Nitrogen Narcosis + Oxygen Toxicity
Under The Influence: A Performance Guide To Managing Narcosis
By Barry Fowler, Ph.D.*
This paper was originally prepared for habitat divers using nitrogen-oxygen mixes, where narcosis becomes a key limiting factor to performance. - Ed.
Breathing hyperbaric air causes a syndrome of behavioural and subjective effects called nitrogen narcosis, which limits the work efficiency of divers and is ultimately life-threatening. Table 1 presents the classic view of the progressive effects of nitrogen narcosis based on descriptions in a number of current textbooks (Bennett, 1981; Miller, 1979; Edmonds et al., 1983). This view emphasizes the growing helplessness of the diver to combat narcosis until eventually stupification sets in at 295 fsw. The image of helplessness is reinforced by Cousteau’s well-known description of narcosis as "raptures of the deep" and his accompanying warnings about a loss of self control, which is exemplified by the urge a diver might have to give his mouthpiece to a passing fish.
Given the assumption of helplessness, it is not surprising that the usual advice to divers is to avoid narcosis by not descending too deep, or to ascend immediately when symptoms are encountered. This is excellent advice: narcosis should be avoided if possible. On the other hand, this advice is not helpful to those divers who must work while narcotic.
The purpose of this paper is twofold. First, to highlight recent advances in behavioural research on narcosis which suggest that it might be possible to develop training procedures to improve the work effectiveness and safety of divers exposed to narcosis. The second purpose is to propose some principles which could serve as a guide for he development of these procedures. For more detail on the experiments mentioned in this paper, the reader is referred to a recent review covering the last fifteen years of behavioural research on narcosis (Fowler, et al., 1985).
Narcosis - A Slowing Of Responding
Recently, a theory called the slowed processing model has been proposed which suggests that, prior to unconsciousness, the primary effect of narcosis on performance arises from a single fundamental deficit in the Central Nervous System. This deficit is thought to be a decrease in arousal which slows responding but does not cause perceptual distortions of either vision or audition. The claim that narcosis does not cause perceptual distortions is counterintuitive because narcosis typically decreases the accuracy of responding as well as increasing response time on a variety of cognitive, perceptual-motor and manual dexterity tasks.
To explain how the slowed processing model accounts for these decreases in accuracy, it is useful to consider an example of the research that is being conducted on narcosis with the hyperbaric facilities at Defence and Civil Institute of Environmental Medicine in Toronto. One of the tasks used to study narcosis is called the Serial Choice Reaction Timer. It consists of a set of push buttons arranged so that a finger can rest comfortably on each one. Adjacent to each button is a light-emitting diode. The task is to extinguish a lighted diode as quickly as possible by pressing the appropriate button. This lights another diode randomly which must then be extinguished and so on for a specified period of time - usually 90 seconds. A computer controls this sequence of events and also records reaction time and the number of times an unlit button is pressed - this is defined as an error and reflects the level of accuracy. Subjects performing this task in a hyperbaric chanber at the equivalent of 295 fsw show an increase in reaction time and in the number of errors committed, but they are not stupefied as the classic view would suggest. Moreover, it turns out that these errors can be eliminated by training the subjects to slow down. In other words, the loss in accuracy can be controlled at the expense of speed. Generally speaking, it appears that this is true for many tasks where a loss of accuracy is not necessarily part of the performance breakdown due to narcosis. To summarize, the slowed processing model holds that decreased accuracy on many tasks is due to untrained individuals working too quickly and being willing to take more risks than usual.
Two training principles are suggested by this research. First, disorganized behaviour is not necessarily part of narcosis and can be overcome by training. Second, errors can be avoided by slowing down. Conversely, when time is at a premium and the diver is hurrying, an increase in errors will be unavoidable. The potential costs of these errors in terms of work efficiency and safety must be weighed against the possible gains. For example, it might be acceptable to hurry and make an assembling a piece of apparatus. It would not be acceptable to hurry and make an incorrect decision resulting in loss of orientation with respect to an anchor or guideline.
The Effects Of Narcosis On Memory
Tasks involving long-term memory and learning are one area where slowed processing model is unsuccessful in explaining decreases in accuracy by a failure to slow down. Narcosis causes forgetting which can be so severe that it was evident to early observers. Even before World War II, it was noticed that, after surfacing, divers were unable to recall all the events that had taken place under water. More recently, research has demonstrated another effect. During a dive, Material learned beforehand may not be recalled.
Quite clearly, these forms of amnesia raise a number of potential problems. During the dive, there is the possibility of forgetting previously learned instructions and the learning of new material will be impaired. This latter effect will contribute to difficulty in solving new problems. After surfacing, events during the dive may not be recalled.
Two training principles could be employed to counter these amnesic effects. First, the diver should rely on memory as little as possible. Second, when memory must be relied on, the material should be highly overlearned and memory cues used to minimize forgetting. Examples of procedures relating to the first principle include preparing and using a check-off list which details every stage of the dive and recording all interesting observations during the dive. With respect to the second principle. divers must overlearn any emergency procedure which is to be executed quickly in a precise sequence. In addition, an obtrusive alarm system should serve as a cue for critical items, such as bottom departure time.
The Subjective Symptoms Of Narcosis And Their Relationship To Performance
The term "raptures of the deep" was coined to highlight a striking characteristic of narcosis - the subjective sensations of euphoria which may induce rash behavior. However, the point was made earlier that divers can be trained to act rationally under narcosis. One may feel euphoric without necessarily acting these feelings. The emphasis on euphoria has obscured the fact that there are other subjective sensations induced by narcosis. These have been documented by asking experienced divers to identify adjectives describing their feelings. In all, four clusters of adjectives have been identified. These relate to euphoria (e.g., more carefree and cheerful), consciousness (e.g., more fuzzy and hazy), work capability (e.g., less effective and efficient) and inhibitory state (e.g., less cautious and self-controlled).
For training purposes, it is important to note that, apart from inducing rash behavior, subjective symptoms have the potential to influence performance in two ways. First, performance may be disrupted because the diver pays attention to the internal sensations of narcosis at the expense of maintaining concentrations on the environment and the task. This is because a fairly strong relationship has been demonstrated between subjective ratings of the severity of narcosis and the degree of performance impairment. It should be noted that this study was performed under ideal conditions in a dry hyperbaric chamber and possibly these results could not be replicated under water. This is because a variety of other factors, e.g., cold, anxiety and fatigue, could all produce sensations which might mask narcosis.
The potential influence of the subjective symptoms of narcosis on performance suggests three training principles. First, the diver must become familiar and comfortable with the sensations of narcosis. Second, the diver must learn to allocate attention between the task and the symptoms of a manner appropriate to the situation. The object here is to prevent a performance deficit due to inattention, but at the same time, not to ignore the symptoms entirely. The reason for not ignoring symptoms becomes apparent in the third principle. This states that a diver should be taught to use the intensity and type of symptoms to estimate performance capability. For example, in the event of inadvertantly exceeding the depth limit during an excursion dive, subjective symptoms could be the first warning if the development of a life-threatening situation.
Adaptation To Narcosis
It is generally agreed by divers that frequent exposure to narcosis leads to adaptation. The problem is that research on this question has not clarified what kind of adaptation is taking place (Fowler, et al., 1985).
There is some evidence of adaptation that is specific to narcosis. This means that, over successive exposures, performance under narcosis improves at a greater rate than a surface control - this is true adaptation. On the other hand, this kind of adaptation has not been found in some experiments where the improvement in performance is identical for narcosis and the surface control. This is a case of non-specific learning, but it is important to note that there is still an improvement in performance under narcosis,. Figure 1 illustrates these two cases.
Three conclusions are suggested by these results. First, true adaptation to narcosis may occur but only under certain circumstances which are not presently understood. Second, it is possible that divers may sometimes mistake non-specific learning for true adaptation. Third, it is not clear what the relationship is between the adaptation of subjective symptoms and the adaptation of objective performance. It is possible that divers may be basing their opinions about adaptation largely on subjective symptoms. To date, researchers have ignored this possibility and focussed on measuring the adaptation of objective performance.
It is clear that a good deal more research is required before the issues raised here about adaptation are resolved. In the absence of clear-cut research results, it is difficult to offer specific training principles which take advantage of adaptation or non-specific learning. Until these results become available, a generally useful training principle is to provide the diver with as much practice as possible prior to the dive on the tasks to be performed under water. If these tasks can be practiced under narcosis prior to the dive, so much the better. There are some techniques which might be useful for this purpose, but it is beyond the scope of this paper to discuss them.
Some Final Caveats
The eight training principles which have been proposed are aimed at controlling and possibly ameliorating the effects of narcosis when it cannot be avoided. Underlying these principles is a view of narcosis, expressed in terms of the slowed processing model, which differs from that presented in current textbooks. However, it must be emphasized that these principles are only tentative and must be tested by controlled research. There is definitely no suggestion that current maximum depth guidelines for sports divers should be violated.
Finally, for the purposes of this paper, the whole question of predicting performance in the underwater environment has been over-simplified. There are a variety of other stressors which coexist with narcosis and which, in combination with it, have the potential to place severe limits on performance. These include hypercapnia, cold, anxiety, perceptual disorders and weightlessness (Fowler, et al., 1983; Godden and Baddeley, 1979). This has been demonstrated clearly in the case of anxiety (Baddeley and Fleming, 1967), but information about other combinations is virtually nonexistant. If deep diving on air is to be carried out with a maximum of safety and efficiency, training procedures must not only be guided by the effects of narcosis on performance, but also by the effects of any additional stressor which may be present in combination with narcosis.
Barry Fowler, Ph.D., is one of the leading researchers in the field of inert gas narcosis. He can be reached at York University, 4700 Keele Street, New York, Ontario, M3J 1P3, Canada.
Baddeley, A.D. and Flemming, N.C. (1967). "The Efficiency of Divers Breathing Oxy-helium." Ergonomics 10, 311-319.
Bennett, P.B. (1982). "Inert Gas Narcosis and the High Pressure Syndrome." In: Hybaric and Undersea Medicine. Vol 1. (J.C. Davis, ed.). Lesson No. 16. Medical Seminars, Inc. San Antonio, Texas
Edmonds, C., Lowry, C. and Pennefeather, J. (1983) Diving and Scubaquatic Medicine. (Revised second edition). Chap. 9. Diving Medical Centre, Mosman, NSW.
Fowler, B., Ackles, K.N. and Porlier, G. (1985). "Effects of Inert Gas Narcosis on Behavior - A Critical Review." Undersea Biomed. Res. 12, 369-402
Godden, D. and Baddeley, A. (1979). "The Commercial Diver." In: Compliance And Excellence. The Study Of Real Skills. Volume 2. (W.T. Singleton, ed.). MTP Press, Lancaster.
Miller, J.W., ed. (1979). NOAA Diving Manual. Diving For Science And Technology. (Second edition). Sections 2-20-2-23. U.S. Government Printing Office, Washington, D.C.
Pilmanis, A.A., Given, R.R. and Borgh, B.C. (1984). "Unique Design of the New NOAA/USC Saturation Diving System." Proc of Oceans. September 10-12.
[Table 1 text, p. 48]
TABLE 1: A summary of the classic view of the progressive effects of nitrogen narcosis.
4ATA (98 fsw) Mild euphoria, delayed responses
6ATA (164 fsw) Sleepiness, hallucinations, impaired judgement; laughter and loquacity may be overcome by self control.
8ATA (230 fsw) Convivial group atmosphere, severe impairment of intellectual performance, uncontrolled laughter or terror reaction in some.
10ATA (299 fsw) Stupifecation, mental abnormalities, euphoria, almost total loss of intellectual faculties.
[Figure 1 text, p. 49]
Adaptation To Narcosis
Number of Exposures to Narcosis
Figure 1. True adaptation to narcosis is illustrated in the top figure and non-specific learning in the bottom one. It is assumed that learning is occuring on the task but adaptation could occur without learning.
[Box text. p. 50]
Fowler On Narcosis
• Disorganized behavior is not a necessary part of narcosis and can be overcome by training. Errors can be avoided by slowing down.
• Divers should rely on memory as little as possible. When memory must be relied on, the material should be highly overlearned and memory cues used to minimize forgetting.
• Divers must become familiar and comfortable with the sensations of narcosis, and learn to allocate attention between the task and the symptoms in a manner appropriate to the situation. Divers can l;earn to use the intensity and type of symptoms to estimate performance capability.
• Divers should practice as much as possible prior to the dive on the tasks to be performed underwater.
Oxygen metabolism is the primary energy source in higher life forms, but when oxygen enters the metabolic process prematurely, reactive oxygen species can form which interfere with normal function and cause convulsions or other symptoms of oxygen toxicity. Immersion, exercise, and inspired carbon dioxide increase susceptibility to oxygen toxicity by elevating cerebral blood flow and oxygen delivery to the brain. The risk of toxicity is reduced by limiting oxygen exposure, but exposure limits are based on limited data. Limits for oxygen in mixed gas appear shorter than for pure oxygen. Open-water experience indicates that convulsions can occur near the accepted exposure limits. The risk of oxygen toxicity can be modelled statistically but with uncertain accuracy. The choice of "safe" exposure limits depends upon the risk of convulsions one is willing to accept. The maximum "safe" oxygen partial pressure for air or nitrox diving in the water appears to be in the range of 1.2-1.4 bar though some individuals set the limits as high as 1.6 bar. For pure oxygen, 1.6 bar has been used safely for in-water decompressions of up to 30 min.
Knowledge of central nervous system (CNS) oxygen toxicity is unnecessary in order to breathe oxygen underwater safely at a partial pressure of one bar or less. Considerably more knowledge is needed at higher partial pressures or when the oxygen pressure changes with time. The real questions are; how much oxygen can be used safely given our current knowledge, and how can oxygen be used more effectively without sacrificing safety?
The Biochemistry of Oxygen Toxicity (Stryer 1988)
Oxygen metabolism is the primary energy source in higher life forms. Because heat energy produced by oxygen reactions such as fire would damage tissue, metabolic pathways have evolved that safely capture small packets of reusable chemical energy. This energy is stored in molecules called adenosine triphosphate (ATP).
Figure 1 illustrates some features of ATP production during the breakdown of sugar at normal oxygen partial pressures. The biochemical processes known as glycolysis use no oxygen and produce relatively little ATP. The major product of glycolysis, pyruvic acid, enters the Krebs cycle which releases carbon dioxide and supplies electrons needed to form ATP. Most ATP is produced in a series of electron transport reactions called the respiratory chain.
Oxygen usually does not enter the respiratory chain until the very end where it reacts with hydrogen to form water. Should oxygen enter the respiratory chain prematurely, molecules like the superoxide anion (O2-) and hydrogen peroxide (H2O2) can form. These reactive species of oxygen are potentially toxic but are deactivated by protective enzymes such as superoxide dismutase and catalase.
When the oxygen partial pressure is raised (Fig. 2), the production of reactive oxygen species increases and may overwhelm the protective mechanisms. This can initiate biochemical and physiological changes that interfere with normal function and cause signs and symptoms we know as oxygen toxicity.
Signs and Symptoms of CNS Oxygen Toxicity (Donald 1992; Clark 1993)
Convulsions are the most spectacular and objective signs and symptoms of CNS oxygen toxicity, but there is no evidence they lead to permanent damage if the oxygen exposure is discontinued promptly. This assumes, of course, that drowning or physical injury are avoided. Experimental oxygen exposures are often terminated by less specific symptoms including abnormal breathing, nausea, twitching, dizziness, incoordination, and visual or auditory disturbances. These symptoms do not necessarily precede convulsions.
Factors which elevate cerebral blood flow, thereby augmenting oxygen delivery to the brain, appear to increase susceptibility to oxygen toxicity. These factors include immersion, exercise, and carbon dioxide. Carbon dioxide may be present in the inspired gas or may be retained due to inadequate ventilation. Inadequate ventilation can be caused by high gas density, external breathing resistance, or poor ventilatory response to carbon dioxide by "CO2 retainers" (Lanphier 1982; Warkander et al. 1990).
Oxygen Exposure Limits
Oxygen exposure limits like those of Fig. 3 were established to decrease the risk of convulsions for divers breathing pure oxygen or oxygen in mixed gas. Figure 3 shows three sets of pure oxygen limits and two sets of mixed gas limits. The U.S. Navy limits from the 1973 Diving Manual (USN 1973) were published in the 1979 NOAA Diving Manual (NOAA 1979). The Navy has since modified its pure oxygen limits (Butler and Thalmann 1986) while NOAA has modified both the pure oxygen and mixed gas limits for its 1991 Diving Manual (NOAA 1991). Compared with the 1973 Navy/1979 NOAA limits for pure oxygen, Fig. 3 shows that the 1986 Navy limits are less conservative while the 1991 NOAA limits are more conservative. For mixed gas, the 1991 NOAA limits are less conservative than the 1973 Navy/1979 NOAA limits.
The changes to the exposure limits of Fig. 3 reflect uncertainty concerning which limits are best and suggest an examination of the type of data upon which oxygen limits are based. These data are shown in Fig. 4 and represent most of the CNS toxicity episodes that have occurred in U.S. experiments during wet, working dives at a single depth for pure oxygen or for oxygen in mixed gas (Lanphier and Dwyer 1954; Lanphier 1955; Piantadosi et al. 1979; Vann 1982; Schwartz 1984; Butler and Thalmann 1984, 1986; Butler 1986; Lanphier 1992). The squares represent convulsions, and the triangles represent symptoms. The 1991 NOAA limits are shown for comparison. While the discussion below is confined to U.S. data, Donald (1992) has recently published a large body of British data which will be very important.
The mixed gas incidents occurred at lower oxygen partial pressures than the pure oxygen incidents. Lanphier, who conducted oxygen research for the Navy in the 1950's, postulated that high breathing resistance during deeper mixed gas dives caused carbon dioxide retention which potentiated oxygen toxicity by increasing cerebral blood flow (Lanphier and Dwyer 1954). This led him to propose more restrictive limits for mixed gas than for pure oxygen. In subsequent studies, the lowest partial pressure and shortest exposure time at which a mixed gas convulsion occurred was 1.6 bar for 40 min (Vann 1982; Vann and Thalmann 1993). The corresponding exposure for pure oxygen was 1.76 bar for 72 min (Butler and Thalmann 1984).
The mixed-gas convulsion occurred after 40 min at 100 fsw during a wet, working nitrox chamber dive with a 1.6 bar oxygen set-point in a rebreather (Vann 1982). Heavy exercise and high breathing resistance appeared to be contributing factors. Upon decreasing the breathing resistance and reducing the oxygen pressure to 1.4 bar, 110 dives were conducted with no further oxygen incidents during 60 min exposures at 100 and 150 fsw with both nitrox and heliox.
Is an oxygen partial pressure of 1.4 bar sufficiently conservative given the potential for depth control error, the unpredictability of carbon dioxide retention, and the minimal mixed-gas exposure data? The Navy is leaning towards a set-point of 1.2-1.3 bar for rebreathers where the oxygen partial pressure fluctuates during control around a pre-determined set-point (Thalmann, personal communication).
The data of Fig. 4 suggest a need for separate mixed gas and pure oxygen limits but are insufficient to conclusively prove this need. As a convulsion underwater is potentially fatal, however, a cautious diver might wish to use separate oxygen and mixed gas limits until further data firmly establish they are unnecessary.
What can we learn about oxygen toxicity from open-water diving with mixed gas and pure oxygen? The incidents described below took place within the past year.
A mixed gas fatality occurred in a southeastern U.S. cave where two divers breathed air for 15 min and EAN 40 (40% O2, balance N2) for 45 min at depths of 80-105 fsw (Menduno 1992). The oxygen partial pressure was mostly 1.4 bar but occasionally reached 1.5-1.7 bar. After 45 min of hard swimming on enriched air nitrox, one diver convulsed and lost his regulator. His buddy could not reinsert the regulator, and the diver drowned after a failed attempt to swim him out of the cave. The oxygen exposure was, for the most part, less than the 1991 NOAA limit of 1.6 bar for mixed gas diving.
Another enriched air diver who drowned after an apparent convulsion had told friends that the NOAA limits did not apply to him (Menduno 1992). His oxygen partial pressure was estimated at 1.7-2.0 bar for a bottom time of 45-50 min.
An incident involving pure oxygen occurred in a southeastern U.S. lake (Menduno 1992). After an 8 min exposure at 300 fsw on a trimix 14/33 (14% O2, 33% He, and 53% N2) a diver decompressed on EAN 32 to 20 fsw where he switched to pure oxygen. Prior to breathing oxygen at 20 fsw (1.6 bar PO2), his PO2 was 1.4 bar except for 7 min at 1.5-1.7 bar. After 20 min on oxygen, he unclipped from his decompression line to visit a nearby diver but drifted down to 35 fsw (2.05 bar PO2) and dozed off. (An Emergency Medical Technician, he had slept only 2 hrs the previous night.) He was awakened by abnormal breathing and the onset of convulsions but inflated his buoyancy compensator before losing consciousness. He recovered from near drowning after rescue on the surface.
It is commonly assumed that convulsions do not occur at oxygen pressures of less than about 1.6 bar, but this assumption depends on a normal seizure threshold. Figure 5 shows the depth-time profile of an 80 fsw dive that terminated with a convulsion at 34 min (Vann et al. 1992). The diver breathed EAN 33 with an oxygen partial pressure of 1.26 bar. After rescue, he was found to have an unreported history of convulsions and to be on anti-convulsant medication. While such a situation is rare, it emphasizes the uncertainty of our knowledge, the need to expect emergencies such as oxygen convulsions or decompression illness, and the necessity for emergency management plans.
Do these open-water incidents over emphasize rare events? What is the risk of a rare event? We can estimate this risk by statistical modeling of oxygen exposure data (Vann 1988).
Suppose the risk of oxygen toxicity increased with the concentration of the reactive oxygen species produced during hyperoxic metabolism (Fig. 2) and represented below by "X". Suppose also that the rate of change of the local concentration of X were equal to its production minus its removal. If X were produced in proportion to the local oxygen tension (c •PO2) and removed at a fixed rate (k), its rate of change would be
dX/dt = c•PO2 - k
where c and k are constants. When integrated, this first order differential equation gives
X = (c•PO2 - k)•t (1)
The risk of toxicity is specified by a separate function of X.
Equation 1 defines a family of rectangular hyperbolas proposed empirically for the pressure-time relationship of pulmonary and CNS oxygen toxicity (Clark 1974). Statistical modelling derives this relationship theoretically and finds the constants c and k directly from experimental data (Vann 1988). This allows the risk of toxicity to be estimated for any oxygen exposure.
Figure 6 shows three rectangular hyperbolas for 2%, 5%, and 8% risks of either symptoms or convulsions. These were estimated from data on 773 pure oxygen exposures. The convulsions, represented by black dots in Fig. 6, occurred at estimated risks of 2-8%. In a context of risk, an oxygen exposure limit is the depth and time at the level of risk which is judged to be acceptable. In Fig. 6, for example, the limit for a pure oxygen exposure at 25 fsw (1.76 bar) would be 49 min if a 2% risk of either symptoms or convulsions were judged acceptable. The level of acceptable risk for a chamber dive where immediate rescue is possible after a convulsion is greater than for an open-water dive where drowning is the likely outcome.
Statistical modeling can track the resolution of risk as well as its development. In Fig. 7, for example, a pure oxygen diver spends 120 min at 20 fsw, 15 min at 40 fsw, and 105 min at 20 fsw. His risk increases gradually to 0.2% while at 20 fsw and rapidly to 4.1% at 40 fsw. The maximum risk of 4.3% occurs just before surfacing after which the risk resolves in 10 min.
Unfortunately, the accuracy of the risk estimates of Figs. 6 and 7 is uncertain because human oxygen exposure data are limited and their results variable (Donald 1992; Clark 1993). This uncertainty encourages conservative exposure limits, at present, instead of permitting the oxygen exposure to be adjusted continuously such that the estimated risk never exceeds the risk judged to be acceptable. For mixed gas, even less data are available than for pure oxygen, and the potential for carbon dioxide retention introduces further uncertainty which makes modeling of mixed gas risk even more problematic.
What Are "Safe" Oxygen Exposure Limits?
The choice of "safe" oxygen exposure limits depends upon the risk of convulsions that one is willing to accept. This subjective judgment is rendered all the more difficult because so few data are available from which to estimate risk and because there is so much variability in the response to oxygen exposure. Variability can be due to exercise, carbon dioxide retention, gas analysis error, oxygen set-point control, and susceptibility to oxygen toxicity from inter- and intra-individual differences.
For air or enriched air diving, a maximum exposure limit of 1.2 bar would appear to be conservative while allowing a "cushion" for oxygen partial pressure increases due to unplanned depth excursions. Perhaps 1.4 bar would be acceptable if depth could be carefully controlled. On the other hand, there are those who testify to diving safely at 1.6 bar. This may well be true, but skepticism is appropriate until these divers document their claims in the form of computer-recorded depth-time profiles with certified breathing mixtures (Fig. 5). Denoble et al. (1993) describe a project and data acquisition software which might help to provide such documentation.
For pure oxygen, commercial (Imbert and Bontoux 1987) and scientific experience (Fife et al. 1992) suggests that at least 30 min of in-water oxygen decompression may be possible at 1.61 bar (20 fsw) with little risk of CNS toxicity. Experimental data (Fig. 4) also suggest a low risk at 1.76 bar (25 fsw), but a small depth excursion can cause large increases in oxygen pressure. Pure oxygen diving at depths below 20 fsw is more hazardous.
Improvements in our ability to manage oxygen exposure are expected as basic studies illuminate the fundamental biochemistry and physiology, as additional exposure data become available, and as statistical modeling methods develop. Basic studies have already led to pharmacological methods for extending oxygen exposure in mice (Oury et al. 1992), but further work is needed before such methods are applied to humans. The diving community itself can provide some of the necessary exposure data should it adopt a rigorous approach to data collection. Statistical modeling and computer tracking of oxygen exposure may eventually lead to guidelines for variable oxygen partial pressures to supplement single stage oxygen limits (Fig. 3). A particularly important advance that might eliminate much of the current unpredictability would be a mouthpiece sensor for measuring end-inspired and end-expired carbon dioxide. In the meantime, a patient and conservative approach to oxygen exposure management is appropriate to minimize the frequency of mishaps such as those of the past year.
After graduating in 1965 with a B.S. in mechanical engineering from Columbia University, Richard Vann worked as a diving engineer at Ocean Systems, Inc. on Sealab III life support systems and as a research subject during experimental dives. Following four years in the Navy, two as Diving Officer for Underwater Demolition Team TWELVE, he attended Duke University graduating in 1976 with a Ph.D. in biomedical engineering. Since then, he has conducted research at the F.G. Hall Hypo/Hyperbaric Center of Duke Medical Center on bubble formation, inert gas exchange, decompression procedures, oxygen toxicity, breathing apparatus design, and biomaterials. Dr. Vann is currently an Assistant Research Professor in Anesthesiology, Director of Applied Research at the Hall Center, and Research Director of the Divers Alert Network. He can be contacted at: Box 3823, Duke University Mediacal Center, Durham, NC 27710, fax: 919-684-6002
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1. The production of ATP during the breakdown of sugar at normal (normoxic) oxygen partial pressures.
2. The production of ATP and reactive oxygen species during the breakdown of sugar at elevated (hyperoxic) oxygen partial pressures.
3. Oxygen exposure limits for pure oxygen and for oxygen in mixed gas as published by NOAA and the U.S. Navy. The 1973 U.S. Navy limits (USN 1973) were adopted for the 1979 NOAA Diving Manual (NOAA 1979). These are indicated as USN/NOAA 1979. The Navy revised its pure oxygen limits in 1986 (Butler and Thalmann 1986). NOAA revised its pure oxygen and mixed gas limits in 1991 (NOAA 1991). Exposure times and pressures are connected by line segments to facilitate comparisons. Field application requires step changes at the indicated points. The original references should be consulted for operational use.
4. CNS oxygen toxicity data (convulsions and symptoms) from U.S. experiments with wet, working divers exposed to constant oxygen partial pressures (Lanphier and Dwyer 1954; Lanphier 1955; Piantadosi et al. 1979; Vann 1982; Schwartz 1984; Butler and Thalmann 1984, 1986; Butler 1986; Lanphier 1992). The 1991 NOAA exposure limits for pure oxygen and mixed gas are shown for comparison (NOAA 1991).
5. The depth-time profile recorded by a dive computer for an exposure on 32.8% nitrox at a nominal depth of 80 fsw and oxygen partial pressure of 1.26 bar. The dive was terminated at 34 min by a convulsion. After rescue, the diver was found to have an unreported history of convulsions and to be on anti-convulsant medication.
6. Estimates of CNS oxygen toxicity risk based upon a statistical model (Vann 1988). The model was fitted to experimental data from 773 pure oxygen exposures which resulted in 11 convulsions and 33 incidents of symptoms. Exposures for estimated risks of 2, 5, and 8% are shown with the observed convulsions.
7. The development and resolution of CNS oxygen toxicity risk according to the model of Fig. 6 during a multi-level dive on pure oxygen.
copyright Mark Ellyatt