The models listed have all been developed for the North America and European healthcare markets and have mostly been manufactured in quantities ranging from 10,000 to 360,000 over several years. Two models from different manufacturers were stated to be recent modifications with improved performance, based upon earlier models.

The machines in Table 2 have similar overall specifications and are all approximately the same size and weight. They are available in any different voltage and frequency versions, typically 120V 60Hz, 220 V 60Hz or 230V 50Hz within +/- 10% voltage range, 50Hz versions may deliver slightly lower oxygen concentrations than 60Hz versions. The specifications and costs are given below.

Specifications and costs

(1) Recently modified.

(2) Information is also given about a suitable DC/AC inverter at a cost of US$ 1200.

All except the HBO model are stated to deliver 90% oxygen at a flow of 5 litres/min with acceptable tolerances and the maximum concentration is reached within 10 minutes of switching on. Oxygen is delivered at a typical pressure of about 60KPa and a back pressure of 7KPa has no effect on output. This means that they can be used to deliver oxygen through 6mm ID nylon tubing for upto 10m if required. The AirSep Intensity has an outlet pressure of 140KPa, which is stated to be suitable for driving nebulizers and venti-masks.

Sound levels at 4 litres/min are in the range 42-50 dB(A) for each machine, except the Kroeber AeroPlus 600, which is stated to be 38 dB(A). All are acceptable sound levels.

All the models are fitted with life long sensors to measure oxygen concentration and are not dependent upon fuel cells. These analysers have an accuracy of about +/- 3% at the working concentration. Appropriate alarms to indicate machine malfunction, low and high pressure, power failure and low oxygen concentration are fitted to these machines.

The operating temperature range and the maximum relative humidity for normal functioning are important performance indicators. The temperature range should be as wide as possible and the maximum humidity as high as possible. High humidity is particularly likely to impair performance. It cannot be assumed that the machine will be installed in an air-conditioned envi4ronment.

Perhaps the most important characteristics which are common to thee models are that they comply with the relevant regulatory requirements. They all comply with ISO 8359 (the particular Standard for oxygen concentrators of this type). They have all been certified to comply with IEC 60601-1 (the relevant electrical safety Standard) and finally they are all CE marked. The CE marking process involves compliance with ISO 9001, EN 46001 and the European Medical Device Directive (93/42/EEC) with third party inspection annually.

Some aspects of the information provided for each model listed in Table 2 are highlighted below.

AirSep Elite The cheapest machine with the largest number manufactured, 5-year warranty on parts and low cost filters keep the running costs low. The widest range of running temperatures (10-43 ^oC) is also claimed at highest humidity.

AirSep Intensity This model has 2 flowmeters and a higher pressure output than other machines. This could be useful to drive ancillary devices. With a 4-way flowsplitter on each flowmeter this model could deliver 1 L/min to 8 patients at the same time.

DeVilbiss This model is a re-design of the earlier model that met the previous WHO specification. A special system is available for interrogating the condition of the machine for maintenance purposes. Running costs are higher than average. An estimated but expensive flowsplitter is available for paediatric use, which permits flows of 0.5 or 1 L/min.

HBO This is a comparatively new design, which is also stated to have a flowsplitter for paediatric use. It gives a higher pressure output than most machines (90Kpa) and is stated to perform at up to altitudes of 4000 m.

Kroeber This is the quietest machine and also has a system for downloading machine data to a personal computer for maintenance purposes. It is stated to be compatible with both a generator and a DC/AC inverter. Spare parts are the most expensive but it has a cheap flowsplitter.

Nidek Mark 5 and Dual The Nidek models are similar in price and performance to the AirSep Elite. It is a well-established, reliable design. The cost is low and the specification good. The Dual has two flowmeters that can both be fitted with flowsplitters but the total flow is limited to 5 L/min.

Respironics Millennium This model is competitively priced and has low running costs but had the narrowest range of operating temperatures. It uses a different compressor than most other models. It has a 4-way flowsplitter and is stated to be generator compatible.

Clearly some machines may be more appropriate in certain circumstances than others, and this may be a matter for local decision. A key factor is whether the concentrator is likely to be used on more than one patient at the same time.

There are machines with 2 flowmeters (at a small extra cost), which can be used for 2 patients, available from 2 manufacturers. The Nidek Dual machine is basically the 5 litre model fitted with 2 flowmeters. The AirSep Intensity machine has an 8 litre/140 KPa specification, which is clearly better suited to serving 2 flowmeters.

For both the Nidek and the AirSep Dual machines the user will have to ensure that the total flow does not exceed the manufacturers specified flow.

All companies offer flowsplitters for up to 4 patients with fixed flows of 0.5 or 1 L/min. These systems are intended for use to treat pneumonia in infants. In all cases the maximum flow produced by the concentrators must not be exceeded. Table 3 lists the total cost for the concentrator for 40,000 hours of use together with the cost of the flowsplitter.

(1) Total Flow = 5 Litres/min

(2) Total Flow = 8 Litres/min

It can be seen that machines with dual flowmeters and dual flowsplitters could treat patients for a capital cost of about 0.5 US cents per hour. For machines with single flowmeters and flowsplitters the capital cost per hour per patient for different models is in the range of 0.6 to 2 US cents.

The revised questionnaire asked specific questions about the availability of re-usable, sterilizable humidifiers with concentrators.

All concentrators can be supplied with humidifiers. For single patient use, it is standard practice to supply “disposable” humidifiers. These are usually bought-in items that are not made by the concentrator manufacturers. Some types of disposable humidifier can be disinfected in boiling water and re-used several times for the same patient.

None of the humidifiers that are normally supplied with oxygen concentrators are stated to be sterilizable or suitable for use with different patients. It is however known that sterilizable humidifiers exist and possibly they should be purchased as separate items, if they are required.

The revised questionnaire asked manufacturers to state the make, model and cost of a generator suitable for running their concentrator. The replies pointed out that the requirements for a suitable generator depended upon the specification of the compressor motor because this had to be matched exactly to the generator output. In general terms, the recommendations were for a 100Watt generator to allow for start-up surges, with a true sine wave output. Honda, Tecumseh and Generac were mentioned as possible sources of generators, with typical costs of up to US$500.

One manufacturer (Kroeber) also mentioned the use of a DC/AC inverter, which could be used with a wind or solar power source and adequate battery storage. The Inverter would cost US$ 1200 with a further, unquantified cost for the DC source and battery storage. Kroeber also indicate that systems involving car batteries for storage have proved to be reliable and cost-effective.

There were large variations between manufacturers in the quoted cost of spares for 40,000 running hours. The variations were due to differences in the guarantee times (1-5 years), in the quantities of parts used) only one manufacturer quoted for replacement of sieve beds) and in the actual cost of equivalent items ($31 to $250 for the cost of filters).

In spite of these variations, the costs of spare parts is not considered to be excessive and it is essential that the recommended quantities of spare parts are purchased at the outset, i.e. the budget for purchase should include the cost of spares for 40,000 hours. This will typically cover 8 to 10 years of use and will require at least two changes of compressor seals by local technicians who will need to be trained.

All manufacturers say that they are willing to participate in the training of technicians.

This report covers 8 models of oxygen concentrator on the basis of the information supplied by their manufacturers. It concludes that all the machines could be considered for use in Developing World locations. Their respective advantages and disadvantages are discussed together with their suitability for different users.

All but one of the models was stated to cost less than US$2000 for 40,000 running hours (approximately 8 years of actual use). The cost per hour to treat multiple patients could be as low as 0.5 US cents per patient.