Deficiency in the amount of oxygen reaching body tissues.
hypoxic hy·pox'ic adj.
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Neurological Disorder:
Hypoxia
Definition
Hypoxia generally refers to a lack of oxygen in any part of the body. In a neurological context, it refers to a reduction of oxygen to the brain despite adequate amounts of blood.
Description
A decrease in oxygen supply to the brain can occur due to choking, strangling, suffocation, head trauma, carbon monoxide poisoning, cardiac arrest, and as a complication of general anesthesia. A failure to deliver oxygen and glucose to the brain causes a cascade of abnormal events. The extent of damage is directly proportional to the severity of the injury. The severity of cerebral ischemia, a low-oxygen state caused by arterial obstruction or lack of blood supply, and the duration of blood-flow loss in the brain determine the extent of brain damage. The neurons can suffer temporary dysfunction, or there may be irreversible damage to nerve cells that are sensitive to minute changes in oxygen levels. Severe damage involving extensive areas can occur (cerebral infarction). Cerebral hypoxia/ischemia can be caused by a broad spectrum of diseases that affect the cardiovascular pumping system or the respiratory system. There are four types of disorders to consider: focal cerebral ischemia, global cerebral ischemia, diffuse cerebral hypoxia, and cerebral infarction.
Focal cerebral ischemia
Focal cerebral ischemia (FCI) is often results from a blood clot in the brain. The blood flow in the affected area is reduced. The reduction could be severe or mild but usually FCI causes irreversible injury to sensitive neurons. The clinical signs and symptoms last approximately 15–30 minutes.
Global cerebral ischemia
Global cerebral ischemia (GCI) is a serious condition caused by ventricular fibrillation or cardiac asystole, which stops all blood flow to the brain. If the GCI lasts more than five to ten minutes, then it is likely the person will have suffered a loss of consciousness that makes recovery doubtful.
Diffuse cerebral hypoxia
Diffuse cerebral hypoxia (DCH) is limited to conditions that cause mild to moderate hypoxemia, or low arterial-oxygen content due to deficient blood oxygenation. Pure cerebral hypoxia causes cerebral dysfunction but not irreversible brain damage. Pure cerebral hypoxia can occur due to pulmonary disease, altitude sickness, or severe anemia.
Cerebral infarction
Cerebral infarction (CI) is a severe condition caused by a focal vascular occlusion in an area of the brain. This causes an area of destruction resulting from a lack of oxygen delivery.
Pathology of cerebral ischemia
Lack of oxygen causes neurons in the brain to die in several ways. Autolysis can occur, which results from the digestion of nerve tissues by enzymes. Cerebral infarction causes the death of neurons; transient cessation of the cerebral circulation for a few minutes causes selective areas of ischemic necrosis. This type of necrosis is especially evident in highly vulnerable neurons that are sensitive to abrupt oxygen deprivation. More prolonged periods of moderate-to-severe hypoxemia or carbon monoxide poisoning can cause a loss of the outer sheath of neurons.
Molecular mechanisms of cerebral hypoxia
In cases of severe ischemia to brain tissue, the tissue loses structural integrity within a few seconds or a few minutes. Soon after there is an abnormal exchange of ions in neurons through a process called depolarization; this is characterized by an influx of sodium and calcium ions inside the neuron, and a simultaneous efflux of potassium ions outside the neuron.
Cerebral edema refers to abnormal increases in water content in the brain and occurs with all types of cerebral ischemia and hemorrhagic
Symptoms
Symptoms vary depending on the severity of damage. Symptoms of mild cerebral hypoxia can include poor judgment, memory loss, inattentiveness, and a decrease in motor coordination. In more severe cases, there can be permanent neurologic deficits, coma,
Treatment
Treatment depends on the cause and availability of equipment. Treatment is urgent and includes basic and advanced life-support measures. It is important to maintain breathing, dispense intravenous fluids and medications, and maintain stability with blood products and medications that control blood pressure and seizures. The outlook depends on the extent of cerebral ischemia.
Resources
BOOKS
Goldman, Lee, et al. Cecil's Textbook of Medicine, 21st ed. Philadelphia: W. B. Saunders Company, 2000.
ORGANIZATIONS
Brain Injury Association. 8201 Greensboro Drive, Suite 611, McLean, VA 22102. (703) 761-0750 or (800) 444-6443; Fax: (703) 761-0755. FamilyHelpline@biausa.org. http://www.biausa.org.
National Rehabilitation Information Center (NARIC). 4200 Forbes Boulevard, Suite 202, Lanham, MD 20706-4829. (301) 562-2400 or (800) 346-2742; Fax: (301) 562-2401. naricinfo@heitechservices.com. http://www.naric.com.
Laith Farid Gulli, MD
Robert Ramirez, DO
Sci-Tech Encyclopedia:
Hypoxia
The failure of oxygen to gain access to, or to be utilized by, the body. Although the term anoxia is commonly used, a more precise term, hypoxia, is more often applicable because there is seldom a complete oxygen defect.
Oxygen deprivation may result from interference with some stage of the inspiration, lung diffusion, blood transport, cellular absorption, and final utilization by enzyme systems. A defect at any one or more of these major stages quite often induces a decreased ability of other related mechanisms to survive. This is seen most dramatically in any form of hypoxia in which the brain is deprived of the necessary oxygen for more than a few minutes. Nerve cell degeneration begins quickly, and although the original cause of hypoxia is removed, damage to the respiratory centers prevents resumption of breathing. See also Respiratory system.
The term anoxia is used by many authorities to indicate an oxygen deficiency at the tissue level, and failure of cellular respiration may be designated histotoxic anoxia. There are other terms employed to differentiate the type of oxygen deficiency or the stage in the total respiratory process where defects occur.
World of the Body:
hypoxia
Hypoxia means a shortage of oxygen — as compared to anoxia, which means a total lack of it. In common with other mammals, humans have evolved with a system of breathing and blood circulation, which allows intake of oxygen from the air and its transport throughout the body. The tissues need to extract oxygen from the blood constantly at a basic rate for their metabolism, along with the extra needed for work and exercise, and also are accustomed to a certain level of oxygen in their immediate environment. The body can compensate to some extent for a decreased level, but life depends on maintainence of the supply of oxygen. Different organs and tissues can survive lack of oxygen for different lengths of time: the brain is the most rapidly and irrevocably damaged. Because the brain regulates breathing and the circulation — the means by which oxygen is supplied to the whole body, including itself — deprivation of the brain prevents restoration of the supply; a potentially lethal vicious circle.
Hypoxia occurs (i) when there is less than the normal amount of oxygen in the air inhaled; (ii) when breathing is obstructed, is inadequate, or stops; (iii) when oxygen is not transferred normally from the lungs to the blood; (iv) when the blood cannot carry its normal quota of oxygen; (v) when the flow of blood is inadequate, or stops.
The air inhaled may provide insufficient oxygen either because the atmospheric pressure is low (at high altitude) or when the supply of fresh air is restricted. At high altitude the air is ‘thinner’ in that every molecule of the gas occupies a larger volume. The blood leaves the lungs carrying less oxygen than normal, therefore the tissues are exposed to a lower oxygen level. If this is not too profound, they can still obtain oxygen at the required rate, at least for resting metabolism, because the rate of flow of blood can increase. The tissues are living at a lower level but are still getting a sufficient oxygen supply.
When the supply of fresh air is restricted — with a bag over the head, in a closed cupboard, or in a larger enclosed space crowded with people — oxygen is progressively depleted and exhaled carbon dioxide accumulates. In some circumstances there may be displacement of air by other gases, and the effect of irritant or toxic gases, such as smoke, chlorine, or sulphur dioxide, can complicate the effects of displacement of oxygen.
Disturbance of breathing Obstruction to breathing can occur either externally (smothering, strangulation, compression of the chest in a crowd disaster) or internally (choking, allergic swelling of the upper airways, asthmatic attacks). In other less drastic ways breathing can become inadequate to keep the oxygen level up to normal, and carbon dioxide down to normal, in the lungs and blood: when breathing becomes mechanically difficult in some types of lung disease; when there is damage to the brain stem or to the upper spinal cord where the nerves arise which activate the muscles of breathing; or when the muscles themselves are weak. Breathing may be depressed by drugs acting on the control centres in the brain, and it may stop entirely in collapse from various causes (concussion, near-drowning, heart attack, electric shock).
If obstruction or cessation of breathing is total, the condition is known as asphyxia: it rapidly causes death by depriving the brain of oxygen. A lesser degree of inadequate breathing is called hypoventilation, characterized by a lowered level of oxygen and a raised level of carbon dioxide in the lungs, blood, and tissues; a person suffering from chronic lung disease, for example, can live for many years in a state of moderate hypoxia. The term suffocation is less precisely defined, but is commonly applied either to obstructed breathing or to lack of fresh air supply.
Oxygen transfer from the lungs to the blood can be impaired in some types of lung disease because the barrier it has to diffuse across is thickened. So despite breathing as much or more than normal, the blood and tissue oxygen level is below normal, although, again, an adequate supply may be maintained by increased blood flow. There are also conditions (including some congenital heart defects) in which the blood is not routed properly through the lungs, so that some blood bypasses the oxygen supply, with the result that arterial blood is hypoxic.
In all the types of hypoxia described so far, the haemoglobin in the arterial blood is less than fully saturated with oxygen. The redness of blood depends on this saturation. In hypoxia it becomes more blue, and cyanosis is the outward and visible sign of this when blueness tinges the skin.
The oxygen-carrying capacity of the blood is lowered when red blood cells, and haemoglobin, are in short supply (anaemia): the blood carries less oxygen than normal, although there is sufficient oxygen in the air and in the lungs, and all available haemoglobin is fully saturated. There are also conditions in which the haemoglobin is not all in its normal form. Carbon monoxide poisoning acts by combining with haemoglobin, making it unable to carry oxygen.
Deprivation of blood flow makes organs and tissues hypoxic: the state of ‘ischaemia’. This can occur either as part of whole-body deprivation in heart failure, or locally where blood vessels are obstructed by arterial disease or by clots, or constricted as in the skin in cold exposure.
Defences against hypoxia
The body has ways to defend itself against hypoxia at each stage of the process of oxygen acquisition: by breathing harder, to get more into the lungs; by crowding more red cells into the blood so that it can carry more in every circulating millilitre; by pumping the blood around at a greater rate; and by widening the blood vessels which supply the vital organs. Most of these adjustments can be made very rapidly.
When oxygen is low — but tolerably so — in inhaled air, and hence in the blood, the arterial chemoreceptors — minute structures in the neck — sense this and, via the brain, cause a reflex increase in breathing. This brings the oxygen concentration in the lungs closer to that of the outside air — it remains low, but not as low as it would be if the breathing did not increase. Stimulation of breathing occurs more dramatically when carbon dioxide is accumulating in the blood whilst oxygen is decreasing, such as in the example of breathing in a confined space.
If hypoxia of a tolerable degree is sustained for weeks, the bone marrow produces extra red blood cells, resulting in polycythaemia. The greater density of red cells brings the oxygen concentration in the blood back towards normal despite their haemoglobin carrying less than it ideally could. The down side is that the thicker blood gives extra work to the pumping heart. This defence mechanism cannot of course operate against anaemia, when the fault itself lies in a deficient production of red blood cells.
The heart compensates for hypoxia by pumping out more blood per minute so that the actual delivery rate of oxygen to the tissues can be kept up despite its lower concentration in the blood.
These automatic attempts at self-preservation operate unless the oxygen lack becomes too profound to sustain brain functions, including that of maintaining breathing itself. At worst, the heart weakens, the blood pressure falls, breathing stops, and cessation of the heartbeat soon follows.
— Sheila Jennett
See also altitude; breathing; cyanosis; lungs; oxygen; suffocation.
Food and Fitness:
hypoxia
An inadequate supply of oxygen to the tissues.
Britannica Concise Encyclopedia:
hypoxia
For more information on hypoxia, visit Britannica.com.
Sports Science and Medicine:
hypoxia
A condition in which there is an inadequate supply of oxygen to respiring tissues.
Veterinary Dictionary:
hypoxic
A state of hypoxia.
- h. cell sensitizers — compounds that selectively sensitize hypoxic tumor cells to the effects of radiation.
- h. vasoconstriction — reduced oxygen supply to tissues causes local vasoconstriction and diversion of the blood to other tissues.
- For other uses of the term "hypoxia", see hypoxia.
Hypoxia is a pathological condition in which the body as a whole (generalised 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 distinguished from apoxemia. Apoxemia is an abnormally low partial pressure of oxygen (PO2) in arterial blood [1]. A frequent error is to use the term hypoxemia to mean low oxygen content in arterial blood. It is possible to have a low oxygen content (e.g. due to anemia) but a high PO2 also, and incorrect use can lead to confusion.
Generalised hypoxia occurs in healthy people when they ascend to high altitude, where it causes altitude sickness, and the potentially fatal complications of altitude sickness, high altitude pulmonary edema (HAPE) and high altitude cerebral edema (HACE). Hypoxia also occurs in healthy individuals when breathing mixtures of gases with a low oxygen content, for example while diving underwater, especially with closed-circuit rebreather systems that control the amount of oxygen in the air breathed in. Altitude training uses mild hypoxia to increase the concentration of red blood cells in the body for increased athletic performance.
Symptoms
Symptoms of generalized hypoxia depend on its severity and speed of onset. In the case of altitude sickness, where hypoxia develops gradually, the symptoms include headaches, fatigue, shortness of breath, a feeling of euphoria and nausea. In severe hypoxia, or hypoxia of very rapid onset, changes in levels of consciousness, seizures, coma and death occur. Severe hypoxia induces a blue discolouration of the skin, called cyanosis (haemoglobin is a darker red when it is not bound to oxygen (deoxyhaemoglobin), as opposed to the rich red colour that it has when bound to oxygen (oxyhaemoglobin), and 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 be 'cherry red' instead of cyanotic.
Types of hypoxia
- Hypoxic hypoxia is a generalized hypoxia, an inadequate supply of oxygen to
the body as a whole. The term "hypoxic hypoxia" refers to the fact that hypoxia occurs as a consequence of low partial pressure of oxygen in arterial blood, in contrast to the other causes of hypoxia that follow,
in which the partial pressure of oxygen in arterial blood is normal. Hypoxic hypoxia may be due to:
- Low partial pressure of atmospheric oxygen such as found at high altitude [2] or by replacement of oxygen in the breathing mix either accidentally as in the modified atmosphere of a sewer or intentionally as in the recreational use of nitrous oxide.
- Either Sleep apnea or Hypopnea causing a decrease in oxygen saturation of the blood.
- 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). Normal anatomical shunt occurs in everyone, because of the Thebesian vessels which empty into the left ventricle and the bronchial circulation which supplies the bronchi with oxygen.
- Anemic hypoxia in which arterial oxygen pressure is normal, but total oxygen content of the blood is reduced.[3]
- Hypemic hypoxia when there is an inability of the blood to deliver oxygen to
target tissues.
- Carbon monoxide poisoning which inhibits the ability of haemoglobin to release the oxygen bound to it.
- Methaemoglobinaemia in which an abnormal version of haemoglobin accumulates in the blood
- Histotoxic hypoxia in which quantity of oxygen reaching the cells is normal, but the cells are unable to effectively use the oxygen due to disabled oxidative phosphorylation enzymes.
- Ischemic, or stagnant hypoxia in which there is a local restriction in the flow of otherwise well-oxygenated blood. The oxygen supplied to the region of the body is then insufficient for its needs. Examples are cerebral ischemia, ischemic heart disease and Intrauterine hypoxia, which is an unchallenged cause of perinatal death.
Pathophysiology
After mixing with water vapour and expired CO2 in the lungs, oxygen diffuses down a pressure gradient to enter arterial blood around where its partial pressure is 100mmHg (13.3kPa).[2] 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 bacteria-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 (hypoxia), 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.[4] If severe or prolonged it could lead to cell death.
Vasoconstriction and vasodilation
In most tissues of the body, the response to hypoxia is vasodilation. By widening the blood vessels, the tissue allows greater perfusion.
By contrast, in the lungs, the response to hypoxia is vasoconstriction. This is known as "Hypoxic pulmonary vasoconstriction", or "HPV".
Treatment
To counter the effects of high-altitude diseases, the body must return arterial P02 toward normal. Acclimatization, the means by which the body adapts to higher altitudes, only partially restores P02 to standard levels. Hyperventilation, the body’s most common response to high-altitude conditions, increases alveolar P02 by raising the depth and rate of breathing. However, while P02 does improve with hyperventilation, it does not return to normal. Studies of miners and astronomers working at 3000 meters and above show improved aveolar P02 with full acclimatization, yet the P02 level remains equal to or even below the threshold for continuous oxygen therapy for patients with chronic obstructive pulmonary disease (COPD).[5] In addition, there are complications involved with acclimatization. Polycythemia,in which the body increases the number of red blood cells in circulation, thickens the blood, raising the danger that the heart can’t pump it.
In high-altitude conditions, only oxygen enrichment can counteract the effects of hypoxia. By increasing the concentration of oxygen in the air, the effects of lower barometric pressure are countered and the level of arterial P02 is restored toward normal capacity. A small amount of supplemental oxygen reduces the equivalent altitude in climate-controlled rooms. At 4000 m, raising the oxygen concentration level by 5 percent via an oxygen concentrator and an existing ventilation system provides an altitude of 3000 m, which is much more tolerable for the increasing number of low-landers who work in high altitude.[6] In a study of astronomers working in Chile at 5050 m, oxygen concentrators increased the level of oxygen concentration by 6 percent (that is, from 21 percent to 27 percent). The result was increased worker productivity, less fatigue, and improved sleep.[7]
Oxygen concentrators are uniquely suited for this purpose. They require little maintenance and electricity, provide a constant source of oxygen, and eliminate the expensive, and often dangerous, task of transporting oxygen cylinders to high, remote areas. Offices and housing already have climate-controlled rooms, in which temperature and humidity are at a constant level of comfortableness. Oxygen can be added to this system easily and relatively cheaply. Advances by Pacific Consolidated Industries LLC using vacuum swing adsorption technology has translated into smaller, mobile or portable oxygen concentrators that are more energy- and cost-efficient for the market.[1]
See also
- Asphyxia
- Cerebral hypoxia
- Drowning
- Hypoxic tumor
- Hyperoxia
- Time of Useful Consciousness
- Shallow water blackout for special case of hypoxia via self-induced hypocapnia
- Deep water blackout for special case of latent hypoxia
- Altitude training for beneficial use of mild hypoxia
- 1999 South Dakota Learjet crash for notable instance of a hypoxia-caused aircraft crash
- Helios Flight 522 crashed when the pilots passed out due to gradual decompression.
- Oxygen Concentrators
Footnotes
- ^ West J. "Pulmonary Pathophysiology: The Essentials" 1977 Williams & Wilkins p22
- ^ a b Kenneth Baillie and Alistair Simpson. Altitude oxygen calculator. Apex (Altitude Physiology Expeditions). Retrieved on 2006-08-10. - Online interactive oxygen delivery calculator
- ^ Kenneth Baillie and Alistair Simpson. Oxygen content calculator. Apex (Altitude Physiology Expeditions). Retrieved on 2006-08-10. - A demonstration of the effect of anaemia on oxygen content
- ^ Hobler KE, Carey LC (1973). "Effect of acute progressive hypoxemia on cardiac output and plasma excess lactate" (scanned copy). Ann Surg 177 (2): 199-202. PMID 4572785.
- ^ West, John B. (2004), "The Physiologic Basis of High-Altitude Diseases", Annals of Internal Medicine 141(10): 791.
- ^ West, John B. (1995), "Oxygen Enrichment of Room Air to Relieve the Hypoxia of High Altitude", Respiration Physiology 99(2):230.
- ^ West, "The Physiologic Basis of High-Altitude Diseases", 793.
Bibliography
- Hypoxia - An invisible enemy, Fast, Airbus technical magazine, #38 : presentation for non specialists of hypoxia and related safety procedures in civil airplanes
| Consequences of external causes (T15-T35, T66-T98, 930-959, 990-995) | |
|---|---|
| General external causes | Foreign body - Burn - Frostbite |
| Other external causes | Radiation poisoning - Hyperthermia - Hypothermia - Immersion foot - Chilblain - Aerosinusitis - Hypoxia -
Barotrauma - Altitude sickness - Chronic mountain sickness - Decompression
sickness - Asphyxia - Starvation -
maltreatment (Physical abuse, Sexual abuse,
Psychological abuse) - Motion sickness
(Airsickness, Sea-sickness) - |
| Certain early complications of trauma | Air embolism - Fat embolism - Crush syndrome/Rhabdomyolysis - Compartment syndrome - Volkmann's contracture |
| Complications of surgical and medical care | Serum sickness - Malignant hyperthermia |
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Neurological Disorder. Gale Encyclopedia of Neurological Disorders. Copyright © 2005 by The Gale Group, Inc. All rights reserved. Read more |
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| Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved. Read more |
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| World of the Body. The Oxford Companion to the Body. Copyright © 2001, 2003 by Oxford University Press. All rights reserved. Read more |
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| Food and Fitness. Food and Fitness: A Dictionary of Diet and Exercise. Copyright © 1997, 2003 by Oxford University Press. All rights reserved. Read more |
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| Dental Dictionary. Mosby's Dental Dictionary. Copyright © 2004 by Elsevier, Inc. All rights reserved. Read more |
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| Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved. Read more |
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| Sports Science and Medicine. The Oxford Dictionary of Sports Science & Medicine. Copyright © Michael Kent 1998, 2006, 2007. All rights reserved. Read more |
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| Veterinary Dictionary. The Veterinary Dictionary. Copyright © 2007 by Elsevier. All rights reserved. Read more |
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| Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Hypoxia (medical)". Read more |
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