The main diagnostic criteria for type 1 respiratory failure is that the partial pressure of oxygen in the arteries is below 60 mmHg, but the partial pressure of carbon dioxide may be normal or below 50 mmHg, which is called type 1 respiratory failure. Type II respiratory failure is also called type II respiratory failure when the partial pressure of oxygen is below 60 mmHg, but the partial pressure of carbon dioxide is severely below normal, usually below 50 mmHg. Respiratory failure is characterized by hypoxia or carbon dioxide retention, dyspnea, shortness of breath, psychiatric and neurological symptoms, and the development of pulmonary, cardiac, and encephalopathic changes.