Basic HTML Version
44
|
SPRING 2012
Brubakk:
Antioxidants seem to reduce bubble formation. It may also
be that antioxidants will reduce inflammatory responses that may play a
role in DCI. This is an area that needs further study, but it is a promising
approach. At present, we know too little about the effects of antioxidants
on healthy people.
Rune Djurhuus:
NO is a small signalling molecule that causes relaxation
and dilatation of the blood vessels. Animal studies have indicated
administration of a pharmacological agent (e.g., nitroglycerin) that releases
NO in the blood stream may reduce gas-bubble formation and increase
survival after decompression. Conversely, inhibiting the enzyme nitric
oxide synthase (NOS) that generates NO in the endothelial layer lining
the inside of blood vessels markedly aggravated the symptoms of DCI.
Moreover, physical exercise is also known to stimulate the generation of
NO in the endothelium. A prevailing hypothesis has therefore been that
NO generation plays a role in protecting the vascular system against adverse
effects of gas bubbles during decompression.
Diving usually implies an elevated partial pressure of oxygen. We recently
showed that such hyperoxic conditions had no effect on NOS’s capability
for generation of NO in isolated human endothelial cells. However, to
function normally, the enzyme is dependent on several cofactors, in
particular tetrahydrobiopterin (BH4). This compound is easily oxidized,
and the oxidized form does not support NO synthesis. Exposing human
endothelial cells to hyperoxic conditions (approximately three times the
partial pressure of oxygen at sea level) caused the BH4 concentration to
drop approximately 50 percent. The consequence of exposure to hyperoxic
conditions while diving may therefore be a decreased level of BH4, which in
turn limits NO generation by NOS and potentially increases the risk of DCI.
It should be emphasized these results were obtained in an experimental
model, but if the detailed mechanism can be elucidated and verified in
animals (preferably humans), remedial actions seem possible. These might
include counteraction of hyperoxic effects by additional supplies of BH4 or
by administration of antioxidants that stabilize the cofactor in the reduced,
active form. A simple antioxidant such as vitamin C has been shown to help
sustain the level of BH4 in experimental models. As research progresses
other factors may turn out to be more crucial.
Several other factors such as whole-body vibration before diving,
oxygen prebreathing, work-up dives and predive sauna have also
been proposed for preconditioning against DCI. Have any practical
applications emerged from these proposals?
Balestra:
These preconditioning techniques are directly related to moderate
cardiac activity (sauna) or increasing lymphatic activity (whole body vibration,
oxygen prebreathing). In some cases the oxygen prebreathing was performed
too long before the dive for any denitrogenation effect to be considered.
All the techniques listed are thought to be more related to moderate
demicronucleation than denitrogenation.
Bennett:
All these proposed measures constitute attempts to reduce the
chance of DCI through preconditioning against bubble formation. The only
one commonly applied to divers is work-up dives, whereby divers aiming
Maldives
•
Thailand
•
Philippines
Timor-Leste
•
Indonesia
Palau
•
Andamans
•
Malaysia
Make Siren Fleet
your No.1 choice
Recreational
Rebreather
Courses
Siren Fleet
6 Luxury Liveaboards diving
across Asia and the Pacific
1-866-258-6398
www.worldwidediveandsail.com
info@worldwidediveandsail.com