AD 2016 Q4

How and when did you become a diver?
I have always lived by the sea, and one high school I
attended was very close to a rocky coastline, where I
would regularly spend my lunch break snorkeling. One
day I watched the bubble trails recede from two scuba
divers in the water, and I thought, “I want to do that.” In
1979 on the day I turned 16 years old (then the minimum
age to learn scuba), I enrolled in a scuba course. I have
been involved with diving, for work and play, ever since.
Why did you develop the health survey tool, and
what does it do for decompression research?
In the early 1990s a diver-intensive fish-farming industry
arose in South Australia. Bluefin tuna were caught at sea
and transported to coastal pens, where they were fattened
for the sashimi market. At first the industry employed
divers with recreational training, and there were a large
number of incidents of serious decompression sickness
(DCS). I worked with Derek Craig, a diver and health
and safety inspector, to characterize the decompression
practices. I needed a standardized method for collecting
postdive health-outcome information, particularly the
symptoms of DCS, from a large number of divers in
the field. The ideal method would be having medically
trained personnel go into the field to monitor the divers.
That was not practical, so I developed and validated the
diver health survey, a one-page questionnaire used for
self-reporting health status relevant to DCS.
The incidence of DCS in recreational diving is
relatively low, and divers are surprised when it occurs
because they and their buddies previously dived
similar profiles many times without any problem.
Could you tell us about variability of decompression
outcomes and how that affects your research?
It is because of this question we are having this interview.
In 1985 I had finished my undergraduate studies in
physiology and was working as a scuba instructor. One
day two friends and I were doing a wreck dive; it was a
fairly deep air decompression dive, and afterward one of
my buddies, who had done thousands of dives previously,
developed DCS. That got me thinking, “Why him, and why
that day?” and sparked my interest in DCS.
Similar dives may actually be different enough to
have a different risk of DCS even if the depth/time
profile is the same (which is unlikely outside of research
settings). For instance, if divers work hard while at
depth or are cold during decompression, their risk of
DCS is greatly increased compared to being at rest and
warm — as much as if they had doubled their bottom
time. Even in a laboratory setting, however, where dives
are conducted in a pressurized wet pot where the dive
profile, breathing gas, water temperature and work
performed can be identical for all practical purposes,
we see variability in DCS outcome. In other words,
identical dives with the same diver can result in DCS on
some occasions and not others.
Since 2000 the Navy has conducted a number of
experiments in which an individual dives the identical
dive profile numerous times, sometimes resulting in
DCS and sometimes not. That is clear evidence that an
individual’s susceptibility to DCS changes on a day-to-day
basis. There are two important messages for divers: 1)
Dives are not completely “safe” or “unsafe” — rather they
have a higher or lower risk of DCS; and 2) having done
similar dives in the past without incident is not a reason
to discount DCS. Modern decompression science is
concerned with measuring and predicting DCS risk.
How do dive computers handle the variability of
decompression outcomes?
Dive computers and decompression tables handle the
person-to-person, day-to-day variability in susceptibility
to DCS by providing a choice of relatively conservative
decompression guidance, so that most people on most
days will be safe if they dive properly.
How should dive computers be evaluated in the
development phase?
A dive computer should implement a decompression
algorithm that has been subjected to human testing
so that the DCS risk within the no-stop limits and
decompression schedules is well characterized. The
dive computer should then undergo validation testing
to make sure it faithfully produces no-stop limits and
Doolette inspects a fossil
in the Wakulla-Leon Sinks
underwater cave system.
DAVID RHEA
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