High-altitude breathing apparatus is a breathing apparatus which allows a person to breathe more effectively at an altitude where the partial pressure of oxygen in the ambient atmospheric air is insufficient for the task or to sustain consciousness or human life over the long or short term.
High-altitude breathing sets may be classified by type in several ways: by application: aviation breathing apparatus and mountaineering breathing apparatus, by breathing gas source: self-contained gas supply, or remotely supplied gas, by breathing circuit type: open, semi-closed, or closed circuit, by gas supply type: constant flow, supply on demand, or supplemental, by ventilatory driving force: the breathing effort of the user, or mechanical work from an external source, by gas mixture: air, oxygen-enriched, or pure oxygen. The user respiratory interface is the delivery system by which the breathing apparatus guides the breathing gas flow to and from the user. Some form of facepiece, hood or helmet is usual.
Any given unit is a member of several types.
Breathing at high altitude High-altitude breathing apparatus is used for unpressurised (ambient pressure) aeronautical and mountaineering activities where the oxygen content of the natural atmosphere is insufficient for maintaining physical activity, consciousness, or life, but the atmospheric pressure is sufficient that a pressure suit is not needed.
Both rebreather and open-circuit equipment have been used in this application, where either pure oxygen or supplemental oxygen is provided by the equipment. Minor leakage in either direction usually only affects efficiency and gas endurance, as the ambient air is usually only hypobaric due to low ambient pressure.
Altitude zones 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.
Above 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 using pressure suits and full pressurisation in space suits.
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 ) exceeds outside air pressure. Above , known as the Armstrong limit, exposed fluids in the throat and lungs start boiling away at normal body temperature, and pressure suits are needed. Generally, 100% oxygen is used to maintain an equivalent altitude of .
The death zone altitude is or above.
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 .
In aviation, there is generally no opportunity to acclimatise, and the necessity for breathing apparatus generally assumes that the user starts the flight at or near sea level.
Equipment use 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 with less than 100% oxygen 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 8000 ft) 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.
Drop tanks To avoid carrying the weight of all the cylinders all the way up and down a peak, one or more cylinders can be cached along the route for retrieval on the return. This implies the use of two or more cylinders to carry the oxygen supply for the summit day, which may last up to 18 hours, making an endurance of about 6 hours for a full cylinder a practical choice. This strategy is not possible if a single cylinder is used.
Types of apparatus High-altitude supplemental oxygen Mountaineering breathing apparatus provides oxygen at a higher concentration than available from atmospheric air in a naturally hypoxic environment. It needs to be lightweight and to be reliable in severe cold, including not getting choked with deposited frost from the exhaled gas, which is saturated with water vapour at body temperature.
Both chemically generated and compressed gas oxygen have been used in experimental closed-circuit oxygen mountaineering systems, but open circuit has mostly been used, although relatively wasteful, as the equipment is more reliable. For mountaineering at high altitudes where the user has to carry the stored oxygen, open-circuit demand or closed circuit may be used to maximise endurance of the set.
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 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 is vulnerable to freezing during periods when it is not in active use.
Unpressurised aircraft and high-altitude parachuting have similar requirements and working environment to mountaineering, but weight is less of a problem.
Closed-circuit oxygen rebreather
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.
An additional potential advantage of a rebreather is that the carbon dioxide scrubbing reaction is exothermic and keeps the gas in the breathing circuit warm if sufficiently insulated. It also conserves humidity, reducing dehydration. The disadvantages include the weight of the scrubber and the problems of the humidity condensing in the circuit and freezing, which can block the gas passages and choke the scrubber. If the scrubber freezes, it must be defrosted before the reaction can resume, taking some time to warm up to a temperature at which the reaction is sufficient.
Open-circuit dilutor demand regulator The dilutor demand regulator was developed for high-altitude flying during the Second World War. 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. 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 do 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.
Oxygen sources
Bottled oxygen