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Nitrogen Narcosis: A Critical Conversation

Photo: Lee Newman

Text by Bret C. Gilliam

The following discussion picks up from the current issue of DIVER Magazine – Volume 37 Number 4 – now on newsstands. Against the background of an historical perspective and today’s ardent debate on this subject author Bret Gilliam advocates the dissemination of more accurate information on nitrogen narcosis and a more enlightened attempt at proper education.

Limits and Opinions
Today’s diver has the advantage of extremely well engineered, high performance scuba gear that can significantly increase his performance.  Design evolutions in buoyancy compensating devices (BCDs), scuba regulators, instrumentation, diving computers, less restrictive and more efficient thermal suits, etc., all contribute to his ability to work deeper more safely.

I would like to emphasize that deep air diving below 218 fsw (60.6m) is generally not recommended given the alternatives available in today’s industry. This depth represents the outer limits of recommended oxygen exposures at 1.6 ATA of 02. For high risk or particularly demanding dives this depth should be adjusted shallower.  Many veteran air divers now opt for mixed gas that virtually eliminates narcosis and oxygen toxicity problems.  What is the cut-off depth on air?  This is clearly subjective and must be answered by the individual diver who considers his own narcosis susceptibility, his dive objective and his access and financial commitment to mixed gas equipment.

Wes Skiles, who died in 2010, a highly experienced and respected cave diver, expressed his preference for mixed gas on any penetrations below 130 fsw (39.4m) primarily because of his admitted low tolerance for narcosis.  This was back in 1990. Members of the scientific diving community still practice air dives to 190 fsw (57.6m) officially with far deeper dives reported ‘unofficially’.  Mount and I have long suggested practical air limits of between 250 and 275 fsw (75.7-83.3m) for properly trained and adapted professionals. But it is necessary to understand that such depths exceed the typical ‘working depth’ guidelines for oxygen and place the diver in the 02 exceptional exposure zone. The reader is directed to references specifically on oxygen toxicity to better understand various 02 exposure theories and phenomena. Mixed gas solves some problems for some people, but it adds several new problems and operational considerations to the equation:  expense, heat loss, extended deco times, etc., Deep air diving remains a viable choice for many with this experience because it remains a viable technique when proper disciplines and training are observed, which is to say the level of risk is acceptable. But new divers venturing beyond traditional sport limits must be fully aware of the risks, and that deep diving reduces the margin for error and increases the chance of injury or death.  Diving within one’s limitations should be etched firmly in the deep diver’s memory.  Depths below 130 fsw (39.4 m) can be safely explored but such diving cannot be taken lightly.

Predisposing Factors
Factors contributing to narcosis onset and severity include:

  • Increased partial pressures of CO2 (hard work, heavy swimming, etc.)
  • Cold
  • Alcohol use or ‘hangover’ conditions
  • Fatigue
  • Work of breathing, e.g. inherent resistance within the breathing system on inhalation/exhalation cycles
  • Anxiety or apprehension: Fear
  • Effects of motion sickness medications
  • Rate of descent (speed of decompression)
  • Vertigo or spatial disorientation caused by no ‘up’ reference such as in bottomless clear ‘blue water’ or in severely restricted visibility
  • Task loading stress
  • Time pressure stress
  • Another lesser-known contributory factor is increased oxygen partial pressure

Narcosis can be controlled to varying degrees specific to individuals but tolerances can change from day to day.  Almost any experienced deep diver will tell you that “adaptation” to narcosis takes place.  Bennett (1990) notes, “the novice diver may expect to be relatively seriously effected by nitrogen narcosis, but subjectively at least there will be improvement with experience.  Frequency of exposure does seem to result in some level of adaptation.”  The actual mechanics of adaptation are not clearly understood or proven but most deep divers agree that they will perform better with repeated progressively deeper penetrations on a cumulative basis.

During a series of experimental dives in 1990, I had no significant impairment at 452 fsw (137 m) for my brief exposure, approximately 4.5 minutes in the critical zone (especially for O2 tox) below 300 fsw (91 m).  I was able to successfully complete a series of higher math and thought/reasoning problems while suspended at the deepest level.  But this is probably the extreme end of adaptation; I dove every week for over a year with never more than a six-day lay-off.  My 627 dives during this period included 103 below 300 fsw (91 m).

For the diver who regularly faces deep exposures, a tolerance far in excess of the unadapted diver will be exhibited.  A gradual work-up to increasing depths is the best recommendation. I refer to making each first dive of the day progressively deeper than the day before to build tolerances, i.e. Day 1: first dive to 150 fsw, Day 2: first dive to 175 fsw etc. Subsequent dives on Day 1 and Day 2 would be shallower than the first.)  This process should be over several days’ time if the diver has been away from deep diving for more than two weeks.  Adaptation appears to be lost exponentially as acquired so no immediate increased narcosis susceptibility will necessarily be evident but divers are cautioned to exercise great conservatism if any lay-off is necessitated.

The Diving Reflex
Back in the mid-1800’s Paul Bert observed pronounced brachycardia (lowered heartbeat) in ducks while diving.  Suk Ki Hong (1990) describes “a reflex phenomenon that is accompanied by an intense peripheral vasoconstriction, a drastic reduction in the cardiac output, and a significant reduction of 02 consumption”.  Hickey and Lundgren (1984) further noted aspects of the mammalian diving reflex to include “muscular relaxation, astonishing levels of brachycardia, e.g., heart rates 13{c383baab7bef8067e8c9786a45d8006c492489841a98fe37723e304bb1ddd030} of pre-dive levels in harbor seals… and depressed metabolism.  All of these adaptations conserve the body’s energy stores.”  Simply put, this reflex serves to apparently slow down most vital, internal functions such as heartbeat and shunt blood from the extremities enabling the diving seal or dolphin to more effectively utilize its single breath oxygen load while underwater.

Similar responses have been noted in human subjects.  Several divers stumbled onto this in the late 1960s and began to effectively incorporate facial immersion breathing periods prior to diving.  Exley and Watson practiced such techniques and I became a leading proponent of surface and ten-foot depth (3.03 m) level extended breathing with my diving mask and hood removed before dives below 300 fsw (91 m) in 1971.  I have recorded dramatic reductions in my heart rate and respiration rate by following a protocol of ten minutes facial immersion breathing at the surface, then five minutes at ten to fifteen fsw (3.03 to 4.5 m) from a pony bottle.  My pulse has been measured at twelve to fifteen beats per minute and respiration rate dropped to two a minute at deep depths (dive to 405 fsw/122.7 m 1977). Other divers have adopted varying uses of the diving reflex technique in conjunction with meditation disciplines with significant success. Of the divers using this technique, many report pronounced reduction of narcosis, reduced air consumption and better coordination at depth.  Regardless of the scientific proof challenges, the technique is becoming more widespread and its subjective benefits certainly bear closer scrutiny.

At depth the air we breathe has far greater density and can be an operational problem if the scuba regulator is not carefully selected to comfortably deliver adequate volumes upon demand.  Breathing resistance can markedly increase onset and progression of narcosis.  Until the 1990s many so-called “professional” regulator models fell sadly short on performance below 200 fsw (60.6 m).Exhalation resistance is a prime factor in breathing control, perhaps more so than inhalation ease.  Studies have shown exhalation detriments to be the most significant fatigue element in underwater breathing tests.  So how do you choose between the dozens of models offered?  Some benchmark can be derived from perusal of U.S. Navy test reports but sometimes results can offer inconclusive appraisals.  Back in the late 1980s, the Tekna 2100 series unit basically failed the Navy tests for high performance due its unique second stage design, but was popular regulator with many experienced deep divers since its introduction. I used it on my record setting 452 fsw (137 m) dive in Roatan and had complete satisfaction. But remember that the numbers of regulators that are genuinely suited for deep diving are contained on a very short list. (I personally use the superlative Titanium series from Atomic since 1996.)

Now is a good time to insure that you select comparable quality instruments compatible with the depths you anticipate exploring.  Keep in mind that many depth gauges and dive computers have depth limitations that will render them useless much over normal sport diving ranges.  Make certain that the information is displayed in an easily understood format.  If you have a hard time deciphering what you are looking at on the surface, imagine the problem at 250 fsw (75.8 m) under the influence of narcosis.

On the Dive
Wreck and drop-off wall divers should use descents undertaken with a negative glide to the desired operational depth and thee BCD used to quickly attain neutral buoyancy.  Do not waste energy and generate CO2 using leg kicking to maintain position in the water column.  Slow, deep ventilations with minimal exertions will keep C02 down and reduces onset and severity of narcosis.  Narcosis has been reported subjectively to be most strong when first arriving at depth.  Allow yourself a stop-activity period to monitor your instruments and let the initial narcosis effects stabilize.

Diving deep properly is more a mental exercise than a physical one.  The diver must constantly be aware of his own limitations to narcosis and not hesitate to abort a dive if impairment becomes unreasonable.  If narcosis is severe on descent, slow the rate or stop completely until symptoms are controlled.  If possible face an “up” reference at all times such as anchor line or face the drop-off to orient the wall perpendicularly to the surface.  This affords more accurate references if you are sinking or rising.  If necessary, hold on to the descent line or a drop-off wall outcropping to insure of control of depth while narcosis can be evaluated.

In spite of the warnings of various academicians, it is unlikely that the diver will experience “rapture” or the uncontrollable desire to kiss a fish or dance with an imaginary mermaid.  However, there is a wide range of individual susceptibility.  Almost all divers will be impaired eventually.  This will manifest in many ways.

Most divers are acquainted with traditional depictions of narcosis symptomatology (lightheadedness, slowed reflexes, euphoria, poor judgment, even numbness etc.).  But many early symptoms are more classically subtle.  Initially divers will notice, in many cases, a reduced ability to read fine graduations in a depth gauge diving computer, or watch along with increased awareness of sensitivity to sound such as exhalation and inhalation noise.  Perceptual narrowing may limit some divers to successful execution of only limited task loading.  Short-term memory loss and perceptions of time can be affected.  With experience, divers can learn to control these deficits to some extent.  But these very real dangers cannot be underestimated.  A diver unaware of his depth, bottom time or remaining air volume is about to become a statistic!

Narcosis Symptoms

  • Lightheadedness
  • Euphoria
  • Drunkenness
  • Impaired neuromuscular coordination
  • Hearing sensitivity or hallucination
  • Slowed mental activity
  • Decreased problem solving capacity
  • Overconfidence
  • Short-term memory loss or distortions
  • Improper time perceptions
  • Fine work deterioration
  • Exaggerated movements
  • Numbness and tingling in lips, face and feet
  • Stupor
  • Sense of impending blackout
  • Levity or tendency to laughter
  • Depressive state
  • Visual hallucination or disturbances
  • Perceptual narrowing
  • Less tolerance to stress
  • Exaggerated (oversized) handwriting
  • Amnesia
  • Loss of consciousness
  • Retardation of higher mental processes
  • Retardation of task performances
  • Slurred speech
  • Poor judgment
  • Slowed reaction time and reflex ability
  • Loss of mechanical dexterity

Underwater Awareness
Buddy teams need to be more aware of each other in deep dives.  Just as frequent scanning of instruments is mandated so is confirmation of your buddy’s status.  Generally, you should look for him about every three breaths and observe him for any overt signs of impairment.  Quick containment of a problem situation in its development is vital to prevent a stressful rescue event that may be difficult to perform at depth.

In 1972 I offered an effective underwater narcosis check between divers. We were frequently diving very deep with long working bottom times on this contract in the Virgin Islands.  I had a secret dread of one of our team’s divers being overcome without our immediate knowledge.  So I came up with a childishly simple hand signal response exercise for use at depth to detect narcosis.  If one diver flashed a one-finger signal to another diver, it was expected that the diver would answer with a two-finger signal.

A two-fingered signal was answered with three-fingers; if you really wanted to screw a guy up you gave him all five fingers and then he had to use two hands to come up with a six-finger response.  We reasoned that if a diver was not able to respond quickly and correctly to the signal given, then sufficient impairment was presumed to abort his dive.  It worked great for us then and I still use it today. Over the years, scores of divers have reported using the “Gilliam narcosis signals” (also known as “The Finger”) with success.

Although narcosis effects are generally eliminated by ascent, it is important to understand that many divers will experience some degree of amnesia of their performance at depth.  Commercial divers have reported successful completion of a work project to the diving supervisor upon ascent, only to learn later that the objective was not completed at all!  Less experienced deep divers will typically not remember their greatest depth or bottom time unless disciplined to record it on a slate prior to ascent.  Again, the experienced deep diver will sharply focus on his job objectives and constantly monitor his instruments.  Modern devices such as dive computers greatly improve safety controls with maximum depth and time memories as well as decompression planning models.

The Mount-Milner Test
In 1965 a research project was conducted by professional diver Tom Mount and psychiatrist Dr. Gilbert Milner to determine the effects of anticipated behavior modeling in diving students with respect to narcosis. Three control groups of four students with equal male/female ratios were trained in identical dive classes except:

Group One was taught that a diver will get narcosis at 130 fsw, and much emphasis was placed on the high probability of narcosis impairment with severe symptoms.

Group Two was taught of the existence of narcosis, the symptoms and depths of occurrence cited as beginning at 100 fsw, but were not as intimidated with narcosis manifestations.

Group Three was well educated on narcosis with three full hours of lecture on symptoms, risk, danger and known research. They were told that divers with strong will power as postulated by Miles (1961) could mentally prepare themselves and greatly reduce the effects.

Prior to the open water deep dives all students were given two dives to 30 fsw and two dives to 100 fsw to develop good breathing techniques.

Before the actual dives for testing purposes, the students were taken on a 50 fsw dive where the tests were performed so a mental/dexterity familiarity could be achieved with the format of the test problems. Changes were then made in the test so they could not be performed from memory. The tests consisted of handwriting evaluations, pegboard testing, math, and ball bearing placement in a long-necked narrow bottle etc.

In the initial test depth of 130 fsw, divers in Group One had minor-to-above-average narcosis problems while Group Two and three divers had little affect on test scores.

At the 180 fsw test depth, two Group One divers dropped from the exercise due to severe narcosis problems and were removed from the dive. All Group Two divers were affected although still functioning at about 50{c383baab7bef8067e8c9786a45d8006c492489841a98fe37723e304bb1ddd030} test levels. Group Three divers had minor impairment.

At the 200 fsw test depth, all divers in Group One and two from Group Two were dropped due to severe narcosis and apprehension. Group Three divers actually showed slight improvement in test scores.

At the 240 fsw test depth, one diver was dropped from Group Two and one from Group Three due to severe narcosis. The remaining Group Two diver and three Group Three divers showed levels of impairment but again scores and performance showed improvement over the previous depth level. One diver, a female from Group Three, registered her highest scores on all tests at the 240 fsw level.

Concurrent testing of experienced deep divers showed seven out of ten divers with no decrease in performance or scores at the 200 fsw test level. The three divers with decreased performance finished the testing (two with perfect scores) but required additional time than was usual. At 240 fsw, five out of ten performed all tests with no decreased performance. One diver had problems with the ball bearing test but perfect scores on the pegboard, math and handwriting. The other two showed up to 42{c383baab7bef8067e8c9786a45d8006c492489841a98fe37723e304bb1ddd030} deficits and had problems completing the tests.

The obvious conclusions include a subjective validation to both “adaptation” and the negative influence of “modeling” behavior in those groups of divers pre-conditioned that narcosis was inevitable and severe. The Group Three divers with little prior diving experience were satisfactorily still performing at the 200 fsw level and three divers continued to perform (with one showing improvement still) at the 240 fsw test level.

If we teach our children that all dogs will bite, we can safely assume that when presented with a specimen even as lowly as a toy poodle (which should probably be shot on sight anyway), we can expect a high fear index. Likewise, if we teach our dive students that narcosis is a finite, unyielding biophysical wall… then we can logically expect such conditioning to impair their performance beyond a more realistically educated diver lacking pre-conceived phobias and suggestions. Education is the key to performance and safety.

Depth limitation largely becomes a decision then based upon narcosis levels and gas supply (until the O2 toxicity range is entered).  Most divers will be able to function well in excess of the so-called 130 fsw (39.4 m) limit with even a little practice.

Interestingly, the first edition of the NOAA Diving Manual published in the mid-1970s contained this notation on narcosis:  “Experience, frequent exposure to deep diving, and a high degree of training may permit divers to dive on air as deep as 200 fsw (60.6 m) . . .” Although scientific diving programs and university based research groups generally advocated air diving to around this recommended limit, a significant proportion of dives were conducted in far deeper depths if necessary for observation or collection purposes including dives beyond 300 fsw. The proliferation of “Do as I say, not as I do” mentalities still dominate all factions of the industry primarily for fear of critical condemnation by less realistic “experts”.

All divers should exercise prudence and reasonable caution in all aspects of deep diving but particularly so when it comes to narcosis.  Experience is vital before attempting progressively deeper dives. Ideally, the diver should be seeking out the benefit of training by a competent, well-experienced deep diving instructor before a penetration below “entry level/open water” training diving depths.  Don’t try to obtain field experience on your own or with another buddy.  The historical record provides too many fatalities or near misses due to narcosis to warrant such a risk.

Many critics condemned even the discussion of practical operational narcosis planning and dismissed those of us who advocated more realistic guidelines as members of the “lunatic fringe”. Happily, most of that misguided ultra-conservatism has been withdrawn. I contend that by professionally addressing the questions of the real risks and real experiences associated with narcosis and deep diving, we will more responsibly serve today’s diver who, in many cases, is already undertaking dives beyond his ability, training and operational physiology because no proper advanced deep diver training is offered through the traditional national training agencies. Truth in education is critical to any learning process and especially with diving. Let’s not shy away from our responsibilities as diving educators by holding fast to the naive belief that all sport diving stops at 130 fsw. For many divers 130 fsw is a reasonable limit… but others will go deeper. They will be safer and more likely to observe a practical limit if we provide the training to better identify the real hazards and the required commitments to plan deeper diving.


Author Notes: Bret Gilliam has had a 40-year career in professional diving, logging over 18,000 dives in military, commercial, scientific, filming, and technical diving operations. He is one of the diving industry’s most successful entrepreneurs with investments in publishing, training agencies (TDI/SDI), manufacturing, resorts, dive vessels, cruise ships, and film production companies. The aggregate sale value of his multiple multi-national companies totaled over $80 million when he retired in 2005 at the age of 54.

Author of over nearly 1000 published articles, his photos have graced over 100 magazine covers, and he is principal author or contributor to over 50 books & manuals. His writing and photography has been published worldwide. He also has worked as location director, cameraman, and operations manager on scores of Hollywood movies, television series, documentaries (including National Geographic and the Cousteau series), and IMAX films.

He is a Fellow National of the elite Explorers Club and the ex-world record holder as the deepest scuba diver on conventional scuba equipment. He is also the recipient of numerous awards.

He continues a limited practice as a widely sought litigation consultant and expert witness for diving and maritime legal cases. After nearly 30 years living in the Caribbean and equatorial regions worldwide, he now lives in Maine where he divides his time between three homes and a motor yacht. He is still active in special film and publishing projects. His latest book, a large hardbound coffee table style volume, Diving Pioneers & Innovators: An In-Depth Series of Interviews, has been met with widespread enthusiasm by reviewers internationally.

 Bret Gilliam, President OCEAN TECH

2 Comments Leave A Reply

2 Responses to “Nitrogen Narcosis: A Critical Conversation”

  1. MJJ

    Interesting article. You mean to say “bradycardia,” not “brachycardia.”


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