1. Deep capability or deep trouble?
Associate Professor Simon Mitchell
MB ChB, PhD, FUHM, FANZCA
Department of Anaesthesiology
University of Auckland
2013 BSAC Meeting
Decompression diving
Deep capability or deep trouble?

2. decompression algorithm?
What is
decompression
diving?
Why do we
do it?
What is the best
decompression algorithm?
How do we
do it?

3. What is
decompression
diving?

4. When an inert gas is breathed at increased pressure, then more
is dissolved in blood, carried to
tissues and absorbed into those
tissues

5. relationship between:
Manipulation of
relationship between:
Tissue gas pressure
Ambient pressure
Tissue gas pressure
Spend bottom time
at this depth
Descent
Ambient pressure (~depth)

6. How high can the tissue gas pressure go? Tissue gas pressure Bottom
= ambient
How high can
the tissue gas
pressure go?
Tissue gas pressure
Bottom
time
Ambient pressure (~depth)

7. “saturated” with inert gas at this depth Tissue gas pressure
= ambient
The tissue is now
“saturated” with inert
gas at this depth
Tissue gas pressure
Depending on their
kinetics, other “slower”
tissues will have lower
pressures
Ambient pressure (~depth)

8. Ambient pressure (~depth)
Tissue
pressure
= ambient
Tissue gas pressure
Ambient pressure (~depth)

9. diver begins ascending? Tissue gas pressure
= ambient
What happens when the
diver begins ascending?
Tissue gas pressure
Super-
saturation
Ambient pressure (~depth)

10. Decompression diving is when tissue super- saturation reaches a level
pressure
= ambient
Slower tissue
Decompression diving is
when tissue super-
saturation reaches a level
requiring we stop and
wait for tissue gas
pressure to fall – called
“decompression stops”
Tissue gas pressure
Super-
saturation
Ambient pressure (~depth)

11. Why do we do it?
Insert your own motivational
story here…..

12. Discovery
Exploration
Getting to places
we otherwise
couldn’t reach

13. How do we
do it?

14. You need: a dive plan
a gas plan
a decompression regimen

15. Some features of deep decompression diving
For deep phase:
For decompression:
Substitute helium
for nitrogen:
- Less narcotic
- Lighter
Progressive switch
back to richer
nitrox mixes
Completion of
decompression on
100% oxygen
Reduce oxygen
content below that
of air to maintain
a safe inspired PO2
eg 1.3 ATA maximum

16. Decompression strategies

17. Obvious question… Tissue gas pressure Decompression diving is
= ambient
Obvious
question…
Decompression diving is
when tissue super-
saturation reaches a level
requiring we stop and
wait for tissue gas
pressure to fall – called
“decompression stops”
Tissue gas pressure
Ambient pressure (~depth)

18. Two approaches Gas content models
How do we decide on the allowable supersaturation
and therefore the safe ascent depth?
Two approaches
Gas content
models
Tissue supersaturation is
tracked and allowed
up to limits derived by
“experiment” and adjusted
based on outcomes

19. Supersaturation limit
Tissue
pressure
= ambient
Supersaturation limit
Most famous “limits”
are the ZHL16 ascent
rules proposed by
AA Buhlmann
Tissue gas pressure
Supersaturation limits were calibrated to prevent symptoms, and by adhering to them you prevent symptoms MOST of the time, but unexpected DCS still occurs…..
Ambient pressure (~depth)

20. supersaturations didn’t usually cause symptoms, they often resulted in
While these
supersaturations didn’t
usually cause symptoms,
they often resulted in
bubble formation
Since bubbles are the
cause of DCS, it was
reasoned that an approach
that produced less
bubbling might be safer.
This led to….

21. Two approaches Gas content models Bubble models
How do we decide on the allowable supersaturation
and therefore the safe ascent depth?
Two approaches
Gas content
models
Bubble
models
Allow tissue supersaturation
up to limits derived by
“experiment” and adjusted
based on outcomes
Limit tissue supersaturation
to minimize bubble formation
and growth according to
models of bubble behaviour

22. Supersaturation limit
Tissue
pressure
= ambient
Supersaturation limit
Bubble models
predicted the initial
supersaturations were
too great and likely
to initiate bubble
formation whereas
smaller initial
supersaturations
would prevent this
Tissue gas pressure
Ambient pressure (~depth)

23. It has become an article of faith among
technical divers that bubble model / deep stop
decompressions are superior
Gas content models have been derided as
“bend and mend”

24. For example “GF 20:90” Supersaturation limit Tissue gas pressure
= ambient
Supersaturation limit
For example
“GF 20:90”
20%
Tissue gas pressure
Divers began using
“gradient factors” to
force gas content models
to impose deeper stops
90%
Ambient pressure (~depth)

25. There are NO reliable data that support the view
that deep stop approaches are superior
Available human data suggest caution

26. The evidence

27. Gas content model compared to 3 experimental
deep stop protocols for 50 – 60m air dives
Outcome: venous bubbles
None of the deep stop protocols were superior
One of the deep stop protocols was inferior

28. NOT a comparative study
Compliance with a bubble model did not prevent
consistently high grade VGE in decompression
from m trimix dives.
NOT a comparative study

29. The
NEDU
study

30. Deco from 170’ for 30 min on air
Gas Content (traditional shallow stops) vs Bubble Model (deep stops)
Deep stops
Shallow stops

31. - 115 watts exercise at bottom
- Water temp 30oC
- Outcomes:
DCS
VGE

32. DCS Incidence

33. Maximum bubble grades Navy declined to adopt bubble model
decompressions for routine operations

34. Bubble model advocates: “the Navy bubble model deco does not look anything like VPM!”
Deep stops
Shallow stops

35. Actually, its different, but not that different…
Green = VPM-B profile with enough
conservatism to be the same
length as the NEDU study deco

36. Is this study really irrelevant to
technical diving???
Why did the NEDU deep stops fail?

37. Different effect of deep stops on supersaturation
in fast and slow tissues

38. Supersaturation limit
Tissue
pressure
= ambient
Supersaturation limit
Tissue gas pressure
Ambient pressure (~depth)

39. The profiles
Supersaturation in a
fast tissue
Supersaturation in a
slow tissue

40. less supersaturation of
Deeper distribution
of stop time will cause
less supersaturation of
fast tissue and more
supersaturation in
slow tissue

41. in the deep stops profile
The Navy deep stops profile did reduce supersaturation of fast tissues early in the decompression but this did not seem to be protective
In contrast, increased supersaturation of slower tissues late in the decompression must have been the cause of ↑ in DCS
in the deep stops profile

42. So, if deep stops cause increased supersaturation of slower tissues late in the ascent, and if this increases risk of DCS, then how should we decompress? Should we omit deep stops entirely??!!
Rational answer: be guided by the expected
effect of the decompression plan on supersaturation in
fast and slow tissues during the ascent

43. Deep stops skew: the area under the decompression curve
Greater the deep stops emphasis, the greater the DSS

44. Effect of deep stop emphasis on
supersaturation in fast and slow tissues
Shallow stops
profile
Deep stops
profile

45. Conclusions
These data are not presented as conclusive proof that deep stop decompression is inferior, or does not work across the range of technical diving depths and gas combinations, but…..
They make a strong case for arguing against the current perception that deep stop decompression is superior or safer

46. Conclusions
Deep stops may fail because supersaturation in slower tissues may be more important in DCS than faster tissues in at least some time / depth combinations
On the basis of presently available evidence, divers may consider de-emphasizing deep stops in configuration of their decompression algorithms.
Doolette DJ, Mitchell SJ. Decompression from technical
dives. Diving Hyperbaric Med 2013:43(2):96-104