We just published a paper on the effects of controlling inspired carbon dioxide during consecutive breathlessness challenges using respiratory loading.
Isocapnia can be used to reduce effects of PETCO2 fluctuations on BOLD response during respiratory loading, but the obligatory hypercapnia might then also alter dyspnoea perception. In this study, we observed a small increase in baseline (unloaded) unpleasantness during hypercapnia (at +0.6kPa), but none during loading (i.e. it is not additive). We observed no habituation effects with repeated trials (four consecutive days). In short, isocapnic respiratory loading can induce respiratory unpleasantness in a stable and FMRI-friendly manner, even in multi-session studies, but does not provide the additional benefit of increased unpleasantness for the participant at such a low level.
Also (on a more personal note), maintaining said hypercapnia manually is quite a task. Coffee should be consumed, focus should be applied, training is always good.