The most common cause of elevated blood carbon dioxide is pulmonary ventilation dysfunction, mainly seen in chronic obstructive pulmonary disease. Because human exhalation is a passive process, coupled with the lack of airway cartilage and smooth muscle in the small airways, the small airways will be prematurely trapped and closed at the end of exhalation, resulting in the inability to expel gas from the alveoli in a timely manner, which will cause carbon dioxide retention over time. Mild elevation of carbon dioxide will stimulate the respiratory center, causing respiration to deepen and accelerate, and promote carbon dioxide expulsion. When the partial pressure of carbon dioxide exceeds 80 mmHg, respiratory depression may occur, and the patient may show drowsiness and coma, which is clinically known as pulmonary encephalopathy and carbon dioxide anesthesia.