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Mark,
As to HPNS, in Bennett & Elliott they state:
"All these neurophysiological studies have demonstrated the complexity
of HPNS. The variables of the environment have differential actions on the
different nervous structures which react according to their own sensitivity.
In general, the results obtained during experiments carried out both in
animals and man suggest that there is alteration by PRESSURE of neurochemical
mechanisms."
"Further, in the mouse and rat, the use of various anesthetics under pressure
has shown different effects (Halsey, 1982). Although their does had to be
increased at pressure in order to maintain anesthesia, their effects on the
HPNS differed and the results indicate that the interaction between
anesthetics and pressure is not a simple antagonism but involved more than
one site of action. These different effects show that in addition to the
physical effects on the membrane, there are specific actions of anesthetics
and narcotics."
"When such substances bind to specific protein sites, some types of
neurotransmission may be affected. These protein sites may show differential
sensitivity to pressure depending on their physico-chemical characteristics.
It should be noted that anesthetic substances having a marked anti-HPNS
action, such as Ketamine, when administered at 50% of its anesthetic dose
to rats, suppressed HPNS to 1000meters (3281ft) and are known to enhance
GABA-mediated neurotransmission and antagonize N-methyl-D-aspartate (NMDA)
neurotranmission. One can postulate that narcotic or anesthetic substances,
and by extrapolation probably nitrogen and hydrogen, reduce some HPNS
symptoms by increasing inhibitory neurotransmission (e.g. GABA) and by
lessening or blocking the excitatory neurotransmission (e.g. NMDA type)."
As to ways to suppress HPNS, Bennett and Elliott mention slowing the
compression rate (symptoms appear shallower and much more intense
with rapid compression rates):
Also "These studies show that at these very slow rates of compression, which
virtually eliminate teh effects of compression, the hydrostatic pressure itself
induces severely incapacitating HPNS between 1400ft and 1800ft."
"....Nevertheless, from the view of both delays in getting to the work shite
and the economics involved, plus the questionable functional ability of the
diver on a regular operational basis, the limits of helium-oxygen only diving
with excursions or trimix would seem to be about 1500 ft. However, a German
dive was made to 2000ft in 1990 which showed that it is possible for some
divers to be relatively fit for such a deep heliox exposure".
As to individual susceptability, Bennett and Elliott have this to report:
"Experimentation, both human and animal, has shown that there are important
individual differences in susceptibility to the hyperbaric environment. Thus
for the same depth and compression schedule, the intensity of the symptoms may
be greater in some subjects than in others. Likewise, some individuals may
have more symptoms than others. "
"These results indicate that some subjects are more resistant than others to
the effect of the hyperbaric environment. This individual susceptibility
is a stable characteristic for each subject and thus a given subject always
has the same symptoms during identical exposures."
As to ways to suppress HPNS, Bennett and Elliott have this to say:
"A further way to supress the HPNS is to add small amounts of an anesthetic
or nitrogen or hydrogen {to the breathing mix}. Some pilot studies were made
in man by the COMEX company in France. Vigreux examined nitrogen-helium-
oxygen mixtures with almost equal parts of nitrogen and helium (O2 18%, N2 52%,
He 40%). This produced a reduction in the cost of the dive, and increased
thermal confort together with an improvement in the distortion of speech due
to helium."
"Zal'tsman has also reported an 'antagonism' of the narcotic action of nitrogen
in the presence of helium. For simplicity of operational diving, air-helium
mixtures were suggested, with at depths from 60 to 100 meters 50% by volume
of Helium and at depths from 101 to 160 m 67% Helium. This gave a maximum
nitrogen parital pressure of 4.5 ATA and the oxygen did not exceed 2.0 ATA."
"...in 1973, a series of human investigations were initiated by Bennett
at the F.G. Hall Laboratory at Duke Medical Center to utilize this phenomenon
in the reverse way - that is, to use the addition of nitrogen to helium-oxygen
to ameliorate or prevent HPNS."
".... The trimix suppressed the nausea, dizziness, intention and postural
tremors noted with heliox alone. ........Parallel studies in animals
by Brauer confirmed nitrogen, hyrdrogen and nitrous oxide antagonism of
HPNS in mice."
I refer anyone interested to the new edition of Bennett and Elliott,
"The Physiology and Medicine of Diving", 4th edition.
John
Submariner Research, Ltd.
(johncrea@de*.co*)
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