanged under ACZ (vs. placebo). peak power was positively related to cardiac output () and in hyperoxia (P < 0.01), in hypercapnia (P < 0.001) and under ACZ (P < 0.001). period was negatively related to and in hyperoxia (P < 0.01 and P < 0.001, respectively), in hypercapnia (P < 0.05 and P < 0.01, respectively) and under ACZ (P < 0.05 and P < 0.01, respectively). Total respiratory cycle time was the main factor responsible for changes in period. In conclusion, exercise, hypoxia, and hypercapnia increase ventilatory oscillations by increasing and , whereas ACZ decreases ventilatory instability in part by a contrasting action on O₂ and CO₂ sensing. An intrinsic oscillator might modulate ventilation through a complex system where peripheral chemoreflex would play a key role.

Periodic breathing in healthy humans at exercise in hypoxia

Periodic breathing is frequent in heart failure or ventilatory disorders during sleep, and common during sleep at high altitude, but has been rarely studied in wakefulness and during exercise. A retrospective analysis of ventilation from hypoxia exercise tests was realized in 82 healthy subjects separated into two groups with either high or low ventilatory response to hypoxia at exercise (HVRe). A fast Fourier transform spectral analysis of the breath-by-breath ventilation (V̇e) signal, O2 saturation, and end-tidal PCO2 evidenced a periodic pattern with a period of 11.1 to 12.0 s. The peak power of the V̇e spectrum was higher in the high HVRe group (P < 0.001). A prospective study (25 subjects) was performed to evaluate the influence of cardiorespiratory factors on the amplitude and period of oscillations in various conditions of exercise (20 to 40% maximal aerobic power) and hypoxia (0 to 4,000 m altitude). The period of V̇e was shorter at exercise (vs. rest, P < 0.001) and hypoxia (vs. normoxia, P < 0.001), and inversely related with cardiac output and V̇e (P < 0.001). V̇e peak power was higher at exercise (P < 0.001) and hypoxia (P < 0.001), and was positively related with cardiac output and V̇e (P < 0.001). V̇e peak power in hypoxia was positively related with the ventilatory response to CO2 (HCVR). This novel observation suggests that healthy subjects demonstrate a spontaneous periodic breathing, not clearly observable at rest and in normoxia, but triggered by hypoxic exercise. The periodic pattern is enhanced in subjects with high HVRe and high HCVR, suggesting that oxygen and CO2 play synergistic roles in the modulation of these oscillations.