54 / 116
52
|
FALL 2016
RESEARCH, EDUCATION & MEDICINE
RESEARCHER PROFILE
decompression schedules exactly as intended. This is the
approach taken by the U.S. Navy.
What should divers consider when choosing a
dive computer?
Unfortunately recreational divers probably do not have
many choices of dive computers that implement a
decompression algorithm subjected to the same level of
human testing as the U.S. Navy dive computer. I am not
an expert on recreational dive computers, but I know
many implement variants of the ZH-L16 (Bühlmann)
decompression algorithm, and the human testing of
ZH-L16 is well documented.
Deep stops have been of interest to the diving public
— technical divers in particular. The disputes may not be
settled yet, but your team contributed important evidence.
Where do we stand regarding the efficacy of deep stops?
We need to consider three types of deep stops. First is the
use of deeper than traditional safety stops for recreational
no-stop dives, where the total dive time, including the
safety stop, is less than accepted no-stop limits. These
deep safety stops probably do no harm, but the evidence is
conflicting as to whether they are of any benefit compared
to traditional safety stops at 10-15 feet of seawater.
Second is the practice that was popular in the
early days of technical diving of adding some brief,
unscheduled decompression stops deeper than the first
prescribed decompression stop and then recalculating
(or letting the dive computer recalculate) the additional
required decompression time. This will result in a longer
total decompression time and, if the stops are not too
deep, should be safer than the original schedule, but
how much safer has never been rigorously tested.
The third type of deep stop is when a decompression
algorithm is designed to redistribute time from shallow
decompression stops to deep stops; in other words,
compared to a conventional decompression schedule,
there are additional deep stops but the total decompression
time is the same (or shorter). The theoretical premise is
that the deep stops result in fewer and smaller bubbles and
so the resulting deep-stops schedule should have lower
risk of DCS than a conventional schedule. There is now
considerable experimental evidence that these types of
deep-stops schedules do not impart a lower risk of DCS
than conventional schedules.
How does body temperature affect decompression?
Being very warm on the bottom or being very cold during
decompression increases the risk of DCS. Presumably this
results from increased blood flow to superficial tissues
and therefore faster uptake of inert gas when warm and,
conversely, reduced blood flow and slower removal of inert
gas when cold. This is probably not of great consequence
for divers conducting no-stop dives and certainly not
worth divers making themselves deliberately cold on the
bottom and risking hyperthermia. For divers conducting
decompression dives, however, it is worth considering.
If a diver becomes very cold during decompression, the
time required for decompression is increased. If divers
have active heating, such as electrically heated drysuit
undergarments, they should use these only enough to stay
comfortable while on the bottom and conserve the battery
to ensure they can use the heat during decompression.
What should recreational divers do when dive
conditions make them exert themselves more
than usual?
Work on the bottom increases blood flow and results in
faster uptake of inert gas. This will increase the risk of DCS
for a no-stop dive. Recreational divers who exert themselves
more than usual on the bottom should add some safety by
ascending before they reach their no-stop limit.
We now have more ways to study the venous gas
bubbles that may occur after diving. What are some
of the tools and methods that allow this research?
Venous gas emboli (VGE), the bubbles that occur in
the body’s tissues, are transported by venous blood and
can be detected in large veins or in the right side of the
heart. The number of bubbles is commonly described
semiquantitatively (i.e., by a grade on an ordinal
scale). Many dives result in detectable VGE but do not
result in DCS. VGE after a dive in no way indicates
whether the diver will get DCS. In large compilations of
experimental dives, however, there is a higher incidence
of DCS among dives that resulted in high-grade VGE
than in dives that resulted in low-grade VGE. This
relationship leads people to use VGE as measure of
Doolette helps deploy equipment for dye-tracing the water flow in
the Wakulla-Leon Sinks underwater cave system.
JACKIE BOOTH