hypoxia

American Heritage Dictionary:

hy·pox·i·a

(hī-pŏk'sē-ə, hĭ-) pronunciation

n.
Deficiency in the amount of oxygen reaching body tissues.

hypoxic hy·pox'ic adj.

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hypoxia

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Condition in which tissues are starved of oxygen. The extreme is anoxia (absence of oxygen). There are four types: hypoxemic, from low blood oxygen content (e.g., in altitude sickness); anemic, from low blood oxygen-carrying capacity (e.g., in carbon monoxide poisoning); distributive, from low blood flow (e.g., generally in shock or locally in atherosclerosis); and histotoxic, from poisoning (e.g., with cyanide) that keeps cells from using oxygen. If not reversed quickly, hypoxia can lead to necrosis (tissue death), as in heart attack.

For more information on hypoxia, visit Britannica.com.

McGraw-Hill Science & Technology Encyclopedia:

Hypoxia

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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.

Oxford Food & Fitness Dictionary:

hypoxia

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An inadequate supply of oxygen to the tissues.

Oxford Companion to the Body:

hypoxia

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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

Oxford Dictionary of Sports Science & Medicine:

hypoxia

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A condition in which there is an inadequate supply of oxygen to respiring tissues.

McGraw-Hill Dictionary of Aviation:

hypoxia

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A state of oxygen deficiency in the body sufficient to cause an impairment of body function. It is caused by the reduction in partial pressure of oxygen, inadequate oxygen transport, or the inability of the tissue to use oxygen. In flying, hypoxia is mostly caused by a lower partial pressure of oxygen.

Oxford Dictionary of Biochemistry:

hypoxia

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the presence of less than normal amounts of dioxygen in a vertebrate or in its blood. See ARNT, HIF. Compare hyperoxia.
—hypoxic adj.

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Saunders Veterinary Dictionary:

hypoxic

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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.

Mosby's Dental Dictionary:

hypoxia

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(hī-pok'sē-ə)
n

Low oxygen content or tension.

Random House Word Menu:

categories related to 'hypoxia'

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Random House Word Menu by Stephen Glazier

For a list of words related to hypoxia, see:

Physiology - hypoxia: oxygen deficiency at tissue level

Wikipedia on Answers.com:

Hypoxia (medical)

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Hypoxia
ICD-9	799.02
MeSH	D000860

Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole (generalized hypoxia) or a region of the body (tissue hypoxia) is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise. A mismatch between oxygen supply and its demand at the cellular level may result in a hypoxic condition. Hypoxia in which there is complete deprivation of oxygen supply is referred to as anoxia.

Hypoxia differs from hypoxemia in that, in the latter, the oxygen concentration within the arterial blood is abnormally low.^[1] It is possible to experience hypoxia and have a low oxygen content (e.g., due to anemia) but maintain high oxygen partial pressure (pO₂). Incorrect use of these terms can lead to confusion, especially as hypoxemia is among the causes of hypoxia (in hypoxemic hypoxia).

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 (HAPE) and high altitude cerebral edema (HACE).^[2] Hypoxia also occurs in healthy individuals when breathing mixtures of gases with a low oxygen content, e.g. while diving underwater especially when using closed-circuit rebreather systems that control the amount of oxygen in the supplied air. A mild and non-damaging intermittent hypoxia is used intentionally during altitude trainings to develop an athletic performance adaptation at both the systemic and cellular level.^[3]

Hypoxia is also a serious consequence of preterm birth in the neonate. The main cause for this is that the lungs of the human foetus are among the last organs to develop during pregnancy. To assist the lungs to distribute oxygenated blood throughout the body, infants at risk of hypoxia are often placed inside an incubator capable of providing continuous positive airway pressure (also known as a humidicrib).

Contents

Classification

Hypoxemic hypoxia is a generalized hypoxia, an inadequate supply of oxygen to the body as a whole. The term "hypoxemic hypoxia" specifies hypoxia caused by low partial pressure of oxygen in arterial blood. In the other causes of hypoxia that follow, the partial pressure of oxygen in arterial blood is normal. Hypoxemic hypoxia may be due to:
- Hypoventilation. Inadequate pulmonary minute ventilation (e.g., respiratory arrest or by drugs such as opiates)
- 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 decreased).
  - Normal anatomical shunt occur due to Thebesian veins which empty into the left ventricle and the bronchial circulation which supplies the bronchi with oxygen.
  - Normal physiological shunts occur due to the effect of gravity. The highest concentration of blood in the pulmonary circulation occurs in the bases of the pulmonary tree compared to the highest pressure of gas in the apices of the lungs.
- V/Q mismatch. When the ventilation does not match the perfusion through the paranchyema of the lung. This can occur for a variety of reasons, the commonest being a Pulmonary embolism
- Diffusing defects such as pulmonary fibrosis where the Aa gradient has increased.
- Decreased concentration of oxygen in inspired air. Low partial pressure of atmospheric oxygen such as found at high altitude^[4] or by reduced replacement of oxygen in the breathing mix.
  - Low partial pressure of oxygen in the lungs when switching from inhaled anaesthesia to atmospheric air, due to the Fink effect, or diffusion hypoxia.
Anaemia in which arterial oxygen pressure is normal, but total oxygen content of the blood is reduced. This is due to a decreased total carrying capacity.^[5]
Hypoxia when the blood fails to deliver oxygen to target tissues.
- Carbon monoxide poisoning which inhibits the ability of hemoglobin to release the oxygen bound to it.
- Methaemoglobinaemia in which an abnormal version of hemoglobin accumulates in the blood
Histotoxic hypoxia in which quantity of oxygen reaching the cells is normal, but the cells are unable to use the oxygen effectively, due to disabled oxidative phosphorylation enzymes. Cyanide toxicity is one example.

Signs and symptoms

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 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, priapism, and death occur. Severe hypoxia induces a blue discolouration of the skin, called cyanosis. Because hemoglobin is a darker red when it is not bound to oxygen (deoxyhemoglobin), as opposed to the rich red colour that it has when bound to oxygen (oxyhemoglobin), 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.

Pathophysiology

After mixing with water vapour and expired CO₂ in the lungs, oxygen diffuses down a pressure gradient to enter arterial blood where its partial pressure is around 100 mmHg (13.3 kPa).^[4] 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. The fuel's carbon is oxidized to CO₂, which diffuses down its partial pressure gradient out of the cells into venous blood to be exhaled finally by the lungs. Experimentally, oxygen diffusion becomes rate limiting (and lethal) when arterial oxygen partial pressure falls to 40 mmHg (5.3 kPa) 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.^[6] 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 pO₂ toward normal. Acclimatization, the means by which the body adapts to higher altitudes, only partially restores pO₂ to standard levels. Hyperventilation, the body’s most common response to high-altitude conditions, increases alveolar pO₂ by raising the depth and rate of breathing. However, while pO₂ does improve with hyperventilation, it does not return to normal. Studies of miners and astronomers working at 3000 meters and above show improved alveolar pO₂ with full acclimatization, yet the pO₂ level remains equal to or even below the threshold for continuous oxygen therapy for patients with chronic obstructive pulmonary disease (COPD).^[7] 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 pO₂ 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 equivalent of 3000 m, which is much more tolerable for the increasing number of low-landers who work in high altitude.^[8] In a study of astronomers working in Chile at 5050 m, oxygen concentrators increased the level of oxygen concentration by almost 30 percent (that is, from 21 percent to 27 percent). This resulted in increased worker productivity, less fatigue, and improved sleep.^[9]

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 remote areas. Offices and housing already have climate-controlled rooms, in which temperature and humidity are kept at a constant level. Oxygen can be added to this system easily and relatively cheaply.

References

^ West, John B. (1977). Pulmonary Pathophysiology: The Essentials. Williams & Wilkins. pp. 22. ISBN 0683089366.
^ Cymerman, A; Rock, PB. Medical Problems in High Mountain Environments. A Handbook for Medical Officers. USARIEM-TN94-2. US Army Research Inst. of Environmental Medicine Thermal and Mountain Medicine Division Technical Report. http://archive.rubicon-foundation.org/7976. Retrieved 2009-03-05.
^ *Nonhematological mechanisms of improved sea-level ... - PubMed Med Sci Sports Exerc. 2007 Sep;39(9):1600-9.
^ ^a ^b Kenneth Baillie and Alistair Simpson. "Altitude oxygen calculator". Apex (Altitude Physiology Expeditions). http://www.altitude.org/oxygen_levels.php. Retrieved 2006-08-10. - Online interactive oxygen delivery calculator
^ Kenneth Baillie and Alistair Simpson. "Oxygen content calculator". Apex (Altitude Physiology Expeditions). http://www.altitude.org/oxygen_carriage.php. Retrieved 2006-08-10. - A demonstration of the effect of anaemia on oxygen content
^ Hobler, K.E.; L.C. Carey (1973). "Effect of acute progressive hypoxemia on cardiac output and plasma excess lactate" (scanned copy). Ann Surg 177 (2): 199–202. doi:10.1097/00000658-197302000-00013. PMC 1355564. PMID 4572785.
^ West, John B. (2004). "The Physiologic Basis of High-Altitude Diseases". Annals of Internal Medicine 141 (10): 791. PMID 15545679.
^ West, John B. (1995). "Oxygen Enrichment of Room Air to Relieve the Hypoxia of High Altitude". Respiration Physiology 99 (2): 230. doi:10.1016/0034-5687(94)00094-G. PMID 7777705.
^ West, John B. (2004). "The Physiologic Basis of High-Altitude Diseases". Annals of Internal Medicine 141 (10): 793. PMID 15545679.

Bibliography

Hypoxia - An invisible enemy, Fast, Airbus technical magazine, #38 : presentation for non specialists of hypoxia and related safety procedures in civil airplanes
Human Exposure to Vacuum web page, discussing the effects of vacuum on an unprotected human.

Respiratory system, physiology: respiratory physiology

Lung volumes

VC · FRC · Vt · dead space · CC · PEF

calculations: respiratory minute volume · FEV1/FVC ratio

methods of lung testing: spirometry · body plethysmography · peak flow meter · nitrogen washout

Airways/
ventilation (V)

positive pressure ventilation · breath (inhalation, exhalation) · respiratory rate · respirometer · pulmonary surfactant · compliance · hysteresivity · airway resistance · bronchial hyperresponsiveness · bronchoconstriction/bronchodilation

Blood/
perfusion (Q)

pulmonary circulation · hypoxic pulmonary vasoconstriction · pulmonary shunt

Interactions/
ventilation/perfusion ratio (V/Q)

ventilation/perfusion scan · zones of the lung · gas exchange · pulmonary gas pressures · alveolar gas equation · alveolar-arterial gradient · hemoglobin · oxygen-haemoglobin dissociation curve (Oxygen saturation, 2,3-DPG, Bohr effect, Haldane effect) · carbonic anhydrase (chloride shift) · oxyhemoglobin · respiratory quotient · arterial blood gas · diffusion capacity (DLCO)

Control of respiration

pons (pneumotaxic center, apneustic center) · medulla (dorsal respiratory group, ventral respiratory group) · chemoreceptors (central, peripheral) · pulmonary stretch receptors (Hering-Breuer reflex)

Insufficiency

high altitude · oxygen toxicity · hypoxia

M: RES

anat(n, x, l, c)/phys/devp

noco(c)/cong/tumr, sysi/epon, injr

proc, drug(R1/2/3/5/6/7)

v · d · eConsequences of external causes (T66–T78, 990–995)

Temperature/radiation	elevated temperature: Hyperthermia · Heat syncope reduced temperature: Hypothermia · Immersion foot syndromes (Trench foot • Tropical immersion foot • Warm water immersion foot) · Chilblains · Frostbite · Cold intolerance • Acrocyanosis • Erythrocyanosis crurum radiation: Radiation poisoning · Radiation burn · Chronic radiation keratosis • Eosinophilic, polymorphic, and pruritic eruption associated with radiotherapy • Radiation acne • Radiation cancer • Radiation recall reaction • Radiation-induced erythema multiforme • Radiation-induced hypertrophic scar • Radiation-induced keloid • Radiation-induced morphea

Air	Hypoxia/Asphyxia · Barotrauma (Aerosinusitis, Decompression sickness) · High altitude (Altitude sickness/Chronic mountain sickness, HAPE)

Food	Starvation

Maltreatment	Physical abuse · Sexual abuse · Psychological abuse

Emesis	Motion sickness · Seasickness · Airsickness · Space adaptation syndrome

Adverse effect	Hypersensitivity (Anaphylaxis, Angioedema, Allergy, Arthus reaction) Adverse drug reaction

Other	Electric shock · Drowning · Lightning injury

Ungrouped skin conditions resulting from physical factors	Dermatosis neglecta • Pinch mark • Pseudoverrucous papules and nodules • Sclerosing lymphangiitis • Tropical anhidrotic asthenia • UV-sensitive syndrome environmental skin conditions: Electrical burn • frictional/traumatic/sports (Black heel and palm • Equestrian perniosis • Jogger's nipple • Pulling boat hands • Runner's rump • Surfer's knots • Tennis toe • Vibration white finger • Weathering nodule of ear • Wrestler's ear • Coral cut • Painful fat herniation ) • Uranium dermatosis iv use (Skin pop scar • Skin track • Slap mark • Pseudoacanthosis nigricans • Narcotic dermopathy)

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