Breathing apparatus may be used to providing gas suitable for breathing in a range of applications where the ambient environment does not provide suitable breathing gas:
Underwater breathing apparatus Underwater breathing apparatus is any breathing apparatus intended to allow the user to breathe underwater, and includes open circuit scuba, diving rebreathers and surface supplied diving equipment, and both ambient pressure and controlled pressure single atmosphere systems. The major categories of
ambient pressure underwater breathing apparatus are: •
Scuba set, any breathing set that is carried entirely by an underwater diver and provides the diver with breathing gas at the ambient pressure: •
Open circuit scuba, where the diver carries the gas supply, and exhaled gas is exhausted to the environment •
Diving rebreathers, where the diver carries the gas supply, and exhaled gas is partly or entirely recycled for further use, and •
Surface supplied diving equipment, where the gas supply is provided from the surface through a hose in a
diver's umbilical This may be free-flow open circuit, demand open circuit, semi-closed circuit
gas extenders, or closed circuit
helium reclaim. Two other types may also be identified: •
Escape sets provide a limited amount of breathing gas to allow the user to reach the surface from a disabled vessel or vehicle, such as a disabled submarine, a sunken armoured vehicle, or a ditched helicopter. These may also be open or closed circuit. • Atmospheric pressure underwater breathing apparatus is also used, in the form of armoured
atmospheric diving suits, which maintain an internal pressure approximating surface pressure. Their breathing apparatus tend to be closed circuit
rebreathers.
Industrial breathing apparatus Breathing gas must be supplied for work in unbreathable normobaric atmospheres, which may be toxic, irritant, narcotic or hypoxic, and may include firefighting, damage control, exploration, and rescue work, and in
normobaric environments where contamination of the person (
hazmat environments) must be avoided. Open circuit and rebreather systems can be used, and self-contained (SCBA) and remotely supplied systems are used depending on the requirement for mobility. Positive or negative pressure equipment may be appropriate, depending on what is to be protected from contamination. A supplied-air respirator (SAR), also called an airline respirator, is a type of respiratory protection equipment used where the ambient atmosphere is unsuitable to breathe directly or after filtering at the user. The equipment may provide air on demand, at positive pressure, or may supply a constant flow at a rate greater than the user's peak demand rate. Depending on the nature of the hazardous atmosphere, the user may need to wear personal protective equipment to isolate the entire body from the environment (
hazmat suit).
Emergency and escape breathing sets (EEBD) Escape breathing apparatus are a class of self contained atmosphere supplying or air purifying breathing apparatus for use in emergencies, intended to allow the user to pass through areas without a breathable atmosphere to a place of relative safety where the ambient air is safe to breathe. These are ambient pressure systems, and include: •
Helicopter escape set •
Mine escape set •
Submarine escape set •
Amphibious Tank Escape Apparatus •
Smoke hood Early escape sets were often
rebreathers and were typically used to escape from
submarines that were unable to surface. Escape sets are also used ashore, in the
mining industry, and by the military for escape from tanks. The small open-circuit scuba
Helicopter Aircrew Breathing Device has the similar purpose of providing breathing gas to escape from a ditched helicopter. Another type of emergency breathing set, which is remotely supplied, is
built-in breathing systems in submarines and hyperbaric chambers. A
built-in breathing system is a source of
breathing gas installed in a confined space where an alternative to the ambient gas may be required for medical treatment, emergency use, or to minimise a hazard. They are found in
diving chambers,
hyperbaric treatment chambers, and
submarines. The use in hyperbaric treatment chambers is usually to supply an oxygen rich treatment gas which if used as the chamber atmosphere, would constitute an unacceptable
fire hazard. In this application the
exhaust gas is vented outside of the chamber. In
saturation diving chambers and
surface decompression chambers the application is similar, but a further function is a supply of breathable gas in case of toxic contamination of the chamber atmosphere. This function does not require external venting, but the same equipment is typically used for supply of oxygen enriched gases, so they are generally vented to the exterior by default. In submarines the function is to supply a breathable gas in an emergency, which may be contamination of the ambient internal atmosphere, or flooding. In this application venting to the interior is both acceptable and generally the only feasible option, as the exterior is typically at a higher pressure than the interior, and external venting is not possible by passive means. The
emergency oxygen supplied to passengers in commercial airliners that have lost cabin pressure is also a basic form of built-in breathing system, where the oxygen is generated and supplied as a constant flow for a limited period, which should be sufficient to allow the aircraft to safely descend to an altitude where the ambient air oxygen content is sufficient to support consciousness. These systems vent to the interior.
Smoke hoods and other
escape respirators are used in many industrial environments where they may be needed to evacuate a building in a fire or other incident which may compromise the ambient air quality but there is likely to be sufficient oxygen remaining to sustain the necessary activity. Emergency and escape breathing apparatus may provide purified ambient air where it has sufficient oxygen and it is reasonably practicable to purify it, or may supply stored breathing gas that is known to be respirable.
Supplemental oxygen provision Supplemental oxygen is oxygen additional to that available from atmospheric air at the ambient pressure. This may be necessary or desirable in hypobaric environments, or for medical purposes in any pressure regime. With supplemental oxygen the flow rate is often stipulated, but it is the partial pressure in the alveoli that is important to achieve the desired result, and that is strongly dependent on the delivery system of the breathing apparatus and the ambient pressure. Systems providing a constant flow rate of open circuit oxygen at the nose or mouth will waste a lot of the gas to dead space and during exhalation.
Oxygen conserving devices A closed circuit rebreather is highly effective at conserving stored oxygen, but it makes no use of ambient oxygen, so its effectiveness at minimising use of stored oxygen depends on where it is used. It is most applicable where it is not possible to use enriched ambient gas, either because there is none (underwater and in space), because its pressure is too low (extreme altitude), because it does not contain a useful partial pressure of oxygen, or because the contaminants make the risk unacceptable. The delivery of open circuit supplemental oxygen is most effective if it is made at a point in the breathing cycle when it will be inhaled to the alveoli, where gas transfer occurs. This is during the first part of inhalation. Oxygen delivered later in the cycle will be inhaled into
physiological dead space, where it serves no useful purpose as it cannot diffuse into the blood. Oxygen delivered during stages of the breathing cycle in which it is not inhaled is also wasted, unless it is stored temporarily. A continuous constant flow rate delivered to the mouth and nose uses a simple regulator, but is inefficient as a high percentage of the delivered gas does not reach the alveoli, and over half is not inhaled at all. A system which accumulates free-flow oxygen during resting and exhalation stages, (
reservoir cannulas,
partial rebreather masks and
non-rebreather masks) makes a larger part of the oxygen available for inhalation, and it will be selectively inhaled during the initial part of inhalation, which reaches furthest into the lungs, and may also recover the volume inhaled into dead space for re-use on the next breath if it can be accommodated by the reservoir bag. The flow rate must be matched to the breathing interface storage volume and the user's breathing tidal volume and breathing rate for best efficiency, and the tidal volume and breathing rate can vary considerably over a short period with changes in exertion, so these methods are not very effective for an active user. Delivery by demand valve avoids wastage of oxygen when the user is not actively inhaling, and when combined with a suitably calibrated dilution orifice can conserve a large proportion of the stored oxygen, but it still wastes oxygen to fill the anatomical and mechanical dead spaces, and it requires some physical effort by the user. Since the 1980s, devices have been available which conserve stored oxygen by delivering it during the stage of the breathing cycle when it is more effectively used. This has the effect of stored oxygen lasting longer, or a smaller, and therefore lighter, portable oxygen delivery system being practicable. This class of device can also be used with portable oxygen concentrators, making them more efficient. A
pulse dose oxygen conserving device, (or demand pulse device) senses the start of inhalation and provides a metered bolus, which, if correctly matched to requirements, will be sufficient and effectively inhaled into the alveoli. Such systems can be pneumatically or electrically controlled. Adaptive demand systems are a development in pulse demand delivery. They are devices that automatically adjust the volume of the pulsed bolus to suit the activity level of the user. This adaptive response is intended to reduce desaturation responses caused by exercise rate variation. The exhaled gas from these devices is discharged to the environment, and the oxygen is lost, so they are less gas-efficient than closed circuit rebreathers, but do not have a carbon dioxide scrubber or counterlungs, which is a saving on weight and bulk, and make use of the oxygen available in the ambient air, so their efficiency is better at lower altitudes.
High altitude supplemental oxygen Mountaineering breathing apparatus provides oxygen at a higher concentration than available from atmospheric air in a naturally hypoxic environment. Breathing pure oxygen results in an elevated partial pressure of oxygen in the blood: a climber breathing pure oxygen at the summit of Mt. Everest has a greater arterial oxygen partial pressure than breathing air at sea level. This results in being able to exert greater physical effort at altitude. The equipment must be lightweight and reliable in severe cold, including not getting choked with deposited frost from the exhaled gas, which is saturated with water vapour at body temperature. For mountaineering at high altitudes where the user has to carry the stored oxygen, it is desirable to maximise endurance of the set by efficient use of the gas. The theoretically available delivery systems are: a constant flow system without reservoir, which is simple and reliable, but extremely wasteful, a constant flow system with reservoir, which when matched to the user demand is more efficient than simple constant flow, and is also relatively simple and reliable, a demand valve system, which automatically follows user demand, but also wastes a significant part of inhaled gas on dead space, a pulse dose demand system, which wastes less gas on dead space, but relies on a relatively complex control system which introduces reliability issues, or a closed circuit system, which is very efficient, but requires a carbon dioxide scrubber. The exothermic carbon dioxide absorption reaction of a rebreather helps keep the scrubber contents from freezing while it is in use, and helps reduce heat loss from the user, but it is bulky and heavy, and is sensitive to freezing when not in constant use. Both chemically generated and compressed gas oxygen have been used in experimental closed-circuit oxygen mountaineering systems, but open circuit constant flow using a reservoir mask has usually been used in the field, although relatively wasteful, as the equipment is reliable. Although there is considerable similarity in the basic conditions in which aviation and mountaineering breathing apparatus is used, there are differences sufficient to make directly transferable use of equipment generally impracticable. One of the major considerations is that, unlike the aviator, the mountaineer cannot quickly descend to a safe altitude if the equipment fails, so it must be reliable. Another is that the mountaineer must personally carry the breathing apparatus, so the advantage gained by breathing supplemental oxygen must exceed the disadvantage of carrying the extra bulk and weight of the equipment. Other requirements are that the added work of breathing must be low, the equipment must function at low temperatures, and conservation of heat and moisture are desirable. The altitude range for mountaineering is also limited, there are no requirements for pressurisation.
Oxygen therapy Oxygen therapy is the use of supplemental oxygen as
medical therapy. Acute indications for therapy include
hypoxemia (low blood oxygen levels),
carbon monoxide toxicity,
cluster headache and
decompression illness. It may also be prophylactically given to maintain blood oxygen levels during the induction of
anesthesia. Oxygen therapy is often useful in chronic hypoxemia caused by conditions such as severe
COPD or
cystic fibrosis. Partial pressures administered range from low flow rates giving slight increases over ambient air up to 2.8 bar absolute used in
hyperbaric oxygen treatment of decompression illness and some other indications. Oxygen can be delivered to spontaneously breathing patients via
nasal cannula,
face mask,
artificial airway, or by
built-in breathing system demand mask or
oxygen hood in a
hyperbaric chamber. Delivery may be by continuous flow, by
bag reservoir mask, on demand, or on
pulse demand. Patients who are not able to breathe sufficiently for themselves are provided with breathing gas by ventilator or resuscitator.
Medical breathing apparatus An
anaesthetic machine (
British English) or
anesthesia machine (
American English) is a
medical device used to generate and mix a fresh gas flow of medical gases and
inhalational anaesthetic agents for the purpose of inducing and maintaining
anaesthesia.
Anaesthetic machines The anaesthetic machine is commonly used together with a
mechanical ventilator,
breathing system,
suction equipment, and
patient monitoring devices; strictly speaking, the term "anaesthetic machine" refers only to the component which generates the gas flow, but modern machines usually integrate all these devices into one combined freestanding unit, which is colloquially referred to as the anaesthetic machine for the sake of simplicity. In the developed world, the most frequent type in use is the
continuous-flow anaesthetic machine, which is designed to provide a supply of medical gases mixed with an accurate concentration of anaesthetic vapour, and to deliver this continuously to the patient at a safe
pressure and flow. This is distinct from
intermittent-flow anaesthetic machines, which provide gas flow only on demand when triggered by the patient's own inspiration.
Mechanical ventilators and resuscitators Mechanical ventilation is the provision of breathing gas to the user by the ventilator or resuscitator, when the user is unable to provide the driving forces to induce gas flow. Such
artificial ventilation is a characteristic of resuscitation and may be provided by medical ventilators when needed. Two basic types of mechanical ventilation may be distinguished by the limiting mechanism. Some are pressure controlled, in which the delivery stops when a limiting pressure is reached, and others are volume controlled, in which a set volume is delivered for each breath. Both of these methods have limitations and may work sub-optimally in some circumstances. A
ventilator is a type of equipment that provides
mechanical ventilation by moving breathable air into and out of the
lungs, to deliver breaths to a patient who is physically unable to breathe, or breathing insufficiently. Ventilators are
computerized
microprocessor-controlled machines, but patients can also be ventilated with a simple, hand-operated
bag valve mask. Ventilators are chiefly used in
intensive-care medicine,
home care, and
emergency medicine (as standalone units) and in
anesthesiology (as a component of an
anesthesia machine). A
resuscitator is a device using positive pressure to inflate the lungs of an
unconscious person who is
not breathing, in order to keep them
oxygenated and alive. There is considerable overlap between
ventilator and
resuscitator. The difference may mainly be in the way the equipment is used. There are three modes of mechanical ventilation, which are the ways in which a breath is delivered by a medical ventilator: In control mode, each breath is mechanically delivered, but may be triggered by a timing mechanism or by patient effort. These breaths may be volume or pressure controlled. In supported or spontaneous mode, each breath is triggered by the patient, and supported by ventilator. In combination mode, there is a combination of controlled and supported breaths, and there may be a combination of volume controlled and pressure supported or controlled breaths.
High altitude breathing apparatus High altitude breathing apparatus is used in aviation as standard equipment in unpressurised aircraft capable of high altitude flight, as emergency equipment in unpressurised aircraft, and in high altitude mountaineering.
Environmental influence At
high altitude, from there are physiological effects of the reduced oxygen partial pressure which include reduced exercise performance and increased respiratory rate.
Arterial oxygen saturation is generally still over 90% in healthy people, but arterial PO2 is reduced. At
very high altitude, from arterial oxygen saturation falls below 90% and arterial PO2 is reduced to the extent that extreme
hypoxemia may occur during exercise and sleep, and if
high altitude pulmonary edema occurs. In this range severe altitude illness is common. At
extreme altitude, above , one can expect significant hypoxemia,
hypocapnia and
alkalosis, with progressive deterioration of physiological function, which exceeds acclimatisation. Consequently, there is no human habitation in this altitude range.
Physiological effects In the region from sea level to around , known as the
physiological-efficient zone, oxygen levels are usually high enough for humans to function without
supplemental oxygen and
altitude decompression sickness is rare. The
physiological-deficient zone extends from to about . In this zone there is an increased risk of
hypoxia, trapped-gas
dysbarism (where gas trapped in the body expands), and evolved-gas dysbarism (where dissolved gases such as nitrogen may form in the tissues, i.e.
decompression sickness). Above approximately oxygen-rich
breathing mixture is required to approximate the oxygen available in the lower atmosphere, while above oxygen must be provided under positive pressure. Above , respiration is not possible because the pressure at which the lungs excrete carbon dioxide (approximately 87 mmHg) exceeds outside air pressure. Above , known as the
Armstrong limit, exposed fluids in the throat and lungs will boil away at normal body temperature, and pressure suits are needed. Generally, 100% oxygen is used to maintain an equivalent altitude of .
Physiogical acclimatisation People can become acclimatised to an altitude of if they remain at high altitude for long enough, but for high altitude rescue work, rescue teams must be rapidly deployed, and the time necessary to acclimatise is not available, making oxygen breathing equipment necessary above approximately .
Theoretical solutions An oxygen partial pressure equivalent to sea level can be maintained at an altitude of with 100% oxygen. Above , positive pressure breathing with 100% oxygen is essential, as without positive pressure even very short exposures to altitudes above lead to loss of consciousness. Oxygen conservation devices may be used with open circuit breathing apparatus to improve efficiency of gas use at lower altitudes where ambient pressure breathing is viable.
Management At high enough altitudes the partial pressure of oxygen in the air is insufficient to support useful work and consciousness, even after acclimatisation, and at even higher altitudes it cannot support human life. At altitudes where the problem is hypoxia, breathing gas with a higher oxygen content at ambient pressure is a viable solution. Supplemental oxygen sufficient to provide an equivalent altitude of a pressurised aircraft cabin (about 8000ft) is sufficient for many purposes, but higher concentrations, such as sea level equivalent (PO2 of about 0.21 bar), can allow a greater capacity for aerobic work. Balanced against this is the need to conserve oxygen and to minimise the weight carried by the user of breathing apparatus.
Practical aspects Where the user must carry the supplementary oxygen supply, and also perform significant work over a fairly long period, as in mountaineering and rescue work, the efficiency of oxygen use and the reliability of the breathing apparatus are more important, and there is a trade-off of these characteristics with the weight that must be carried. The amount of supplementary oxygen needed to bring the inhaled partial pressure to sea level equivalent, or any other fixed value greater than that of the ambient atmosphere is a function of the altitude, and increases with an increase in altitude in direct proportion to pressure drop. The amount of supplementary oxygen actually used is also proportional to
respiratory minute volume, which depends on the level of exertion.
Oxygen concentrators When there is no limitation on power use and the work is to be done at a fixed location, oxygen concentrators may be an effective solution. An oxygen concentrator is a device that concentrates the
oxygen from a gas supply (typically ambient air) by selectively removing nitrogen to supply an oxygen-enriched product gas stream. They are also used industrially and as
medical devices for
oxygen therapy. Two methods in common use are
pressure swing adsorption and
membrane gas separation. They are most efficient when the supplemental oxygen does not need to be at a high percentage. Pressure swing adsorption oxygen concentrators use a
molecular sieve to adsorb gases and operate on the principle of
rapid pressure swing adsorption of atmospheric
nitrogen onto
zeolite minerals at high pressure. This type of adsorption system is therefore functionally a nitrogen scrubber, leaving the other atmospheric gases to pass through, with oxygen as the primary gas remaining. Gas separation across a membrane is also a pressure-driven process, where the driving force is the difference in pressure between inlet of raw material and outlet of product. The membrane used in the process is a generally non-porous layer, so there will not be a severe leakage of gas through the membrane. The performance of the membrane depends on permeability and selectivity. Permeability is affected by the penetrant size. Larger gas molecules have a lower diffusion coefficient. The membrane gas separation equipment typically pumps gas into the membrane module and the targeted gases are separated based on difference in diffusivity and solubility. Product gas can be delivered directly to the user through a suitable breathing apparatus. Pulse dose (also called intermittent-flow or on-demand)
portable oxygen concentrators are the smallest units, which may weigh as little as Their small size enables the user to waste less of the energy gained from the treatment on carrying them. The unit administers a set volume (bolus) of oxygen enriched air at the start of each breath, which is the part of the breath most likely to reach the gas exchange regions of the lung beyond the physiological dead space. Their ability to make efficient use of oxygen is key to keeping the units compact.
Closed circuit oxygen rebreathers In a closed circuit system, any unused oxygen is retained and rebreathed, so the utilisation is close to 100%, with some losses possible due to expansion on increased altitude and incidental leakage from the breathing loop. There is a risk of
pulmonary oxygen toxicity if the pressure of the oxygen exceeds about 0.5 bar for extended periods, which could happen at altitudes below 5500 m, where atmospheric pressure is about half of the value at sea level. A closed circuit oxygen rebreather is the most efficient in terms of oxygen use, but is relatively bulky and requires the use of a carbon dioxide absorbent, which must either be sufficient for the oxygen supply, or must be periodically replaced. If the oxygen supply fails, the loop gas can become more hypoxic than ambient atmosphere if the loop was not adequately purged or if it gets contaminated by ambient air. In the absence of oxygen monitoring the user may not notice the reduction in oxygen concentration. A closed-circuit oxygen system was tested by
Tom Bourdillon and
Charles Evans during the 1953 British expedition to Mount Everest.
Open circuit dilutor demand regulator The
dilutor demand regulator was developed and extensively used for high altitude flying during WWII. A dilutor demand regulator draws ambient air into the mask through an orifice in the regulator, while concurrently being fed with pure oxygen through a demand valve in the regulator. For aeronautical use the size of the ambient air orifice is controlled by an aneroid valve operator and is directly proportional to atmospheric pressure. As the altitude increases, the pressure decreases and the orifice gets smaller, so the user is provided with a higher proportion of oxygen, and when correctly calibrated, the partial pressure of oxygen in the mixture remains fairly consistent at a value similar to the 0.21 bar at sea level. This system makes efficient use of a combination of ambient and stored oxygen. The function of the aneroid valve operator can be substituted for terrestrial use by a simpler, lighter, and more rugged manually operated orifice selector knob, giving a stepwise range of concentrations which is lighter, more reliable, a bit less efficient, and requires appropriate selection by the user. It also allows the user to manually adjust the mixture to match personal needs. As it is manually selected, It is less suitable for flying, and more suitable for pedestrians who will not change altitude rapidly. The flow rates through the orifice and regulator are sensitive to flow rate of inhalation, and can be designed to provide a somewhat higher oxygen partial pressure at higher inhalation flow rates, which helps compensate for higher exertion.
Obligatory pressurisation zone This is the zone where 100% oxygen at ambient pressure is insufficient, and some form of pressurisation is required to provide a viable inhalation oxygen pressure. The options are partial pressurisation and full pressurisation. A pressure suit is a
protective suit worn by high-altitude pilots who may fly at altitudes where the
air pressure is too low for an unprotected person to survive, even breathing pure oxygen at
positive pressure. Such suits may be either full-pressure (e.g., a
space suit) or partial-pressure (as used by
aircrew). Partial-pressure suits work by providing mechanical counter-pressure to assist breathing at altitude. worn by astronaut
Buzz Aldrin on
Apollo 11, with completely self-contained life support for lunar excursions. worn by astronaut
Michael Fincke outside the
International Space Station, which has a remote supply via the umbilical. A
space suit is a garment worn to keep a human alive in the harsh environment of
outer space, primarily as protection from
vacuum and temperature extremes. The breathing gas is pure oxygen, which allows the lowest suit pressure. Space suits are often worn inside
spacecraft as a safety precaution in case of loss of cabin
pressure, and are essential for
extravehicular activity (EVA). Modern space suits augment the basic pressure garment with a complex system of equipment and environmental systems designed to keep the wearer comfortable, and to minimize the effort required to bend the limbs, resisting a soft pressure garment's natural tendency to stiffen against the vacuum. A self-contained
oxygen supply and environmental control system may be used to allow greater freedom of movement, independent of the spacecraft. Three types of space suits exist for different purposes: IVA (intravehicular activity), EVA (extravehicular activity), and IEVA (intra/extravehicular activity). IVA suits are meant to be worn inside a pressurized spacecraft, and are therefore lighter and more comfortable. IEVA suits are meant for use inside and outside the spacecraft, such as the
Gemini G4C suit. They include more protection from the harsh conditions of space, such as protection from
micrometeoroids and extreme temperature change. EVA suits, such as the
EMU, are used outside spacecraft, for either planetary exploration or spacewalks. They must protect the wearer against all conditions of space, as well as provide mobility and functionality. ==Safety==