Facteurs limitants centraux et périphériques de la performance physique en haute altitude (rôle modulateur de la chémosensibilité à l'hypoxie)

Le cadre général de notre travail a été de mettre en évidence le rôle primordial de la chémosensibilité à l hypoxie dans les facteurs centraux et périphériques susceptibles de moduler la performance aérobie en hypoxie aiguë. L étude chez neuf sujets de l influence des niveaux d hypoxie et d intensité d exercice sur les réponses à l hypoxie a permis de démontrer que les réponses ventilatoires et cardiaques à l exercice utilisées pour évaluer la tolérance à l hypoxie sont robustes aux conditions de réalisation du test. Elles peuvent être considérées comme des invariants de la chémosensibilité individuelle. Les caractères protecteur du vieillissement et délétère de l entraînement en endurance sur le risque d apparition de pathologies d altitude nous ont amenés à étudier l influence de ces deux facteurs sur les réponses physiologiques à l hypoxie. L analyse des réponses de 4675 sujets a mis en évidence une majoration des réponses ventilatoires avec l âge, ce qui permet de maintenir le niveau de désaturation artérielle. En revanche les sujets entraînés présentent une désaturation artérielle à l exercice plus importante que les sédentaires malgré de meilleures réponses ventilatoires. Une altération de la chémosensibilité ne semble donc pas pouvoir être un des facteurs majorant la désaturation artérielle à l effort chez les sujets entraînés. L exploration de facteurs périphériques du transport de l O chez six sujets ayant réalisé un entraînement en endurance pendant six semaines suggère en revanche qu une limitation de l extraction tissulaire puisse expliquer en partie la diminution plus importante de VO max en hypoxie chez les sujets entraînés. Il apparaît en effet que les sujets entraînés mobilisent en normoxie leur volume sanguin musculaire et leur capacité d extraction maximaux. Contrairement aux sédentaires, ils n ont donc plus la possibilité d augmenter ces deux facteurs, ce qui pourrait favoriser la désaturation à l exercice en condition hypoxique.Our work forcused on the primary role of chemosensitivity to hypoxia in the central and peripheral factors susceptible to modulate the aerobic performance in acute hypoxia. We studied in nine subjects the influence of altitude and exercise intensity on responses to hypoxia and showed that ventilatory and cardiac responses at exercise used to evaluate the tolerance to hypoxia are robust to the testing conditions. They can be considered as intrinsic physological characteristics of chemosensitivity to hypoxia. Since a young age and aerobic exercise training are correlated with a higher risk of severe high altitude illness, we studied the influence of these two factors on physiological responses hypoxia. The data collected in 4675 subjects showed an increased ventilatory response with ageing, leading to a maintained arterial desaturation. On the other hand, trained subjects had an increased arterial desaturation at exercise despite higher ventilatory responses. Also an altered chemosensitivity doesn't seem to be a factor able to increase the arterial desaturation during exercise in hypoxia in trained subjects. The study of O transport peripheral factors in six subjects exposed to acute hypoxia before and after an aerobic training session suggests that a limitation of O muscular extraction could partly explain the greater decrease in VO max in hypoxia in trained subjects. It appears that endurance trained subjects already use in normoxia their maximal local blood volume and O extraction capacity. On the contrary to sedentary subjects, they cannot increase these two factors anymore in hypoxia, wich could lead to a greater desaturation during exercise in hypoxia.PARIS13-BU Sciences (930792102) / SudocSudocFranceF

Cardiovascular physiology and pathophysiology at high altitude

Oxygen is vital for cellular metabolism; therefore, the hypoxic conditions encountered at high altitude affect all physiological functions. Acute hypoxia activates the adrenergic system and induces tachycardia, whereas hypoxic pulmonary vasoconstriction increases pulmonary artery pressure. After a few days of exposure to low 47 48 49 50 51 Key points 52 53 • Acute exposure to high altitude stimulates the adrenergic system, 54 increasing heart rate and cardiac output; although blood pressure remains 55 stable, pulmonary artery pressure increases owing to hypoxic pulmonary 56 vasoconstriction. 57 58 • Prolonged exposure to high altitude induces a decrease in maximal heart 59 rate through desensitization of the adrenergic pathway, as a protective 60 mechanism against environmental conditions of low oxygen availability. 61 62 • Long-term exposure to high altitude results in cardiac adaptations with 63 no obvious dysfunction; stroke volume is slightly reduced owing to 64 decreased left ventricular filling volume secondary to right ventricular 65 overload. 66 67 • High-altitude natives can develop chronic mountain sickness, associated 68 with erythropoiesis, pulmonary hypertension and right heart failure, 69 although genetic adaptations to hypoxia have been found in Tibetan and 70 Ethiopian populations. 71 72 • Patients with cardiovascular diseases can be at increased risk of adverse 73 events at altitudes above 2,500 m, owing to hypoxaemia, high adrenergic 74 activity and pulmonary hypertension. 75 76 • Intermittent, moderate hypoxia might be useful in the conditioning of 77 patients with cardiovascular diseases, such as coronary heart disease and 78 heart failure

Low frequency ventilatory oscillations in hypoxia are a major contributor of the low frequency component of heart rate variability Running head: Cardiorespiratory coupling at exercise in hypoxia

International audienceHeart rate variability (HRV) may be influenced by several factors, such as environment (hypoxia, hyperoxia, hypercapnia) or physiological demand (exercise). In this retrospective study, we tested the hypothesis that inter-beats (RR) intervals in healthy subjects exercising under various environmental stresses exhibit oscillations at the same frequency than ventilatory oscillations. Methods Spectra from RR intervals and ventilation (E) were collected from 37 healthy young male subjects who participated in five previous studies focused on ventilatory oscillations (or periodic breathing) during exercise in hypoxia, hyperoxia and hypercapnia. Bland & Altman test and multivariate regressions were then performed to compare respective frequencies and changes in peak powers of the two signals. Results Fast Fourier analysis of RR and E signals showed that RR was oscillating at the same frequency than periodic breathing, i.e. ~0.09 Hz (11 seconds). During exercise, in these various conditions, the difference between minimum and maximum HRV peak power was positively correlated to the same change in ventilation peak power (P < 0.05). Low Frequency (LF) peak power was correlated to tidal volume (P < 0.01) and breathing frequency (P < 0.001). Conclusions This study suggests that low frequency ventilatory oscillations in hypoxia are a major contributor of the LF band power of heart rate variability

A Step Test to Evaluate the Susceptibility to Severe High-Altitude Illness in Field Conditions

International audienceA laboratory-based hypoxic exercise test, performed on a cycle ergometer, can be used to predict susceptibility to severe high-altitude illness (SHAI) through the calculation of a clinicophysiological SHAI score. Our objective was to design a field-condition test and compare its derived SHAI score and various physiological parameters, such as peripheral oxygen saturation (SpO2), and cardiac and ventilatory responses to hypoxia during exercise (HCRe and HVRe, respectively), to the laboratory test. A group of 43 healthy subjects (15 females and 28 males), with no prior experience at high altitude, performed a hypoxic cycle ergometer test (simulated altitude of 4,800 m) and step tests (20 cm high step) at 3,000, 4,000, and 4,800 m simulated altitudes. According to tested altitudes, differences were observed in O2 desaturation, heart rate, and minute ventilation (p < 0.001), whereas the computed HCRe and HVRe were not different (p = 0.075 and p = 0.203, respectively). From the linear relationships between the step test and SHAI scores, we defined a risk zone, allowing us to evaluate the risk of developing SHAI and take adequate preventive measures in field conditions, from the calculated step test score for the given altitude. The predictive value of this new field test remains to be validated in real high-altitude conditions

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

International audiencePeriodic 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 urn:x-wiley:2051817X:media:phy212446:phy212446-math-0001 evidenced an increase in urn:x-wiley:2051817X:media:phy212446:phy212446-math-0002 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. urn:x-wiley:2051817X:media:phy212446:phy212446-math-0003 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). urn:x-wiley:2051817X:media:phy212446:phy212446-math-0004 peak power was positively related to cardiac output (urn:x-wiley:2051817X:media:phy212446:phy212446-math-0005) and urn:x-wiley:2051817X:media:phy212446:phy212446-math-0006 in hyperoxia (P < 0.01), in hypercapnia (P < 0.001) and under ACZ (P < 0.001). urn:x-wiley:2051817X:media:phy212446:phy212446-math-0007 period was negatively related to urn:x-wiley:2051817X:media:phy212446:phy212446-math-0008 and urn:x-wiley:2051817X:media:phy212446:phy212446-math-0009 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 urn:x-wiley:2051817X:media:phy212446:phy212446-math-0010 period. In conclusion, exercise, hypoxia, and hypercapnia increase ventilatory oscillations by increasing urn:x-wiley:2051817X:media:phy212446:phy212446-math-0011 and urn:x-wiley:2051817X:media:phy212446:phy212446-math-0012, 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

Ordonnances - Activité Physique : 90 prescriptions

International audienceLes effets bénéfiques de la pratique d’une activité physique à des fins thérapeutiques sont désormais démontrés tout au long du parcours de soin.Les médecins jouent un rôle central dans la prescription de l’activité physique et ont besoin d’outils appropriés à leur exercice médical quotidien. Prescrire l’activité physique est ainsi conçu comme un guide pratique destiné à tous les médecins souhaitant accompagner de façon éclairée leurs patients dans une activité physique raisonnée, régulière et durable : généralistes, spécialistes et internes.L’action de prescription d’une activité physique doit être prise au sérieux tant elle peut s’avérer complexe et faire appel à des compétences multiples conjuguées dans le temps limité d’une consultation. Aussi, 90 ordonnances pratiques sont proposées non seulement dans des affections de longue durée mais aussi dans des pathologies fréquemment rencontrées et pour lesquelles un bénéfice de l’activité physique est reconnu. Caractéristiques des maladies en lien avec l’activité physique, bilans et contre-indications, précautions à prendre avant de prescrire, modalités pratiques (fréquence, intensité, temps, type), intervenants à qui confier ses patients, conseils oraux à donner au patient et démarche motivationnelle à engager, sont traités de manière à offrir au médecin l’ensemble des éléments pour la réussite de sa prescription

Prescription de l'activité physique adaptée par les médecins : enjeux en termes de formation et de profession

National audienc

Exponential Relationship Between Maximal Apnea Duration and Exercise Intensity in Non-apnea Trained Individuals

International audienceIt is well known that the duration of apnea is longer in static than in dynamic conditions, but the impact of exercise intensity on the apnea duration needs to be investigated. The aim of this study was to determine the relationship between apnea duration and exercise intensity, and the associated metabolic parameters. Ten healthy active young non-apnea trained (NAT) men participated in this study. During the first visit, they carried out a maximum static apnea (SA) and a maximal progressive cycle exercise to evaluate the power output achieved at peak oxygen uptake (PVO 2 peak). During the second visit, they performed four randomized dynamic apneas (DAs) at 20, 30, 40, and 50% of PVO 2 peak (P20, P30, P40, and P50) preceded by 4 min of exercise without apnea. Duration of apnea, heart rate (HR), arterial oxygen saturation (SpO 2 ), blood lactate concentration [La], rating of perceived exertion (RPE), and subjective feeling were recorded. Apnea duration was significantly higher during SA (68.1 ± 23.6 s) compared with DA. Apnea duration at P20 (35.6 ± 11.7 s) was higher compared with P30 (25.6 ± 6.3 s), P40 (19.2 ± 6.7 s), and P50 (16.9 ± 2.5 s). The relationship between apnea duration and exercise intensity followed an exponential function ( y = 56.388e –0.025 x ). SA as DA performed at P20 and P30 induces a bradycardia. Apnea induces an SpO 2 decrease which is higher during DA (−10%) compared with SA (−4.4%). The decreases of SPO 2 recorded during DA do not differ despite the increase in exercise intensity. An increase of [La] was observed in P30 and P40 conditions. RPE and subjective feeling remained unchanged whatever the apnea conditions might be. These results suggest that the DA performed at 30% of VO 2 peak could be the best compromise between apnea duration and exercise intensity. Then, DA training at low intensity could be added to aerobic training since, despite the moderate hypoxia, it is sufficient to induce and increase [La] generally observed during high-intensity training

Impact of EBM Fabrication Strategies on Geometry and Mechanical Properties of Titanium Cellular Structures

International audienceManufacturing well controlled cellular materials by conventional methods is strongly difficult 2 (foaming, replication...). Additive Manufacturing appears to be a promising answer for such structures. The EBM technology is indeed theoretically able to produce freeform metallic open cell materials or lattice structures 3. However, due to the process, differences in strut geometries are observed between the CAD model and the manufactured parts, which affect mechanical properties. This study is focused on the determination and improvement of parameters which tailor the mesostructure of titanium cellular structures made by EBM. Mechanical properties are determined by compressive tests on various cellular structures (Cubic, Octet-truss...). Results indicate that the desired relative density (as defined by the CAD model) is not relevant for giving a correct estimation of the mechanical properties. Different sizes, shapes and orientations of struts have been quantitatively studied using X-Ray tomography. Geometry and residual porosity are analysed. An effective cross-section is defined and compared to the desired one. This effective cross-section is strongly related to the strut orientations and fabrication strategies. It allows to explain discrepancies in mechanical properties (stiffness and strength) between desired and real structures. The comparison with tomography results shows the necessity to adapt Gibson and Ashby's laws4 for the mechanical properties prediction of cellular solids made by EBM