Supplemental oxygen and dypsnoea in interstitial lung disease: absence of evidence is not evidence of absence.

It is with great interest that we read the recent systematic review by BELL et al. [1] concerning the effects of oxygen therapy on dyspnoea and exercise capacity in patients with interstitial lung disease (ILD). The authors report that, while supplemental oxygen increases exercise capacity, it does not improve dyspnoea. Overall, this is a well-executed systematic review that accurately reflects the current literature; however, we believe the conclusion regarding the lack of benefit of supplemental oxygen on dyspnoea in ILD is misleading. This opinion is not based on the quality of the systematic review, but rather on the quality of the existing literature that was evaluated. We base our opinion on two important lines of evidence. First, previous studies that have evaluated the effect of supplemental oxygen on exertional dyspnoea in ILD patients only report peak or end-exercise dyspnoea ratings, thereby ignoring important clinically and physiologically relevant changes occurring at submaximal exercise intensities. Secondly, the exercise testing modalities employed are variable, such as self-paced walk tests [2–5] or incremental cycle exercise tests [6], which are often insensitive to changes in dyspnoea. An additional concern relates to the measures in place to reduce experimental bias, as the authors have addressed. Indeed, the only study included in their review that showed a beneficial effect of oxygen on dyspnoea did not have an appropriate control condition [5], making it impossible to rule out the placebo effect.