The primary objective of this study is to estimate the efficacy and safety of NBO on 3-month functional outcome after acute ischemic stroke

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:

1. Age at least 18 years old;
2. 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);
3. No prior stroke in the last 3 months;
4. Time from stroke onset (last seen well) to randomization is within 9 hours;
5. (1) Baseline NIHSS score at 6 or more and Intracranial ICA or MCA-M1 or MCA-M2 dominant occlusion, with or without tandem cervical carotid stenosis or tandem cervical occlusion, confirmed by preoperative CTA (or MRA, DSA) and consistent with signs and symptoms; or (2) Baseline NIHSS score at 6 or more with a hyperdense MCA sign on non-contrast CT; or (3) Baseline NIHSS score at 12 or more;
6. NIHSS score 0 or 1 in the section of level of consciousness;
7. No significant pre-stroke disability (pre-stroke mRS 0--1);
8. ASPECTS at least 6 on non-contrast CT;
9. Patient is planned for transfer to a EVT-capable hospital for EVT;
10. Signed informed consent from the patient or the legally authorized representative (LAR).

Exclusion Criteria:

1. Known history of severe chronic obstructive pulmonary disease (FEV1 less than 1.0), New York Heart Association (NYHA) Heart Failure Class III or IV, acute pulmonary infection or aspiration pneumonia, prior to enrollment;
2. Respiratory rate <= 10 or >= 30 breaths per minute;
3. Oxygen-dependence at baseline to maintain SaO2 > 95% or intubation at baseline;
4. Seizure at stroke onset;
5. Exhibiting symptoms of vomiting, or severe headache, or unconscious;
6. Rapidly improving neurological deficits or transient ischemic attack prior to consent;
7. Signs and symptoms suggestive of subarachnoid hemorrhage, even if CT scan is normal;
8. Evidence of intracranial tumor (except small meningioma) or arteriovenous malformation;
9. Woman of childbearing potential known to be pregnant or with a positive pregnancy test;
10. Life expectancy < 90 days due to comorbidity;
11. Unlikely to complete the 90-day follow-up;
12. Participating in another clinical treatment trial, or completed participation within prior 30 days;
13. Receiving other cerebral protective agent (e.g., edaravone dexborneol, n-butylphthalide);
14. Evidence of acute intracranial hemorrhage on CT/MRI;
15. Significant mass effect with midline shift, defined as any deviation of midline structures such as the septum pellucidum, is observed on CT/MRI scans.