A purple and black mouth is a sign of cyanosis, which represents a lack of oxygen to the blood in the body, and an arterial blood gas analysis can be done to clarify the diagnosis. If the arterial blood gas analysis indicates that the partial pressure of oxygen in arterial blood is below 60 mmHg or the blood pressure saturation is below 90%, type I respiratory failure can be diagnosed. In respiratory failure, the patient may have purple and black lips and end extremities, which represent the lack of oxygen in the body and need to give oxygen therapy. According to the type of respiratory failure, if it is type I respiratory failure, it is simple hypoxia and high flow oxygen can be given. In case of type II respiratory failure, hypoxia is accompanied by carbon dioxide retention and low-flow oxygenation is required.