Hypoxia (medicine)

Hypoxia is a medical condition where the body or a specific region is deprived of an adequate oxygen supply at the tissue level. Oxygen is essential for the metabolic processes that keep cells alive. It is important to distinguish hypoxia from related terms. Hypoxemia refers specifically to low oxygen levels in the arterial blood. Anoxemia refers to a state with no oxygen in the blood. Anoxia is the term used when there is a complete absence of oxygen supply to the tissues. While hypoxia is often a pathological condition, it can sometimes be a normal part of physiology. For instance, arterial oxygen concentrations can vary during strenuous physical exercise.

Scientists classify hypoxia based on its cause. Hypoxic hypoxia, or generalized hypoxia, occurs when there is insufficient oxygen in the breathing gas. This can happen at high altitudes due to low ambient pressure. It can also happen to divers if they use malfunctioning rebreather systems. Another type is hypoxemic hypoxia. This occurs when the lungs cannot sufficiently oxygenate the blood. This might be caused by a ventilation-perfusion mismatch, known as V/Q mismatch. In this case, the ratio between air reaching the lungs and blood flowing through them is unbalanced. This can result from mucus plugs, bronchitis, or pulmonary edema.

Other forms of hypoxia involve how blood moves or how it carries oxygen. Ischemic hypoxia, also called stagnant hypoxia, happens when blood flow to the tissues is too low. This can be caused by cardiac arrest or severe congestive heart failure. In these cases, the blood has oxygen, but the circulation is too slow to deliver it. Anemic hypoxia occurs when the blood lacks the capacity to carry normal oxygen levels. Carbon monoxide poisoning is a dangerous cause of this. The carbon monoxide binds to hemoglobin to form carboxyhemoglobin, which prevents oxygen transport. Finally, histotoxic hypoxia occurs when cells cannot use the oxygen provided to them. Cyanide poisoning is a known cause, as it inhibits enzymes required for cellular respiration.

Hypoxia can be categorized by its extent in the body. Generalized hypoxia affects the entire system and all perfused tissues. This is common in healthy people ascending to high altitudes. Such elevation can lead to altitude sickness. This condition may cause high-altitude pulmonary edema (HAPE) or high-altitude cerebral edema (HACE). Localized hypoxia affects only a specific organ or limb. This is usually caused by ischemia, which is a restriction in blood supply. Ischemia can be a partial or total blockage. It can result from vascular occlusion, such as a thrombosis or an embolism.

Specific organs are highly sensitive to oxygen deprivation. The brain is especially vulnerable to cerebral hypoxia. This can take several forms, including diffuse cerebral hypoxia and focal cerebral ischemia. If oxygen deprivation is prolonged, it can lead to neuronal cell death through apoptosis. This results in a hypoxic brain injury. In newborns, a condition called hypoxic ischemic encephalopathy (HIE) may occur. This happens when the entire brain is deprived of adequate oxygen, often due to birth asphyxia. Because lungs develop late in pregnancy, preterm infants are at a higher risk. Doctors may use incubators with supplemental oxygen or continuous positive airway pressure (CPAP) to help.

Physical signs of hypoxia can be quite distinct. A common sign is cyanosis, where the skin takes on a blue discoloration. This happens because deoxyhemoglobin, or blood without oxygen, reflects blue light through the skin. However, if oxygen is displaced by carbon monoxide, the skin might appear cherry red instead. In cases of localized ischemia, the affected area may feel cold or appear pale.

If the hypoxia is severe enough, the tissue may eventually become gangrenous. The severity of symptoms in generalized hypoxia can range from fatigue and nausea to confusion, hallucinations, and even death.

Understanding hypoxia is vital for medicine and athletics. While it is often a dangerous medical complication, mild intermittent hypoxia is used intentionally. Athletes use altitude training to develop adaptations at both the systemic and cellular levels. This is considered a method to improve performance rather than a medical condition. However, in extreme environments like freediving, acute cerebral hypoxia can lead to sudden blackouts. This is a serious risk for even highly trained athletes. By studying these different mechanisms, medical professionals can better treat the various causes of oxygen deprivation.