The effects of oxygen therapy in patients presenting to an emergency department with exacerbation of chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is a common disorder affecting up to 2.6 million Australians, with affected patients occupying 1000 hospital beds daily in Australia.1 Management of the exacerbations that characterise COPD is described in several international guidelines. Common to these guidelines is the recommendation for controlled oxygen therapy to treat hypoxaemia.2-4 The COPDX Plan published in this Journal in 2003 recommends administering a fraction of inspired oxygen (Fio2) of 0.28 (about equal to 2 L per minute of oxygen via nasal prongs, or a Venturi mask set at an Fio2 of 0.28) or less until the arterial tension of respiratory gases is determined by arterial blood gas (ABG) sampling.3 This recommendation is based on the knowledge that increasing arterial oxygen tension in patients with hypercapnic respiratory failure causes deleterious changes to alveolar ventilation and gas exchange, resulting in worsening acidosis5,6 that, in turn, leads to higher morbidity and mortality.7,8

A study from the United Kingdom has shown that controlled oxygen delivery is not emphasised in ambulance and nursing staff education, and is often overlooked by emergency and intensive care physicians.9 Our anecdotal experience was similar, and we undertook an audit of patients presenting to our university teaching hospital with an acute exacerbation of COPD (AECOPD) to determine: (i) the proportion of patients who received care consistent with international guidelines; and (ii) whether not adhering to such guidelines resulted in differing outcomes for patients.

We undertook a retrospective audit of patients admitted after presenting to the emergency department (ED) with AECOPD. Patients discharged between 1 June and 30 September 2005 with a diagnosis of AECOPD were identified. Included patients had respiratory function test (RFT) results (either before or after admission) consistent with the British Thoracic Society (BTS) definition of COPD (ie, forced expiratory volume in 1 second [FEV1] less than 80% of predicted, and forced expiratory ratio less than 70%).4

Data on oxygen delivery, triage category, ABG test results, length of stay, admission destination, mortality, and patient demographic information were recorded. Patients who were included were classified as retaining carbon dioxide if they had a partial pressure of arterial CO2 (Paco2) of greater than 45 mmHg recorded either on admission or previously. Patients with chronic hypercapnia or hypercapnia associated with respiratory acidosis were included.

One hundred potentially eligible patients were identified, and 35 of these were excluded (20 had not had RFTs, nine did not have RFT results consistent with BTS guidelines, and six were not admitted through the ED). The demographic characteristics of the remaining 65 patients are shown in Box 1.

There are a number of potential biases in our audit. Because of its retrospective nature, we were reliant on accurate documentation for our results. The charting of oxygen delivery methods and respiratory rate was variable, which made analysing oxygen administration according to length of exposure or minute ventilation difficult. Use of discharge diagnosis as an identifier creates a selection bias by which patients with milder exacerbations of COPD, managed as outpatients, were excluded. Although we cannot prove causation between the parameters measured, several observations can be made.

Our findings confirm that uncontrolled oxygen therapy is commonplace at our university teaching hospital. The use of oxygen flow rates greater than 4 L per minute in patients with high CO2 levels was associated with a significant increase in length of stay (P = 0.034), a greater need for NIV (P = 0.033), and a higher admission rate to the HDU (P = 0.033). Use of more than 4 L of oxygen per minute was associated with severe acidosis in patients with AECOPD known to retain CO2 (P = 0.034). This association has been shown previously in a similar group of patients.9

Increased oxygen delivery promotes retention of CO2 through numerous mechanisms (Box 5). The most important of these are generally considered to be an increase in ventilation–perfusion mismatch secondary to reduced hypoxic vasoconstriction11-13 and a reduction in ventilation on removal of a hypoxic stimulus.13,14

Our audit shows that the administration of high-flow oxygen usually begins in the ambulance. At the time of writing, no specific guideline exists for oxygen administration by ambulance officers.15 High-flow oxygen therapy is continued in the ED, where more than 80% of patients with AECOPD receive oxygen at more than 2 L per minute. This occurs in an ad-hoc fashion, with no formal method for prescribing oxygen. Furthermore, 22/65 patients receiving oxygen at more than 2 L per minute (34%; 95% CI, 24%–46%) did not have an ABG test. This highlights the need for more judicious use of ABG tests in monitoring patients who present with AECOPD at our university teaching hospital.

Emergency staff often use high-flow oxygen on the basis that patients presenting with AECOPD are profoundly hypoxic. Our audit showed that patients who achieved a higher Pao2 were significantly more likely to have NIV on admission, significantly more likely to require HDU admission, and to stay longer in hospital. More research is needed to further delineate the relationship between high Pao2 and patient oucomes.

We used a collaborative approach between respiratory and emergency physicians to disseminating information about the treatment of AECOPD, to improve outcomes by:

• liaising with ambulance services about oxygen delivery guidelines;

• educating patients about oxygen delivery;

• establishing an alert system in the ED for at-risk patients; and

• repeating the audit after implementing the above strategies.

The delivery of oxygen to patients with AECOPD and monitoring the effects with ABG testing is still a major issue at our university teaching hospital. Results of our retrospective audit suggest that administering high-flow oxygen to patients with AECOPD may contribute to the observed differences in length of stay, need for NIV and admission to the HDU, highlighting the importance of controlled oxygen therapy in managing this common disease.

2 Patients classified as retaining carbon dioxide (grey shading)

5 Mechanisms contributing to retention of carbon dioxide in acute exacerbation of chronic obstructive pulmonary disease