A failure in the respiratory system is any process that limits the lung’s ability to exchange gas, oxygenating the body and/or eliminating carbon dioxide. These failures are often classified as either hypoxaemia or hypercapnoea. They are dysfunctions of one or more essential components of the respiratory system, including:

• Chest wall and diaphragm
• Airways
• Alveolar-capillary exchange
• Pulmonary circulation
• Nerves, central nervous system, brain stem

When interpreting a patient’s blood gas result, it is useful to understand the patient’s underlying pathology. If a patient has a disease that affects their alveolar-capillary membrane, for example pulmonary fibrosis, the ability for gases to move from the lung into the blood are going to be impaired, and therefore the blood oxygen levels are going to be reduced. Similarly, if there is poor blood perfusion of the lungs, for example in pulmonary hypertension, the movement of gases is going to be impaired. Another factor is the number of red blood cells in the body. Oxygen is predominantly bound to haemoglobin, so if the number of red blood cells is reduced, for example in anaemia, or the oxygen carrying capacity of red blood cells is reduced, for example in sickle cell anaemia, even blood saturated with oxygen will not be able to carry a large volume of the gas.

Type I respiratory failure, commonly called hypoxic respiratory failure, is characterised by a low PaO₂, with a normal or abnormally low PaCO₂. Type II respiratory failure, commonly called hypercapnic respiratory failure, is characterised by a low PaO₂, with an abnormally high PaCO₂.