We inspire air and get oxygen, and the storaging conditions of the oxygen in the blood are gas and solid (Hb-O2) type. To measure the gas type oxygen in the blood, we use blood gas analyzing meters, and we also can measure solid type oxygen by this method. From this method, we can get arterial partial pressure of oxygen (PaO2), arterial partial pressure of carbon dioxide (PaCO2), pH, HCO3(-), and so on. By the way, pulse oximeter is an equipment to measure percutaneous arterial oxygen saturation (SpO2). Although the PaO2 and SpO2 are essential parameters to know the clinical conditions of COPD patients, we should be aware of the primary differences between them, and use these parameters effectively.