TEK-Tauchen Foren

Technical Diving, without any doubt, is one of the most risky and challenging activities people can take part in. It requires discipline, precision and profound knowledge in a wide variety of topics.

Pushing diving to the limits, leaves a lot space for speculations and fuzziness anyway. Of those things we are certain about, we should talk very clearly and specificly.

Having a closer look at the language use in current pubilcations, software manuals, forums and magazines we find the widespread expression: CNS-clock or even worse CNS%. Both terms imply that the risk of central nervous system (CNS) oxygen toxicity has a timed or accumulative component. That is, as we all know, not true! CNS toxicity is exclusively caused by high oxygen partial pressure (influenced by some contributing factors as level of carbon dioxide, workload, gas density a.o.), and does not depend on exposure times.

Problem is, obviously the term CNS-clock (CNS%) doesn`t refer to CNS toxicity, but pulmonary oxygen toxicity. This is the other form of threat, causes by breathing oxygen under pressure. But there the similarities are ending.

Pulmonary oxygen toxicity has its own set of determining factors, and is based on a total different mechanism. It has total diffenent signs and symptoms. In spite of CNS toxicity, POT (pulmonary oxygen toxicity) has a timed component and an accumulative aspect. It can damage the respiratory system irreversibly.

Let`s have a look at some basics again, cited from Field Manual (FM) 20-11 as an authorized reprint of United States (US) Navy diving
manual SS521-AG-PRO-010.

"Oxygen Toxicity. Partial pressure of oxygen in excess of that encountered at normal atmospheric conditions may be toxic to the body.[...] The two types of oxygen toxicity experienced by divers are pulmonary oxygen toxicity and central nervous system (CNS) oxygen toxicity."

"Central nervous system (CNS) oxygen toxicity. The nervous system coordinates all body functions and activities. The nervous system comprises the brain, spinal cord, and a complex network of nerves that course through the body. The brain and spinal cord are collectively referred to as the central nervous system (CNS). "

"High pressure oxygen poisoning, or central nervous system (CNS) oxygen toxicity, is most likely to occur when divers are exposed to more than 1.6 atmospheres of oxygen.[...] In general, oxygen partial pressures at or below 1.4 ata are unlikely to produce CNS toxicity. Closed-system oxygen rebreathing systems require the lowest partial pressure limits, whereas surface-supplied helium-oxygen systems
permit slightly higher limits."

"Symptoms of CNS Oxygen Toxicity. The most serious direct consequence of oxygen toxicity is convulsions. Sometimes recognition of early symptoms may provide sufficient warning to permit reduction in oxygen partial pressure and prevent the onset of more serious symptoms. The warning symptoms most often encountered also may be remembered by the mnemonic VENTIDC:

V: Visual symptoms: Tunnel vision, a decrease in diver’s peripheral vision,
and other symptoms, such as blurred vision, may occur.
E: Ear symptoms. Tinnitus, any sound perceived by the ears but not resulting
from an external stimulus, may resemble bells ringing, roaring, or a
machinery-like pulsing sound.
N: Nausea or spasmodic vomiting. These symptoms may be intermittent.
T: Twitching and tingling symptoms. Any of the small facial muscles, lips, or
muscles of the extremities may be affected. These are the most frequent
and clearest symptoms.
I: Irritability: Any change in the diver’s mental status including confusion,
agitation, and anxiety.
D: Dizziness. Symptoms include clumsiness, incoordination, and unusual
fatigue.
C: Convulsions. The first sign of CNS oxygen toxicity may be a convulsion
that occurs with little or no warning.

Symptoms may not always appear and most are not exclusively symptoms of oxygen toxicity. Twitching is perhaps the clearest warning of oxygen toxicity, but it may occur late, if at all.[...] The actual mechanism of CNS oxygen toxicity remains unknown in spite of many theories and much research. Preventing oxygen toxicity is important to divers."

Pulmonary oxygen toxicity. Low pressure oxygen poisoning, or pulmonary oxygen toxicity, can begin to occur if more than 60 percent oxygen is breathed at one atmosphere for 24 hours or more [...] resulting in permanent lung damage or pneumonia." [end of quote]

Symptoms of pulmonary oxygen toxicity include, but are not limited to:
Chest pain, difficulties of breathing, burning sensation in throat and lungs...

There is not the slightest hint, that even excessive exposure times of low pressure oxygen could end in a CNS intoxication.

Two problems, two solutions

Avoiding CNS toxicity seems to be quite simple. Keeping the oxygen partial pressure below acceptable limits, by adjusting depth and oxygen fraction in the gas mixture.

Avoiding pulmonary oxygen toxicity needs a more complex and scientific approach. Tables which are balancing partial pressure and allowable exposure time are the standard. NOAA`s (National Oceanic & Atmospheric Administration) exposure time table and the OTU (oxygen tolerance/toxicity units) procedure in particular.

But, both are different problems and must not be confused.

What the hell is a CNS-clock???

We came this far: If pulmonary oxygen toxicity got nothing to do with CNS, and on the other hand CNS got nothing to do with time or percentage, what is a CNS-clock (CNS%) then ???

Maybe, I am just to stupid to understand. Or maybe, we are forwarding a wrong expression without questioning it anymore. I personally find it confusing, counterproductive and unnecessary. I think we should name things precisely, what they are. By accepting these terms, we are perpetuating a misconception.

So what do you think, guys? Why don`t we kiss the good old CNS-clock good bye and name it what it is:

POT-clock / POT% or AOE-clock (allowable oxygen exposure) or OEL-clock (oxygen exposure limit) LPO% (low pressure oxygen) or what ever !?! Come up with your ideas.


Oliver Wolf

PS: If you find this topic worth a consideration, feel free to post this text in other forums.

Es darf auch in deutsch geantwortet werden.