There are several types of hypoxia (a reduced level of oxygen in the tissue) and the condition can be caused by a number of external and internal factors.

Some of the broad classes of hypoxia include:

Hypoxic hypoxia or generalized hypoxia

This refers to hypoxia resulting from an inadequate saturation of blood oxygen due to a reduced supply of oxygen in the air, decreased lung ventilation or respiratory disease. With this type of hypoxia, the partial pressure of oxygen in the arterial blood (paO2) is lower than normal. Some of the causes of hypoxic hypoxia include:

• A high altitude, where the concentration of atmospheric oxygen is decreased.
• Deep sea diving if there is an inadequate supply of oxygen in the breathing gas or if a rusting cylinder has extracted oxygen, for example.
• The inhalation of nitrous oxide or laughing gas on a repeated basis for recreational purposes can decrease oxygen availability while increasing carbon dioxide levels.
• Sleep apnea or obstructive sleep apnea can interrupt airflow to the lungs.
• Certain diseases such as bronchial asthma, respiratory arrest, chronic obstructive pulmonary disease causing inadequate ventilation of the lungs.

Anemic hypoxia

This refers to when the capacity of the blood to carry oxygen is reduced and inadequate levels of oxygen are therefore circulated around the body.

Examples of causes include anemia and a decreased amount of oxygenated haemoglobin; carbon monoxide poisoning where the receptors that usually carry oxygen are blocked by the carbon monoxide; and the use of certain medications which can alter the receptors present on red blood cells and affect their ability to carry oxygen.

Histotoxic hypoxia

This refers to when oxygen is delivered to the tissues but they fail to utilize it effectively because the cells are damaged and cannot extract and absorb oxygen from circulating blood. This may occur with the overuse of alcohol or drugs and is also seen in cyanide poisoning. Cyanide disrupts cytochrome oxidase, an important enzyme in cell respiration.

Stagnant hypoxia

This arises from a decrease in blood flow preventing adequate blood supply to tissues. Heart attack, heart failure, or cardiac arrest, for example, can slow the circulation of blood meaning inadequate oxygen is delivered to important tissues and organs.