Hypoxia (medical)

For other uses of the term "hypoxia", see hypoxia (disambiguation).

Hypoxia is a pathological condition in which the body as a whole (generalized hypoxia) or region of the body (tissue hypoxia) is deprived of adequate oxygen supply. Hypoxia in which there is complete deprivation of oxygen supply is referred to as anoxia.

Hypoxia is often associated with high altitudes, where it is called altitude sickness. Hypoxia can also occur while diving underwater, especially with closed-circuit rebreather systems that control the amount of oxygen in the air breathed in.

Symptoms

Symptoms of generalized hypoxia depend on its severity and speed of onset. They include headaches, fatigue, shortness of breath, nausea, unsteadiness, and sometimes even seizures and coma. Severe hypoxia induces a blue discolouration of the skin (the red blood cells become darker as they lose oxygen, which intensifies the blue appearance of the veins).

Categories and causes of hypoxia

Generalised hypoxia

This may be due to low levels of oxygen in the blood (Hypoxemia) or where tissues throughout the body are unable to utilise the oxygen supplied.

Hypoxic hypoxia when there is an inadequate supply of oxygen. This may be due to:
- Low partial pressure of atmospheric oxygen (e.g. at high altitudes or as may be caused during scuba diving).
- Inadequate pulmonary ventilation (e.g. in chronic obstructive pulmonary disease or respiratory arrest).
- Shunts in the pulmonary circulation or a right-to-left shunt in the heart. Shunts can be caused by collapsed alveoli that are still perfused or a block in ventilation to an area of the lung. Whatever the mechanism, blood meant for the pulmonary system is not ventilated and so no gas exchange occurs (the ventilation/perfusion ratio is zero). Standard existing shunts include the thebesian vessels which empty into the left ventricle and the bronchial circulation which supply the bronchi with oxygen.
Carbon monoxide poisoning which inhibits hemoglobin's ability to bind oxygen.
Anemic hypoxia in which arterial oxygen pressure is normal, but total oxygen content of the blood is reduced. This may be due to:
- Reduced hemoglobin content in erythrocytes. Since hemoglobin carries oxygen and carbon dioxide (and/or carbon monoxide), the quantity (volume) of oxygen carried is affected by how much hemoglobin is present in the red blood cells. For example iron deficiency anemia lowers hemoglobin levels in red blood cells and therefore hinders their carrying capacity.
- Decreased hematocrit e.g. from blood loss (blood loss anemia).
Hypemic Hypoxia when there is an inability of the blood to carry oxygen.
Histotoxic hypoxia in which quantity of oxygen reaching the cells is normal, but the cells are unable to effectively use the oxygen.

Localised tissue hypoxia

Ischemic, or stagnant hypoxia in which there is a local restriction in the flow of otherwise well-oxygeneated blood. The oxygen supplied to the region of the body is then insufficient for its needs. Examples are cerebral ischemia to the brain and ischemic heart disease.
Cerebral hypoxia in which the brain is deprived of oxygen despite normal blood flow.
Intrauterine hypoxia, which is an unchallenged cause of perinatal death.

Pathophysiology

The measurement of oxygen supply involves calculating its Partial pressure by multiplying atmospheric pressure (e.g. 760 mmHg minus the 47mmHg of water vapor) by the gas's fraction in air (e.g., 713mmHg x 21% = 150mmHg). After mixing with expired CO2 in the lungs, oxygen diffuses down a pressure gradient to enter arterial blood around where its partial pressure is 100mmHg. Arterial blood flow delivers oxygen to the peripheral tissues, where it again diffuses down a pressure gradient into the cells and into their mitochondria. These bacterial like cytoplasmic structures strip hydrogen from fuels (glucose, fats and some amino acids) to burn with oxygen to form water. Released energy (originally from the sun and photosynthesis) is stored as ATP, to be later used for energy requiring metabolism. The fuel's carbon is oxidized to CO₂, which diffuses down its partial pressure gradient out of the cells into venous blood to finally be exhaled by the lungs. Experimentally, oxygen diffusion becomes rate limiting (and lethal) when arterial oxygen partial pressure falls to 40mmHg or below.

If oxygen delivery to cells is insufficient for the demand (hyoxia), hydrogen will be shifted to pyruvic acid converting it to lactic acid. This temporary measure (anaerobic metabolism) allows small amounts of energy to be produced. Lactic acid build up in tissues and blood is a sign of inadequate mitochondrial oxygenation, which may be due to hypoxemia, poor blood flow (e.g. shock) or a combination of both. If severe or prolonged it could lead to cell death.

References

Pulmonology | Diving medicine | Aviation medicine