Serum carbon dioxide 20.9 is a low carbon dioxide value, suggesting the possibility of hyperventilation or compensatory metabolic acidosis, requiring close monitoring of blood gas analysis and adjustment of ventilator parameters. Serum carbon dioxide value is expressed in the form of partial pressure of carbon dioxide or carbon dioxide binding capacity. The normal range of partial pressure of carbon dioxide is 35-45 mmhg; the normal range of carbon dioxide binding capacity is 22-31 mmol/L. If serum carbon dioxide value is 20.9, it means low carbon dioxide value. If serum carbon dioxide 20.9 indicates the value of serum partial pressure of carbon dioxide, it is a serious reduction of carbon dioxide, considering the existence of hyperventilation, there is the possibility of compensatory respiratory alkalosis and metabolic acidosis, and it is necessary to further combine with the bicarbonate and the PH value for the adjustment of the ventilator parameters, to reduce the hyperventilation as well as the active treatment of the primary pathology. If serum carbon dioxide 20.9 indicates the value of carbon dioxide binding capacity, it is a mildly low carbon dioxide binding capacity, combined with other blood gas analysis values, if it is in the normal range, it may be a transient hyperventilation, and can be temporarily observed, and the level of blood gas analysis should be closely rechecked.