Hypoxia (medicine)

Hypoxia is a condition in which the body or a region of the body is deprived of an adequate oxygen supply at the tissue level. Hypoxia may be classified as either generalized, affecting the whole body, or local, affecting a region of the body. Although hypoxia is often a pathological condition, variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise.
Hypoxia differs from hypoxemia and anoxemia, in that hypoxia refers to a state in which oxygen present in a tissue or the whole body is insufficient, whereas hypoxemia and anoxemia refer specifically to states that have low or no oxygen in the blood. Hypoxia in which there is complete absence of oxygen supply is referred to as anoxia.
Hypoxia can be due to external causes, when the breathing gas is hypoxic, or internal causes, such as reduced effectiveness of gas transfer in the lungs, reduced capacity of the blood to carry oxygen, compromised general or local perfusion, or inability of the affected tissues to extract oxygen from, or metabolically process, an adequate supply of oxygen from an adequately oxygenated blood supply.
Generalized hypoxia occurs in healthy people when they ascend to high altitude, where it causes altitude sickness leading to potentially fatal complications: high-altitude pulmonary edema and high-altitude cerebral edema. Hypoxia also occurs in healthy individuals when breathing inappropriate mixtures of gases with a low oxygen content, e.g., while diving underwater, especially when using malfunctioning closed-circuit rebreather systems that control the amount of oxygen in the supplied air. Mild, non-damaging intermittent hypoxia is used intentionally during altitude training to develop an athletic performance adaptation at both the systemic and cellular level.
Hypoxia is a common complication of preterm birth in newborn infants. Because the lungs develop late in pregnancy, premature infants frequently possess underdeveloped lungs. To improve blood oxygenation, infants at risk of hypoxia may be placed inside incubators that provide warmth, humidity, and supplemental oxygen. More serious cases are treated with continuous positive airway pressure.

Classification

Hypoxia exists when there is a reduced amount of oxygen in the tissues of the body. Hypoxemia refers to a reduction in arterial oxygenation below the normal range, regardless of whether gas exchange is impaired in the lung, arterial oxygen content is adequate, or tissue hypoxia exists. The classification categories are not always mutually exclusive, and hypoxia can be a consequence of a wide variety of causes.

By cause

Hypoxic hypoxia, also referred to as generalised hypoxia, may be caused by:
*Hypoventilation, which is insufficient ventilation of the lungs due to any cause.
*Low-inspired oxygen partial pressure, which may be caused by breathing normal air at low ambient pressures due to altitude, by breathing hypoxic breathing gas at an unsuitable depth, by breathing inadequately re-oxygenated recycled breathing gas from a rebreather, life-support system, or anesthetic machine.
*Hypoxia of ascent in freediving and rebreather diving.
*Airway obstruction, choking, drowning.
**Chronic obstructive pulmonary disease
**Neuromuscular diseases or interstitial lung disease
*Malformed vascular system such as an anomalous coronary artery.
Hypoxemic hypoxia is a lack of oxygen caused by low oxygen tension in the arterial blood, due to the inability of the lungs to sufficiently oxygenate the blood. Causes include hypoventilation, impaired alveolar diffusion, and pulmonary shunting. This definition overlaps considerably with that of hypoxic hypoxia.
is hypoxia from hypoxemia due to abnormal pulmonary function, and occurs when the lungs receive adequately oxygenated gas which does not oxygenate the blood sufficiently. It may be caused by:
*Ventilation–perfusion mismatch, which can be either low or high. A reduced V/Q ratio can be caused by impaired ventilation, which may be a consequence of conditions such as bronchitis, obstructive airway disease, mucus plugs, or pulmonary edema, which limit or obstruct the ventilation. In this situation there is not enough oxygen in the alveolar gas to fully oxygenate the blood volume passing through, and P_{a_O₂} will be low. Conversely, an increased V/Q ratio tends to be a consequence of impaired perfusion, in which circumstances the blood supply is insufficient to carry the available oxygen, P_{a_O₂} will be normal, but tissues will be insufficiently perfused to meet the oxygen demand. A V/Q mismatch can also occur when the surface area available for gas exchange in the lungs is decreased.
*Pulmonary shunt, in which blood passes from the right to the left side of the heart without being oxygenated. This may be due to anatomical shunts, in which the blood bypasses the alveoli, via intracardiac shunts, pulmonary arteriovenous malformations, fistulas, and hepatopulmonary syndrome, or physiological shunting, in which blood passes through non-ventilated alveoli.
*Impaired diffusion, a reduced capacity for gas molecules to move between the air in the alveoli and the blood, which occurs when alveolar–capillary membranes thicken. This can happen in interstitial lung diseases such as pulmonary fibrosis, sarcoidosis, hypersensitivity pneumonitis, and connective tissue disorders.
, also known as ischemic hypoxia or stagnant hypoxia, is caused by abnormally low blood flow to the lungs, which can occur during shock, cardiac arrest, severe congestive heart failure, or abdominal compartment syndrome, where the main dysfunction is in the cardiovascular system, causing a major reduction in perfusion. Arterial gas is adequately oygenated in the lungs, and the tissues are able to accept the oxygen available, but the flow rate to the tissues is insufficient. Venous oxygenation is particularly low.
Anemic hypoxia or hypemic hypoxia is the lack of capacity of the blood to carry the normal level of oxygen. It can be caused by anemia or:
*Carbon monoxide poisoning, in which carbon monoxide combines with the hemoglobin, to form carboxyhemoglobin preventing it from transporting oxygen.
*Methemoglobinemia, a change in the hemoglobin molecule from a ferrous ion to a ferric ion, which has a lesser capacity to bind free oxygen molecules, and a greater affinity for bound oxygen. This causes a left shift in the O₂–Hb curve. It can be congenital or caused by medications, food additives or toxins, including chloroquine, benzene, nitrites, benzocaine.
Histotoxic hypoxia or occurs when the cells of the affected tissues are unable to use oxygen provided by normally oxygenated hemoglobin. Examples include cyanide poisoning which inhibits cytochrome c oxidase, an enzyme required for cellular respiration in mitochondria. Methanol poisoning has a similar effect, as the metabolism of methanol produces formic acid which inhibits mitochondrial cytochrome oxidase.

Intermittent hypoxic training induces mild generalized hypoxia for short periods as a training method to improve sporting performance. This is not considered a medical condition. Acute cerebral hypoxia leading to blackout can occur during freediving. This is a consequence of prolonged voluntary apnea underwater, and generally occurs in trained athletes in good health and good physical condition.

By extent

Hypoxia may affect the whole body, or just some parts.

Generalized hypoxia

The term generalized hypoxia may refer to hypoxia affecting the whole body, or may be used as a synonym for hypoxic hypoxia, which occurs when there is insufficient oxygen in the breathing gas to oxygenate the blood to a level that will adequately support normal metabolic processes, and which will inherently affect all perfused tissues.
The symptoms of generalized hypoxia depend on its severity and acceleration of onset. In the case of altitude sickness, where hypoxia develops gradually, the symptoms include fatigue, numbness / tingling of extremities, nausea, and cerebral hypoxia. These symptoms are often difficult to identify, but early detection of symptoms can be critical.
In severe hypoxia, or hypoxia of very rapid onset, ataxia, confusion, disorientation, hallucinations, behavioral change, severe headaches, reduced level of consciousness, papilloedema, breathlessness, pallor, tachycardia, and pulmonary hypertension eventually leading to the late signs cyanosis, slow heart rate, cor pulmonale, and low blood pressure followed by heart failure eventually leading to shock and death.
Because hemoglobin is a darker red when it is not bound to oxygen, as opposed to the rich red color that it has when bound to oxygen, when seen through the skin it has an increased tendency to reflect blue light back to the eye. In cases where the oxygen is displaced by another molecule, such as carbon monoxide, the skin may appear 'cherry red' instead of cyanotic. Hypoxia can cause premature birth, and injure the liver, among other deleterious effects.

Localized hypoxia

Hypoxia that is localized to a region of the body, such as an organ or a limb, is usually the consequence of ischemia, the reduced perfusion to that organ or limb, and may not necessarily be associated with general hypoxemia. A locally reduced perfusion is generally caused by an increased resistance to flow through the blood vessels of the affected area.
Ischemia is a restriction in blood supply to any tissue, muscle group, or organ, causing a shortage of oxygen. Ischemia is generally caused by problems with blood vessels, with resultant damage to or dysfunction of tissue i.e. hypoxia and microvascular dysfunction. It also means local hypoxia in a given part of a body sometimes resulting from vascular occlusion such as vasoconstriction, thrombosis, or embolism. Ischemia comprises not only insufficiency of oxygen, but also reduced availability of nutrients and inadequate removal of metabolic wastes. Ischemia can be a partial or total blockage.
Compartment syndrome is a condition in which increased pressure within one of the body's anatomical compartments results in insufficient blood supply to tissue within that space. There are two main types: acute and chronic. Compartments of the leg or arm are most commonly involved.
If tissue is not being perfused properly, it may feel cold and appear pale; if severe, hypoxia can result in cyanosis, a blue discoloration of the skin. If hypoxia is very severe, a tissue may eventually become gangrenous.