Summary
Home oxygen therapy should be prescribed only for patients with symptoms and signs of chronic hypoxaemia. The patients should be thoroughly assessed and reversible factors treated. Objective evidence of hypoxaemia at rest and its increase on exertion and at night is readily obtained.If a patient requires oxygen therapy for more than 4 hours a day at a flow rate of 2 L/minute (1 x E size cylinder a week), a concentrator rather than a cylinder should be prescribed. Patients and families should be instructed in the care and maintenance of the equipment, and the precautions to take when using oxygen.Home oxygen therapy is expensive, but correctly used, it can add quality, as well as years, to life

History
Oxygen is a colourless, odourless, tasteless gas which constitutes approximately 21% of the air at sea level.

Although it was first prepared by Stephen Hale in 1727, its importance as a normal component of air was not identified until the work of Priestley in 1777. Lavoisier and his colleagues demonstrated that oxygen was absorbed from the lungs and, after metabolism in the body, excreted as carbon dioxide and water.

The need for oxygen
Oxygen is necessary to provide energy, not only for the metabolism of food, but also for all cellular activity. It is always indicated when patients experience acute hypoxaemia to prevent death or irreversible brain damage, while efforts are made to reverse the cause of the hypoxaemia.

The use of oxygen therapy, especially at home, for patients with chronic hypoxaemia has been the subject of more discussion. Guidelines have been developed by the Thoracic Society of Australia and New Zealand to limit the inappropriate use of what can be a valuable but expensive form of treatment.1

Transport of oxygen in the blood
Oxygen is carried in the blood in two forms:

  • in physical solution in the plasma
  • combined with haemoglobin

The oxygen in physical solution in the plasma determines the oxygen tension in the arterial blood, which is usually 80-100 mmHg. This in turn determines the amount of oxygen that combines with haemoglobin for transport to the tissues. In normal patients breathing air, the oxygen saturation of haemoglobin is at least 97% at sea level.

The objective of oxygen therapy is to ensure that the oxygen tension is maintained at 60 mmHg at rest and the oxygen saturation of haemoglobin at 90% during exercise.

Indications for oxygen therapy in chronic disease
Supplemental oxygen is indicated when it can be shown that the patient's disability is related to arterial hypoxaemia which can be reversed by its administration. Patients who may benefit include those with

  • chronic airflow obstruction, especially if the PaCO2 is >45 mmHg
  • diffuse interstitial lung disease
  • advanced pulmonary malignancy
  • advanced cystic fibrosis
  • severe congestive cardiac failure
  • congenital cyanotic heart disease.

In the absence of hypoxaemia, oxygen therapy is unlikely to benefit patients with dyspnoea, angina or heart failure, although the placebo effect is recognised.

Criteria to be met
Before oxygen therapy is prescribed, there should be a thorough assessment of the patient's respiratory and cardiac function to determine the cause of the hypoxaemia.

The use of simple objective tests of respiratory function, such as ventilatory capacity and forced expiratory volume in the first second (FEV1), and arterial blood gases will provide a baseline against which to assess improvement or deterioration. Many authorities regard the assessment for home oxygen as requiring a period of time after any acute illness e.g. one month, and then having two separate blood gases demonstrating the hypoxaemia. Clinical, radiological and cardiac assessments will determine the severity of right heart failure and pulmonary hypertension.

Reversible conditions must be investigated and treated e.g. correcting anaemia. The patient must be on optimum drug treatment and should not smoke.

Prescription of home oxygen should be based on blood gas measurements, except for patients who have had episodes of life-threatening asthma despite appropriate and supervised maintenance therapy. Home oxygen may be prescribed for continuous or intermittent use.

Continuous use
Under the guidelines developed by the Thoracic Society of Australia and New Zealand, long-term continuous oxygen therapy (at least 15 hours a day) should be prescribed when the arterial oxygen tension is consistently <55 mmHg or the oxygen saturation is 88% or less. There is still debate whether or not measurement of oxygen saturation alone will suffice. As oximetry has a recognised error/variability of ±3%, most clinicians favour measurement of oxygen tension as well as oxygen saturation. When there is polycythaemia or corpulmonale, an oxygen tension <60mmHg is an indication for continuous oxygen. This shortens the periods of hypoxaemia and reduces the pulmonary hypertension.2

If the patient's mental function is impaired by episodes of hypoxaemia, the treatment is continuous oxygen therapy. If there is any concern about the patient's comprehension and memory, the carer should take the responsibility for the administration of oxygen. It is always wise to ensure that the carer understands not only the method of administration, but also all the necessary safety precautions.

Intermittent use
When activity is restricted by hypoxaemia associated with obstructive or fibrotic lung conditions or advanced malignancy, supplemental oxygen may limit hypoxaemia and improve exercise tolerance. An exercise challenge, preferably in hospital, will show not only the hypoxaemia, but also whether or not the use of supplemental oxygen is beneficial in terms of functional improvement and improvement in oxygen saturation.

Intermittent supplemental oxygen may also be indicated for asthmatics for whom medical help may be delayed. The patient and carer must be responsible and accept the need to seek medical help promptly and not delay because oxygen is available. Oxygen therapy at night can also be prescribed if sleep studies reveal hypoxaemia that is corrected by oxygen therapy. Nocturnal nasal CPAP (continuous positive airways pressure) is used to reverse apnoea and atelectasis. It should not be used to reverse hypoxaemia.

Review
The timing for review will be determined by the patient's underlying condition and prognosis. In general, the prescribing physician should review the patient within 6 months and record blood gas measurements and oximetry. Subsequent review at 12-monthly intervals is always desirable.

Equipment
Home oxygen may be provided from cylinders, oxygen concentrators or liquid oxygen systems. The last are not widely available in Australia, but in the future they may offer several advantages including portability, quietness and the ability to nebulise medications.

Cylinders
Oxygen for medical use is supplied in black cylinders with white shoulders. The pin index system prevents application of an incorrect regulator to an oxygen cylinder, or a cylinder containing other than oxygen to an oxygen regulator.

Cylinders are appropriate for the:

  • delivery of continuous high flow or intermittent low flow of oxygen to patients who require <5 hours oxygen therapy per day
  • emergency back-up supply for patients using oxygen concentrators.

For home use, sizes are the D size (1500 L) and the E size (4200 L). At 2 L/minute, a D cylinder will last approximately 11 hours and an E cylinder 30 hours. Portable C size (490 L) and BL (224 L) cylinders are available so that a patient can leave home and still have oxygen available.

Essential accessories for delivery of oxygen from the cylinder to the patient include a regulator to reduce the high cylinder pressure to a working pressure of 400 kPa or 60 p.s.i., a flow meter, spanner and key wheel, together with tubing and a face mask or soft nasal cannula. Special regulators are available for C size cylinders and for use in infants and children.3 A trolley will facilitate movement of D and E size cylinders.

Charges of up to $6.00 are made for cylinder hire and up to $30.00 for filling, depending on cylinder size. Filling of cylinders should be undertaken only by official suppliers - BOC Gases and Medical Gases Australia. In remote areas, special arrangements may be made to permit decanting from large to small cylinders. Special training is required. Some hospitals provide both instructions and facilities for decanting. Standards Australia has strict requirements for testing cylinders.

Oxygen from cylinders should not be used to nebulise medications when a nebuliser pump would be equally effective.

Oxygen conservation equipment - Conservation devices can be placed between the cylinder and the patient to ensure that oxygen is delivered as a pulse during inspiration so that oxygen is not wasted during expiration. This can make the oxygen in a cylinder last at least twice as long. Conservation equipment is not used with concentrators.

Oxygen concentrators
If a patient uses oxygen for more than 4 hours a day at a flow rate of 2 L/minute, an oxygen concentrator should be prescribed.

Oxygen concentrators are floor-standing, electrically driven machines with a compressor that draws in room air and transfers it under pressure to containers, called sieve canisters, filled with zeolite. Under pressure, the zeolite traps molecules of nitrogen while allowing oxygen to pass through. When the pressure in the sieve is relieved, the nitrogen is released. The availability of the two sieve canisters permits continuous concentration of oxygen from room air.

Concentrators deliver oxygen with a purity of 90-95% and can operate at flow rates up to 5 L/minute. The purity and thus the fraction inspired oxygen (FiO2) may fall at high flow rates.

Two oxygen concentrators in series at 2 L/minute will be more efficient than using one concentrator at 4 L/minute. Paediatric units have specially calibrated flow meters for low flow prescriptions. In order to permit a constant flow of oxygen, the oxygen produced from air is stored in a reservoir - this will be available for a few minutes after the concentrator is turned off.

The oxygen concentrator should always be located in a well -ventilated area, away from curtains, drapes or clothing. The cord should have enough slack to prevent accidental disconnection.

Oxygen concentrators can be wheeled around a home, but are heavy (21-26 kg), and there is always noise associated with the compressor and the cycling from one sieve canister to the other. Some patients find the noise of the concentrator especially intrusive and distressing. Location of the compressor in an adjacent room with longer tubing may suffice, or, occasionally, return to cylinder use, despite its greater cost, will improve patient care. Concentrators must be serviced regularly, with changing of filters and maintenance of alarm systems. Charges vary, but the cost of a new concentrator is approximately $4000.00 and a unit can be hired for approximately $140.00 per month. If the anticipated need is longer than two years, it is cheaper to buy rather than hire a concentrator. The cost of electricity must be added, but is usually less than $2.00 per week. At the time of the 3-monthly service, the fee of $50.00 includes the provision of tubing and nasal prongs or masks.

Humidifiers
Humidifiers are not required for home oxygen. They add to the risk of infection.

Precautions

Operating precautions
Because oxygen cylinders are filled under high pressure and oxygen aids combustion, strict safety precautions are indicated (see box).

Care of oxygen equipment

  • Change oxygen cannula or face mask after prolonged use e.g. 4-5 hours.
  • Wash thoroughly and allow to dry.
  • When oxygen is not in use, ensure all valves are turned off.

Ordering of oxygen
Place orders in adequate time, assessing the needs for weekends and public holidays.

Summary
Home oxygen adds significantly to the comfort and quality of life of patients in whom reversible hypoxaemia is the cause of discomfort. The objective is to provide a resting PaO2 of
60 mmHg using the lowest flow rate that will achieve this - usually 2 L/minute. When indicated, the flow rate should be increased by 1 L/minute during exercise and during sleep to maintain an oxygen saturation of 90%.

Safety precautions with use of oxygen cylinders
  • Store oxygen cylinders in a cool, ventilated room
  • Do not cover cylinders with cloth or plastic
  • When a cylinder is almost empty, close the valve and mark the cylinder as empty
  • Do not store full and empty cylinders together
  • Handle cylinders carefully to avoid dropping
  • Do not permit oxygen use near an open flame
  • Keep oxygen equipment and delivery point at least 3 metres from radiators or other heat sources
  • Do not permit smoking near oxygen equipment
  • Do not permit alcoholic solutions, oil or grease to come in contact with oxygen supply devices
  • Use the correct regulator and pressure gauge
  • Before connecting the regulator, open the cylinder momentarily, with the valve pointing away, to remove any dust which may be in the outlet
  • Check with the State Department of Transport regarding the transport of oxygen cylinders in cars as the safety standards may vary from state to state, but always secure a cylinder carefully if transporting in a car to prevent it becoming a projectile or explosive device
  • If electrical appliances, e.g. electric blankets, are in use, ensure their regular maintenance

For most patients, soft oxygen cannulae provide a cheap and efficient means of achieving these aims. Venturi style masks delivering 24-28% oxygen are acceptable, but many patients find them intolerable, especially in summer. The nature of the oxygen supply - cylinder or concentrator - will be determined by the patient's condition.

Successful oxygen therapy requires that the prescriber has a clear understanding of when it is indicated, and is prepared to review the patient frequently. The patient should be given simple, concise instructions about the use of the equipment and the essential precautions for its safe operation.

Further reading
Queensland Health. Guidelines for domiciliary oxygen prescription.

Department of Veterans Affairs. Guidelines for provision of domiciliary oxygen through the Rehabilitation Appliances Programme (RAP).

Self-test questions

The following statements are either true or false.

1. Home oxygen is of no benefit if it does not reverse the patient's arterial hypoxaemia.

2. Patients receiving continuous oxygen require the gas to be humidified.

Answers to self-test questions

1. True

2. False

References

  1. Breslin AB, Colebatch HJ, Engel LA, Young IH. Adult domiciliary oxygen therapy. The Thoracic Society of Australia and New Zealand. Med J Aust 1991;154:474-7.
  2. Medical Research Council Working Party. Long-term domiciliary oxygen therapy in chronic hypoxic cor pulmonale complicating chronic bronchitis and emphysema. Lancet 1981;1:681-5.
  3. Masters IB, Asher MI. Domiciliary oxygen therapy in children. J Paediatr Child Health 1993;29:259-62.