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Ambulatory and training oxygen: a review of the evidence and guidelines for prescription.

ABSTRACT

The evidence for prescription of Long Term Oxygen Therapy is well established whereas the evidence for supply of ambulatory and training oxygen is less robust. There is increasing evidence that the use of these latter therapies is beneficial although guidelines for supply are limited. This paper aims to review the evidence base for ambulatory and training oxygen and to suggest guidelines for assessment and prescription based on our local research and experience in Aotearoa/New Zealand. Young P (2005). Ambulatory and training oxygen: a review of the evidence and guidelines for prescription. New Zealand Journal of Physiotherapy 33(1) 7-12. Key Words: Ambulatory oxygen, training oxygen, exercise, pulmonary rehabilitation.

Key Points

* There is increasing evidence that the use of training and ambulatory oxygen improves rehabilitation outcomes.

* It is difficult to predict which patients will benefit.

* A standardised assessment process should establish individual need and benefit.

* A standarised assessment process is outlined with the view to optimising clinical benefit and resource use.

INTRODUCTION

The evidence for prescription of Long Term Oxygen Therapy (LTOT) is well established (Medical Research Council Working Party 1981, Nocturnal Oxygen Therapy Trial Group 1980) in contrast to that for ambulatory and training oxygen. With regard to the latter forms of oxygen therapy, patients with chronic respiratory disease who desaturate significantly on exertion may benefit from the provision of oxygen during activity through improved exercise tolerance and reduced dyspnoea during exercise or activities of daily living. Supply of oxygen in this context has been variably described. The American Thoracic Society (1995) criteria for provision of oxygen during exercise suggest that supplemental oxygen should be provided for patients who demonstrate arterial oxygen saturation at or below 88% during exercise. The Royal College of Physicians guidelines (1999) define ambulatory oxygen as oxygen therapy during exercise and activities of daily living and recommend provision when arterial saturations fall below 90% on exertion. It is important to clarify the distinction between an oxygen supply which is available for the patient at home or out of the home and that which is provided as part of a pulmonary rehabilitation programme. For purposes of this discussion, training oxygen is that supplied for use during the exercise component of pulmonary rehabilitation and ambulatory oxygen is that which is supplied for activities of daily living, which might include exercise outside the home environs.

There is increasing evidence that the use of training and ambulatory oxygen is beneficial although guidelines for supply are limited. This paper aims to review the evidence base for training and ambulatory oxygen and to suggest guidelines for assessment and prescription based on the Auckland research group experience.

Exercise and respiratory disease

Pulmonary rehabilitation is an integral component of the medical management of patients with chronic obstructive pulmonary disease (COPD). It has been shown to reduce disability and handicap and hence improve quality of life. There is strong supporting evidence to demonstrate significant improvement in exercise performance and reduction in the perception of dyspnoea (ACCP/AACVPR 1997, Lacasse et al., 1996). Although the majority of evidence is related to COPD it seems reasonable that this could be extrapolated to other respiratory diseases such as pulmonary fibrosis and bronchiectasis.

Exercise training is the cornerstone of pulmonary rehabilitation (American Thoracic Society 1999). Muscle weakness and atrophy are common in patients with chronic respiratory disease and significantly contribute to impaired exercise performance (Gosselink et al., 2000). The training effect depends on the specificity of exercise and the intensity of exercise where only a load higher than baseline will achieve a training effect. Although improvements in quality of life and exercise tolerance have been achieved with low intensity exercise programmes or symptom limited programmes (Clark et al., 1966, Normandin et al., 2002, Ries et al., 1995, O'Donnell et al., 1998) high intensity exercise appears to yield improved training results (Gimenez et al., 2000, Coppoolse et al., 1999, Casaburi et al., 1991). It is accepted that high intensity training at a heart rate at or above anaerobic threshold or at a modified Borg dyspnoea score of above four (Mejia et al., 1999) is recommended.

Casaburi et al (1991) suggest a training intensity of 80% maximal oxygen consumption although the patient group in this study was younger than those generally entering pulmonary rehabilitation programmes. The British Thoracic Society (2001) recommends exercise should commence at a level commensurate with 60% of maximal oxygen peak obtained from an incremental shuttle walk test. Similarly, the American Thoracic Society (1999) suggests 60--75% of maximum work load or if this cannot be achieved, interval training of two --three minute training at high intensity (60- 80% of maximum) with equal periods of rest. One of the key messages from Emtner et al (2003) is that to achieve significant clinical benefits from training, the exercise intensity needs to be as high as possible. The training effects are only maintained if exercise is continued and it is essential that patients are able to continue exercising in the home environment.

It is recommended that oxygen requirements are established when the patient is medically stable and prior to rehabilitation. Physiotherapists are arguably ideally placed to perform the appropriate assessment and prescription.

Field walk tests for assessment

An exercise test is considered mandatory before commencing pulmonary rehabilitation to establish baseline exercise tolerance and physiological parameters. This test is chosen according to its ability to reflect the aims of the exercise programme and may also be used to assess the need for training or ambulatory oxygen. Field walk tests are highly suitable as they assess requirements during this functional activity. The options for testing include:

a. The incremental shuttle walk test (ISWT)

The ISWT (Singh et al., 1992) was developed as a standardised walk test, which is externally paced and incremented by increasing the walk speed at minute intervals. It allows a measure of peak maximal oxygen consumption to be estimated and has been validated in COPD patients. Garrod et al (2000) used the ISWT as an outcome measure for rehabilitation with training oxygen and demonstrated only minimal gains. In the editorial accompanying this paper Calverley (2000) emphasised that the greatest effects of oxygen are with endurance exercise and hence the six minute walk test or the endurance shuttle walk test (ESWT) may be more sensitive measures.

b. Six minute walk test (6 MWT)

The 6 MWT is a self paced test which is performed according to a defined protocol (American Thoracic Society, 2002, Steele, 1996)). It is highly reproducible when delivered in a standardised format, has been shown to be sensitive, and is responsive to change with a validated minimal clinically important difference of greater than 53 metres (Radelmeier et al., 1997) The Auckland research group (Eaton et al., 2002) used the 6 MWT to test the clinical utility of ambulatory oxygen and demonstrated both an acute and short term response. Turner et al (2004) reported that greater oxygen desaturation was observed during field walk tests and suggested that both the 6 MWT and the ISWT are more sensitive than cycle ergometry in detecting exercise induced hypoxaemia and in assessing ambulatory oxygen therapy needs.

c. The endurance shuttle walk test (ESWT)

The ESWT was developed by Revill et al (1999) and uses the ISWT to establish the walking pace. It is performed with externally controlled pacing at a pace commensurate with 85% of the ISWT estimated maximal oxygen consumption. It provides the same relative exercise intensity for all patients and was shown to be more sensitive to change following pulmonary rehabilitation than the ISWT. The Auckland research group (Eaton et al., 2004) also found that the ESWT was more sensitive to change following pulmonary rehabilitation when compared with the 6 MWT. The ESWT has been used in a trial of ambulatory oxygen and an ambulatory ventilator and demonstrated sensitivity to the acute application of oxygen (Revill et al., 2000).

TRAINING OXYGEN

The provision of training oxygen during exercise can be used to achieve higher training intensity. Patients who are hypoxemic at rest and who are using LTOT are advised to exercise on oxygen, the flow rate titrated to prevent saturations falling below 90% if possible. However some patients with normal oxygen tension at rest profoundly desaturate on exercise or during activities of daily living. In addition to ventilation perfusion mismatch, in some patients with COPD, resting hyperinflation, as reflected by increased residual functional capacity and total lung capacity is present. Tidal breathing occurs at a less advantageous portion of the diaphragmatic length--tension curve and accessory muscle use is increased at rest. During exercise it is difficult to increase tidal volume and hence an increased respiratory rate is used to augment ventilation. This results in a decreased expiratory time and consequent air trapping and further hyperinflation. There is an increased work of breathing and respiratory muscle fatigue occurs. Dynamic hyperinflation has been identified as a possible mechanism which limits exercise in patients with COPD (O'Donnell and Webb, 1993). Oxygen has been shown to decrease respiratory frequency, minute ventilation and dynamic hyperinflation and improve exercise performance (Somfay et al., 2001).

The evidence for training oxygen

Although laboratory studies (Somfay et al., 2001, Davidson et al., 1988, Woodcock et al., 1981) showed application of training oxygen increased exercise endurance, randomised controlled rehabilitation studies (Garrod et al., 2000, Rooyackers et al., 1997) using training oxygen for patients with exercise desaturation did not demonstrate improved outcomes. However both of the latter studies had few patients and may have failed to detect change. Also exercise training was at lower intensity. More recently, Emtner et al (2003) unequivocally demonstrated that the application of training oxygen delivered at three litres/min during a seven week training programme resulted in improved physical performance and health status. Non-hypoxemic, physically inactive patients with severe COPD were able to train with a more rapid progression of exercise intensity and increase of endurance in a steady state exercise.

The British Thoracic Society (2001) suggest training oxygen should be provided during exercise where clinically important desaturation, defined as being less than 90%, has been found at the training load in the preliminary test. It should be continued for similar activity at home. Applying this guideline would need an increase in resource for both ambulatory and training oxygen in New Zealand. Other guidelines (American Thoracic Society 1999, ACCP/AACVPR 1997) do not address the issue.

Emtner et al (2003) attempted to predict patients who would benefit from oxygen during exercise. There was a significant correlation between improvement in endurance time induced by oxygen versus room air in the pre training tests, however it was not sufficiently high to predict benefit accurately in individual subjects.

In carefully controlled study conditions it appears that training oxygen may be of benefit but it remains to be established how this would translate into routine clinical practice and guidelines remain unsatisfactory. The suggestion is that the testing process establishes individual need and benefit.

Assessment for training oxygen

The assessment protocol is the same regardless of the field walk test selected.

1. A baseline field walk test is performed on room air according to a standard protocol to ensure reproducibility and to establish the need for assessment for oxygen.

It is important to allow sufficient recovery time between tests. Recommended recovery times vary for the 6 MWT. American Thoracic Society guidelines (1999) recommend one hour, while others (Steele, 1996, Sciurba and Slivka, 1998) recommend 15-30 minutes. For the ESWT tests it is recommended that 40 minutes be allowed (Revill et al., 1999)

American Thoracic Society (1995) advise that if the field walk test shows a fall in saturation below 88% at maximum load then oxygen should be provided. However it would seem that the degree of dyspnoea and fatigue should also be considered (Griffiths et al., 2000). In most rehabilitation programmes, continuous flow oxygen would be the delivery of choice. The flow rate used during the test should be high enough to maintain saturations above 90%.

2. Two further tests are performed on cylinder air and cylinder oxygen. As there is a significant placebo effect to applying oxygen, the tests are randomised and blinded with oximeters and cylinder labels obscured from the patient.

* The patient rests for 10 minutes

* The gas is applied via nasal prongs at 4 litres per minute flow rate and the patient rests for a further five minutes

* Baseline dyspnoea and oxygen saturation is measured. The most usual measure of dyspnoea is the Modified Borg (1982) score of perceived breathlessness. Continuous oximetry is employed during the test and recorded at minute intervals

* The walking distance is measured and post test dyspnoea score determined

* The recovery time to baseline saturation and dyspnoea is also recorded, as the rate at which breathlessness resolves may be a relevant outcome (Calverley, 2000).

* The test is repeated with the second gas after an appropriate interval.

Prescription of training oxygen

Oxygen should be prescribed for exercise if oxygen saturations can be maintained close to 90% and there is a significant increase in walk distance or a significant decrease in the dyspnoea score on cylinder oxygen versus cylinder air. Royal College of Physicians (1999) arbitrarily suggest a positive change of 10% over the baseline walking distance and/or breathlessness score on room air after walking with cylinder oxygen. Eaton et al (2002) used a distance greater than 53 metres or a decrease in the Borg dyspnoea score of one point to identify an acute response to ambulatory oxygen. The flow rate should be titrated according to the test results and further tests may be required. Snider (2002) suggests that a flow rate of 6 litres/minute may be necessary in some patients.

AMBULATORY OXYGEN

Provision of training oxygen is based on an acute response and the oxygen is used while exercising during pulmonary rehabilitation or exercising at home. Ambulatory oxygen is indicated for patients who are established on LTOT who are mobile and need to leave the home on a regular basis. It can also be prescribed for patients who do not fulfil the criteria for LTOT but who desaturate severely on exertion, show improvement in exercise capacity on oxygen, are mobile and are motivated to use ambulatory oxygen outside the home.

In New Zealand, ambulatory oxygen is supplied by small portable cylinders equipped with a demand delivery device that delivers the flow rate only on inspiratory demand thus extending cylinder usage. There are several different devices available all shown to improve arterial oxygenation with lower flow rates than continuous oxygen. However some devices appear to be more effective than others (Fuhrman et al., 2004). The portable cylinders weigh approximately 2.7 kilograms and are generally carried in a backpack. There is a measurable negative impact from carrying the weight of the cylinder and an alternative may be to use a trolley. Liquid oxygen has the advantage of being lighter to carry but is not available in New Zealand.

The evidence for ambulatory oxygen

McDonald et al (1995) assessed the effects of ambulatory oxygen used during activity at home on quality of life and concluded that although there were small improvements in exercise performance, this did not translate into improved quality of life. As studies show poor compliance with the use ambulatory oxygen (Lock et al., 1991, Vergeret, et al., 1989) careful patient selection is necessary. The Auckland research group (Eaton et al., 2002) examined the clinical utility of ambulatory oxygen and its effect on quality of life on those patients with COPD and exertional desaturation who did not fulfil the criteria for provision of LTOT. It was demonstrated that ambulatory oxygen is associated with modest but clinically significant improvements in quality of life particularly in the mastery domain and an increased walking distance with decreased dyspnoea. The research also showed that the benefits of ambulatory oxygen could not be predicted by the acute response to cylinder oxygen and despite either an acute response or a short term (6 weeks) response, a considerable number of patients (41%) declined to continue with ambulatory oxygen due to poor tolerability. As a result of this work, a rigorous screening and assessment protocol has been developed.

Assessment for ambulatory oxygen

An essential prerequisite is that the patient should have completed a pulmonary rehabilitation programme ensuring dyspnoea coping strategies and fitness are maximised. It also acts as a convenient surrogate measure of adherence and willingness to improve exercise tolerance. Patients must be sufficiently ambulant, defined as being able to walk further than 200 metres in six minutes, and have dyspnoea of sufficient severity that it impacts on the ability to perform activities of daily living outside the home. Patients must understand that ambulatory oxygen is to be used to facilitate activity outside the home on a regular basis. It needs to be emphasised that it is not for short burst use i.e. pre-oxygenation before activity, breathlessness during recovery from activity or control of breathlessness at rest. Oxygen used in this manner has been shown to be generally ineffective (Lewis et al., 2003, Stevenson and Calverley, 2004).

Assessment Protocol

Assessment takes place over three visits.

1. On the initial visit, the patient completes two field walk tests on room air to ensure reproducibility and to ensure they fulfil the criteria for further assessment. They are shown the equipment and educated on the use of ambulatory oxygen so that they can make a more informed decision to proceed with further assessment. If so, they are asked to complete an activity diary for two weeks, which documents the activities they have completed during that time, the time taken to do them and the Modified Borg dyspnoea score on completion of each activity. The patient's needs should be established and transparent so that an alternative supply of oxygen for use inside the home to facilitate activity may be considered.

2. After two weeks, the patient completes

* a further field walk test on room air

* a randomised, blinded trial of cylinder air versus cylinder oxygen using the portable equipment

* a baseline Chronic Respiratory Questionnaire (Guyatt, et al., 1993) using activities in the dyspnoea domain for which the patient will use ambulatory oxygen

* a Hospital Anxiety and Depression score (Zigmond and Snaith, 1983)

The diaries are scrutinised and individual uses of ambulatory oxygen identified.

As the acute response is not a predictor of longer term benefit, this is followed by a six week clinical trial of cylinder oxygen. Patients are instructed to continue with the activity diary.

3. After six weeks all of the baseline tests as performed at visit two are repeated.

The main outcome measures of the clinical trial are a significant improvement in quality of life as measured by the Chronic Respiratory Questionnaire, improved exercise tolerance or dyspnoea and an appropriate pattern of usage as demonstrated by the patient diary. As most cylinders last a maximum of three hours, adequate usage has been defined as being at least two cylinders per week.

Prescription of ambulatory oxygen

Careful scrutiny of all outcome measures and a willingness by the patient to use the equipment outside the home is essential before ambulatory oxygen is prescribed for long term use. In some cases the patient's expectations of benefits are not met by the clinical reality resulting in poor adherence and inappropriate use.

The prescription of flow rate is difficult as it is a balance between maintaining oxygenation and the constraints of cylinder hours available. The Auckland research group (Eaton et al., 2002) found that even at four litres per minute, the maximum available with demand device used in the trial, correction of hypoxaemia was only achieved in 54% of patients and this factor did not predict short term response. The Royal College of Physicians (1999) state that correction of hypoxaemia should be achieved before provision of ambulatory oxygen. However this may not be entirely realistic as correction of hypoxaemia may not be the only mechanism, which improves exercise tolerance.

Patient follow up is imperative to determine the ongoing appropriateness of ambulatory oxygen in the event of symptomatic deterioration. It is also essential to check that the equipment and prescribed flow rate continue to be appropriate for the patient. The Auckland research group suggest an initial three-month follow up and then six monthly thereafter.

CONCLUSION

Provision of training oxygen may enhance pulmonary rehabilitation outcomes and maximise patient fitness levels by assisting the patient to achieve a higher level of intensity during exercise training. Ambulatory oxygen may assist patients to remain active within the community and so improve their quality of life and exercise tolerance. The provision of both modalities has considerable implication for resourcing and therefore the assessment process should reflect the outcomes anticipated in supplying these forms of oxygen therapy. The benefits will only be realised if each decision is correct and appropriate to each individual patient. Further studies are recommended to refine the guidelines and assessment process. The Auckland research group has attempted to standardise assessment processes in order to optimise clinical benefit and ensure the best possible use of an often-scarce resource.

Acknowledgement

The author wishes to thank Dr. Tam Eaton for assistance, advice and support during the preparation of the manuscript and in furthering the practice of physiotherapy in the management of patients with COPD.

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Pamela Young MCSP, Dip TP, NZRP Physiotherapist, Physiotherapy Department, Greenlane Clinical Centre, Auckland 3

Address for Correspondence Pamela Young MCSP, Dip TP, NZRP, Physiotherapist, Physiotherapy Department, Green lane Clinical Centre, Auckland 3.
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Publication:New Zealand Journal of Physiotherapy
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Date:Mar 1, 2005
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