Ambulatory oxygen in idiopathic pulmonary fibrosis: of what benefit?

To the Editors:

We read with interest the Letter by Visca et al. [1], recently published in the European Respiratory Journal, which described improvement in 6-min walk distance with ambulatory oxygen in patients with interstitial lung disease (ILD). We have looked specifically at the effects of ambulatory oxygen on walk distance in patients with idiopathic pulmonary fibrosis (IPF), and here describe a practical way of ensuring patients are prescribed an optimum flow rate of ambulatory oxygen.

Between 2004 and 2007, we conducted a retrospective review of anonymised data, studying the effect of ambulatory oxygen on the distance walked in patients with IPF in the ILD clinic of the University Hospital of South Manchester (Manchester, UK). 70 patients performed an adapted 6-min walk test (6MWT) on air or their usual flow rate of oxygen. If their oxygen saturation fell to <90%, the test was terminated and repeated with a 2 L·min⁻¹ increase in oxygen flow rate. This continued until patients did not desaturate to <90% or reached a 6 L·min⁻¹ flow rate. Diagnosis of IPF was based on the American Thoracic Society (ATS)/European Respiratory Society (ERS) consensus statement [2] and the 6MWT was conducted as per the ATS protocol [3]; however, once the patient had stopped, they were not allowed to continue. The distance patients walked was compared using the Wilcoxon signed-rank test. A paired t-test was used to compare oxygen saturations and Borg scores, and an unpaired t-test was used to compare characteristics of the patients who initially performed the walk test on air and those who performed it on their usual flow rate of oxygen.

Characteristics were as follows in patients who performed the walk test initially on their usual flow rate of oxygen versus those who performed it on air: 62.8 versus 61.5 yrs of age (p=0.62); 80.5 versus 65.5% males (p=0.16); 83 versus 65.5% ever-smokers (p=0.16); 51.2 versus 58.4 forced vital capacity % predicted (p=0.15); 27.7 versus 32.4 diffusion capacity of the lung for carbon monoxide % pred (p=0.05); 8.7 versus 8.5 kPa ear lobe gas partial oxygen pressure at rest (p=0.5).

The 29 patients not on oxygen therapy prior to testing walked a mean distance of 81.2 m further using optimal ambulatory oxygen (p<0.01). The 41 patients already on home oxygen walked a mean distance of 16.9 m extra with optimised flow rates (p=0.02) (table 1). Borg scores were not significantly different, suggesting that patients walked further with the same degree of breathlessness.

As well as its retrospective nature, limitations to this study are that patients who performed their baseline walk test on air did not carry a placebo air cylinder, and we did not document whether the increase in distance walked reflected improved quality of life.

Our study demonstrates that ambulatory oxygen significantly improves exercise capacity in IPF patients. In patients already using oxygen, titrating the flow rate of oxygen appropriately also significantly increased the distance they were able to walk. We have described a step-wise method of increasing ambulatory oxygen in 2 L·min⁻¹ increments and suggest this is an objective method of ensuring patients are prescribed an optimum flow rate.

• ©ERS 2012