Hypoxia (medical)
13292
225504407
2008-07-14T01:17:38Z
68.193.104.198
/* Symptoms */
{{for|other uses of the term "hypoxia"|hypoxia}}
{{SignSymptom infobox |
| Name = Hypoxia
| Image =
| Caption =
| DiseasesDB = <!-- no entry -->
| ICD10 =
| ICD9 = {{ICD9|799.02}}
| ICDO =
| OMIM =
| MedlinePlus =
| eMedicineSubj =
| eMedicineTopic =
| eMedicine_mult =
| MeshID = D000860
}}
Chronic '''Hypoxia''' is a [[Pathology|pathological]] condition in which the body as a whole ('''generalized hypoxia''') or region of the body ('''tissue hypoxia''') is deprived of adequate [[oxygen]] supply. However variations in arterial oxygen concentration can be part of normal physiology eg during strenuous physical exercise due to mismatch between supply and demand for oxygen at the cellular level. Hypoxia in which there is complete deprivation of oxygen supply is referred to as '''anoxia'''.
Hypoxia is distinguished from apoxemia, which is an abnormally low concentration of oxygen in arterial blood.<ref>{{cite book |last=West |first=John B. |title=Pulmonary Pathophysiology: The Essentials |year=1977 |publisher=Williams & Wilkins |pages=22}}</ref> A frequent error is to use the term hypoxia to mean low oxygen content in arterial blood. The correct term for low oxygen content in arterial blood is [[hypoxemia]]. It is possible to have a low oxygen content (e.g., due to anemia) but a high PO<sub>2</sub>. Incorrect use of these terms can lead to confusion.
Generalized hypoxia occurs in healthy people when they ascend to [[high altitude]], where it causes [[altitude sickness]] and its potentially fatal complications, [[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, such as while diving underwater, especially when using closed-circuit [[rebreather]] systems that control the amount of oxygen in the supplied air. [[Altitude training]] intentionally uses mild, non-damaging intermittent hypoxia in order to produce beneficial for athletic performance adaptations that are evident at both systemic and cellular level <ref> *[http://www.ncbi.nlm.nih.gov/pubmed/17805094] Med Sci Sports Exerc. 2007 Sep;39(9):1600-9.</ref>.
==Symptoms==
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 [[headache]]s, fatigue, shortness of breath, a feeling of euphoria and [[nausea]]. In severe hypoxia, or hypoxia of very rapid onset, changes in levels of consciousness, [[seizure]]s, [[coma]], [[priapism]], and death occur. Severe hypoxia induces a blue discolouration of the skin, called [[cyanosis]]. Because [[haemoglobin]] 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 ([[oxyhaemoglobin]]), 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.
==Types of hypoxia==
*'''[[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:
**Low [[partial pressure]] of [[atmosphere|atmospheric]] oxygen such as found at high altitude <ref name="02_calc">{{cite web | author=Kenneth Baillie and Alistair Simpson | title=Altitude oxygen calculator | url=http://www.altitude.org/calculators/oxygencalculator/oxygencalculator.htm | publisher= Apex (Altitude Physiology Expeditions) | accessdate=2006-08-10}} - Online interactive oxygen delivery calculator</ref> 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]].
** A decrease in [[oxygen saturation]] of the blood caused by [[sleep apnea]] or [[hypopnea]]
** Inadequate pulmonary ventilation (e.g., in [[chronic obstructive pulmonary disease]] or [[respiratory failure|respiratory arrest]]).
** [[Shunt (medical)|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 (heart)|ventricle]] and the bronchial circulation which supplies the [[bronchi]] with oxygen.
*[[Anaemia|'''Anemic hypoxia''']] in which arterial oxygen pressure is normal, but total oxygen content of the blood is reduced.<ref>{{cite web | author=Kenneth Baillie and Alistair Simpson | title=Oxygen content calculator | url=http://www.altitude.org/calculators/oxygencontent/oxygencontent.htm | publisher=Apex (Altitude Physiology Expeditions) | accessdate=2006-08-10}} - A demonstration of the effect of anaemia on oxygen content</ref>
*[[Hypoxemia|'''Hypoxemic hypoxia''']] when the blood fails 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.
*[[Ischemia|'''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 [[Carbon dioxide|CO2]] in the lungs, oxygen diffuses down a pressure gradient to enter [[artery|arterial]] blood around where its partial pressure is 100mmHg (13.3kPa).<ref name="02_calc"/> 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 [[cytoplasm]]ic structures strip [[hydrogen]] from fuels ([[glucose]], [[fat]]s and some [[amino acid]]s) to burn with oxygen to form [[water]]. Released energy (originally from the [[sun]] and [[photosynthesis]]) is stored as [[Adenosine triphosphate|ATP]], to be later used for energy requiring metabolism. The fuel's carbon is oxidized to CO<sub>2</sub>, 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.<ref>{{cite journal | last=Hobler |first=K.E. |coauthors=L.C. Carey | title=Effect of acute progressive hypoxemia on cardiac output and plasma excess lactate | journal=Ann Surg | year=1973 | pages=199–202 | volume=177 | issue=2 | pmid= 4572785 | url=http://www.pubmedcentral.gov/articlerender.fcgi?tool=pubmed&pubmedid=4572785 | format=scanned copy | doi = 10.1097/00000658-197302000-00013 <!--Retrieved from CrossRef by DOI bot-->}}</ref> 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 PO<sub>2</sub> toward normal. [[Acclimatization]], the means by which the body adapts to higher altitudes, only partially restores PO<sub>2</sub> to standard levels. [[Hyperventilation]], the body’s most common response to high-altitude conditions, increases alveolar PO<sub>2</sub> by raising the depth and rate of breathing. However, while PO<sub>2</sub> does improve with hyperventilation, it does not return to normal. Studies of miners and astronomers working at 3000 meters and above show improved alveolar PO<sub>2</sub> with full acclimatization, yet the PO<sub>2</sub> level remains equal to or even below the threshold for continuous oxygen therapy for patients with [[chronic obstructive pulmonary disease]] (COPD).<ref>{{cite journal |last=West |first=John B. |year=2004 |title=The Physiologic Basis of High-Altitude Diseases |journal=Annals of Internal Medicine |volume=141 |number=10 |pages=791}}</ref> 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 PO<sub>2</sub> 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.<ref>{{cite journal |last=West |first=John B. |year=1995 |title=Oxygen Enrichment of Room Air to Relieve the Hypoxia of High Altitude |journal=Respiration Physiology |volume=99 |number=2 |pages=230 | doi = 10.1016/0034-5687(94)00094-G <!--Retrieved from CrossRef by DOI bot-->}}</ref> 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). This resulted in increased worker productivity, less fatigue, and improved sleep.<ref>{{cite journal |last=West |first=John B. |year=2004 |title=The Physiologic Basis of High-Altitude Diseases |journal=Annals of Internal Medicine |volume=141 |number=10 |pages=793}}</ref>
[[Oxygen concentrator]]s 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.
==See also==
* [[Altitude training]] for beneficial use of mild hypoxia
* [[Asphyxia]]
* [[Cerebral hypoxia]]
* [[Deep water blackout]] for special case of latent hypoxia
* [[Drowning]]
* [[g-LOC]] cerebral hypoxia induced by excessive g-forces
* [[Hypoxic tumor]]
* [[Hypoxicator]] a device intended for hypoxia acclimatisation in a controlled manner
* [[Hyperoxia]]
* [[Shallow water blackout]] for special case of hypoxia via self-induced hypocapnia
* [[Time of Useful Consciousness]]
For aircraft decompression incidents at altitude see:
* [[1999 South Dakota Learjet crash]]
* [[Helios Flight 522]]
* [[Soyuz 11]] fatal spacecraft decompression on re-entry
==Footnotes==
<!-- ----------------------------------------------------------
See http://en.wikipedia.org/wiki/Wikipedia:Footnotes for a
discussion of different citation methods and how to generate
footnotes using the <ref>, </ref> and <reference /> tags
----------------------------------------------------------- -->
<div class="references-small">
<references />
</div>
==Bibliography==
* [http://www.airbus.com/store/mm_repository/pdf/att00007996/media_object_file_FAST38_Hypoxia.pdf ''Hypoxia - An invisible enemy''], ''Fast'', [[Airbus]] technical magazine, #38 : presentation for non specialists of hypoxia and related safety procedures in civil airplanes
{{Respiratory physiology}}
{{Consequences of external causes}}
[[Category:Oxygen]]
[[Category:Pulmonology]]
[[Category:Diving medicine]]
[[Category:Aviation medicine]]
[[bg:Хипоксия]]
[[cs:Hypoxie]]
[[da:Hypoxia]]
[[de:Hypoxie (Medizin)]]
[[el:Υποξία]]
[[es:Hipoxia]]
[[eo:Hipoksio]]
[[fr:Hypoxie]]
[[id:Hipoksia]]
[[it:Ipossia]]
[[lt:Hipoksija (fiziologija)]]
[[hu:Hypoxi]]
[[nl:Hypoxie (medisch)]]
[[ru:Гипоксия]]
[[sr:Хипоксија]]
[[sv:Hypoxi]]
[[uk:Гіпоксемія]]