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09-SEPTEMBER-2008 02:07:44 - Hypoxia medical For other uses of the term hypoxia, see hypoxia. Symptom/Sign: Hypoxia Classifications and external resources ICD-10 ICD-9 799.02 MeSH D000860 Chronic Hypoxia is a 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.1 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 PO2. 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 effects for athletic performance adaptations that are evident at both systemic and cellular level 2. Contents 1 Symptoms 2 Types of hypoxia 3 Pathophysiology 4 Vasoconstriction and vasodilation 5 Treatment 6 See also 7 Footnotes 8 Bibliography 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 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 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 atmospheric oxygen such as found at high altitude 3 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 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.4 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. The effects of drinking alcoholic beverages is a common example. 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.3 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.5 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.6 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.7 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.8 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. 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 ^ West, John B. 1977. Pulmonary Pathophysiology: The Essentials. Williams Wilkins, 22. ^ 1 Med Sci Sports Exerc. 2007 Sep;399:1600-9. ^ a b Kenneth Baillie and Alistair Simpson. Altitude oxygen calculator. Apex Altitude Physiology Expions. Retrieved on 2006-08-10. - Online interactive oxygen delivery calculator ^ Kenneth Baillie and Alistair Simpson. Oxygen content calculator. Apex Altitude Physiology Expions. Retrieved on 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. PMID 4572785. ^ West, John B. 2004. The Physiologic Basis of High-Altitude Diseases. Annals of Internal Medicine 141: 791. ^ West, John B. 1995. Oxygen Enrichment of Room Air to Relieve the Hypoxia of High Altitude. Respiration Physiology 99: 230. doi:10.1016/0034-56879400094-G. ^ West, John B. 2004. The Physiologic Basis of High-Altitude Diseases. Annals of Internal Medicine 141: 793. Bibliography Hypoxia - An invisible enemy, Fast, Airbus technical magazine, #38 : presentation for non specialists of hypoxia and related safety procedures in civil airplanes v d e Respiratory system, physiology: respiratory physiology Volumes lung volumes - vital capacity - functional residual capacity - respiratory minute volume - closing capacity - dead space - spirometry - body plethysmography - peak flow meter - thoracic independent volume Airways ventilation V positive pressure - breath inhalation, exhalation -respiratory rate - respirometer - pulmonary surfactant - compliance - hysteresivity - airway resistance - bronchial hyperresponsiveness - bronchial challenge test Blood pulmonary circulation - perfusion Q - hypoxic pulmonary vasoconstriction - pulmonary shunt Interactions ventilation/perfusion ratio V/Q and scan - zones of the lung - gas exchange - pulmonary gas pressures - alveolar gas equation - alveolar-arterial gradient - hemoglobin - oxygen-haemoglobin dissociation curve 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 v d e Consequences of external causes T15-T35, T66-T98, 930-959, 990-995 General external causes Foreign body - Burn - Frostbite Other external causes radiation/heat/light: Radiation poisoning - Hyperthermia - Heat syncope reduced temperature: 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 Electric shock - Anaphylaxis - Angioedema Hypersensitivity Allergy, Arthus reaction Subcutaneous emphysema Certain early complications of trauma embolism Air, Fat - Crush syndrome/Rhabdomyolysis - Compartment syndrome/Volkmann's contracture Complications of surgical and medical care Serum sickness - Malignant hyperthermia - Transfusion hemosiderosis - Herxheimer reaction - Graft-versus-host disease Retrieved from http://en..org/wiki/Hypoxia_medical Categories: Respiratory therapy | Oxygen | Pulmonology | Diving medicine | Aviation medicine Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages БългарÑ?ки ÄŒesky Dansk Deutsch Ελληνικά Español Esperanto Français Bahasa Indonesia Italiano Lietuvių Magyar Nederlands Polski РуÑ?Ñ?кий СрпÑ?ки / Srpski Suomi Svenska УкраїнÑ?ька This page was last modified on 19 August 2008, at 05:23
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