Use of Oxygen Therapy in COPD

The British Thoracic Society defines Chronic Obstructive Pulmonary Disease (COPD) as a "chronic, slowly progressive disorder characterised by airflow obstruction (reduced FEV₁ and FEV₁/VC ratio) that does not change markedly over several months". This definition is shared by the NICE guidelines on COPD from February 2004.¹ Airflow obstruction is defined as a reduced FEV₁ and a reduced FEV₁/FVC ratio; such that FEV₁ is less than 80% of that predicted, and FEV₁/FVC is less than 0.7.

There is further guidance from Clinical Knowledge Summaries in 2006² but this relies heavily on evidence from NICE and largely reinforces the message.

COPD is a major cause of morbidity, with high usage of resources in Primary and Secondary Care. It costs the NHS about £500 million a year and 95% is caused by smoking.³ The cost per patient in the UK is probably in excess of £1,600 per annum.⁴

In normal people the respiratory drive is largely initiated by PCO₂ but in COPD hypoxia can be a strong driving force and so if this is corrected the respiratory drive will be reduced. Patients with COPD are sometimes divided into pink puffers and blue bloaters. The former work hard to maintain a normal pO₂ which is why they puff away. They tend to have a barrel-shaped, hyperinflated chest and breath through pursed lips. The latter are blue because of hypoxia and polycythaemia. They are often obese and have water retention. This is why they are bloated. The blue bloaters are dependent upon hypoxia for their respiratory drive and to give oxygen and deprive them of this will lead to signficant hypercapnia and acid base imbalance. Although pink puffers "fight" more to maintain their blood gases, they are no more disabled than blue bloaters⁵ and they have a better prognosis.

Hence oxygen therapy in COPD must be used with care but it can have distinct benefits. Chronic hypoxaemia causes slowly progressive pulmonary hypertension with the development of right ventricular hypertrophy and possible cor pulmonale with secondary polycythaemia. The last increases blood viscosity and hence resistance to flow. There is also sludging and a tendency to thrombosis.

When oxygen is used in patients who are not thought to have COPD, especially those with terminal cancer, such caution is not required.⁶ If a general practitioner thinks that a patient with COPD may benefit from oxygen, that patient should first be assessed by a respiratory physician or a specialist respiratory team.

The potential dangers of giving oxygen in COPD have long been known but the potential benefits were not appreciated until the 1980s.^7,8 In the following section, levels of evidence are given and the reader is referred to the article on different levels of evidence or to the NICE explanation of its grading of evidence.

NICE states that the following are indications for considering long term oxygen therapy (LTOT):

• Patients with a PaO₂ of less than 7.3kPa when stable or PaO₂ of 7.3 to 8.0kPa when stable but an additional risk feature such as secondary polycythaemia, nocturnal hypoxaemia, peripheral oedema or pulmonary hypertension. This is given as level of evidence A.
• Oxygen should be used for at least 15 hours a day as there is no benefit for shorter durations.⁹ Evidence level A.

NICE suggests assessment of the need for oxygen therapy in patients with the following: Evidence level D.

• Severe airflow obstruction with FEV₁ of less than 30% of predicted.
• Cyanosis.
• Polycythaemia.
• Peripheral oedema.
• Elevated jugular venous pressure.
• Oxygen saturation less than 92% when breathing air.
• Consider assessment for moderate airflow obstruction (FEV₁ 30 to 49% of predicted). Evidence level D
• All practices should have a pulse oximeter to identify patients needing LTOT. Evidence level D
• Oxygen concentrators should be used to supply the LTOT at home.
• Ambulatory oxygen therapy may be of benefit in those who wish to continue therapy outside the home. Evidence level D.
• Short bursts of oxygen therapy for episodes of breathlessness should only be used if all other methods fail. Evidence level C.

A Cochrane review of randomized controlled trials (RCTs) of long-term oxygen therapy for COPD⁹ had the main outcome measure of survival on home oxygen therapy. The conclusions from the 5 trials were:

• Long-term oxygen therapy improved survival in COPD patients with severe hypoxaemia but few co-morbidities.
• Long-term oxygen therapy did not improve survival in patients with moderate hypoxaemia or in those with mild to moderate hypoxaemia and arterial desaturation at night.
• Those with desaturation at night were probably getting benefit from nocturnal oxygen and did not need it for a full 15 hours a day.
• LTOT reduces mortality from secondary vascular complications but does not affect the progression of the airways disease.

Prescribing oxygen is discussed in the drug section texts but some aspects are repeated here.

The acute episode

• Do not prescribe oxygen at >28% (via venturi mask) or 2 l/min (via nasal prongs) in patients over 50 with a history of COPD until arterial blood gases (ABGs) have been checked.¹⁰
• Measure ABGs within 60 minutes of starting supplemental oxygen or changing its concentration. If pO2 improves and the pH is relatively unaffected (pH >7.26) then the concentration of the supplemental oxygen may be increased to maintain pO2 >7.5 kPa.
• If acidosis develops (falling pH) with a rising pCO2, other therapeutic interventions need to be discussed with a respiratory physician: NIPPV; IPPV; doxapram. This is a grave situation and neither age nor pCO₂ predict survival.
• Check ABGs on air before discharge in those who presented with a low pO₂ and/or hypercapnia to guide later formal assessment for LTOT.
• 4 to 6 weeks follow up should include consideration of LTOT assessment.

Clinical Knowledge Summaries recommend² that if the patient will not stop smoking, that oxygen therapy should be withheld. There is a real risk of fire and burns to the face and any benefit relating to polycythaemia is counteracted by smoking.

Prescription of long term oxygen therapy

• Patients need to be assessed first by a specialist team. The supply of home oxygen has been transferred from community pharmacies to regional oxygen supply companies in England and Wales.^11,12 These companies are responsible for supplying cylinders, concentrators and liquid oxygen as part of an integrated service. Oxygen should be ordered directly from one of four regional supply companies via the Home Oxygen Order Form (HOOF)¹³ which has replaced prescribing of oxygen on FP10 prescriptions. These arrangements do not apply to Scotland and Northern Ireland. As a general rule, it is more economical to use an oxygen concentrator rather than cylinders if oxygen is required for more than 8 hours per day or if prescriptions exceed 21 cylinders per month.
• Smokers should stop smoking or benefit is unlikely. There is also the very significant risk of burns and fire.
• Get optimum benefit from other forms of medication including inhalers.
• Measure ABGs when the patient is clinically stable and on optimal therapy on at least two occasions three weeks apart.
• Measure ABGs with oxygen therapy, to ensure pO2 is >8.0 kPa without unacceptable hypercapnia.
• There is no benefit from LTOT for less than 15 hours a day.
• LTOT is best provided with a concentrator, via nasal prongs at 2-4 l/min (depending on ABGs).
• It is important to maintain 6 monthly follow up with reassessment for early recognition of problems. The BTS recommends domiciliary assessment by a respiratory health worker.

Travel

Travel by land or sea presents few problems:

• Reduced pO2 in airline cabins will increase hypoxia in those patients with hypoxia at sea level. The BTS suggests that flying is relatively contraindicated in those with hypercapnia or gross hypoxia breathing air (pO2 <6.7 kPa)
• Most major airlines can provide supplemental in-flight oxygen and assistance with embarkation if arranged in advance.
• It is usually possible to arrange temporary provision of LTOT from a local chemist during a holiday but many patients can manage well without LTOT for several days.

Ambulatory oxygen therapy

The BTS Working Party which developed the guidance for clinicians on assessment of patients for oxygen allocated categories for prescription of ambulatory oxygen:¹²

• Grade 1 oxygen requirements (LTOT low activity). These patients are mainly housebound or do not leave the home without assistance and will need only an occasional portable cylinder. The oxygen flow rate will be the same as the patient uses for LTOT as they will be sedentary. The average time of use will be approximately 1 hour per day but may vary.
• Grade 2 oxygen requirements (LTOT active group). This includes patients receiving LTOT who are mobile and need to leave the home on a regular basis. They will require referral to specialist care for full assessment for ambulatory oxygen. This will involve the determination of the oxygen flow rate required to abolish desaturations below 90% during exercise. At present there is no reliable tool to predict which patients will be compliant users of ambulatory oxygen and so the working group suggests that patients are first provided with ambulatory oxygen sufficient for 2 hours use daily and asked to monitor their daily usage. All patients should be seen after 2 months when their true requirement can be determined and the oxygen order adjusted if necessary.
• Grade 3 requirements (non-LTOT patients who are exercise desaturators). They will require referral to specialist care for more detailed assessment as to whether they meet the criteria for ambulatory oxygen.

The benefits of ambulatory therapy include:

• It can improve exercise tolerance and breathlessness but there is no evidence of benefit from oxygen before or after exercise in most patients with COPD.¹⁴
• It may allow increased daily oxygen use and/or better compliance
• There is not a great deal of evidence for benefit (NICE level D). A specialist assessment is required first and the team can authorise the equipment so that the GP does not have to issue a prescription. This represents a recent change in the terms and conditions of service. GPs can continue to prescribe oxygen for symptomatic relief in palliative care or where a patient needs short burst of oxygen. If a patient requires long term or ambulatory oxygen, the GP can refer patients to the specialist team to assess and prescribe for particular needs.
• A portable cylinder, size DD provides 2 l/min oxygen for just under 4 hours and at 4 l/min for just under 2 hours. NICE notes that many of the devices in use are not currently available on prescription.¹

Most specialists perform assessments for LTOT, but relatively few have been assessing patients for ambulatory oxygen therapy and assessment services need to be expanded to include this. Appropriate assessment of patients is essential so that the right patients are treated with home oxygen for the right period of time and with appropriate flow rates to obtain optimal benefits and reduce the chance of adverse effects.¹²

NICE notes that¹ short-burst oxygen therapy is one of the most expensive therapies used in the NHS yet its evidence base is poor. It may simply be an expensive placebo that cools the face rather than correcting hypoxia. Short burst oxygen is often prescribed for patients who do not meet the criteria for LTOT but who remain breathless after minimal exertion despite other therapy. It is usually provided from cylinders. The NICE recommendation is that short-burst oxygen therapy should only be considered for episodes of severe breathlessness in patients with COPD not relieved by other treatments and it should only continue to be prescribed if an improvement in breathlessness following therapy has been documented.