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Hello,
First let me apologize for being very late with the continuation of the
oxygen post. I have no excuse, I just put it on the "back burner" for too
long.
I will cover the very basics of pulmonary toxicity first, so that we
are all on the same page, and then, in a separate post, cover some of the
physiology behind, enzymes, etc., that is associated with this.
Pulmonary oxygen toxicity can be defined as a progressive failure of
lung ventilation that is brought about by a prolonged exposure to oxygen
partial pressures over 0.6 ATA, eventually leading to hypoxia by decreased
oxygen tension in the blood supply, and an increase in the distance that
oxygen must diffuse across the alveoli. While it is a concern, it is not
normally seen within the time frames of most divers; however, repeated
exposures in a short period, breathing oxygen at the surface between dives,
and chamber rides after diving will have a cumulative effect and can lead
to pulmonary problems.
This prolonged exposure to oxygen damages the lungs by causing alveolar
edema, which is responsible for the increased diffusion distance. The
edema will also cause a "stiffness" in the lungs that will lead to
dyspnea. If the breathing of oxygen is not abated or stopped the damage
will continue and cause alveolar necrosis leading to subsequent death by
hypoxia, despite the high oxygen content. However, once the oxygen
concentrations return to normal this damage is usually reversed.
Signs and symptoms
Coughing
Wheezing
Sternal pain (reported as being behind the sternum)
Decreased tidal volume and vital capacity
Dyspnea (shortness of breath)
Burning sensation during respiration
Pulmonary edema
Wheezing
Lower partial pressures of oxygen (both during dive and deco) can be
tolerated for longer periods of time without the ill-effects noted
above. One must look at the entire diving plan and account for dive
duration, deco, and contingency factors, when determining best mix for the
dive. The operational maximum that has been set forth for us is 1.3 PO2
for normal dives and (obviously) lower depending on the intended duration
of the dive. The maximum set for our deco is 1.6 P02. 1.6 only occurs at
the gas switches and the PO2 is proportionally lower throughout the ascent,
until the next gas switch. The entire deco is not run at 1.6 PO2 as
proposed by some. This would dramatically increase one's oxygen exposure
and unnecessarily increase the risk of problems, all in an effort to avoid
some deco.
The method that falls in line behind using the appropriate gases in
management of pulmonary oxygen toxicity is the use of back gas
breaks. There has been some mention to them in the past posts and even
some mild discussion on the subject. This topic will be expanded upon and
covered in better detail in the future.
Pulmonary oxygen tolerance appears to be a limiting factor for dives of
extreme duration. One could lower the PO2 of the bottom gas to derive more
exposure time before the onset of problems, but then the decompression
obligation might be increased to an unrealistic point. The addition of 25%
He to deco gases, except oxygen, has decreased the reported incidences of
problems and discomfort that is traditionally associated with pulmonary
oxygen toxicity; however, this step has the potential to increase thermal
considerations and might extend decompression times--hence it would not be
recommended for "average" dives.
There are many hazards that are associated with the use of oxygen, I
hope to cover most of these in future, as well as the methods we have
devised to handle this necessary gas. Like most of you I have 9,127 prongs
in the fire and often have to shuffle the schedule to meet demands. I ask
that you be patient, and as always, if you have questions feel free to
ask. If I do not know the answer I will find it.
Sincerely,
Scott Hunsucker
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<html>
Hello,<br>
First let me apologize for being very late with the
continuation of the oxygen post. I have no excuse, I just put it on
the "back burner" for too long.<br>
I will cover the very basics of pulmonary toxicity first, so
that we are all on the same page, and then, in a separate post, cover
some of the physiology behind, enzymes, etc., that is associated with
this.<br>
<br>
Pulmonary oxygen toxicity can be defined as a progressive
failure of lung ventilation that is brought about by a prolonged exposure
to oxygen partial pressures over 0.6 ATA, eventually leading to hypoxia
by decreased oxygen tension in the blood supply, and an increase in the
distance that oxygen must diffuse across the alveoli. While it is a
concern, it is not normally seen within the time frames of most divers;
however, repeated exposures in a short period, breathing oxygen at the
surface between dives, and chamber rides after diving will have a
cumulative effect and can lead to pulmonary problems.<br>
This prolonged exposure to oxygen damages the lungs by
causing alveolar edema, which is responsible for the increased diffusion
distance. The edema will also cause a "stiffness" in the
lungs that will lead to dyspnea. If the breathing of oxygen is not
abated or stopped the damage will continue and cause alveolar necrosis
leading to subsequent death by hypoxia, despite the high oxygen
content. However, once the oxygen concentrations return to normal
this damage is usually reversed.<br>
<br>
<b>Signs and symptoms </b> <br>
Coughing<br>
Wheezing<br>
Sternal pain (reported as being behind the sternum)<br>
Decreased tidal volume and vital capacity<br>
Dyspnea (shortness of breath)<br>
Burning sensation during respiration<br>
Pulmonary edema<br>
Wheezing<br>
<br>
Lower partial pressures of oxygen (both during dive and
deco) can be tolerated for longer periods of time without the ill-effects
noted above. One must look at the entire diving plan and account
for dive duration, deco, and contingency factors, when determining best
mix for the dive. The operational maximum that has been set forth
for us is 1.3 PO2 for normal dives and (obviously) lower depending on the
intended duration of the dive. The maximum set for our deco is 1.6
P02. 1.6 only occurs at the gas switches and the PO2 is
proportionally lower throughout the ascent, until the next gas
switch. The entire<b> </b>deco is not run at 1.6 PO2 as proposed by
some. This would dramatically increase one's oxygen exposure and
unnecessarily increase the risk of problems, all in an effort to avoid
some deco.<br>
The method that falls in line behind using the appropriate
gases in management of pulmonary oxygen toxicity is the use of back gas
breaks. There has been some mention to them in the past posts and
even some mild discussion on the subject. This topic will be
expanded upon and covered in better detail in the future. <br>
Pulmonary oxygen tolerance appears to be a limiting factor
for dives of extreme duration. One could lower the PO2 of the
bottom gas to derive more exposure time before the onset of problems, but
then the decompression obligation might be increased to an unrealistic
point. The addition of 25% He to deco gases, except oxygen, has
decreased the reported incidences of problems and discomfort that is
traditionally associated with pulmonary oxygen toxicity; however, this
step has the potential to increase thermal considerations and might
extend decompression times--hence it would not be recommended for
"average" dives.<br>
There are many hazards that are associated with the use of
oxygen, I hope to cover most of these in future, as well as the methods
we have devised to handle this necessary gas. Like most of you I
have 9,127 prongs in the fire and often have to shuffle the schedule to
meet demands. I ask that you be patient, and as always, if you have
questions feel free to ask. If I do not know the answer I will find
it.<br>
Sincerely,<br>
Scott Hunsucker<br>
</html>
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