Posted by Steve on June 14, 2001 at 17:27:11:
In Reply to: STEVE- Meyer-Overton hypothosis posted by John Walker on June 14, 2001 at 08:55:49:
JW, thanks for the hundred year old study, after your RBCR post I'm inclined not take you seriously. I need to see how this post looks and get credit for the Doctor that wrote it. If you and Kendall think that PO2 in the 1.4 range creates narcosis you are buying into a unproven myth, IMO. Plus how do you explain the hot trimix blends that you guys use if you believe this myth.
Oxygen Narcosis: Fact or Fiction?
"In recreational divers, if oxygen has any narcotic effect at all, it is far less narcotic than nitrogen."
At Underwater Canada this year I gave a three hour workshop on the medical aspects of nitrox diving. During the discussion at the end of the session, I was asked a very unusual question. I was asked if nitrox was more or less narcotic than air? I was completely baffled as I had just spent a fair amount of time showing that one of the main advantages of nitrox was that it caused less narcosis than air (when some of the nitrogen had been replaced with oxygen, pO2 > 21%). I answered as best I could and probably would have forgotten all about it but the next day I noticed in the January/February 1996 issue of Alert Diver an article by Cathie Cush entitled "It's a GasNitrox" in which she talks about oxygen narcosis. A few days later I received an e-mail from Gerry Wall. Wall had been at my workshop and he was also puzzled by the question. He mentioned the Alert Diver article and suggested I write a column on the topic.
In the June 1995 issue of DIVER Magazine I wrote on nitrox diving and presented the standard concept that the narcotic effect of nitrox can be calculated by determining the partial pressure of nitrogen in the mixture at a specific depth and comparing it to the depth at which air would have the same partial pressure of nitrogen. For example, if a diver was diving Nitrox II at 99 fsw, they would be exposed to a partial pressure of nitrogen (pN2) of 4 ATA X 0.64 (64% N2, 36% O2) = 2.56 ATA. This would be the same as diving on air at a depth of 2.56 ATA/0.79 (79% N2, 21% O2) = 3.24 ATA total pressure or 2.24 X 33 = 74 fsw. Therefore, the diver would actually be diving at 99 fsw breathing Nitrox II but would be exposed to the same pN2 and presumably the same level of narcosis as if they were diving at 74 fsw on air. On thousands of mixed gas dives, divers have noted that the subjective effect of the narcosis seems to match these calculations.
In the article in Alert Diver, Cush claims that research does not support this concept. She quotes Dr. Bill Hamilton of Hamilton Research as saying "We have a small dilemma, there's not much data, but the little data there is says that a difference in performance could not be detected when they switched between a high and a low oxygen in an oxygen-enriched air mix. That suggests, as do the properties of the gases, that oxygen is just as narcotic as nitrogen."
Let us discuss Dr. Hamilton's comment in reverse order. Although no one knows exactly why some gases are narcotic, it is believed that the physical properties of the gases play a role (recent evidence suggests that the narcotic gases might be exerting their effect through binding to intracellular enzymes). An increase in molecular weight is roughly associated with an increase in narcotic effect but this is certainly not true for the lighter gases (see table) and could be a result of the increasing density of the gas. It has been clearly shown that increased levels of carbon dioxide (CO2 ) increase the narcotic effect. As the gas becomes more dense/viscous as a result of increasing molecular weight, the gas becomes harder to move in and out of the lungs. This causes an increase in the level of CO2 in the body and thereby an increased narcotic effect. Increased CO2 does not fully explain the differences in narcotic effect of the various inert gases. Increasing solubility in lipid (fat) seems to be associated with increasing narcotic effect (the more fat soluble, the greater the narcosis) and oxygen is between nitrogen and argon in lipid solubility. The oil-water (fat-water) solubility ratio reflects a similar property such that as lipid solubility increases in comparison to water solubility, narcotic potency increases (the more fat soluble in comparison to water soluble, the greater the narcosis). Oxygen has a ratio almost identical to nitrogen and argon. Although oxygen does have some physical properties that make it comparable to nitrogen, the physical properties discussed so far, and many others, do not fully explain the narcotic effect of the various gases. Therefore, the physical properties of oxygen should not be used to support a claim that oxygen is as narcotic as nitrogen.
* least narcotic ** most narcotic
(Table from Bennett and Elliot, 4th edition, pg 176)
In the first part of his statement, Dr. Hamilton says that a difference in narcosis "could not be detected when they switched between a high and a low oxygen in an oxygen-enriched air mix". I do not know exactly what data Dr. Hamilton was referring to but there are many problems with the research that has been done trying to determine if oxygen is narcotic or if it increases the narcotic effects of other gases. As mentioned above, CO2 increases narcosis (related to breathing resistance, density, viscosity, level of anxiety, arousal and workload). Therefore, the experiment must try to control for changes in end tidal CO2. Measuring the level of narcosis has also proven to be very difficult to do in a reproducible, reliable fashion.
Many of the experiments that have been done have used different testing protocols, thereby making it very difficult to compare their findings. Often one test will show narcosis while under the same conditions another test will not show narcosis. So far, the most sensitive tests will not reliably show narcosis breathing air at less than 132 fsw (some recent tests might be reliable as shallow as 100 fsw) even though most experienced divers can notice the effect at much shallower depths. Therefore, for nitrogen, the narcotic effect can only be reliably measured when the pN2 is greater than 3.95 ATA (132 fsw = 5 ATA X 0.79). It is not possible to conduct research with a partial pressure of oxygen this high because of the dangers of oxygen toxicity. Therefore, even if oxygen was as narcotic as nitrogen, it would be difficult to measure reliably. To further confuse the issue, there are several factors that are known to alter an individual's sensitivity to narcosis. Anxiety, cold, fatigue, sedatives, alcohol and other central nervous depressants all increase the narcotic effect.
Intelligence, motivation, experience, task learning and increased arousal all reduce the effect of narcosis. The reality and significance of "acclimatization" has not yet been clarified. The combination of all of these factors make narcosis research very difficult to conduct in an accurate and reliable fashion so that the results are trustworthy.
In the Alert Diver article, Cush goes on to say, in "The Physiology and Medicine of Diving", DAN Executive Director Peter Bennett, Ph.D., D.Sc., cites a 1963 study which "showed that oxygen significantly potentiated nitrogen narcosis". In fact, Dr. Bennett refers to several old studies that suggest that oxygen might increase the narcotic effect of nitrogen or even be narcotic itself. One of the main researchers in narcosis (on a continuing basis) for more than 20 years has been Dr. Barry Fowler of York University in North York, Ontario. Dr. Fowler was also the author of several of the papers referred to by Dr. Bennett in "The Physiology and Medicine of Diving".
In 1985, Dr. Fowler (with Drs. Ackles and Porlier from DCIEM) published a review paper in "Undersea Biomedical Research" critically evaluating the research that had been conducted on the effects of inert gas narcosis on behavior. They concluded, "The evidence concerning the effects of hyperbaric oxygen on performance and its relationship to narcosis is too contradictory to draw any conclusions". In plain English, this means that the research up to 1985 did NOT support a conclusion that oxygen was narcotic nor even that it potentiated nitrogen narcosis.
In the Alert Diver article, Cush continues by saying, a 1990 study by Sweden's Karolinska Institute and Hamilton Research supports this finding, concluding that "the degree of narcosis is not significantly ameliorated when a substantial part of the N2 is substituted with O2This study has received very wide press and has been referred to by Chris Allen, the Secretary of the British Sub-Aqua Club, Dr. Richard Moon, Medical Director of DAN, and in a article in Aqua Corps, a technical diving magazine (June 1993). The study referred to was an add-on pilot with only a few subjects. It was presented at a meeting in Amsterdam in 1990 (at which I was present) but was published only as an abstract in the meeting program (never in a peer-reviewed or scientifically credible journal). Dr. Fowler exchanged several letters with the author, Dr. Linnarsson, and kindly allowed me the opportunity to review them. To the best of my knowledge, there has been no further work in this area. The proper study to answer this question, as outlined by Dr. Fowler in one of his letters to Dr. Linnarsson, would require so much time and resources that it will probably never be conducted.
So is oxygen narcosis fact or fiction? The scientific data available do NOT support the conclusion that oxygen is narcotic. They also do not and can not show that oxygen has no narcotic effect. Oxygen might somehow be involved in the entire question of narcosis and performance but it is clearly not more narcotic than nitrogen. The narcotic gases are all chemically inert in the body. In contrast, oxygen is one of the most chemically active substances in nature. Finally, divers are able to reliably detect narcosis at partial pressures of nitrogen far less than those required to produce reliable scientific results. Thousands of divers claim that high pO2 nitrox results in less narcosis than air at the same depth.
Although this has been an interesting academic discussion, what does this mean for recreational divers? I am satisfied that if oxygen has any narcotic effect at all, it is far less narcotic then nitrogen and therefore has absolutely no significance to recreational divers.