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High Pressure Neurological Syndrome

Diving Medicine

By Dr. David Sawatzky

In the last two columns I reviewed inert gas narcosis.  High Pressure Neurological (Nervous) Syndrome (HPNS) is a similar problem that is experienced on deeper dives.  It is a difficulty that recreational divers will never experience but advanced technical divers are increasingly performing dives where this is a reality.

In the last column I explained how the narcotic effect of the lighter gases correlates best with their lipid solubility.  Based on this observation it was calculated that helium should not be narcotic until a depth of approximately 1,400 fsw (425 msw).  In 1965 dives were done in chambers to 600-800 fsw (180-240 msw) with compression rates of 20-100 feet/min (6-30 m/min).  When they arrived at depth the divers were relatively incapacitated but unlike narcosis, the problem was tremor, dizziness, nausea, and sometimes vomiting.  Also unlike narcosis, intellectual function was largely intact and the problems improved with time spent at depth.  This was the first time what is now known as HPNS had been seen at such shallow depths.

Animal research showed that if the pressure continued to be increased myoclonic jerks developed, then clonic seizures, tonic seizures, coma, and finally death.  Inert gas narcosis is in general a depression of brain function while HPNS seems to be due to an excitation of brain function.

Susceptibility to HPNS is related to the complexity of the nervous system with humans being more susceptible than animals.  It was determined relatively quickly that the depth at which HPNS developed and the severity of the signs and symptoms was related to the rate of compression.  In general, the faster the rate of compression, the shallower the depth where symptoms occur and the more severe they are.

In 1970 two men were compressed to 1,500 fsw (450 msw) for the first time.  Compression was spread over 3.5 days and although the men still had signs and symptoms of HPNS, they were able to function quite well.  At this depth, helium did not seem to cause any signs or symptoms of narcosis.  It was also apparent that susceptibility to HPNS (like narcosis) differed in different individuals.  EEG changes were observed, but they did not correlate with the signs or symptoms of HPNS.  It was also observed that the tremor had a frequency of 8 to 12 Hz (8-12 times per second).  This is similar to the tremor seen in hypothermia, alcoholism and thyrotoxicosis (too much thyroid hormone) but not the same as the tremor seen in Parkinson’s disease or cerebellar disease (3 to 8 Hz).

At the time narcosis seemed to correlate with lipid solubility in the light gases and it seemed to be related to swelling of the cell membranes.  HPNS seemed to be related to shrinkage of the cell membranes and may have been a simple pressure effect (water does compress up to 4 percent if enough pressure is applied).  HPNS and narcosis seemed to be due to opposite effects on the cell membrane.

Therefore, in the early 1970s Peter Bennett suggested that adding nitrogen to heliox might prevent the effects of HPNS and the pressure might prevent the effects of narcosis.  It had previously been demonstrated in tadpoles and mice that if you continued to increase the pressure, the animals experienced narcosis until they became unconscious but then they woke up again (the increased pressure seemed to reverse the effect of the narcosis).  When the pressure was reduced they went back to sleep and then woke up again as the pressure continued to decline.

A series of human dives were conducted in chambers and it was eventually determined that adding approximately 5 percent nitrogen to heliox resulted in a dramatic reduction in the signs and symptoms of HPNS with little or no narcosis.  One dive reached the depth of 2,250 fsw (686 msw) in 1982 with the divers showing HPNS but still able to function.  However, the rate of compression was still very important.

Separate research had shown that divers could be compressed to 2,100 fsw (640 msw) on heliox and remain functional as long as the rate of compression was slow enough (10 days).  However, they still experienced moderately severe signs and symptoms of HPNS.  The optimal approach seemed to involve both a very slow rate of compression (slower as you get deeper) and nitrogen in the range of 5 percent in heliox.

More recent research has investigated using hydrogen instead of nitrogen.  Hydrogen is less narcotic than nitrogen so much higher percentages have to be used.  It is also a much smaller molecule.  At these depths and pressures gas becomes so dense that breathing is hard work.  Hydrogen is much easier to breathe than nitrogen.  One problem however is that hydrogen and oxygen are explosive so it can only be used at great depths where the percentage of oxygen becomes low enough to reduce or eliminate the risk of explosion (less than 6 percent oxygen).

Using 49 percent hydrogen, dives were conducted to as deep as 2,300 fsw (701 msw) in 1992.  They showed that hydrogen was as effective as nitrogen in reducing the signs and symptoms of HPNS and had the added benefit of reducing the work of breathing.

Unfortunately, this kind of research has largely been abandoned.  The research is very expensive to conduct and it is both very expensive and risky to actually perform these kinds of dives in the real world of commercial diving.  As a result, engineering solutions have been found.  The primary requirement for divers to conduct these kinds of dives was to work on wellheads on the ocean bottom.  The wellheads have been redesigned so that the work can be conducted by remotely operated vehicles, mini-submarines and one atmosphere suits with manipulators.  With this technology the diver is never exposed to pressure.

In spite of all the research that has been conducted we still have very little idea what really causes HPNS (this is also true of narcosis).  Inert gases under pressure and pressure itself cause changes in many brain functions, alter the levels of several chemicals, sometimes increasing them in one area of the brain while reducing them in another!  It has been suggested that helium might act directly on ion channels.  A lot more research will have to be conducted before we fully understand narcosis and HPNS.

HPNS might no longer be a factor in deep commercial work.  However, advanced technical divers are now making dives to depths where HPNS is definitely a factor.  In addition, they are using extremely rapid rates of descent and previous experience would suggest that they should experience HPNS at shallower depths and more severely than research and commercial divers have in the past.  Well-publicized deep technical dives seem to support this expectation.

In August 1993 Sheck Exley did a dive in Busmansgat, South Africa.  The purpose of the dive was to reach the bottom of the cave and maybe set a world depth record.  The floor of the cave and been plumbed with a shot line to 866 feet (264 meters) and the current depth record had been set by Sheck in Mante Cave (Mexico) in 1989 at 867 to 881 feet (264-268 meters, part of the line ran at an angle and it was not possible to calculate an exact depth).  Sheck had taken 24 minutes to reach this depth in Mante because of the strong current.  In the still water in Busmansgat he hoped to reach the bottom in only 10 minutes.

Shecks’ major concern was a calculated narcotic depth equivalent to air at 266 feet (81 meters) so he did a series of air dives to a maximum depth of 412 feet (125 meters) in preparation.  He does not seem to have considered HPNS at all!  The article he wrote that was published in Underwater Speleology (Mar/Apr 1994) states that he used Heliair 7 (1/3 helium, 2/3 air) as the bottom mix but this is obviously a printing mistake as mixing 1/3 helium and 2/3 air would result 14 percent oxygen (Heliair 14) so he must have used 2/3 helium and 1/3 air.  This would result in 7 percent oxygen but also 26 percent nitrogen, far more than the 5 percent nitrogen found to be most effective in preventing HPNS at these depths.  However, the research dives took days to reach the bottom while Sheck was planning on doing the same decent in 10 minutes!

During the dive Sheck descended at more than 100 feet (30 meters) per minute reaching 700 feet (210 meters) in less than 7 minutes.  At this depth he reported experiencing the following symptoms.  “Suddenly my field of vision became hundreds of small adjacent concentric circles, each of which had a tiny sparkling dot in the center.  My distance vision seemed to start blurring, and my skin began itching all over, then stinging.”  He attributed the symptoms to HPNS and inert gas counter-diffusion, paused for about a minute at 750 feet (228 meters) and then continued his descent at a slower rate (25 fpm, 7.6 mpm) on the assumption that the symptoms of HPNS were still far away.  “My entire body began trembling …. gradually escalating to uncontrollable shaking by the time I landed fins first on the bottom.”  “The HPNS was sufficiently alarming that I did not consider wandering down the slope to try for 900 feet.” Standing on the bottom it took almost a minute for Sheck to inject enough gas into his wings to become positively buoyant and start to ascend.  I suggest he was descending far faster than 25 fpm (7.6 mpm) and that he would have been unable to stop his descent if he had not landed on the bottom.

“The HPNS had left me by the time I reached 400 feet”.  Sheck had obviously experienced serious HPNS, even with a far higher percentage of nitrogen than had been used in the research chamber dives.  On 6 April, 1994 (about the time this article was published) Sheck died on a dive in Mante attempting to reach 1,000 feet (305 meters).

HPNS is a fascinating problem and one that current deep technical divers will definitely have to try and find some way to deal with.


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One Response to “High Pressure Neurological Syndrome”

  1. Steve Lewis
    19/08/2015 at 12:57 pm #

    David always manages to get to the point in the briefest time possible. Informed and informative. Thanks, mate.

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