In the event of cardiac and respiratory arrest, the tolerance limit of brain cells to hypoxia at room temperature is usually 4 minutes. If more than 10 minutes, the brain cells will undergo irreversible necrosis and cannot be recovered. Even within a robust first aid network community, it is difficult for first responders to ensure that they arrive within 4 minutes of each report. Before the doctor arrives, it is very important that the "first witness" can properly rescue.