Hyperbaric Oxygen Therapy - Hyperbaric Chamber Information ...

Mortality
A selection mortality threat arises when there is a differential non-random dropout of
participants in one of the study arms. This effect may be due to systematic difference
between experimental groups. Therefore it is important for studies to specify their losses,
per treatment arm and to use intention to treat analysis in order to ensure that the
experimental groups are equivalent at the end of the study.
Instrumentation
Instrumentation effects are usually the result of changes in instrument measurement
from pre- to post-test. This effect usually arises when observers become more
experienced with the instruments and the process at the end of the study. Other effects
of instrumentation are the shift in measurement at different points for example if
measurement intervals are narrower at the ends of a scale in comparison of to the midpoints
a ceiling or basement effect can arise. Comparative studies in thermal burns, nondiabetic
wounds, and migraine headaches might be prone to this threat.
External validity
External validity assumes that the results of a study can be generalised to another context
(ie., a different study sample, setting or time). To ensure that the HBOT results are
generalisable, we need to be confident the study samples are representative of the
populations to which the treatment is directed.
Sampling bias
One of the major threats to the external validity of HBOT is sampling bias. Sampling
bias can arise when the study sample is systematically different from those outside the
study population. Since all of HBOT studies were undertaken in a medical setting the
patients participating in these studies may over-represent the more serious end of the
disease spectrum. One of the major questions that needs to be asked is whether the
results form these studies can be generalised to those with less severe conditions.
Repeatability
External validity can also be assumed if studies can be repeated in different settings (such
as different communities) and at different times. For the indication cardiovascular disease
two RCTs were retrieved, one from the UK and the other from Yugoslavia. Both of
these studies recruited a large number of participants with similar age groups and sex
ratios. Although a difference of 25 years separated the publications of these studies, they
both observed similar results. It would be safe to say that these studies have good
external validity.
Biased reporting
Biased reporting can also effect external validity. Many of the studies used in this
assessment only reported results for patients who accepted HBOT or were compliant
with treatment. Therefore our estimates of effectiveness for HBOT can only generalised
to those patients who are similarly compliant. Can we assume the same effect for those
patients who find HBOT unacceptable or fail to comply? Under these circumstances the
effect of HBOT will be biased, since the estimates of effectiveness are only reflective of
HBOT users.
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