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62-547: The DSEA rig chiefly addressed the problem of anoxia threatening a person ascending through water, by providing oxygen; and the associated risk of lung over-pressure injury as underwater pressure reduces with reducing depth, which it addressed by managing oxygen pressures. It also provided assistance with buoyancy, both in the ascent and after reaching the surface. The risk of decompression illness due to ascending too fast could be addressed by associated equipment; any other escape requirements, such as means of summoning help once

124-798: A ventilation-perfusion mismatch , such as a pulmonary embolus , or alterations in the partial pressure of oxygen in the environment or lung alveoli, such as may occur at altitude or when diving. Common disorders that can cause respiratory dysfunction include trauma to the head and spinal cord, nontraumatic acute myelopathies, demyelinating disorders, stroke, Guillain–Barré syndrome , and myasthenia gravis . These dysfunctions may necessitate mechanical ventilation. Some chronic neuromuscular disorders such as motor neuron disease and muscular dystrophy may require ventilatory support in advanced stages. Carbon monoxide competes with oxygen for binding sites on hemoglobin molecules. As carbon monoxide binds with hemoglobin hundreds of times tighter than oxygen, it can prevent

186-411: A barrier to this diffusion , and therefore can cause damage to the corneas. Symptoms may include irritation, excessive tearing and blurred vision . The sequelae of corneal hypoxia include punctate keratitis , corneal neovascularization and epithelial microcysts. Intrauterine hypoxia, also known as fetal hypoxia, occurs when the fetus is deprived of an adequate supply of oxygen . It may be due to

248-434: A common feature that limitation of oxygen availability contributes to the development of the pathology. Cells and organisms are also able to respond adaptively to hypoxic conditions, in ways that help them to cope with these adverse conditions. Several systems can sense oxygen concentration and may respond with adaptations to acute and long-term hypoxia. The systems activated by hypoxia usually help cells to survive and overcome

310-465: A correlation between hypoxic stress and adaptive tracking performance. Arterial oxygen tension can be measured by blood gas analysis of an arterial blood sample, and less reliably by pulse oximetry , which is not a complete measure of circulatory oxygen sufficiency. If there is insufficient blood flow or insufficient hemoglobin in the blood (anemia), tissues can be hypoxic even when there is high arterial oxygen saturation. Oxygen passively diffuses in

372-413: 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 (poor perfusion ) or total blockage. Compartment syndrome is a condition in which increased pressure within one of

434-413: A hypoxic brain injury. Oxygen deprivation can be hypoxic (reduced general oxygen availability) or ischemic (oxygen deprivation due to a disruption in blood flow) in origin. Brain injury as a result of oxygen deprivation is generally termed hypoxic injury. Hypoxic ischemic encephalopathy (HIE) is a condition that occurs when the entire brain is deprived of an adequate oxygen supply, but the deprivation

496-562: A light-sensitive sensor to measure the absorption of red and infrared wavelengths in the extremity, and estimates the SpO 2 from the absorption spectrum. Healthy individuals at sea level usually exhibit oxygen saturation values between 96% and 99%, and should be above 94%. At 1,600 meters' altitude (about one mile high) oxygen saturation should be above 92%. An SaO 2 (arterial oxygen saturation) value below 90% causes hypoxia (which can also be caused by anemia ). Hypoxia due to low SaO 2

558-460: A limb is used, increasing the oxygen demand in the active muscles. Pain may also be felt as a result of increased hydrogen ions leading to a decrease in blood pH ( acidosis ) created as a result of anaerobic metabolism . G-LOC , or g-force induced loss of consciousness, is a special case of ischemic hypoxia which occurs when the body is subjected to high enough acceleration sustained for long enough to lower cerebral blood pressure and circulation to

620-445: 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

682-407: A non-return release valve which allows air to escape from the bag as the user ascends towards the surface and the water pressure decreases. The wearer can close this valve on reaching the surface, the air in the breathing/buoyancy bag then serving as a life preserver. If the bag becomes deflated while the wearer is on the surface awaiting rescue, it can be refilled (for use as a lifejacket ) by opening

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744-405: A normal amount of oxygen. Both too high and too low levels can have adverse effects on the body. An SaO 2 (arterial oxygen saturation, as determined by an arterial blood gas test ) value below 90% indicates hypoxemia (which can also be caused by anemia ). Hypoxemia due to low SaO 2 is indicated by cyanosis . Oxygen saturation can be measured in different tissues: Pulse oximetry

806-434: A protein in red blood cells . The binding capacity of hemoglobin is influenced by the partial pressure of oxygen in the environment, as described by the oxygen–hemoglobin dissociation curve . A smaller amount of oxygen is transported in solution in the blood. In systemic tissues, oxygen again diffuses down a concentration gradient into cells and their mitochondria , where it is used to produce energy in conjunction with

868-464: A second toxic effect, namely removing the allosteric shift of the oxygen dissociation curve and shifting the foot of the curve to the left. In so doing, the hemoglobin is less likely to release its oxygen at the peripheral tissues. Certain abnormal hemoglobin variants also have higher than normal affinity for oxygen, and so are also poor at delivering oxygen to the periphery. Atmospheric pressure reduces with altitude and proportionally, so does

930-415: 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

992-505: A tissue may eventually become gangrenous. Any living tissue can be affected by hypoxia, but some are particularly sensitive, or have more noticeable or notable consequences. Cerebral hypoxia is hypoxia specifically involving the brain. The four categories of cerebral hypoxia in order of increasing severity are: diffuse cerebral hypoxia (DCH), focal cerebral ischemia, cerebral infarction , and global cerebral ischemia. Prolonged hypoxia induces neuronal cell death via apoptosis , resulting in

1054-441: 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. Hypoxia may affect the whole body, or just some parts. The term generalized hypoxia may refer to hypoxia affecting

1116-522: A variety of reasons such as prolapse or occlusion of the umbilical cord , placental infarction , maternal diabetes (prepregnancy or gestational diabetes ) and maternal smoking . Intrauterine growth restriction may cause or be the result of hypoxia. Intrauterine hypoxia can cause cellular damage that occurs within the central nervous system (the brain and spinal cord). This results in an increased mortality rate, including an increased risk of sudden infant death syndrome (SIDS). Oxygen deprivation in

1178-511: Is a condition in which the body or a region of the body is deprived of 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

1240-532: Is a method used to estimate the percentage of oxygen bound to hemoglobin in the blood. This approximation to SaO 2 is designated SpO 2 (peripheral oxygen saturation). The pulse oximeter is a small device that clips to the body (typically a finger, an earlobe or an infant's foot) and displays its reading, or transfers it to another device. Oxygenated and deoxygenated hemoglobin differ in absorption of light of different wavelengths. The oximeter uses light-emitting diodes of different wavelengths in conjunction with

1302-476: Is a switch to anaerobic metabolism at the cellular level. As such, reduced systemic blood flow may result in increased serum lactate. Serum lactate levels have been correlated with illness severity and mortality in critically ill adults and in ventilated neonates with respiratory distress. All vertebrates must maintain oxygen homeostasis to survive, and have evolved physiological systems to ensure adequate oxygenation of all tissues. In air breathing vertebrates this

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1364-543: Is based on lungs to acquire the oxygen, hemoglobin in red corpuscles to transport it, a vasculature to distribute, and a heart to deliver. Short term variations in the levels of oxygenation are sensed by chemoreceptor cells which respond by activating existing proteins, and over longer terms by regulation of gene transcription. Hypoxia is also involved in the pathogenesis of some common and severe pathologies. The most common causes of death in an aging population include myocardial infarction, stroke and cancer. These diseases share

1426-455: Is called methemoglobin and can be made by ingesting sodium nitrite as well as certain drugs and other chemicals. Hemoglobin plays a substantial role in carrying oxygen throughout the body, and when it is deficient, anemia can result, causing 'anaemic hypoxia' if tissue oxygenation is decreased. Iron deficiency is the most common cause of anemia. As iron is used in the synthesis of hemoglobin, less hemoglobin will be synthesised when there

1488-425: Is deoxygenated. At around 90% (the value varies according to the clinical context) oxygen saturation increases according to an oxygen-hemoglobin dissociation curve and approaches 100% at partial oxygen pressures of >11 kPa. A pulse oximeter relies on the light absorption characteristics of saturated hemoglobin to give an indication of oxygen saturation. The body maintains a stable level of oxygen saturation for

1550-410: 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

1612-399: Is indicated by cyanosis , but oxygen saturation does not directly reflect tissue oxygenation. The affinity of hemoglobin to oxygen may impair or enhance oxygen release at the tissue level. Oxygen is more readily released to the tissues (i.e., hemoglobin has a lower affinity for oxygen) when pH is decreased, body temperature is increased, arterial partial pressure of carbon dioxide (PaCO 2 )

1674-957: Is less iron, due to insufficient intake, or poor absorption. Anemia is typically a chronic process that is compensated over time by increased levels of red blood cells via upregulated erythropoetin . A chronic hypoxic state can result from a poorly compensated anaemia. Histotoxic hypoxia (also called histoxic hypoxia) is the inability of cells to take up or use oxygen from the bloodstream, despite physiologically normal delivery of oxygen to such cells and tissues. Histotoxic hypoxia results from tissue poisoning, such as that caused by cyanide (which acts by inhibiting cytochrome oxidase ) and certain other poisons like hydrogen sulfide (byproduct of sewage and used in leather tanning). Tissue hypoxia from low oxygen delivery may be due to low haemoglobin concentration (anaemic hypoxia), low cardiac output (stagnant hypoxia) or low haemoglobin saturation (hypoxic hypoxia). The consequence of oxygen deprivation in tissues

1736-453: Is not total. While HIE is associated in most cases with oxygen deprivation in the neonate due to birth asphyxia , it can occur in all age groups, and is often a complication of cardiac arrest . Although corneal hypoxia can arise from any of several causes, it is primarily attributable to the prolonged use of contact lenses . The corneas are not perfused and get their oxygen from the atmosphere by diffusion. Impermeable contact lenses form

1798-406: Is suitable for saturation diving . As the divers are decompressed , the breathing gas must be oxygenated to maintain a breathable atmosphere. It is also possible for the breathing gas for diving to have a dynamically controlled oxygen partial pressure, known as a set point , which is maintained in the breathing gas circuit of a diving rebreather by addition of oxygen and diluent gas to maintain

1860-399: Is usually associated with highly malignant tumours, which frequently do not respond well to treatment. In acute exposure to hypoxic hypoxia on the vestibular system and the visuo-vestibular interactions, the gain of the vestibulo–ocular reflex (VOR) decreases under mild hypoxia at altitude. Postural control is also disturbed by hypoxia at altitude, postural sway is increased, and there is

1922-617: The blood . The human body requires and regulates a very precise and specific balance of oxygen in the blood. Normal arterial blood oxygen saturation levels in humans are 96–100 percent. If the level is below 90 percent, it is considered low and called hypoxemia . Arterial blood oxygen levels below 80 percent may compromise organ function, such as the brain and heart, and should be promptly addressed. Continued low oxygen levels may lead to respiratory or cardiac arrest. Oxygen therapy may be used to assist in raising blood oxygen levels. Oxygenation occurs when oxygen molecules ( O 2 ) enter

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1984-407: 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 (CPAP). Hypoxia exists when there is a reduced amount of oxygen in the tissues of

2046-425: The tissues of the body. For example, blood is oxygenated in the lungs , where oxygen molecules travel from the air and into the blood. Oxygenation is commonly used to refer to medical oxygen saturation. In medicine , oxygen saturation , commonly referred to as "sats", measures the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen. At low partial pressures of oxygen, most hemoglobin

2108-812: The DSEA, the Amphibious Tank Escape Apparatus (ATEA) was produced for use by the crews of amphibious DD tanks such as those used during the Normandy landings . There were instances, mostly during WWII , of the DSEA being used for swimming down from the surface, i.e. for early scuba diving . In WWII it was also notably used by the Underwater Working Party at Gibraltar led by Lt. Lionel "Buster" Crabb , and worn at times by frogmen piloting 'Sleeping Beauty' Motorised Submersible Canoes . Hypoxia (medical) Hypoxia

2170-456: The affected tissues. This is called 'ischemic hypoxia'. Ischemia can be caused by an embolism , a heart attack that decreases overall blood flow, trauma to a tissue that results in damage reducing perfusion, and a variety of other causes . A consequence of insufficient blood flow causing local hypoxia is gangrene that occurs in diabetes . Diseases such as peripheral vascular disease can also result in local hypoxia. Symptoms are worse when

2232-400: 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,

2294-512: 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 (C a O 2 – which represents the amount of oxygen delivered to the tissues) 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. Intermittent hypoxic training induces mild generalized hypoxia for short periods as

2356-404: The breakdown of glucose , fats , and some amino acids . Hypoxia can result from a failure at any stage in the delivery of oxygen to cells. This can include low partial pressures of oxygen in the breathing gas, problems with diffusion of oxygen in the lungs through the interface between air and blood, insufficient available hemoglobin, problems with blood flow to the end user tissue, problems with

2418-409: The breathing cycle regarding rate and volume, and physiological and mechanical dead space . Experimentally, oxygen diffusion becomes rate limiting when arterial oxygen partial pressure falls to 60 mmHg (5.3 kPa) or below. Almost all the oxygen in the blood is bound to hemoglobin, so interfering with this carrier molecule limits oxygen delivery to the perfused tissues. Hemoglobin increases

2480-436: The carriage of oxygen. Carbon monoxide poisoning can occur acutely, as with smoke intoxication, or over a period of time, as with cigarette smoking. Due to physiological processes, carbon monoxide is maintained at a resting level of 4–6 ppm. This is increased in urban areas (7–13 ppm) and in smokers (20–40 ppm). A carbon monoxide level of 40 ppm is equivalent to a reduction in hemoglobin levels of 10 g/L. Carbon monoxide has

2542-400: The cylinder's valve admits oxygen to the bag and charges it to the pressure of the surrounding water. The canister of CO 2 absorbent inside the breathing bag is connected to a mouthpiece by a flexible corrugated tube; breathing is through the mouth only, the nose being closed by a clip. Goggles are also provided as a standard part of the apparatus. The breathing/buoyancy bag is fitted with

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2604-517: The desired oxygen partial pressure at a safe level between hypoxic and hyperoxic at the ambient pressure due to the current depth. A malfunction of the control system may lead to the gas mixture becoming hypoxic at the current depth. A special case of hypoxic breathing gas is encountered in deep freediving where the partial pressure of the oxygen in the lung gas is depleted during the dive, but remains sufficient at depth, and when it drops during ascent, it becomes too hypoxic to maintain consciousness, and

2666-638: The diver loses consciousness before reaching the surface. Hypoxic gases may also occur in industrial, mining, and firefighting environments. Some of these may also be toxic or narcotic, others are just asphyxiant. Some are recognisable by smell, others are odourless. Inert gas asphyxiation may be deliberate with use of a suicide bag . Accidental death has occurred in cases where concentrations of nitrogen in controlled atmospheres, or methane in mines, has not been detected or appreciated. Hemoglobin's function can also be lost by chemically oxidizing its iron atom to its ferric form. This form of inactive hemoglobin

2728-504: 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. 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

2790-421: The fetus and neonate have been implicated as either a primary or as a contributing risk factor in numerous neurological and neuropsychiatric disorders such as epilepsy , attention deficit hyperactivity disorder , eating disorders and cerebral palsy . Tumor hypoxia is the situation where tumor cells have been deprived of oxygen. As a tumor grows, it rapidly outgrows its blood supply, leaving portions of

2852-474: The human body senses pure hypoxia poorly. Hypoxic breathing gases can be defined as mixtures with a lower oxygen fraction than air, though gases containing sufficient oxygen to reliably maintain consciousness at normal sea level atmospheric pressure may be described as normoxic even when the oxygen fraction is slightly below normoxic. Hypoxic breathing gas mixtures in this context are those which will not reliably maintain consciousness at sea level pressure. One of

2914-479: The hypoxic conditions. Erythropoietin , which is produced in larger quantities by the kidneys under hypoxic conditions, is an essential hormone that stimulates production of red blood cells, which are the primary transporter of blood oxygen, and glycolytic enzymes are involved in anaerobic ATP formation. Oxygen saturation (medicine) Oxygen saturation is the fraction of oxygen -saturated haemoglobin relative to total haemoglobin (unsaturated + saturated) in

2976-422: 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 ( deoxyhemoglobin ), as opposed to the rich red color that it has when bound to oxygen ( oxyhemoglobin ), when seen through the skin it has an increased tendency to reflect blue light back to

3038-446: The level of oxygenation in hypoxic tumor tissues is poorer than normal tissues and it is reported somewhere between 1%–2% O2. In order to support continuous growth and proliferation in challenging hypoxic environments, cancer cells are found to alter their metabolism. Furthermore, hypoxia is known to change cell behavior and is associated with extracellular matrix remodeling and increased migratory and metastatic behavior. Tumour hypoxia

3100-408: The lung alveoli according to a concentration gradient, also referred to as a partial pressure gradient. Inhaled air rapidly reaches saturation with water vapour, which slightly reduces the partial pressures of the other components. Oxygen diffuses from the inhaled air to arterial blood, where its partial pressure is around 100 mmHg (13.3 kPa). In the blood, oxygen is bound to hemoglobin,

3162-606: 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 ( HAPE ) and high altitude cerebral edema ( HACE ). Hypoxia also occurs in healthy individuals when breathing inappropriate mixtures of gases with

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3224-433: The most part by chemical processes of aerobic metabolism associated with breathing . Using the respiratory system , red blood cells, specifically the hemoglobin , gather oxygen in the lungs and distribute it to the rest of the body. The needs of the body's blood oxygen may fluctuate such as during exercise when more oxygen is required or when living at higher altitudes. A blood cell is said to be "saturated" when carrying

3286-407: The most widespread circumstances of exposure to hypoxic breathing gas is ascent to altitudes where the ambient pressure drops sufficiently to reduce the partial pressure of oxygen to hypoxic levels. Gases with as little as 2% oxygen by volume in a helium diluent are used for deep diving operations. The ambient pressure at 190 msw is sufficient to provide a partial pressure of about 0.4 bar, which

3348-404: The non-return valve and blowing through the mouthpiece. The usual Royal Navy DSEA rig also included an emergency buoyancy bag on the front of the main breathing/buoyancy bag to help keep the wearer afloat after reaching the surface even if he had exhausted the air in the breathing/buoyancy bag. This emergency bag was inflated by an "Oxylet" canister inside it - a small steel oxygen cylinder which

3410-469: The oxygen content of the air. The reduction in the partial pressure of inspired oxygen at higher altitudes lowers the oxygen saturation of the blood, ultimately leading to hypoxia. The clinical features of altitude sickness include: sleep problems, dizziness, headache and oedema. The breathing gas may contain an insufficient partial pressure of oxygen. Such situations may lead to unconsciousness without symptoms since carbon dioxide levels remain normal and

3472-420: The oxygen-carrying capacity of blood by about 40-fold, with the ability of hemoglobin to carry oxygen influenced by the partial pressure of oxygen in the local environment, a relationship described in the oxygen–hemoglobin dissociation curve. When the ability of hemoglobin to carry oxygen is degraded, a hypoxic state can result. Ischemia, meaning insufficient blood flow to a tissue, can also result in hypoxia in

3534-515: The point where loss of consciousness occurs due to cerebral hypoxia. The human body is most sensitive to longitudinal acceleration towards the head, as this causes the largest hydrostatic pressure deficit in the head. This refers specifically to hypoxic states where the arterial content of oxygen is insufficient. This can be caused by alterations in respiratory drive , such as in respiratory alkalosis , physiological or pathological shunting of blood, diseases interfering in lung function resulting in

3596-423: The surface was reached, were not considered. The apparatus itself comprises a rubber breathing/buoyancy bag, which contains a canister of barium hydroxide to scrub exhaled CO 2 and, in a pocket at the lower end of the bag, a steel pressure cylinder holding approximately 56 litres of oxygen at a pressure of 120 bar. The cylinder is equipped with a control valve and is connected to the breathing bag . Opening

3658-496: 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

3720-468: The tumor with regions where the oxygen concentration is significantly lower than in healthy tissues. Hypoxic microenvironements in solid tumors are a result of available oxygen being consumed within 70 to 150 μm of tumour vasculature by rapidly proliferating tumor cells thus limiting the amount of oxygen available to diffuse further into the tumor tissue. The severity of hypoxia is related to tumor types and varies between different types. Research has shown that

3782-442: 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,

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3844-603: Was opened by breaking its weakened neck and wrenching sharply. It also had a speed-retarding drogue , which was a rubber apron unrolled and held out horizontally by the wearer as he ascended, dramatically reducing his speed of ascent through water resistance to avoid decompression illness . Adopted by the Royal Navy in 1929, DSEA was used with limited success to assist crew members to escape from several sunken submarines, for example HMS Poseidon in 1931, HMS Thetis in 1939 and HMS Perseus in 1941. A small version of

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