In the decompensated stage of chronic respiratory failure, the diagnosis is not difficult based on the patient’s history of chronic diseases of the respiratory system or other medical conditions causing respiratory dysfunction, clinical manifestations of O2 deficiency and/or CO2 retention, combined with relevant physical signs. Arterial blood gas analysis can objectively reflect the nature and degree of respiratory failure, and is of great value in guiding oxygen therapy, regulation of various parameters of mechanical ventilation, and correction of acid-base balance and electrolytes. Arterial partial pressure of oxygen (PaO2) refers to the pressure generated by the oxygen molecules physically dissolved in the blood. PaO2 in healthy people decreases gradually with age and is affected by physiological effects such as body position. According to the relationship between partial pressure of oxygen and oxygen saturation, the oxygenated hemoglobin dissociation curve has an S shape, when PaO2>8kPa(60mmHg) or above, the curve is in the flat segment, oxygen saturation is above 90%, PaO2 changes 5.3kPa(40mmHg), and oxygen saturation changes very little, indicating that partial pressure of oxygen is far more sensitive than oxygen saturation; but when PaO2<8kPa or below, the curve at the steep and straight section, oxygen partial pressure decreases slightly and oxygen saturation drops sharply, so PaO2 less than 8kPa (60mmHg) is used as a diagnostic index of respiratory failure. 2. Arterial oxygen saturation (SaO2) is the percentage of oxygen per unit of hemoglobin, and the normal value is 97%. When PaO2 is lower than 8kPa (60mmHg) and the hemoglobin oxygen dissociation curve is in the steep and straight section, the oxygen saturation reflects the state of hypoxia, so in the rescue of severe respiratory failure, pulse oximetry is used to help evaluate the degree of O2 deficiency and adjust the concentration of inhalation O2 to make the patient's SaO2 reach 90% or more, so as to reduce traumatic arterial blood sampling for blood gas analysis, which plays a positive role in rational oxygen therapy and assessment of the therapeutic effect. This has a positive effect on the rational oxygen therapy and assessment of the efficacy.