This study is called CHE-OX-ANT (Cerebral Hemodynamic Effects of Oxygen and Antioxidants). It is a single-center, academic research project led by the Department of Intensive Care at Erasme Hospital (Université Libre de Bruxelles).

The study focuses on understanding how the brain and blood vessels react when a person breathes pure oxygen and how these effects may be influenced by giving vitamin C, an antioxidant. Oxygen is one of the most common treatments in hospitals. While it can be life-saving, too much oxygen may sometimes cause harmful effects, such as oxidative stress (an imbalance between damaging molecules called free radicals and the body's defenses). Antioxidants like vitamin C may help counteract these effects.

The goal of the study is to examine how a short period of high oxygen (30 minutes of 100% oxygen through a mask) affects cerebral hemodynamics, microcirculation, microperfusion, blood markers (levels of oxidative stress, antioxidant activity, and microparticles).

The study will also test whether giving vitamin C beforehand changes these responses compared to a placebo (saline solution).

Each volunteer will participate in two sessions, one week apart. Before each session, participants will receive either vitamin C (given intravenously) or a placebo (saline). They will not know which one they receive.

Then, they will breathe 100% oxygen for 30 minutes through a facial mask. Measurements will be taken at three times: before oxygen (t0), just after oxygen (t1), and 1 hour later (t2). These include: ultrasound of the brain's blood flow (transcranial Doppler), measurements of skin perfusion and blood samplings.

Oxygen therapy is widely used in hospitals and critical care, but excessive oxygen levels (hyperoxemia) can have both beneficial and harmful effects. In the brain, high oxygen may improve oxygen delivery, but it can also trigger oxidative stress, inflammation, and vascular dysfunction, potentially worsening outcomes. Antioxidants, such as vitamin C (ascorbic acid), may counteract these harmful effects by neutralizing reactive oxygen species and restoring redox balance. The CHE-OX-ANT study aims to investigate the physiological effects of a short normobaric hyperoxic stimulus (30 minutes of 100% oxygen breathing) in healthy volunteers, with and without prior antioxidant administration. The primary focus is on oxidative stress markers and microparticle production, while secondary objectives include effects on cerebral blood flow, sublingual microcirculation, and skin perfusion.

This is a prospective, interventional, single-center, randomized, single-blind, placebo-controlled physiological trial conducted at Erasme Hospital, Brussels. Each subject will serve as their own control, undergoing two study sessions one week apart. The study will recruit 10-15 healthy adult volunteers. Eligible participants must be >18 years old, in good health, with no cardiovascular, neurological, hematological, or immunological disease. Exclusion criteria include smoking, pregnancy, BMI >25, or contraindications to oxygen or vitamin

At each session, participants will receive one of two randomized pre-treatments, administered intravenously 30-60 minutes before oxygen exposure:

Vitamin C group: 1-3 g ascorbic acid in 500 mL saline infused over 20 minutes. Placebo group: 500 mL of saline infused over 20 minutes. Following this, participants will breathe 100% oxygen for 30 minutes through a reservoir mask. Measurements will be obtained at three timepoints: baseline (t0), immediately after oxygen (t1), and 2 hours post-exposure (t2).

Cerebral hemodynamics: Transcranial Doppler ultrasonography. Skin perfusion: laser Doppler or infrared techniques. Blood samples: venous and arterial blood for oxidative stress, antioxidant status, microparticles, and arterial blood gas analysis.

Blood pressure: continuous invasive monitoring via a radial arterial catheter. Endpoints Primary endpoint: Change in oxidative stress and microparticle levels. Secondary endpoints: Changes in cerebral hemodynamics and skin perfusion. This exploratory study will provide new insights into how oxygen affects the brain and peripheral perfusion, and whether antioxidant administration modifies these effects. The results may contribute to optimizing oxygen therapy in critically ill patients, where oxygen is widely used but may sometimes cause harm.

Inclusion Criteria:

• Adults
• Otherwise healthy, without known pathologies in the cardiovascular, neurological, immunological, renal hematological, or vascular systems.

Exclusion Criteria:

• Smokers
• Pregnant women
• BMI > 25
• Any reason to avoid vit. C
• Any reason to avoid breathing oxygen at high doses

At present, there is no confirmed plan to share individual participant data (IPD). If sharing is pursued, it would involve de-identified datasets limited to physiological parameters, oxidative stress markers, microparticle levels, and circulatory measurements. No personally identifiable information will be shared.

Participants are blinded to the intervention (placebo vs. vitamin C infusion). Investigators and care providers are not blinded due to the nature of the procedures.