P11. Oxygen therapy

Long term oxygen therapy reduces mortality in COPD.^{18,19,121,122,190-192} It may also have a beneficial impact on haemodynamics, haematological status, exercise capacity, lung mechanics and mental state.^120,122,192  Although effective, it is a potentially expensive therapy that should only be prescribed for those in whom there is evidence of benefit (see below). Information on prescribing oxygen therapy is given in Appendix 3.

Long-term continuous oxygen therapy: (at least 15 hours a day) is appropriate for patients who have PaO₂ consistently ≤ 55 mmHg (7.3 kPa; SpO₂ 88%)^18,19 when breathing air, at rest and awake [evidence level I]. If oxygen is prescribed when the patient's condition is unstable (eg, during an exacerbation), then the requirement for it should be reviewed four to eight weeks after initiation. At assessment for ongoing therapy, the patient's condition must be stable, all potentially reversible factors must have been treated and the patient must have stopped smoking at least one month previously.

Polycythaemia (haemoglobin level > 170 g/L), clinical or electrocardiographic evidence of pulmonary hypertension, as well as episodes of right heart failure, are consistent with the systemic effects of chronic hypoxaemia, and continuous oxygen should be supplied if the stable PaO₂ is 55–59 mmHg (7.3–7.9 kPa; Spo2 < 90%)^190,191 [evidence level D]. Continuous oxygen therapy is of most benefit for patients with increased arterial PaCO₂ (> 45 mmHg, or 6 kPa).¹⁹

Government funding is available on the basis that the prescribing doctor is an approved prescriber (usually a respiratory physician). Oxygen is usually supplied to patients meeting specific criteria and means testing by state or regional health departments in Australia and New Zealand.

Intermittent oxygen therapy: Available evidence does not allow any firm conclusions to be made about the effectiveness of intermittent ambulatory domiciliary oxygen therapy in patients with COPD.¹⁹³ However, use of intermittent oxygen therapy may be considered for:

• People who experience oxygen desaturation on exertion.¹⁹³ A Cochrane review of 31 studies found that ambulatory oxygen was efficacious in single assessment studies when comparing an exercise test performed breathing oxygen or air in patients with moderate to severe COPD. Benefits were shown in endurance exercise capacity, dyspnoea at isotime and oxygen saturation.¹⁹⁴ However, the minimum clinically important difference in these variables with oxygen therapy is unknown, Because of the heterogeneity of the studies, subgroup analyses were not possible to determine which patients were more likely to benefit. Benefit cannot be predicted by a resting test. Acute benefit may be established by comparing exercise endurance on a walking test (e.g. six minute walk test, incremental or endurance shuttle walk test or treadmill test) when breathing oxygen and when breathing room air. The oxygen system used in the assessment should be the same as the system the patient would use if oxygen were prescribed (e.g. trolley or shoulder bag to transport the cylinder). A stationary bicycle should not generally be used for the test as oxygen desaturation is significantly greater in COPD patients when walking as compared to cycling; ^{195, 196, 197} Although oxygen may be used during exercise training with a similar aim, a systematic review of the small number of suitable studies reported to date does not allow a conclusion to be drawn about the use of oxygen in this circumstance.¹⁹⁸ As the relationship between single assessments and long-term benefits is unclear, the acute assessment should form only part of the determination and benefit of ongoing ambulatory oxygen therapy. Long-term review and determination of oxygen usage are also important¹⁹⁹;

• Patients living in isolated areas or prone to sudden life-threatening episodes while they are awaiting medical attention or evacuation by ambulance;

• Patients travelling by air. Flying is generally safe for patients with chronic respiratory failure who are on long-term oxygen therapy, but the flow rate should be increased by 1–2 L/minute during the flight (see also below).

It is to be noted that short-burst oxygen i.e. oxygen inhaled immediately prior and/or following exertion with the aim of relieving breathlessness or improving exercise tolerance is not effective²⁰⁰ [evidence level I].

Nocturnal oxygen therapy: Patients with hypoxaemia during sleep may require nocturnal oxygen therapy. Nocturnal hypoxaemia should be considered in patients whose arterial gas tensions are satisfactory when awake, but who have daytime somnolence, polycythaemia or right heart failure. Oxygen is indicated for patients whose nocturnal arterial oxygen saturation repeatedly falls below 88%. Sleep apnoea should be excluded.

P11.1 Fitness to fly

Commercial aircraft operate at altitudes of up to 12 500 metres, with the plane's interior pressurised to 2100–2400 metres. At this "altitude" the alveolar PaO₂ for healthy individuals decreases from 103 mmHg (13.7 kPa) to 64 mmHg (8.5 kPa) and oxygen saturation declines from 97% to 93%.

As a general rule, supplemental oxygen is unlikely to be required if the resting oxygen saturation is 95% or higher, and likely to be required if oxygen saturation is 88% or lower. Patients with oxygen saturation values between these levels might require specialist assessment.

Before flying, patients should ideally be clinically stable. Patients recovering from an acute exacerbation are particularly at risk. Those already on long-term oxygen therapy need an increase in flow rate of 1–2 L per minute during flight. Careful consideration should be given to any comorbidity that may impair delivery of oxygen to the tissues (eg, cardiac impairment, anaemia). Exertion during flight will exacerbate hypoxaemia.

The American Thoracic Society currently recommends that PaO₂ during air travel should be maintained at more than 50 mmHg (6.7 kPa). At altitude, PaO₂ can be estimated from PaO₂ at sea level by means of published nomograms. If the PaO₂ at sea level is less than 70 mmHg (9.3 kPa), PaO₂ at 2300 metres is less than 50 mmHg (6.7 kPa). The natural conclusion is that all patients with a PaO₂ less than 70 mmHg (9.3 kPa) at rest at ground level should receive supplemental oxygen.^191,201

Many lung function laboratories perform assessments for fitness to fly. These may include measurement of arterial blood gas levels or transcutaneous oxygen saturation while breathing a mixture of 15% oxygen and 85% nitrogen, which mimics conditions at 2800 metres.