Is ambulatory oxygen a viable option and if so how should the oxygen be made available

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Ambulatory oxygen is the provision of oxygen therapy during exercise. The main aims of ambulatory oxygen are to reduce effort-induced breathlessness and increase exercise tolerance. With the increasing use of pulmonary rehabilitation, which has been shown to achieve both these aims for patients with COPD, the role of ambulatory oxygen in training programmes also needs to be defined.

The terms ambulatory oxygen and portable oxygen are often confused. Ambulatory oxygen is carried by the patient, while portable oxygen is transported with the patient, but is not 'on their person'. Portable oxygen includes the smaller cylinders that can be carried on a wheelchair or transported by car for severely hypoxaemic patients, who require continuous oxygen to correct dangerous hypoxaemia, rather than to reduce breathlessness and improve walking distances. As ambulatory oxygen needs to be used by mobile patients outside the home, the practicalities of equipment are important.

Randomized controlled trials are essential to evaluate the effects of ambulatory oxygen because of the large placebo effect, the effect of training, and variable individual responses. However, there are relatively few such studies. One study in 26 patients with severe COPD demonstrated that oxygen improved endurance but not maximal work rate on a treadmill [23]. Ambulatory oxygen resulted in a 13% increase in the distance walked during a 6-min walking test and reduced breathlessness on exercise in a randomized controlled study in 10 patients with severe COPD [20]. A further study of 50 patients confirmed that the increase in distance walked with ambulatory oxygen is around 10% when compared to an air cylinder [24]. However, the placebo effect alone of an ambulatory cylinder can be a 6—9% improvement in the distance walked [24,25]. The mechanism for the beneficial effect of oxygen on exercise endurance is, at least in part, by reduction in minute ventilation [26].

Predicting who will walk further with ambulatory oxygen is difficult, as there is no direct relationship between reduction in breathlessness and improved walking distance [20,24,25]. In part, this is explained by the observation that ambulatory oxygen reduces the severity of breathlessness for a given level of exercise [26].

Patients with COPD who desaturate on exercise as a group tend to improve their distance walked with ambulatory oxygen [24,27]. The 1999 UK guidelines recommend that only patients who show arterial oxygen desaturation on exercise of at least 4% below 90% during a baseline walking test breathing air should be assessed for ambulatory oxygen [3]. In one study of 20 patients with COPD who desaturated on exercise, the 6-min distance walked improved by 22% and breathlessness decreased by 36% with ambulatory oxygen, compared to patients who did not desaturate, in whom breathlessness was also reduced (by 47%) but there was no effect on the distance walked [27]. However, the situation is more complex, in that only a proportion of patients who desaturate on exercise will show reduced breathlessness and/or extended walking distances on ambulatory oxygen, and some patients who do not de-saturate on exercise will improve with ambulatory oxygen [24,27]. Hence, oxygen desaturation during exercise is not a reliable criterion for selection for ambulatory oxygen.

Ambulatory oxygen should only be provided for patients who show an improvement of at least 10% in walking distance and/or visual analogue score for breathlessness when walking with an oxygen cylinder compared with an air cylinder. The proportion of patients who will fulfil the criteria for ambulatory oxygen using this protocol will vary, but was 58% in the original study using these criteria [24].

The role of ambulatory oxygen during training, i.e. pulmonary rehabilitation, remains unclear. The argument for using oxygen is that it improves outcomes by allowing training at greater intensity; against is that training under hypoxic conditions allows adaptation. One randomized, placebo-controlled study of 25 patients with COPD (mean PaO2 8.5kPa/63.8mmHg) showed that while ambulatory oxygen during pulmonary rehabilitation reduced breathlessness at the end of rehabilitation, it had no effect on exercise tolerance or health status [28]. Furthermore, the group without oxygen increased their exercise tolerance during pulmonary rehabilitation, with no ill effects from significant desaturation during exercise (minimum SaO2 of 85%). This small study suggests that the extra inconvenience of training with oxygen is not justified [28].

Ambulatory oxygen is provided in the UK at present by small oxygen cylinders that can be carried (capacity 230 L, weight 2.3 kg). These only deliver 80 min of oxygen at 2 L/min in practice [24] and are not currently prescribable.

An alternative method of providing ambulatory oxygen is liquid oxygen, allowing up to 8 h oxygen from a lightweight cylinder. When full, this may weigh 3.4 kg, but the patient only needs to fill it for the required hours of use, making it lighter than this in practice. A large tank of liquid oxygen is kept at the patient's home for refilling the cylinder and is replaced regularly (about every 3weeks). In theory, liquid oxygen is a more practical way of delivering ambulatory oxygen, although it is more expensive. One randomized study in the UK comparing ambulatory oxygen provided by gaseous cylinders with liquid oxygen cylinders found that the majority of patients preferred liquid oxygen and used it more [29]. However, this study did not show a marked increase in the time patients spent away from home.

Availability of the systems for provision of ambulatory oxygen partly determines patterns of use in different countries. In the United States, liquid oxygen is available and much more widely used, based on the criteria of exercise desaturation alone. It is also used to increase the hours of use of LTOT in mobile patients. A few patients with COPD, like many of those with pulmonary fibrosis, require a higher flow rate on exercise to correct desaturation and in these patients liquid oxygen may be the only practical solution, as ambulatory gaseous cylinders only last about half an hour at 4 L/min.

The equipment used to deliver oxygen from an ambulatory source is also important. Most patients use nasal cannulae. However, other systems may be more efficient, including transtracheal oxygen catheters and oxygen-conserving devices [30]. Transtracheal catheters reduce the dead space of the upper airway, and once in place are a much less obtrusive method of oxygen delivery. They are used in the United States, but have not become standard practice in the UK, partly because of the reluctance to use an invasive procedure to deliver oxygen and partly because of complications associated with their use. Oxygen-conserving devices, including reservoirs and pulsed-flow oxygen delivery systems, can be twice to seven times as economical on oxygen use [30]. Studies comparing these different methods of delivering oxygen are lacking.

The issues of efficacy, practicality and cost are closely intertwined when evaluating the rational use of ambulatory oxygen. The case for liquid oxygen is strongest for patients with pulmonary fibrosis who require high flow rates to correct exercise-induced oxygen desaturation. The theoretical advantages of liquid oxygen for patients with COPD still need to be established in large randomized controlled trials. The best method of delivery for ambulatory oxygen during pulmonary rehabilitation needs to be defined, if found to be appropriate.

Evidence that ambulatory oxygen improves exercise tolerance in some patients with COPD is well established [20,24-27]. There is an urgent need for ambulatory oxygen therapy in some form to be prescribable in the UK to appropriate patients with COPD. Even being able to prescribe gaseous cylinders, which are the cheaper alternative at present, would have a big impact on quality of life for many patients with COPD. It is not rational that large cylinders, used for short-burst oxygen therapy, for which there is much less evidence of benefit, are prescribable but that ambulatory oxygen, even when it has been shown to benefit individual patients following assessment, is not.

Should oxygen therapy be prescribed to smokers?

Prescribing oxygen to patients who smoke is controversial. The two issues are whether oxygen therapy is effective and whether it is safe. In the MRC study, more than a quarter of the control subjects and more than half of the treated subjects were smokers [1], providing evidence that LTOT reduces mortality in patients who smoke. From the safety point of view, an oxygen concentrator is a low flow system with a lower risk of fire than a pressurized oxygen cylinder. The UK guidelines suggest that LTOT should not be prescribed to patients who continue to smoke. However, the evidence does not support this recommendation. In practice, smoking is prevalent among patients on LTOT; 14-19% admitted to smoking in recent studies [16,17]. While every attempt should be made at smoking cessation, patients who continue to smoke should not be discriminated against. LTOT via a concentrator should be prescribed for any patient with a PaO2 below 7.3kPa (55mmHg). In contrast, ambulatory oxygen and short-burst therapy, are usually provided by high-pressure cylinders and there is a significant fire hazard associated with smoking; hence they should not be prescribed for smokers.

Conclusions

We still have a long way to go towards the aim of rational and objective prescription of oxygen. We need further well-conducted randomized trials to extend the 20-year-old results of the MRC and NOT Trials. A benefit of LTOT in moderate hypoxaemia has not been shown. The role of ambulatory oxygen therapy is starting to be defined, but large studies are needed, including studies on its role in pulmonary rehabilitation. There is limited evidence for the use of short-burst oxygen therapy. If used, it should be recommended for 5 min before or after exercise in patients who desaturate on exercise and describe symptomatic benefit with its use.

While we await evidence in these areas, there is much that can be done. The evidence for survival benefit of LTOT is clear and guidelines are established, yet prescription is woefully inadequate. We need to increase awareness of the benefits of appropriate oxygen therapy and ensure evidence-based prescribing by all clinicians who care for patients with severe COPD. The practicalities of oxygen provision urgently need to be sorted out. Which equipment can be prescribed needs to be put on a rational basis; in particular, ambulatory oxygen, either as gaseous cylinders or liquid oxygen, should be prescribable for patients who have been formally assessed and found to benefit [3]. Oxygen services need to be defined, organized and funded, including arrangements for detecting hypoxaemic patients in the community, so that LTOT is provided for those patients with severe COPD who could benefit from it.

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