High altitude exposure typically reduces endothelial function, and this is modulated by hemoconcentration resulting from plasma volume contraction. However, the specific impact of hypobaric hypoxia, independent of external factors (e.g. cold, varying altitudes, exercise, diet, dehydration) on endothelial function is unknown. We examined the temporal changes in blood viscosity, shear stress and endothelial function and the impact of plasma volume expansion (PVX) during exposure to hypobaric hypoxia while controlling for external factors. Eleven healthy males (25±4 years; mean±SD) completed two four-day chamber visits (normoxia [NX] and hypobaric hypoxia [HH; equivalent altitude: 3500m]) in a cross-over design. Endothelial function was assessed via flow-mediated dilation in response to transient (reactive hyperemia; RH-FMD) and sustained (progressive handgrip exercise; SS-FMD) increases in shear stress prior to entering and after 1h, 6h, 12h, 48h and 96h in the chamber. During HH, endothelial function was also measured on the last day after PVX to pre-exposure levels (1140±320 ml balanced crystalloid solution). Blood viscosity and arterial shear stress increased on the first day during HH compared to NX and remained elevated at 48h and 96h (P<0.005). RH-FMD did not differ during HH compared to NX and was unaffected by PVX despite reductions in blood viscosity (P<0.05). The stimulus-response slope of increases in shear stress to vasodilation during SS-FMD was preserved in HH and increased by 44±73% following PVX (P=0.023). These findings suggest that endothelial function is maintained in HH when other stressors are absent and that PVX improves endothelial function in a shear stress stimulus-specific manner.

中文翻译：

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低压缺氧时的内皮功能和切应力：男性的时程和血浆容量膨胀的影响。

高海拔暴露通常会降低内皮功能，这是由血浆体积收缩引起的血液浓缩调节的。然而，不依赖于外界因素（例如，寒冷，海拔不同，运动，饮食，脱水）的低压缺氧对内皮功能的具体影响尚不清楚。我们在控制外部因素的同时检查了暴露于低压缺氧期间血液粘度，切应力和内皮功能的暂时变化以及血浆容量膨胀（PVX）的影响。十一名健康男性（25±4岁；平均±标准差）以交叉设计完成了两次为期四天的内室就诊（正常氧[NX]和低压低氧[HH；等效海拔：3500m]）。通过对短暂性（反应性充血; RH-FMD）和持续（渐进式握力练习； SS-FMD）在进入腔室之前和之后1h，6h，12h，48h和96h时，剪切应力会增加。在HH期间，在PVX达到暴露前水平（1140±320 ml平衡晶体溶液）的最后一天也测量了内皮功能。与NX相比，HH期间第一天的血液粘度和动脉切应力增加，并在48h和96h保持升高（P <0.005）。与NX相比，HH期间RH-FMD没有差异，尽管血液粘度降低，但不受PVX的影响（P <0.05）。在SSH-FMD中，剪应力对血管舒张的刺激刺激斜率在HH中得以保留，PVX后增加44±73％（P = 0.023）。