PaCO₂ is the medical term for the partial pressure of arterial blood carbon dioxide, referring to the tension produced by physically dissolved carbon dioxide. The reference value of PaCO₂ at altitude is 35~45mmHg, which is an important indicator of alveolar ventilation, reflecting the respiratory factor in acid-base balance. 1. Low partial pressure of carbon dioxide: common in respiratory alkalosis, due to hyperventilation, carbon dioxide excretion increases, resulting in a lower than normal partial pressure of carbon dioxide, which belongs to respiratory alkalosis. 2. High partial pressure of carbon dioxide: Commonly found in respiratory acidosis, due to insufficient ventilation, carbon dioxide is discharged too little, and is retained in the body, resulting in a higher than normal partial pressure of carbon dioxide, which belongs to respiratory acidosis. When the body has abnormal PaCO2, it should be examined in time to clarify the cause of the disease, and then treated according to the doctor’s instructions.