Why does oxygen supplementation during exercise training in COPD patients with exercise-induced desaturation not consistently improve exercise capacity?

To the Editor:

We would like to congratulate the authors of the recent study entitled “Oxygen compared to air during exercise training in COPD with exercise-induced desaturation” for conducting a methodologically well-designed randomised controlled trial (RCT) [1]. The potential effects of several adjuncts to exercise training during pulmonary rehabilitation, such as supplemental oxygen or inspiratory muscle training, are still a matter for debate [2]. Additional improvements in functional capacity are only likely to occur when the adjunct interventions enable patients to train at higher intensities during their general exercise-training programme. There exists a sound physiological rationale for correcting hypoxaemia as a strategy to improve performance and reduce symptoms during exercise in patients with COPD [3]. However, not all of the previous randomised clinical trials were able to achieve relevant contrasts in training intensities and treatment outcomes between oxygen and air groups [4].

The study by Alison et al. [1] constitutes the largest RCT conducted in this area so far. It confirms that supplemental oxygen was not required for patients to train safely and to derive some benefits from exercise training. When looking closer at the results it appears, however, that the room air group did not succeed in achieving clinically relevant improvements in the shuttle walk test after an intensive 8-week training period. It is surprising that the oxygen group was not able to exceed the improvements achieved by the room air group. The authors explain the absence of between group differences in the primary outcome by concluding that “during treadmill training the oxygen group was not able to achieve a greater training dose per session than the [room] air group, despite a significantly higher measured S_pO2 and significantly lower dyspnoea and RPE [rating of perceived exertion] scores during treadmill training sessions.” This inability to train at higher intensities (despite higher S_pO2 and lower symptom scores) is unexpected given the description of the planned progression of exercise intensity during the training programme. The authors initially aimed to “increase training intensity according to symptoms so that dyspnoea or rate of perceived exertion would be maintained at a “moderate” to “somewhat severe” level (i.e. a score of 3 to 4 on the modified Borg dyspnoea and RPE (0–10) scales)” [5]. First, these target symptom scores appear to be rather low in comparison with previous recommendations [6, 7]. In addition, when looking at actual average Borg scores that were recorded during treadmill walking sessions (table S1 in their supplementary material) it appears that a large part of the oxygen group must have been training at scores well below these pre-specified targets. Reported standard deviations around average Borg scores indicate that about half of the oxygen group must have trained at scores between 1(!) and 3. In contrast, during the initial endurance shuttle walk test subjects were able to walk for longer than 5 min on room air with Borg scores above 4 on average. This of course raises the question as to why these patients were not able to tolerate higher symptom scores during their walking training sessions on the treadmill? Was the fact that training intensity could only be increased every three sessions (once a week) possibly preventing subjects from reaching sufficient increases in training intensity? Or was it the fact that both dyspnoea and RPE scores had to be <3 after a treadmill training session before intensity could be increased? Or were the physiotherapists (who were blinded to group allocation) maybe just overly cautious to increase intensity during the training sessions?

Another interesting observation from the current study is that some differences in training intensity could be achieved between the oxygen and the room air group during exercise on the bicycle in contrast to treadmill walking. Since patients had to perform their exercises as continuous training (without interruptions) we were wondering whether this might have been related to the differences in initial training durations (cycling sessions 10 min versus treadmill 20 min)? Would it in hindsight maybe have been better to subdivide the treadmill training into blocks of 5 to 10 min? Or might cycling training simply be the preferred training modality in this context? We believe that some additional discussion on the points raised above could provide important information for future work in this area.

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