An oxygen concentrator is a device that concentrates the oxygen coming from a particular source (typically ambient air) providing a gas mixture enriched with oxygen. Generally an oxygen concentrator makes use of the technology known as "pressure swing adsorption", a physical process for the separation of gas mixtures by adsorption under pressure. This type of medical device is being used widely for the supply of oxygen in medical applications, in particular when liquid or pressurized oxygen is too dangerous or inconvenient, such as in private homes or in other contexts.
The oxygen concentrators work based on the principle of the rapid pressure swing adsorption (PSA) Atmospheric of nitrogen on a molecular zeolite sieve. The nitrogen is subsequently removed. This type of adsorption system is then essentially a "filter" for the nitrogen that allows other atmospheric gases to pass without problems the zeolite sieve. At the end of this process remains high oxygen concentration, as the main residual gas. The PSA technology is a reliable technique, fast, convenient and economical for the generation of oxygen in small or medium quantities, while the cryogenic separation lends itself more to the production of higher volumes. The porous zeolite, at high pressures, adsorbs large amounts of nitrogen, because of its large contact surface. In a second step, after the separation of oxygen and other gases that are not adsorbed, via a further step the nitrogen is to be deadsorbed by the molecular sieve. An oxygen concentrator is simply constituted by an air compressor, two cylinders filled with pellets of zeolite, a pressure equalization tank, and some valves and pipes. In the first half of the cycle, a first cylinder receives air from the compressor, for a time of about 3 seconds. During this time the pressure in the first cylinder increases from the levels of the atmospheric pressure up to a pressure equal to approximately twice the normal atmospheric pressure (typically 20 psi/138 kPa, ie 2,36 absolute atmospheric pressure) and the zeolite is saturated with nitrogen. Once in the first cylinder, at the end of the first half of the cycle is reached a concentration close to that of pure oxygen (not sieved by the zeolite remain small amounts of argon, CO2, water vapor, radon, and other minors atmospheric components), opens a valve and the oxygen enriched gas flowing to the tank pressure equalization, which is connected to the tube of oxygen to the patient. At the end of the first half of the cycle, occurs another change of position of the valve, so that the air from the compressor is directed toward the second cylinder. In the moment in which the oxygen enriched moves direct to the tank, the pressure in the first cylinder descends, the thing which allows nitrogen to be deadsorbed and to return in the diluted gas. During the second half of the cycle occurs another variation of position of the valve which allows gas present in the first cylinder to vent into the ambient air. At the same time the concentration of oxygen in the tank pressure equalization is maintained and does not descend to below about 90%. The pressure in the pipe that carries oxygen from the expansion tank until the patient is kept constant thanks to the presence of a pressure reducer.
The old units concentrators were characterized by a cycle of a period equal to about 20 seconds, and were able to deliver up to 5 liters per minute of a mixture containing oxygen percentage higher than 90%. Since 1999, have been sold in a unit capable of delivering up to 10 liters per minute, with the same concentration.
The oxygen generators using PSA system represent a cost-effective source of oxygen. They are safer, less expensive, and tend to be more affordable compared to cryogenic oxygen tanks or classical liquid oxygen tanks. They find application in different fields and industries in addition to the medical, for example in the pharmaceutical field, water treatment and in the production of the glass.
Newer concentrator devices are technically designed to deliver oxygen-enriched air up to 90-95%, with variations of up to ± 3% depending on the extent of the flow. They are divided into stationary concentrators (whose power source is represented by the normal electric network) and mobile concentrators (with source of battery power).
For safety reasons, these devices are equipped with an alarm system that is activated when, for whatever reason, the supply of oxygen-enriched air drops below 82% concentration. Again for safety issues the patient that uses the device at home, outside of every health control, is instructed to use a cylinder of oxygen gas in case of emergency and to request qualified help for the control and maintenance of the concentrator. The weight and size of these devices are quite content: the units currently commercially hardly exceed the 25 Kg. These characteristics represent a real limit to the attainment of workflows and particularly high concentrations. However, even in the presence of these limitations, these units provide excellent performance for a wide range of needs. In particular in patients who have reduced ability to mobility, consistent with certain clinical features, the use of concentrators fixed is indicated and particularly favorable either in the home or in some health structure with hospital or territorial valence (for example in some rehabilitative care homes and assisted-RSA nursing homes).
As already mentioned above the therapy with oxygen by dispenser should be considered a treatment with oxygen-enriched air. In other words, the percentage of oxygen differs from that obtained with liquid oxygen (O2 concentration is not less than 99.5%), and generally, with some variations from manufacturer to manufacturer, reaches 90-95% and in any case never drops below 82%. The main building regulations also provide for a tolerance of ± 3% compared to the concentrations reported by the manufacturer.
Beyond the small variations from model to model, it must be noted that the concentration of oxygen provided by these devices, it can also vary depending on which air flow is meant to reach the mouth of the patient. By way of example, if a flow of 1 liter/minute is compatible with a concentration of O2 concentrator equal to approximately 95%, increasing the flow up to 5 liters/minute, the O2 concentration decreases to about 90%. Normally, this decrease in oxygen concentration does not hold any clinical significance, however, is always a good idea to re-evaluate the oxygen saturation (SpO2) and PaO2 after an appropriate period of time (half an hour) breathing with the flow prescribed by a doctor and dispensed from the concentrator at home. The oxygen concentration is to be reduced also due to other factors, mainly including malfunctions or problems with their filters, especially the bacteria filter. For this reason, it is still appropriate to carry out proper maintenance of the unit once or twice a months. The use of the oxygen concentrator obviously entails a cost for the consumption of electricity. This cost is calculated for a 400 watt for a unit that functions 20 hours a day, is around 25-30 € per month. In some European countries this expense is reimbursed by the health service. According to some guidelines for the long term oxygen therapy is not recommended resort to the use of the oxygen concentrator in case the patient needs to flow in excess of 5 liters per minute. Below this limit the use of the oxygen concentrator is certainly possible even if should always be evaluated peculiar clinical characteristics of the patient. According to some studies the best results with this type of device is to have oxygen flows around 2 liters/minute, or streams that allow O2 concentrations not less than 95%.
Technological progress is encouraging the development of several concentrators portable type powered by a rechargeable internal batteries that produce a stream of non-medical oxygen (ie, the concentration of which is less than 99.5%) at intermittent flow, in pulsed mode (ie oxygen delivery in the first part of the inspiration) or on demand (oxygen delivered throughout inspiration).