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is thought to increase the risk, while gentle exercise during
decompression has been advocated to assist offgassing and
reduce the risk. Vigorous exercise after completing a dive
is discouraged because of the potential to promote bubble
formation by mechanical stimulation
Alf Brubakk:
Regular exercise is recommended for divers.
Aerobic exercise prior to diving will reduce vascular bubble
formation. Exercise after diving may increase or decrease
bubble formation; the effects may be dependent upon general
fitness levels. This is an area where information from which
to make firm recommendations is lacking.
Most experts agree severe dehydration might increase
DCI risk, but it has been suggested that mild to moderate
dehydration may reduce the risk. What do you think
about this, and what would you recommend divers do?
Balestra:
There are points of view that assert a “normal” volume
of blood plasma or even a moderately reduced plasma volume
could possibly reduce the nitrogen saturation of the tissues
during a dive. The actual take-home message is not to increase
plasma volume too rapidly or too much as this will increase
urine production and not really hydrate the tissues. My advice
is to drink a glass of water every 15 or 20 minutes to allow the
tissues to be hydrated without increasing plasma volume.
Bennett:
Some work investigating the effect of both exercise
and exposure to heat on the risk of subsequent DCI may be
interpreted as suggesting, somewhat paradoxically, that mild
dehydration is protective. The suggestion is a consequence of
one possible mechanism by which these challenges provide
protection against bubble formation. Blatteau and colleagues
suggested the moderate dehydration and decreased blood
volume (hypovolemia) induced by predive exercise or heat
exposure in a sauna might decrease cardiac output and
reduce the delivery of nitrogen to the tissues. There are a
number of competing theories, however, and I am not aware
of any data that support this particular assertion.
The suggestion is actually rather surprising. Although
the risks associated with dehydration have yet to be well
defined, everything we know suggests predive fluids are a
good thing rather than harmful. For example, in 2008 Gempp
and colleagues published a crossover study that concluded:
“Predive oral hydration decreases circulatory bubbles, thus
offering a relatively easy means of reducing decompression
sickness risk.” In this study, prehydration with 1.3 liters of
a saline/glucose mixture attenuated the dehydration and
prevented the hypovolemia induced by diving but did not
change plasma surface tension. My recommendation is divers
should try to ensure adequate hydration before diving and
actively avoid diving when dehydrated.
Brubakk:
I do not know of any data that support this, and I
do not think theory would suggest this. I recommend divers
be well hydrated.
Some researchers have proposed predive administration
of antioxidants such as vitamin C, other nutrients, or
drugs such as nitroglycerin to reduce the risk of DCI.
How might such agents reduce risk?
Balestra:
Experience shows this approach does not really
interfere with bubble production but with endothelial
function. After a dive, when endothelial function is
transiently impaired, antioxidants can prevent such
impairment, but there is no clear demonstration that bubble
production can be reduced with such agents. Research on
this subject is ongoing.
Bennett:
We have recently seen a growing interest in the
modification of endothelial function by pharmacological
means. In general, most interest is generated by agents that
increase NO availability and the subsequent effect on sites
(presumed to be on the endothelium) where gas bubbles
form. Such bubble formation is likely to cause endothelial
injury and promote both microvascular obstruction and
activation of coagulation cascades — changes that might
be directly responsible for the clinical picture of DCI.
Indeed, although it is early, there is some experimental
evidence from both animals and humans that administration
of compounds such as those listed could significantly
reduce the risk of DCI. Essentially, both NO donors (such
as nitroglycerin) and antioxidants (such as Vitamin C)
counteract the oxidative stress that is the cause of the
endothelial damage that may be the reason bubble-induced
injury produces such widespread effects in divers.
This is a fascinating area of research and may soon
produce some definite recommendations for divers. At
this time, however, we should exercise caution. Many of
these agents have wide-ranging effects — some of which
may result in considerably more harm than good — and as
yet we have no practical evidence that clinical DCI can be
prevented by these agents.
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