Study limitations
One trial that met all the validity criteria was not included
because its raw data could not be obtained.22 This was a small trial
(12 cases, 13 controls), and its results were of a net benefit of
exercise training in exercise tolerance and quality of life. It is
unlikely that the principal findings of our meta-analysis would
have been altered if the raw data had been available.
Exercise training can necessarily only be trialled in open
design studies, and it is important to consider the possibility that
there may have been more vigorous prognostic pharmacotherapy
in one arm than in the other. At baseline there was no
significant difference in treatment pattern between groups. To
assess the plausibility of changes in medical therapy as a cause
for the reduction in mortality, we asked all the investigators
about changes in drugs during the trial. Investigators in six of the
nine trials, covering two thirds of the patients, were able to provide
information. They stated that there was no change in angiotensin
converting enzyme inhibitor, blocker, or antialdosterone
therapies during the trial period. As is normal clinical practice,
however, patients were allowed to vary their dosage of loop diuretic,
but comprehensive data on this are not available.
Since the primary end point was death from any cause
(selected to minimise potential for observer classification bias)
and there was no evidence of differences in prognostically effective
drugs, it seems likely that there is a genuine beneficial effect
and that it arises from the exercise training