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High Pressure Neurological Syndrome

Uppstår vid andning av höga halter av helium på ett större djup, oftast djupare än 100 meter. Det har i viss mån kunnat spåras till en snabb nedstigning/tryckföränding, det är långt ifrån alla som uppvisar syptom på HPNS. Visar sig genom kramp/skakningar och koordinationssvårigheter.

Utdrag från litteraturen:

"U.S. Navy Diving Manual"
Utgåva 6, 2008
Utgivare: U.S. Navy
Sida 3-58:
3-11.1 High Pressure Nervous Syndrome (HPNS).
High Pressure Nervous Syndrome (HPNS) is a derangement of central nervous system function that occurs during deep helium-oxygen dives, particularly saturation dives. The cause is unknown. The clinical manifestations include nausea, fine tremor, imbalance, incoordination, loss of manual dexterity, and loss of alertness. Abdominal cramps and diarrhea develop occasionally. In severe cases a diver may develop vertigo, extreme indifference to his surroundings and marked confusion such as inability to tell the right hand from the left hand. HPNS is first noted between 400 and 500 fsw and the severity appears to be both depth and compression rate dependent. With slow compression, depth of 1000 fsw may be achieved with relative freedom from HPNS. Beyond 1000 fsw, some HPNS may be present regardless of the compression rate. Attempts to block the appearance of the syndrome have included the addition of nitrogen or hydrogen to the breathing mixture and the use of various drugs. No method appears to be entirely satisfactory.
Sida 15-24
"15-20.2 Compression to Storage Depth.
Rapid compression to saturation storage depth may provoke symptoms of High-Pressure Nervous Syndrome (HPNS) and may intensify compression joint pains. To avoid these complications, the slowest rate of compression consistent with operational requirements should be used. Table 15-6 shows the range of allowable compression rates.
Table 15‑6. Saturation Diving Compression Rates.
Depth Range Compression Rate
0–60 fsw 0.5 – 30 fsw/min
60–250 fsw 0.5 – 10 fsw/min
250–750 fsw 0.5 – 3 fsw/min
750–1000 fsw 0.5 – 2 fsw/min
If operational necessity dictates, compression to storage depth of 400 fsw or shallower can be made at the maximum rates indicated in Table 15‑6 with little risk of HPNS. Direct compression at maximum rates to deeper storage depths, however, may produce symptoms of HPNS in some divers. These divers may be unable to perform effectively for a period of 24 to 48 hours. Experience has shown that the appearance of such symptoms can be minimized by slowing compression rates or introducing holds during compression.
The depth and time duration of holds,
(Not: 400 fot = ca 120 m, 500 fot = ca 150 m, 1000 fot = ca 300 m)

"NOAA Diving Manual"
Utgåva 4, 2001
ISBN 0-941332-70-5
Editor: James T. Joiner
Sida 3-21:
" High Pressure Nervous Syndrome (HPNS)
High pressure nervous syndrome (HPNS) occurs at depths greater than 400 fsw (123 msw). First noted in the 1960s, HPNS was initially thought to be an effect of breathing helium, so it was called helium tremors. At that time, helium was the most commonly used diluent gas for diving at deeper depths. HPNS becomes worse with increasing pressure and rate of compression.
HPNS is characterized by dizziness, nausea, vomiting, postural and intention tremors, fatigue and somnolence, sudden muscle twitching (called myoclonic jerking), stomach cramps, intellectual and psychomotor performance decrements, and poor sleep with nightmares.
Adding a small amount (5-10%) of nitrogen into the breathing mix reduces HPNS. At high pressure, nitrogen is a neural depressant. Other methods of preventing or reducing HPNS include slow, steady compression, stage compression with long intervals, exponential compression rates, and careful personnel selection.
Sida 16-5:
" Diving with Trimixes of Nitrogen, Helium, and Oxygen
Helium has been used to avoid nitrogen narcosis since the 1930s for diving beyond the traditional air range (approximately 190 fsw (58.3 msw), or shallower in many situations). Traditionally, such diving has been conducted with helium-oxygen mixtures, ''heliox,'' but lately ''trimixes'' of nitrogen, helium, and oxygen have been used for diving in the surfaceoriented range to about 350 fsw (107.5 msw). The term ''trimix'' has also been applied to very deep saturation diving mixtures with 5-10% nitrogen added to relieve HPNS (see Chapter 3).

"Deeper Into Diving"
Utgåva 2, 2005
ISBN 0-9752290-1-X
Författare: John Lippmann & Dr Simon Mitchell
Sida 435:
One diving medical problem of unique relevance to deep mixed gas diving is the high-pressure neurological syndrome (HPNS). This is a mysterious condition that has the potential to produce a variety of symptoms that include tremor, nausea, disorientation and other neurological problems. Like nitrogen narcosis its onset is very depth dependent and is rarely seen during dives shallower than 150m (495ft) (so its not something many of us have to worry about!). Moreover, like nitrogen narcosis, there is a marked inter-individual variation in susceptibility to its effects.
The cause is still not completely understood, but it may relate to a combined effect of pressure and the gas being breathed on the cell membranes of our nervous system. Paradoxically, we breathe helium in order to avoid nitrogen narcosis, yet breathing pure helium-oxygen mixtures is strongly associated with the HPNS. In another paradox, adding a little nitrogen to the mix often ameliorates the effects, and this constitutes the third reason (alluded to earlier) for leaving some nitrogen in gas mixes for deep diving.
Divers required to work at extreme depths do exhibit a degree of adaptation to the symptoms over the course of the dive, although this is not invariable. Slow compressions have been found to facilitate this adaptation though these would not be feasible in extreme technical bounce dives. Like nitrogen narcosis, there do not appear to be any after effects from the HPNS, although transient alterations in behaviour persisting for a time after a dive have been reported.

"International Textbook Of Mixed Gas Diving"
Utgåva 2, 2001
ISBN 0-941332-50-0
Författare: Heinz K.J. Lettnin
Sida 30-31:
"At greater depths, helium causes a physiological problem known as High Pressure Nervous Syndrome, or HPNS. If divers go deeper than 200 msw and, if additionally, high compression rates are used, disorders of the central nervous system (CNS) may occur. These disorders are manifested by symptoms such as tremors of the hands and torso, visual disorders, micro sleep, dizziness, and nausea. Other abnormal reactions have also been noticed, for example, brain waves which changed significantly in cases of HPNS, according to reference 10].
To understand the complex HPNS process, a simple technical picture may be helpful. The nerves, consisting of nerve cells as conductors and the surrounding lipids as insulation, are exposed to an increasing pressure. This results in compression of the lipids and the insulation, causing ''short-circuits'' of the CNS. A high lipid solubility means a large swelling o the lipid tissues with increasing cell volume, which counteracts the effects of compression. Table 5.3 shows the correlation between lipid solubility and the narcotic potential for different diving gases The solubility of helium in lipid tissues is clearly lower than that of other gases, resulting in less swelling. Raised ambient pressures cause compression of the nerve cell membranes; in the case of helium, the swelling effects are too small to compensate for the pressure effects. To overcome HPNS effects, narcotic gases with corresponding high lipid solubility are added to increase the nerve cell volume. Based on these findings, in U.S. deep-diving operations 5 to 10% nitrogen is added to he heliox ix to suppress HPNS effects, as stated in reference [10, 13]. The mixture with the additional nitrogen is known as trimix. Using trimix, a test dive down to 686 m was completed as part of he ATLANTIS III project at Duke University. In Germany, trimix with 5% nitrogen was used successfully, without any HPNS symptoms, for all dives down to 600 m in the underwater simulator GUSI at Geesthacht.
Sida 147:
"Based on these findings, among others from the U.S. project ATLANTIS at the beginning of the 1980s, there are three possible ways to suppress HPNS effects: adding a narcotic gas to the breathing mixture; reducing the compression rate; and taking longer stops during compression for pressure adaptation. In practice, all three measures are used simultaneously, according to references [13, 54. 55]. Results from 20 deep dives between 150 and 600 m performed at GKSS research center (GUSI), confirmed the methods of suppressing HPNS effects. For all dives, a trimix containing 5% nitrogen, in addition to oxygen and helium, was used. A higher percentage of nitrogen may cause nitrogen narcosis at extreme depths. The compression rate was reduced with increasing depth. While the compression rate was 5 m/min to 180 m, it was reduced from stop to stop until the rate was 0.05 m/min, only one hundredth of the initial rate, at the maximum depth of 600 m."

"Technical Diving From The Bottom Up"
Utgåva 1, 2002
Författare: Kevin Gurr
Sida 150-151:
"High Pressure Nervous Svndrome (HPNS).
Helium has a solubility in the lipid (fat) of 0.015 5 whilst nitrogen has a solubility in the lipid of 0.019. It would therefore be likely that the level of narcosis experienced at 10 Bar breathing compressed air would not occur until 43 Bar with a helium mixture.
This, however, is just maths. Test dives in 1965 to 180 metres and 240 metres showed a marked performance decrement. One difference to nitrogen Narcosis is that the symptoms with helium mixtures began to improve after about one hour. This does not occur with compressed air. Another difference is that with helium mixes, motor skills are more impaired than intellectual tasks.
Helium mixes at depth excite the brain, whereas nitrogen mixes decrease the excitement of the brain.
The rate of compression greatly affects the onset of HPNS. With very rapid compression, symptoms can occur at relatively shallow depths. Avoidance of HPNS is achieved by very slow compression (150m per hour) or by stage descents. In 1970 divers were successfully compressed to 46 Bar by pausing their descent at 19, 31 and 40 Bar for 24 hours. Such methods are obviously not possible for open-circuit dives.
Trimix was first used for the experimental ‘pressure reversal of narcosis theory’. Dives in 1974 to 330 metres with 10% nitrogen with rapid compression (33 minutes) showed no performance loss, no narcosis and no nausea, tremors or EEG changes. Nitrogen provided a ‘buffer’ for the HPNS symptoms.
Both Hasenmayer and Exley’s deep cave explorations on heliox caused mild symptoms of HPNS manifested as a shaking fit. Later dives using trimix were symptomless.
HPNS has been noted in open circuit scuba dives between 120 and 200 metres again dependent on rate of compression (in this case descent rate). Another key variant in occurrence of HPNS is personnel variations in physiology.
Symptoms include muscle tremor, dizziness, vertigo, imbalance, nausea, intermittent somnolence (drowsiness or micro sleep) and loss of appetite.

Relaterade ord: Andningsgas Blandgas EAD END Gasblandning Heliair Heliox Helitrox Helium Helox Helox Inertgas Mix Trimix Triox