Limitations of the review
There are a few limitations with respect to this review.
Firstly, the total number of patients in each study is
sometimes very small, which makes it unable to detect
clinically relevant effect sizes. Secondly, the designs of the
randomized controlled trials used in our review are very
different. Not only have they used different outcome
measures, also the control group, the duration of the
training and the content of the training of each study is
different. If the same outcome measures are used in different
trials, they are often measured in a different way.
The duration of the training used in the individual studies on breathing exercises and physical training is very different.
A range of four to 28 weeks is seen in breathing exercises
(Table 2), for physical exercises six to 16 weeks
(Table 4). In addition, training on physical exercises varied
in duration and frequency for each trial (Table 4). For
example, one group trained 30 min twice a week, another
group trained 90 min three times a week. The studies used
different control groups, such as usual care, education,
other breathing exercises or healthy controls. Through all
these different methods it is difficult to compare the various
trials, therefore we were not able to pool data of different
studies or to make a meta-analysis comparing the
results. We were only able to describe results found in the
different studies.
In daily practice, physiotherapists mainly use combinations
of various techniques. Unfortunately, most randomized
controlled trials investigated individual techniques.
Only three11,15,26 of the 21 included trials in our review
combined various techniques. Shaw et al.15 is the only study
that compared a combination of techniques with usual
care. No significant differences were found between these
two groups. In conclusion, there are too few studies that
have examined the effectiveness of combinations of these
techniques, while this can be very promising.