Adjuvant Cerebroprotection Using Normobaric Hyperoxia in Pre-hospital Patients with Suspected Stroke

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.