Summary
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Introduction
Oxygen is a medication often used in medical emergencies. The gas may also be prescribed for long-term use by patients with chronic respiratory conditions.
Oxygen is indicated for the treatment of hypoxemia, but not for the symptom of dyspnea.
Long-term oxygen therapy is most commonly prescribed for patients with chronic obstructive pulmonary disease (COPD). While oxygen may improve survival, not all patients will benefit, so prescribing oxygen therapy should be guided by evidence from clinical trials. Although results from studies in COPD have been extrapolated to hypoxemic patients with other lung diseases, evidence of benefit is lacking.
Long-term continuous oxygen therapy
The prescription of long-term continuous oxygen therapy is based on two studies that demonstrated improved survival in patients with COPD and severe hypoxemia. In the UK Medical Research Council study, patients were prescribed 15 hours of oxygen per day or no oxygen. Mortality at three years was 66% in the control group and 42.5% in the oxygen group.
2 patients in the US Nocturnal Oxygen Treatment Trial (NOTT) were prescribed continuous oxygen (average of approximately 18 hours/day) or nocturnal oxygen. Mortality in the nocturnal oxygen therapy group was 1.94 times higher than that in the continuous oxygen therapy group (p=0.01). The results of these trials significantly altered the treatment of hypoxemic COPD.
Home oxygen was until recently the only therapy (other than smoking cessation) known to significantly reduce mortality. Most international guidelines are based on the entry criteria for these trials.
- They recommend that oxygen should be considered for patients with stable COPD, who have a partial pressure of oxygen in arterial blood (Pa0 2) of: 55 mm Hg or less at rest when awake and breathing air.
- 56-59 mmHg if they have polycythemia (hematocrit >0.55) or clinical, electrocardiographic or echocardiographic evidence of pulmonary hypertension or right heart failure.
Before prescribing oxygen, the patient’s condition should be stable and all reversible factors, such as underlying lung disease and comorbidities, for example, anemia or sleep apnea, should have been treated to the extent possible.
Continuous oxygen is administered through a stationary concentrator, an electrical device that extracts nitrogen from room air, and should be prescribed for at least 15 hours a day . The flow rate should be adjusted to maintain Pa0 2 above 60 mmHg , at rest. Increasing flow rates by 1 L/minute may be considered during sleep, exertion, and air travel.
Oxygen can be provided from a portable cylinder or a battery-powered portable oxygen concentrator for use outside the home for patients who are physically active and want to maximize the number of hours they receive oxygen.
Ambulatory oxygen therapy
Ambulatory oxygen may be provided to patients who have:
- Severe hypoxemia at rest and are physically active, to maximize survival benefit by increasing the duration of their therapy.
- An improvement in exercise capacity in response to ambulatory oxygen on a laboratory-based functional exercise test (usually a 6-minute walk test).
Despite some small acute benefits during laboratory testing, an Australian double-blind randomized controlled trial of oxygen or air therapy in COPD patients without significant hypoxemia at rest found no greater relief of dyspnea during activities of daily living in the oxygen group. This raises the possibility that the small benefits in both groups were related to a placebo effect or an effect of gas flow to the face.
Oxygen therapy during pulmonary rehabilitation
Oxygen supplementation during pulmonary rehabilitation in COPD patients who desaturate with exertion is no more beneficial than air supplementation. This was demonstrated in a double-blind randomized controlled trial, which compared oxygen and air administered at 6 L/minute. These results are consistent with those of a previous meta-analysis.
nocturnal oxygen therapy
Two small studies, published more than 20 years ago, investigated the impact of nocturnal oxygen therapy in patients with COPD who desaturated below 85% or 90% for more than a third of the night. Although one study showed a trend toward improved pulmonary artery pressures in those randomly assigned to receive nocturnal oxygen, no benefit was seen in the other study.
The International Nocturnal Oxygen (INOX) trial also investigated patients with COPD and nocturnal desaturation. It was designed to evaluate whether supplemental oxygen delivered through a concentrator would delay death or progression to long-term continuous oxygen therapy longer than sham oxygen (air delivered through the identical device). Recruitment and retention difficulties led to premature discontinuation of the trial, after recruitment of only 243 of the projected 600 patients, with no benefits observed.
Overall, the evidence to date does not support the use of nocturnal oxygen in patients without severe daytime hypoxemia.
Oxygen for moderate hypoxemia
The long-term oxygen treatment trial originally aimed to evaluate whether supplemental oxygen would improve survival in patients with COPD and moderate hypoxemia at rest (pulse oximetry: Sp0 2 89–93%). Recruitment difficulties led to extension of entry criteria to include exercise-induced desaturation and modification of the outcome measure to also include first hospitalization for any cause.
Compared with patients who did not use oxygen, there were no differences in any of the trial’s primary or secondary outcomes. The conclusion was that long-term supplemental oxygen in patients with stable COPD and moderate desaturation induced by exercise or at rest has no benefit . These results were consistent with a small study with similar entry criteria that found that oxygen had no mortality benefit in patients with moderate hypoxemia.
Palliative oxygen therapy
Home oxygen is often sought to control intractable dyspnea, but, in the absence of significant hypoxemia, there is no convincing evidence that it provides greater benefit than sham oxygen. Even in the presence of hypoxemia and when underlying therapies have been maximized, oxygen may not relieve dyspnea. Other palliative therapies, including ventilators and opioids, may be more appropriate for symptom control.
Assessment of oxygen needs
Doctors often first become aware of a patient’s hypoxemia when the patient is admitted to the hospital for a COPD exacerbation. Oxygen is then often prescribed upon hospital discharge, but this practice is not evidence-based.
A New Zealand study reported that more than a third of patients who met the criteria for long-term continuous oxygen therapy at the time of hospital discharge no longer met the criteria two months later. Therefore, guidelines recommend reviewing patients 4 to 8 weeks after discharge to assess their oxygen needs.
To determine eligibility for long-term continuous oxygen therapy, the Thoracic Society of Australia and New Zealand (TSANZ) Home Oxygen Therapy Clinical Practice Guideline for Adults recommends arterial blood gas analysis while the patient breathes room air. This is due to the known inherent variability of measuring oxygen saturation with pulse oximetry.
Evaluations should be performed at least one month after the patient has stopped smoking. There should also be periodic reviews to confirm any ongoing need for and appropriateness of oxygen therapy, or the need for patients to use oxygen for exertion to progress to long-term continuous oxygen therapy.
For patients who do not meet criteria for long-term continuous oxygen therapy but become desaturated with exertion, a blinded trial of portable oxygen versus air may be appropriate to determine if there is any improvement in dyspnea or distance walked. Then, after discussion with the patient, a home trial may be necessary, with a review to assess any benefit and the need for continued therapy.
Contraindications, adverse effects and dangers
Oxygen therapy is an absolute contraindication in smoking patients due to the risk of fire. Open flames in the home, such as from gas stoves or open fires, can also present a risk. Identified issues related to therapy burden include decreased mobility, discomfort related to nasal cannulas, and noise related to the device, to name a few.
It is important for patients to be aware that oxygen is a drug and should not be adjusted without consulting the prescribing doctor or therapist.
There is growing evidence regarding the burden on patients and caregivers of oxygen therapy, particularly ambulatory oxygen. Education about the potential benefits (or lack thereof) and burdens should occur when patients undergo evaluation for home oxygen therapy. Patients benefit from discussing their beliefs and concerns, as their beliefs about oxygen influence its use.
Oxygen supplies
In Australia there are three main methods of oxygen delivery. These are stationary concentrators for continuous use and portable cylinders or portable concentrators for use during exertion.
While the TSANZ Guidance provides evidence-based guidance, Australian states and territories vary in their interpretations and applications of this advice and in their provision of oxygen therapy.
Conclusion
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Conflicts of interest: none declared
