The primary objective of this study is to determine the efficacy and safety of inhaled normobaric hyperoxia therapy (NBO) beginning in the pre-hospital setting in patients with suspected acute cerebral ischemia (ACI) due to large vessel occlusion presenting within 6 hours of symptom onset.

Stroke is a leading cause of death and disability globally, with acute ischemic stroke (AIS) patients often benefiting from intravenous thrombolysis and endovascular therapies such as mechanical thrombectomy, which have been shown to improve reperfusion rates. However, despite reperfusion, the proportion of patients with large vessel occlusion achieving a favorable functional outcome, defined as a modified Rankin Scale score of 0-2 at 90 days, remains under 50%.

Normobaric hyperoxia (NBO) emerges as a compelling option for cerebral protection. Its neuroprotective mechanisms are thought to include hypoxic tissue rescue, blood-brain barrier preservation, brain edema reduction, neuroinflammation alleviation, mitochondrial function improvement, oxidative stress mitigation, and apoptosis inhibition. NBO's diffusion properties allow it to reach the penumbra before reperfusion, enhancing aerobic metabolism and potentially reducing infarct volume. Its advantages also include low cost, wide availability, and ease of use, making it accessible across various healthcare settings.

Inclusion Criteria:

• Age 18 to 80 years old;
• Signs and symptoms are consistent with a new acute stroke, with low possibility of stroke mimics (e.g., no sudden coma, prior seizure disorder, suspected hypoglycemia);
• No prior stroke;
• The time from stroke onset/last seen well to randomization is within 6 hours;
• Paramedic-obtained FAST-ED score >= 3 (The enrollment of patients with a FAST-ED score of 3 is prespecified to constitute no more than 10% of the total study population);
• No significant pre-stroke disability (pre-stroke mRS 0-1);
• Signed informed consent from the patient or the legally authorized representative (LAR).

Exclusion Criteria:

• Endotracheal intubation in the field (prior to consent);
• Respiratory rate <= 10 or >= 30 breaths per minute;
• Oxygen-dependence at baseline to maintain SaO2 > 95%;
• Known history of severe chronic obstructive pulmonary disease (FEV1 less than 1.0), New York Heart Association (NYHA) Heart Failure Class III, acute pulmonary infection or aspiration pneumonia, prior to enrollment;
• Seizure at stroke onset;
• Exhibiting symptoms of vomiting, severe headache, or unconscious;
• Known pregnancy: women of childbearing age (18 to 44 years old according to the CDC's Division of Reproductive Health) will be asked about their pregnancy status.
• Participating in another clinical trial, or completed participation within prior 30 days;
• Receiving other neuroprotective agent (e.g., edaravone dexborneol, n-butylphthalide);
• Life expectancy < 90 days due to comorbidities;
• Unlikely to complete the 90-day follow-up visit.