The clinical manifestations of chronic respiratory failure include the original clinical manifestations of the primary disease and the damage to the organs due to hypoxia and carbon dioxide retention. The damage of hypoxia and carbon dioxide retention not only depends on the degree of hypoxia and carbon dioxide retention, but also on the speed and duration of hypoxia and carbon dioxide retention, so when chronic respiratory failure is acutely aggravated, the clinical manifestations are often particularly severe because of the rapid occurrence of hypoxia and carbon dioxide retention. The damage caused by hypoxia and carbon dioxide retention varies, but there is a lot of overlap. For a patient with respiratory failure, the clinical manifestations are often the result of a combination of hypoxia and carbon dioxide retention. Therefore, the clinical manifestations caused by hypoxia and carbon dioxide retention are described together below. 1. is the number of milliliters of oxygen in 100 ml of blood. This includes the sum of hemoglobin-bound oxygen and the physical dissolved oxygen in plasma. CaO2=1.34×SaO2×Hb+0.003×PaO2. The reference value of CaO2 in healthy people is 20ml%. The oxygen saturation of mixed venous blood (SVO2) is 75% and its oxygen content CVO2 is 15 ml%, then there is about 5 ml of oxygen for tissues after every 100 ml of arterial blood through the tissues. Hemoglobin is reduced, SaO2 is lower than normal, and blood oxygen content can still be in the normal range. 2. Arterial blood carbon dioxide partial pressure (PaCO2). It refers to the pressure generated by the physically dissolved CO2 molecules in the blood. Normal PaCO2 is 4.6kPa-6kPa (35-45mmHg), greater than 6kPa is considered hyperventilation, less than 4.6kPa may be hyperventilation. In acute hypoventilation, PaCO2 > 6.6kPa (50 mmHg), the pH is already below 7.20 according to Henderson-Hassellbalch formula, which will affect the circulation and cellular metabolism. In chronic respiratory failure due to the compensatory mechanism of the body, PaCO2>6.65kPa(50mmHg) is used as a diagnostic indicator of respiratory failure. 3. pH is the negative logarithmic value of hydrogen ion concentration in the blood. The normal range is 7.35-7.45, with an average of 7.40. Below 7.35 is decompensated acidosis, above 7.45 is decompensated alkalosis, but it does not indicate what kind of nature of acid-base toxicity. Clinical symptoms are closely related to the pH excursion. 4. Base excess (BE) is the amount of acid-base required to titrate blood to pH 7.4 at 38°C. The partial pressure of CO2 is 5.32 kPa (40 mmHg) and the amount of oxygen saturation is 100%. It is a quantitative indicator of metabolic acid-base imbalance in human body, and a positive value of BE for acid addition is metabolic alkalosis; a negative value of EB for alkalinity addition is metabolic acidosis. The normal range is 0±2.3 mmol/L. It can be used as a reference for estimating the dose of antacid or anti-base drugs when correcting metabolic acid-base imbalance.