The purpose of this study is to find out a serum marker for the evaluation of blood brain barrier damage based on animal experiments and investigate the effect of NBO（Normobaric hyperoxia）on blood brain barrier in the acute ischemic stroke patients who received r-tPA thrombolytic therapy.

Inclusion Criteria:

1. Male or female with age from 18 to 85;
2. Acute ischemic stroke;
3. National Institutes of Health Stroke Scale (NIHSS) score of 4-25
4. mRS score less than 1 before onset of stroke symptom;
5. Onset of stroke symptoms within 4.5h before initiation of intravenous r-tPA thrombolytic therapy;
6. Informed consent obtained

Exclusion Criteria:

1. Stroke or serious head trauma within the previous 3 months
2. Major surgery or severe trauma with in the preceding 3 months
3. Intracranial hemorrhage
4. Systolic pressure greater than 185 mm Hg or diastolic pressure greater than 110 mm Hg, or aggressive treatment intravenous medication) necessary to reduce blood pressure to these limits
5. Symptoms rapidly improving
6. Symptoms suggestive of subarachnoid hemorrhage, even if CT scan was normal
7. Gastrointestinal hemorrhage or urinary tract hemorrhage within the previous 21 days
8. Arterial puncture at a noncompressible site within the previous 7 days
9. Seizure at the onset of stroke
10. Platelet count of less than 100,000 per cubic millimeter
11. Received heparin within the 48 hours preceding the onset of stroke and had an elevated partial-thromboplastin time
12. Received oral anticoagulation therapy preceding the onset of stroke and INR greater than 1.7 or prothrombin times greater than 15 seconds
13. CT showed a multiple infarction (low density area greater than 1/3 cerebral hemisphere)
14. severe hepatic or renal dysfunction
15. active and chronic obstructive pulmonary disease or acute respiratory distress syndrome;
16. >3 L/min oxygen required to maintain peripheral arterial oxygen saturation (SaO2)﹥95% as per current stroke management guidelines; 17 medically unstable;

18. inability to obtain informed consent. 19. Life expectancy<1 years 20. Pregnant or breast-feeding women 21. Unwilling to be followed up or poor compliance for treatment 22. Patients being enrolled or having been enrolled in other clinical trial within 3 months prior to this clinical trial