Microcirculatory reactivity features in apparently healthy individuals during acute moderate hypoxia and hyperoxia modeling

The dynamics of microvascular circulation and tissue oxygenation in response to the hypoxic test and the subsequent hyperoxia have been studied. The microcirculatory and tissue oxygenation parameters (laser Doppler flowmetry and optical tissue oximetry) were recorded in 30 apparently healthy young men during the hypoxic test (HT): breathing a gas mixture with 10% O 2 for 10 min followed by hyperoxia (30% O 2 ) for 3 min. It was established that, during the HT, no change in the relative level of tissue saturation with oxygen ( S O 2 ) occurred, but the relative degree of oxygen extraction by tissues (Δ Sa O 2 - S O 2 ) decreased significantly with a rapid recovery in the hypoxic phase. This is accompanied by the activation of the neurogenic sympathetic-related vasomotor mechanisms (NT), as well as the endothelium-dependent microvascular tone component (EDTC) with a decrease in the shunting index (SI) mostly in the hyperoxic phase but not during hypoxia. The nature of the microcirculatory response to hypoxia depends on the initial level of resistance/sensitivity to hypoxia. In hypoxia-sensitive subjects, the HT causes Sa O 2 and Δ Sa O 2 - S O 2 to decrease in the absence of significant shifts in the microcirculation regulation. Among the subjects resistant to hypoxia, HT leads to the nutritive blood flow activation by increasing the initially decreased EDTC and the neurogenic sympathetic component of microhemodynamics regulation and to a reduction of blood shunting. At the same time, Δ Sa O 2 - S O 2 does not significantly change, and the activation of microcirculation is also retained in the hyperoxic recovery phase. The identified functional criteria of the contribution of the endothelial and neurogenic vasomotor regulatory components of microhemodynamics in the HT substantiate the involvement of the microcirculatory component and confirm the important role of the hyperoxic phase in the adaptive response of the body to acute hypoxia/hyperoxia.