Elevation of Absolute Pressure
High-Pressure Nervous Syndrome
High pressure induces a constellation of symptoms consisting of
tremor, ataxia, nausea, and vomiting, known as the high-pressure nervous syndrome.
[77]
It occurs at an ambient pressure greater than
15 to 20 ATA and was first described during the compression phase of deep dives with
a helium-oxygen atmosphere. High-pressure nervous syndrome is ameliorated by slow
compression and the addition of a narcotic gas (e.g., nitrogen) to the breathing
mix.[29]
Pressure Reversal of Anesthesia
Studies in animals have demonstrated that high pressure has a
tendency to reverse general anesthesia. Elevations in ambient pressure in the absence
of a narcotic inert breathing gas will tend to decrease the effectiveness of both
inhaled and intravenous anesthetics. At 50 ATA, an approximately 20% increase in
the 50% effective dose (ED50
) has been observed in mice for a variety
of inhaled anesthetics. At this same ambient pressure the effective dose of thiopental
increases approximately 30% in rats and mice. Winter and coworkers[79]
demonstrated a reversal of phenobarbital anesthesia in rats under high pressure,
with a 63% increase in ED50
at 103 ATA as compared with 1 ATA. The ED50
for diazepam in rats is markedly reduced at 90 ATA in a helium-oxygen atmosphere.
[80]
At 31 ATA, the effective concentration for
a half-maximal effect (EC50
) of propofol for loss of the righting reflex
in tadpoles is increased by 19%, and at 61 ATA, it is increased by 38%.[81]
Using the same technique, the EC50
for dexmedetomidine at 31 ATA is nearly
double the value at 1 ATA and is increased 2 1/2-fold at 61 ATA.[82]
This effect of pressure reversal of general anesthesia has been reviewed by Wardley-Smith
and Halsey[83]
and Wann and Macdonald.[84]
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