Exposure to Non-Steady-State Oxygen Is Reflected in Changes to Arterial Blood Gas Values, Prefrontal Cortical Activity, and Systemic Cytokine Levels

Abstract

Onboard oxygen-generating systems (OBOGSs) provide increased inspired oxygen (FiO2) to mitigate the risk of neurologic injury in high altitude aviators. OBOGSs can deliver highly variable oxygen concentrations oscillating around a predetermined FiO2 set point, even when the aircraft cabin altitude is relatively stable. Steady-state exposure to 100% FiO2 evokes neurovascular vasoconstriction, diminished cerebral perfusion, and altered electroencephalographic activity. Whether non-steady-state FiO2 exposure leads to similar outcomes is unknown. This study characterized the physiologic responses to steady-state and non-steady-state FiO2 during normobaric and hypobaric environmental pressures emulating cockpit pressures within tactical aircraft. The participants received an indwelling radial arterial catheter while exposed to steady-state or non-steady-state FiO2 levels oscillating ± 15% of prescribed set points in a hypobaric chamber. Steady-state exposure to 21% FiO2 during normobaria produced arterial blood gas values within the anticipated ranges. Exposure to non-steady-state FiO2 led to PaO2 levels higher upon cessation of non-steady-state FiO2 than when measured during steady-state exposure. This pattern was consistent across all FiO2 ranges, at each barometric condition. Prefrontal cortical activation during cognitive testing was lower following exposure to non-steady-state FiO2 >50% and <100% during both normobaria and hypobaria of 494 mmHg. The serum analyte levels (IL-6, IP-10, MCP-1, MDC, IL-15, and VEGF-D) increased 48 h following the exposures. We found non-steady-state FiO2 levels >50% reduced prefrontal cortical brain activation during the cognitive challenge, consistent with an evoked pattern of neurovascular constriction and dilation.