Q4_Fall_2016_AlertDiver_web

55 / 116

Maldives
Turks & Caicos
Galapagos
Saba/St. Kitts
Silver Bank
Unique liveaboard
experiences.
Exceptional value.
Dive with us!
N
USA/Canada: 1.800.322.3577
+1.307.235.0683
N
| 53
decompression stress (an index of DCS risk) in dives
that do not necessarily result in DCS. This can produce
useful information if VGE are the appropriate outcome
measure, if the experiment is carefully designed and well
executed, and if the results are thoughtfully interpreted.
VGE, however, are not the appropriate outcome
measure for all experiments related to decompression.
VGE are only meaningful if the experiment is testing
an intervention that changes bubble formation and
growth in tissue and thereby influences DCS risk.
Examples are tests of decompression schedules, diver
exertion and diver thermal status. VGE are not an
appropriate outcome measure for interventions aimed
“downstream” of bubble formation and growth — at the
pathophysiological responses to bubbles. An example
would be evaluation of methods of treating DCS.
VGE grades following identical dives are quite variable,
both between divers and in the same diver on different
occasions. Therefore, the VGE grade after a single dive in
a single individual is not informative. Only after multiple
repetitions of the same dive profile are VGE grades useful.
The number of repetitions depends on the research
question, but in my opinion there needs to be good
justification for less than 20, and 50 would be preferable.
There are two common experimental designs. VGE
might be used to validate a decompression table by
evaluating selected schedules. Each schedule is dived at
least 20 times and “fails” if more than half of the dives
results in high-grade VGE; otherwise it “passes.” A more
common experimental design, and in my opinion the
best use of VGE, is to compare two or more different
decompression procedures. Each procedure is dived
50 times, and a significant difference in VGE grades
indicates a difference in DCS risk.
There are many technical challenges in executing a
good VGE study, but one very important consideration
is the frequency and timing of VGE measurements after
diving. The only validated index of “decompression
stress” is the peak VGE grade measured after diving.
This peak might occur any time from immediately after
surfacing to several hours later. It is usually practical to
measure VGE only periodically, so these measurements
need to begin soon after surfacing and continue
frequently for two or more hours.
One of many important considerations in interpreting
results of VGE experiments is that while a significant
difference in VGE grades between two procedures is
evidence of a difference in DCS risk, it is not a reliable
indicator of how large the difference is. Similarly, for
many reasons, failure to find a difference in VGE grades
between decompression procedures does not indicate
there is no difference in risk of DCS. AD