CO₂ is usually defined as the partial pressure of arterial blood carbon dioxide (PaCO₂), which is the tension produced by CO₂ molecules dissolved in arterial blood, with a normal value of 35-45 mmHg and a mean value of 40 mmHg. PaCO₂ is an important indicator for determining the type and degree of respiratory failure, the presence or absence of respiratory acid-base balance imbalance and metabolic acid-base imbalance in the compensatory response. When used to identify the type of respiratory failure, type I respiratory failure PaCO₂ can be normal or slightly lower; type II respiratory failure PaCO₂ must be >50mmHg, but it should be noted that only based on the elevation of PaCO₂ can not be diagnosed with respiratory failure, it is necessary to diagnose by a professional physician combined with the clinical manifestations and the relevant examination results. When used to determine whether there is respiratory acid-base balance disorder, PaCO₂＞45mmHg suggests respiratory acidosis; PaCO₂＜35mmHg suggests respiratory alkalosis. Elevated PaCO₂ is mostly caused by insufficient ventilation, such as chronic obstructive pulmonary disease, asthma, respiratory muscle paralysis and other diseases, and should seek medical treatment in time to avoid delay.