VOL: 98, ISSUE: 40, PAGE NO: 50
Lorna McLauchlan, RGN, DNCert, Dip.Prof.Studies (District Nursing), Dip.Asthma Care, is community respiratory nurse specialist, Ashton, Leigh and Wigan Primary Care NHS TrustThe prescribing criteria, assessment and benefits of long-term oxygen therapy in chronic obstructive pulmonary disease (COPD) are well established. While an interest in the use of liquid oxygen has developed over recent years, the Royal College of Physicians (1999) recognises that further research needs to be undertaken in this area.
The prescribing criteria, assessment and benefits of long-term oxygen therapy in chronic obstructive pulmonary disease (COPD) are well established. While an interest in the use of liquid oxygen has developed over recent years, the Royal College of Physicians (1999) recognises that further research needs to be undertaken in this area.
Benefits of long-term oxygen therapy
The Nocturnal Oxygen Therapy Trial Group(1980) examined the benefits of long-term oxygen therapy. Two hundred and three patients were randomly allocated to either continuous oxygen (24 hours a day) or nocturnal oxygen (12 hours a day), and the results showed a 50% decrease in mortality in the continuous oxygen user group.
Further evidence of the benefits of long-term oxygen therapy was found in the Medical Research Council Working Party Study (1981). Of 87 patients with COPD, 42 received oxygen for 15 hours a day, while a control group received no oxygen. Mortality was again significantly reduced in the oxygen user group.
Since this evidence emerged, along with the publication of the British Thoracic Society guidelines for the management of COPD (British Thoracic Society 1997) the cost of oxygen therapy in the UK has steadily risen.
Domiciliary oxygen concentrators
Domiciliary oxygen concentrators have been available on the Drug Tariff since 1985, and in 1999 around 18,000 machines were prescribed in the UK at an approximate cost of £683 rental per machine per year (Thompson, 2002).
An oxygen concentrator draws room air in through a series of filters, removing fine dust particles from the air. This air is then forced through two molecular sieve beds filled with a substance called zeolite. Here nitrogen and carbon dioxide are removed. The oxygen is then directed via a compressor through a flow meter that is regulated to the desired flow rate. Up to three concentrators can be joined to achieve a maximum flow rate of 12 litres per minute. The waste gas from the sieve beds is then expelled back into the room air, thus allowing a continuous supply of oxygen without the fear of running out. It allows the patient to move around the home, and approximately 15 metres of tubing can be used to aid mobility (Fig 1).
Compliance with long-term oxygen therapy
Compliance with the use of LTOT has been found to be variable. Pepin et al (1996), in a prospective multicentred trial of 930 patients with COPD, found that only 419 patients used oxygen therapy effectively. Effective use was defined as a minimum of 15 hours a day, and data was collected from patient questionnaires and meter readings from the concentrator.
Patient overestimation of oxygen use was reported by Ringbaeck (1999) who, in a study of 182 patients, identified 13% who admitted to using oxygen for less than 15 hours a day on patient questionnaires while meter readings highlighted that 34% were non-compliant.
Restrick et al (1993) noted that, out of a sample of 175 subjects, 47 patients actually admitted to not wearing the nasal cannula for a period of two hours a day while the concentrator was switched on. Similar work by Vegeret et al (1986) accepted that meter readings and patient questionnaires revealed only the functioning of the equipment and not the time that patients actually used their oxygen.
A validation study, using an electrode timer that detected when the nasal cannula was in contact with the skin, was carried out by Phillips et al (1994). They found that, while the concentrator clock showed that oxygen was used for a mean of 18.2 hours a day, the electrode timer measured a mean of 12.7 hours a day when the nasal cannula was in contact with the skin. These studies show the difficulty in assessing compliance with LTOT and the problems associated with using meter readings and patient questionnaires.
Long-term oxygen therapy and ambulatory oxygen
The use of ambulatory oxygen has not been as well researched as LTOT, and the British Thoracic Society warns that more evidence is required to support the assessment for ambulatory oxygen.
It suggests that the prescription for ambulatory oxygen should be based on a documented fall in arterial oxygen saturation of more than 4% to below 90% on a standard walking test, and associated with an improvement in exercise tolerance or breathlessness (British Thoracic Society, 1997).
Royal College Of Physicians guidelines suggest that the purpose of assessment is to measure the extent of desaturation, to improve exercise capacity with supplementary oxygen, to maintain oxygen saturation above 90% and to determine the type of delivery system to use (Royal College Of Physicians, 1999).
Portable oxygen cylinders
Portable oxygen cylinders are available on prescription in the UK. However, they have their limitations. They can be heavy and difficult for patients to handle, and the short duration of gas supplied by them can limit patient activities.
The need for repeated prescriptions along with waiting for deliveries of new cylinders can be frustrating. Thompson (2002) provides a detailed outline of the different modalities of portable units and their application and recommends that a more flexible combination of cylinder, concentrator, conserver device and liquid oxygen is required.
Portable oxygen systems
A number of different portable cylinders are available with different performance details (Table 1).
The PD, DD and PA2 cylinders are all available on prescription (regional variances can occur), excluding the carrying case, which needs to be purchased separately (Fig 2). The PD cylinder is the heaviest of the portable systems but is ideal for patients who require only very short amounts of outside use. It provides 150 minutes of gas when the flow rate is set at two litres per minute.
The DD and PA2 cylinders are lighter in weight and provide a longer duration of gas - approximately 230 minutes. They have the advantage of an integral cylinder head that eliminates the need for exchanging cylinder heads, which can prove difficult for some patients. The PA2 cylinder also has the advantage of providing a wider range of flow rates from 0.5-15 litres per minute.
Cryogenic liquid oxygen is produced in air separation units, which use low temperatures to liquefy the gas (Fig 3). Liquid oxygen is far more condensed than its gaseous form, giving a greater amount of oxygen in a smaller container. The liquid is stored and transported in a series of vacuum-insulated containers and tanks. Patients have a base unit (reservoir) of oxygen from which they can refill their own small portable unit. This will give approximately eight hours of oxygen when full.
Patients are able to refill their own portable unit but must be available for delivery of an exchange of their base unit (usually every two weeks). There is also an evaporation rate to take into account when evaluating cost for patients who do not use the system frequently enough.
A comparison of liquid oxygen and gaseous oxygen for portable use was carried out by Lock et al (1991), where the use and acceptability of the different modalities was compared in 15 patients. Results showed that the liquid oxygen was used for longer periods (23.5 hours a week for liquid compared to 10 hours a week for the gaseous cylinders), and the liquid system was preferred due to longer duration, ease of filling and carrying the canister.
Small cylinders are ideal for occasional use, and the duration of time that the gas lasts can be lengthened with the use of an oxygen conserver device. This releases oxygen as a pulsed dose when triggered by the inspiratory effort, and oxygen is delivered during the first two-thirds of the breath.
The work of Garrod et al (1999) found that the oxygen in a portable cylinder and a liquid oxygen tank lasted four times longer when a conserver device was used, compared with a continuous flow of oxygen. Use of a conserver would appear to decrease the cost of the supply of ambulatory oxygen due to the extra duration of time. However, the cost of the conserver device must be taken into consideration.
An evaluation of pulsed-dose oxygen during exercise performed on patients with COPD (Garrod et al, 1999) showed no significant difference in arterial oxygen saturation readings during a shuttle walking test using continuous oxygen at 2litres per minute or via the pulsed-dose delivery system. Contrasting evidence was found using intermittent boluses of oxygen (Garrod et al (1999). Intermittent devices deliver a bolus of oxygen every second breath while the pulse dose delivers oxygen every breath.
Compliance with ambulatory oxygen
Provision of oxygen therapy constitutes one of the largest medical expenses. Evaluating compliance with LTOT and the consumption of mobile oxygen, Ringbaeck (1999) found that the use of LTOT was increased when patients had access to mobile oxygen.
However, caution must be taken, as Ringbaeck found that only 58% of patients with mobile units reported using the oxygen outdoors. While outdoor activity was shown to reduce consumption of the concentrator oxygen, compliance with the use of mobile units was also poor. More than half of the patients had used it for less than two hours a week. It was suggested that small cylinders were difficult to handle for weak patients and that the weight of portable cylinders was the most important factor affecting use. These findings are replicated in the work of Vegeret et al (1989), where 25% of participants never used their portable oxygen, 15% were using it at home and only 60% used their mobile units outside of the house and garden. These findings have important cost implications for the prescribing, assessment and follow-up of patients for ambulatory oxygen.
While guidelines are now available for the assessment of ambulatory oxygen, more evidence is required if we are to have a wider choice of ambulatory systems to choose from. This wider choice of devices would lead to a selection of devices tailored to patients' needs rather than to cost and availability.