When the lung volume is about 30% of the total lung volume, the small airway has a tendency to close, which is due to the small negative pressure in the chest cavity at the bottom of the lung, which can become positive pressure after deep exhalation, so the small airway is pressed closed, and the alveoli at the bottom of the lung will also be compressed. The lung volume at the beginning of small airway closure is called closed air volume. A. Factors affecting the closed air volume 1. Closed air volume is closely related to age, the closed air volume is higher in children, gradually decreases with age, and is at its lowest point at the age of 16 to 19. Airway closure in normal young people occurs only when the lung volume is very low, and the closed air volume is very small, below the functional residual air volume, near the age of 20 to 70 years, and the closed air volume increases with age. In the elderly, due to the reduced elasticity of lung tissue and reduced negative thoracic pressure, airway closure can occur at higher lung volumes, and at 65 years of age, the closed air volume can exceed the functional air volume. 2. Body position. The functional residual air in supine position is 20% less than in upright position, while the closed air volume remains unchanged, so the closed air volume in the supine position can exceed the functional residual air volume at about 45 years of age. 3. Obesity. The respiratory muscle strength decreases, the abdominal pressure rises, the septum rises, the movement is restricted, and the small airway in the lower part of the lung closes early in the expiratory phase. 4.Pregnancy. In the last month of pregnancy, the lower lung airway closes early. 5, smoking. Smoking can cause the small airways to close early. 6, other: pulmonary edema and inflammation can cause swelling of peribronchial and interstitial tissue, chronic bronchitis and asthma are common causes of early airway closure. Any disorders that reduce lung volume, such as pneumoperitoneum, respiratory depression, respiratory muscle paralysis, and shallow breathing after thoracic and abdominal surgery can easily lead to airway closure. Second, the clinical significance of closed air volume Small airway closure at low lung volume has important clinical significance. As a result of early airway closure, in mild cases, the lower lung tissues only expand at inspiratory time intermittently, and airway closure causes gas to be retained in the alveoli, resulting in uneven distribution of gas in the lungs, imbalance of ventilation/blood flow ratio, affecting the gas exchange between alveoli and blood, and decreasing arterial oxygen saturation, which is related to low arterial oxygen saturation in the elderly. Especially when the closed air volume exceeds the sum of functional residual air volume and tidal volume, the airway will be in a closed state throughout the respiratory cycle, resulting in complete loss of alveolar function, and the consequences will be very serious. Long-term retention of gas in the alveoli can be absorbed, causing pulmonary atelectasis, at this time, if the blood flow continues, will produce the results of static arterial mixing. In acute respiratory failure, the total lung volume is reduced due to factors such as intra-alveolar exudate and reduced alveolar elasticity. The compensatory expiration is the volume of air expelled by the expiratory muscles in the late expiration period based on alveolar retraction and continued exertion. In patients with respiratory failure, the expiratory muscle is already involved in the quiet expiration, and there is no longer additional potential to exhale more air volume at the end of the calm expiration, so the compensatory expiration is very small and can be close to zero, and the lung volume is similar to the calm breathing volume (tidal volume). In respiratory failure, the functional residual air volume decreases, in severe cases by half, or even below the expected value of the residual air volume. If the airway is closed early at the same time, the closed air volume can be far above the functional residual air volume, which seriously affects the oxygenation of the lungs. Patients with chronic obstructive pulmonary disease have increased residual air volume, but the increase in closed air volume can exceed the increase in residual air volume when respiratory failure occurs.