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Hello,
This will be the second post in the series about Oxygen. Once we get
the generalized information out, I will move into more specific
discussions. Judging from the plethora of responses resulting from the
last post, I am sure this one will also be entertaining.
Signs/symptoms of CNS toxicity are covered in the acronym CONVENTID:
CON - convulsions
V - visual disturbances like tunnel vision or blurring
E - ear symptoms like tinnitus (ringing) or other sounds, including pulsing
N - nausea or vomiting (self explanatory)
T - twitching of facial muscles, lips or extremities. There is some
thought that facial muscle is indicative of immediate seizures
I - irritability any change in mental status
D - dizziness (one must ask, what about those who propose alcohol
consumption before the dive, where do they fit in? :-))
Possible causes of CNS toxicity include, but are not limited to, the
following:
Prolonged exposures to elevated PO2s. Using 1.6 in back gas, and also
doing deco on 1.6, no back gas breaks (even though the BG is a higher mix,
it is only 1.6 at depth, so it would be lower at depths safe for O2
use). Breathing a mix that has a MOD shallower than the depth that you are
at; the 40% at 130-140 "discussion" we just had is an excellent
example. Let us imagine that the "diver" was breathing 32% at 130 feet,
and had an emergency supply of 40% in another bottle. If this dive were a
little longer than originally planned, and the diver were to run out of gas
in his primary source thereby forcing himself to breath the 40% at depth,
he would have a prolonged exposure of elevated PO2s. This would
dramatically increase the likelihood of CNS toxicity. The above example
does not model after any person, living or dead, and any resemblance is
purely coincidental . . . besides, he already admitted to breathing air at
130 ;-).
How to avoid: do not use bottom mixes with a PO2 higher than 1.3, and deco
mixes with a PO2 higher than 1.6. Do not run the entire dive at a PO2 of
1.6; this has been proposed by some people using electronic
rebreathers. JUDICIOUS USE of back gas breaks, this one act will serve to
lower the PO2, thereby slowing down the "clock" (referred to in last post)
and subsequently decreasing the risk. (There are other benefits of BG
breaks that will be discussed in future post.)
Sudden spike in PO2s. This can result when switching gases, for example,
switching from bottom mix to a deco mix. This is one of the areas of
greatest concern for longer dives. If you have been in the water for some
period of time your CNS exposure has continued to increase, and the switch
to a deco mix at 1.6 can be the "straw that broke the camels back." This
can also occur due to switching to the wrong gas for the current
depth. This one act should have no excuse, but, it has several
causes: complacency, improper bottle markings, poor instruction, lack of
knowledge or experience (i.e. rapid training sequence), peer pressure, etc.
How to avoid: this is some thought that a gradual introduction to a new
gas is best, by this I mean, take a breath off of the next gas, and then
take 3 off of the last gas, then 2 of the new and 2 off of the old, . . .
until you are completely on the next gas. For most dives this step is
probably unnecessary. However, caution should always be used when
switching to make sure you are at the correct depth, have the correct reg,
etc. I should not have to cover the last cause, get proper training, mark
your gear correctly, avoid bad instructors, in short Do It Right.
Use of improper gas at depth. This is another cause that could be
eradicated with proper training! Deep air is an excellent example, as are
"weak trimixes", such as the infamous 17/17 of years ago and EADs greater
than 100' (personally I run my EAD between 75-80). There is a tool for
every job, He and scooters are the tools for deeper diving. I realize that
my mailboxes will be filled by deep air aficionados, those that can't
access Helium/scooters etc. To this I will say two things: I really don't
want your mail I have heard your excuses/reasons, and this is a dangerous
activity, bite the bullet and get the "tools" needed, or take up something
else.
How to avoid: Use the right tool for the job.
The infamous offgas effect. High PO2s cause biochemical changes in the
Central Nervous System that may linger after the high PO2 has
subsided. This may cause a seizure a few minutes after the diver has
stopped breathing the hyperoxic mix. This is more likely to occur if the
diver had experienced one of the "milder" symptoms of toxicity; however,
toxicity can occur with no symptoms, so a watchful eye must still be
employed. This possibility is greatly diminished after several minutes.
How to avoid: Be extremely careful, avoid high PO2s. Very little is known
about this.
Susceptibility to CNS toxicity will vary from diver to diver and from
day to day in the same diver. This is why it is not advised to continue
bad practices, just because they have worked in the past is not adequate
justification that they will continue to work.
Hypercapnia (high CO2), cold, exertion, and breathing resistance can
serve to make the a diver more susceptible to CNS toxicity.
CNS toxicity is one of the most dangerous possibilities we face while
using gas mixes in the water. The onset can be very rapid and present with
little of no warning. The biggest concern for divers is the possibility of
a seizure while submerged, likely to dislodge the regulator, possibly
resulting in the diver drowning when respirations resume. CNS toxicity is
very real, however, like the other dangers of this type of diving, it too
can be mitigated to a manageable level.
Sincerely,
Scott Hunsucker
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<html>
Hello,<br>
This will be the second post in the series about
Oxygen. Once we get the generalized information out, I will move
into more specific discussions. Judging from the plethora of
responses resulting from the last post, I am sure this one will also be
entertaining.<br>
<br>
Signs/symptoms of CNS toxicity are covered in the acronym
<b>CONVENTID</b>:<br>
<b>CON - convulsions<br>
V - visual disturbances</b> like tunnel vision or blurring<br>
<b>E - ear symptoms</b> like tinnitus (ringing) or other sounds,
including pulsing<br>
<b>N - nausea or vomiting</b> (self explanatory)<br>
<b>T - twitching</b> of facial muscles, lips or extremities. There
is some thought that facial muscle is indicative of immediate
seizures<br>
<b>I - irritability </b>any change in mental status<br>
<b>D - dizziness</b> (one must ask, what about those who propose
alcohol consumption before the dive, where do they fit in? :-))<br>
<br>
Possible causes of CNS toxicity include, but are not limited
to, the following:<br>
<br>
<b>Prolonged exposures to elevated PO2s.</b> Using 1.6 in back gas,
and also doing deco on 1.6, no back gas breaks (even though the BG is a
higher mix, it is only 1.6 at depth, so it would be lower at depths safe
for O2 use). Breathing a mix that has a MOD shallower than the
depth that you are at; the 40% at 130-140 "discussion" we just
had is an excellent example. Let us imagine that the
"diver" was breathing 32% at 130 feet, and had an emergency
supply of 40% in another bottle. If this dive were a little longer
than originally planned, and the diver were to run out of gas in his
primary source thereby forcing himself to breath the 40% at depth, he
would have a prolonged exposure of elevated PO2s. This would
dramatically increase the likelihood of CNS toxicity. The above
example does not model after any person, living or dead, and any
resemblance is purely coincidental . . . besides, he already admitted to
breathing air at 130 ;-).<br>
<u>How to avoid:</u> do not use bottom mixes with a PO2 higher than
1.3, and deco mixes with a PO2 higher than 1.6. Do not run the
entire dive at a PO2 of 1.6; this has been proposed by some people using
electronic rebreathers. JUDICIOUS USE of back gas breaks, this one
act will serve to lower the PO2, thereby slowing down the
"clock" (referred to in last post) and subsequently decreasing
the risk. (There are other benefits of BG breaks that will be
discussed in future post.)<br>
<br>
<b>Sudden spike in PO2s. </b>This can result when switching
gases, for example, switching from bottom mix to a deco mix. This
is one of the areas of greatest concern for longer dives. If you
have been in the water for some period of time your CNS exposure has
continued to increase, and the switch to a deco mix at 1.6 can be the
"straw that broke the camels back." This can also occur
due to switching to the wrong gas for the current depth. <i>This
one act should have no excuse, but, it has several causes</i>:
complacency, improper bottle markings, poor instruction, lack of
knowledge or experience (i.e. rapid training sequence), peer pressure,
etc.<br>
<u>How to avoid:</u> this is some thought that a gradual
introduction to a new gas is best, by this I mean, take a breath off of
the next gas, and then take 3 off of the last gas, then 2 of the new and
2 off of the old, . . . until you are completely on the next gas.
For most dives this step is probably unnecessary. However, caution
should always be used when switching to make sure you are at the correct
depth, have the correct reg, etc. I should not have to cover the
last cause, get proper training, mark your gear correctly, avoid bad
instructors, in short Do It Right.<br>
<br>
<b>Use of improper gas at depth. </b>This is another cause that
could be eradicated with proper training! Deep air is an excellent
example, as are "weak trimixes", such as the infamous 17/17 of
years ago and EADs greater than 100' (personally I run my EAD between
75-80). There is a tool for every job, He and scooters are the
tools for deeper diving. I realize that my mailboxes will be filled
by deep air aficionados, those that can't access Helium/scooters
etc. To this I will say two things: I really don't want your
mail I have heard your excuses/reasons, and this is a dangerous activity,
bite the bullet and get the "tools" needed, or take up
something else. <br>
<u>How to avoid:</u> Use the right tool for the job.<br>
<br>
<b>The infamous offgas effect.</b> High PO2s cause
biochemical changes in the Central Nervous System that may linger after
the high PO2 has subsided. This may cause a seizure a few minutes
after the diver has stopped breathing the hyperoxic mix. This is
more likely to occur if the diver had experienced one of the
"milder" symptoms of toxicity; however, toxicity can occur with
no symptoms, so a watchful eye must still be employed. This
possibility is greatly diminished after several minutes.<br>
<u>How to avoid:</u> Be extremely careful, avoid high PO2s.
Very little is known about this.<br>
<br>
Susceptibility to CNS toxicity will vary from diver to diver
and from day to day in the same diver. This is why it is not
advised to continue bad practices, just because they have worked in the
past is not adequate justification that they will continue to work.=20
<br>
Hypercapnia (high CO2), cold, exertion, and breathing
resistance can serve to make the a diver more susceptible to CNS
toxicity. <br>
CNS toxicity is one of the most dangerous possibilities we
face while using gas mixes in the water. The onset can be very
rapid and present with little of no warning. The biggest concern
for divers is the possibility of a seizure while submerged, likely to
dislodge the regulator, possibly resulting in the diver drowning when
respirations resume. CNS toxicity is very real, however, like the
other dangers of this type of diving, it too can be mitigated to a
manageable level.<br>
Sincerely,<br>
Scott Hunsucker</html>
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