Respiratory failure is caused by cardiopulmonary diseases leading to pulmonary ventilation and/or gas exchange dysfunction, manifested by hypoxemia (and) or hypercapnia, and is diagnosed both by an arterial oxygen partial pressure of less than 60 mmHg on blood gas analysis, or by an accompanying carbon dioxide partial pressure of more than 50 mmHg. After diagnosing respiratory failure, it is also necessary to further classify respiratory failure into type I respiratory failure and type II respiratory failure according to the results of blood gas analysis. Type I respiratory failure is hypoxic expiratory failure, blood gas analysis suggests that the partial pressure of oxygen is less than 60mmHg, the partial pressure of carbon dioxide is less than 50mmHg. Type II respiratory failure is hypercapnic expiratory failure, blood gas analysis shows that the partial pressure of carbon dioxide is more than 50mmHg, with or without partial pressure of oxygen is less than 60mmHg, so the blood gas analysis is of great significance in the diagnosis of respiratory failure, classification, guidance of the treatment and determine the prognosis.