How Deep is Too Deep?

How Deep is Too Deep?

By John Lippmann Executive Director DAN S.E. Asia-Pacific
Copyright: John Lippmann
During diver training, dive students are normally drilled to avoid diving beyond 130 feet / 39 meters. However this depth limit recommended by most of the training agencies is not forged in stone. Historically, it appears that it probably emerged from the U.S. Navy, possibly as a result of equipment limitations at that time, and the work restrictions imposed by the relatively short no-stop times available at greater depths.
An increasing number of divers dive beyond the 130-foot limit, some routinely and others occasionally. The advent of dive computers has negated much of the decompression penalty and dive restrictions previously associated with deep diving, and has no doubt encouraged the current trend. In addition, the increased availability of a variety of gas mixtures has enabled more divers to venture deeper and deeper.
Deep diving demands vast amounts of knowledge, experience and discipline, as well as appropriate preparation and equipment, since deep diving is fraught with potential hazards.
There appear to be some inescapable realities of deep diving. These include:
  • the increased potential for certain problems to occur;
  • if a problem does occur, the consequences are often more serious; and
  • the fact that the physiological effects of deep diving are still largely unknown.
An old friend of mine used to teach diving at a tropical resort. The instructors routinely dived on air to depths approaching 300 ft (90m) and beyond on their days off. During such a dive, one instructor became unconscious at about 200 ft (60m) without obvious warning. He fell away and out of reach of the others before anyone could get it together to do anything. His body was never recovered.
Elsewhere, another diver diving at just over 165 ft (50m) on air on a wreck was seen to become unconscious and to convulse. Luckily his buddies managed to rescue and resuscitate him.
These are not isolated stories, and there are many similar reports involving deep air dives and mixed gas dives.
Unconsciousness underwater is often associated with deep diving accident reports. It usually results in drowning. A number of conditions can cause a diver to lose consciousness underwater. Such conditions include, but are not confined to:
  • high blood carbon dioxide levels (hypercapnia);
  • oxygen toxicity;
  • nitrogen narcosis; and
  • decompression illness;
All of which are exacerbated by depth. Blackout underwater may not be due to a single cause, but may result from a combination of physiological or physical factors.
Nitrogen narcosis can become a very serious adversary on deep air dives. Although we can acclimatize ourselves to the affects of narcosis to some extent by regular exposure to depth, it can still sneak up and very quickly overcome us when we don’t expect it. Although conventional wisdom states that the narcosis appears on arrival at a particular depth and usually does not worsen with continued exposure at that particular depth, many divers are aware that it can quickly be precipitated by exertion or stress at depth, without further descent.
Divers who have had to quickly deal with a problem at 200 ft (60m) on air realize the extreme difficulty of reacting rapidly and appropriately. Sometimes the mind-numbing effects of narcosis can strike suddenly and make appropriate reactions almost impossible. Extremely high levels of stress can be precipitated instantaneously and, unless controlled, panic and injury are likely results. Narcosis may be the direct cause of unconsciousness in a diver at depths somewhere in excess of 200 ft. Narcosis can be reduced by using certain gas mixtures. However, this involves the appropriate equipment, preparation, training and care since new potential hazards are introduced.
Carbon dioxide acts as a respiratory stimulant and can cause depression of the central nervous system (CNS). The effect depends on the level of carbon dioxide in the blood. Deep diving produces elevated blood carbon dioxide levels for several reasons, which include:
  • the resistance to breathing caused by breathing denser gas through a regulator and against a higher ambient pressure;
  • reduced ventilation efficiency due to the denser breathing gas; and
  • reduced transport, and, hence, elimination of carbon dioxide.
Hypercapnia increases narcosis and the likelihood of CNS oxygen toxicity. In addition, it may increase heat loss, alter heart rhythm and predispose to decompression illness. If the carbon dioxide level gets too high, and it can on deep scuba dives -- especially if a diver is very anxious and / or exerting him/herself -- the diver may go unconscious without warning. Certain divers are more susceptible to severe hypercapnia for a variety of reasons and are therefore more at risk.
When divers breathe oxygen at partial pressures greater than about 1.5 atmospheres (ata), it may rapidly exert a toxic effect on the brain. A diver breathing air at a depth of around 200 ft is exposed to an oxygen partial pressure of 1.5 ata. CNS toxicity is a limiting factor and a very real danger in deep diving since it can cause a diver to convulse and/or become unconscious with little or no warning. The likelihood of CNS oxygen toxicity increases with exposure time, cold, exertion and hypercapnia, and the depth and time of onset can vary greatly between individuals and from dive to dive.
The high nitrogen load accumulated by the “fast” and “medium” body tissues during a deep air dive can cause substantial bubble formation during or after ascent unless the decompression is properly controlled and conducted. Some of these bubbles may form in or enter the arterial circulation and cause neurological problems. This mechanism may be responsible for some underwater blackouts during ascents from deep dives.
Various data indicate that deeper diving is associated with a substantially increased risk of decompression illness. This risk appears to increase at depths beyond about 80 ft (24m). In addition, using a dive computer to guide decompression from deep air dives appears to increase the risk further due to the greater dive times allowed and the increased unreliability of the algorithms at depth. More and more divers have adopted the use of various gas mixtures in the belief that it will reduce the risk of decompression illness. However, recompression centers still treat a significant number of these divers.
Certain studies suggest that microbubbles are often present after dives, particularly deep dives, especially if ascent has not been appropriately executed but even after what is generally considered to be a safe ascent. Some hyperbaric specialists fear that microbubbles, although asymptomatic, may cause cumulative neurological damage in divers. However, to date, the evidence does not appear to be consistent.
Unless adequately prepared for, deep diving carries a higher likelihood of an air supply emergency. Increased ambient pressure means increased air consumption. In addition, narcosis may hinder a diver’s ability to properly monitor and manage the air supply. Despite the improvements and superior performance of much of the modern diving equipment, malfunctions do occur. The deep divers who value their hides ensure that they have adequate backups of various essential pieces of equipment, including an independent and adequate air supply.
Buoyancy compensation can sometimes become a critical factor on deep dives, especially in cold water where greater insulation is required. Unless compression of the exposure suit is adequately compensated for by BC or dry suit inflation, a diver may become very negatively buoyant at depth.
Wreck divers may sometimes prefer to be negatively buoyant, but problems can develop if the air supply is low and the diver needs to ascend fairly quickly.
Various experiments have demonstrated that, at low cylinder pressures, it is sometimes impossible to inflate a BC (or dry suit) at depths approaching 130 ft, especially while breathing simultaneously from the regulator. This problem would be magnified at greater depths. At times, a negatively buoyant diver who is low on air may find it difficult, or even impossible, to ascend without ditching their weight belt. If the weight belt is ditched, it is unlikely the diver will make it to the decompression line to get some extra air and perform any necessary stops.
Some divers routinely dive to depths in excess of 165 feet/50 meters on air, although over recent years gas mixtures such as heliox and trimix have become far more commonly used for very deep diving as they are less narcotic. These divers are often, but not always, conversant with the substantial risks and demands of these dives and choose to push the limits for their own reasons. Such divers are usually well equipped and well prepared for the dives. Most usually manage to get away with diving to these depths with no apparent problems, others do not. Some of the unfortunate ones are left with permanent disability; others die.
On the other hand, there is the “occasional” deep diver. These divers are generally less experienced than regular deep divers, are on a dive trip with a group, and are drawn into diving deeper than they normally do because of the more relaxed holiday atmosphere and because “everyone’s doing it.” Such divers are often not sufficiently trained, mentally prepared and appropriately equipped to deal with a problem should it occur on a deep dive.
It becomes obvious that there is no safe depth limit that applies to all divers all of the time. A diver’s ability to cope with depth depends on a number of highly variable factors. The depth of the onset of the effects of the exotic cocktail of elevated pressures of nitrogen, carbon dioxide and oxygen, coupled with the sensory deprivation and stress associated with diving, are not always predictable. A dive to 80 feet in cold, dirty water can be far more hazardous than a dive to twice the depth in warm, clear waters. Factors such as visibility, water temperature and diver experience and preparedness greatly affect a diver’s comfort and safety, rather than depth alone.
Divers in remote locations must also be aware of the complications associated with medical evacuation. These can include significant delays in retrieval due to lack of current availability of an aircraft and and/or medical team, the distances involved, as well as the accessibility of some airstrips in darkness or adverse weather conditions. Such delays can impact the amount and the effectiveness of the subsequent recompression treatment, and the likelihood of residual injury.
In addition, once a diver has been evacuated and/or treated for DCI, they will be advised to avoid air travel or driving to altitude for between three days and six weeks post treatment to avoid recurrence of symptoms. This can certainly impinge upon the diver’s travel and work commitments.
As with many things in life, one must balance the risks against the benefits and make a decision. However, it is essential to have a real understanding and appreciation of the risks.
Other General Articles
The Ups and Downs of Buoyancy Control
How Deep is Too Deep?
Motion Sickness - Updated 2003
Your DAN Card: Why It's One of Your Most Valuable Pieces of Dive Gear
Guide to Health & Fitness in Scuba Diving
DAN’s Diving Tips for the New Diver
Scanning for Blebs
Diving & The Body Systems
Motion Sickness
Transderm Scop (The Patch)

DAN'S Motion Sickness

Motion Sickness

Some salty & sage advice on an age-old problem
By G. Yancey Mebane, M.D.
My experience with seasickness is that at first you are afraid you will die, then after a few hours you are afraid you will not.
Seasickness, or motion sickness, ruins diving trips, vacations and travel for many. Everyone is susceptible, and motion sickness can be produced in anyone if the circumstances are right. A lot is known about motion sickness, but total understanding of the cause is not clear. There are individuals who are resistant to motion sickness, but sufficient angular acceleration will induce motion sickness in anyone.
Even astronauts are annoyed by this problem. Approximately 70 percent of all crew members experience motion sickness of some degree during the first 72 hours of orbital flight on the space shuttle.
Cause If you have experienced motion sickness, you probably think of it as primarily nausea. One theory says that this symptom is the result of your brain's inability to resolve the conflicting signals that it is receiving from the ears, eyes and body.
The vestibular balance apparatus of the ears detects motion and is stimulated by the repeated angular acceleration of the dive boat. If you are in a compartment or have lost visual contact with the horizon, your eyes signal the brain that there is no motion. The sensors of body position are sending still another signal, and your brain is unable to resolve the conflict.
Anxiety, confusion and dismay result, leading to the first symptoms of yawning, pallor (paleness) and headache. They are followed by nausea and vomiting, and frequently a "fear" response. That is the time you are afraid you will not die.
There is more to the cause than mismatching of sensory inputs. Other hypotheses under intense study include the role of Coriolis forces (forces due to the earth's rotation), other nonphysiological stimuli, the cerebrospinal fluid and the cerebellum.
A ship moves in a complex fashion depending on the size and construction of the vessel and the condition of the sea it is sailing. Among the hundreds of research studies on the cause of motion sickness, an interesting study from 1988 reports on sophisticated measurements of vessel motion and consequent seasickness among passengers on six ships, two hovercraft and a hydrofoil. This study showed that the occurrence of motion sickness was closely related to the magnitude of the vertical acceleration experienced. There was low correlation between roll and pitch acceleration magnitude and vomiting.
This information won't cure seasickness, but it does tell us to find the part of the vessel with the least vertical acceleration and stay there. Usually that will be in the center of the vessel, and we want to stay as low as possible while maintaining eye contact slightly above the horizon. If visual contact is not possible, keep your eyes closed. It is prudent to stay away from individuals who are actively ill, though psychological support and reassurance from companions are helpful for the individual and the group.
Have you ever advised a seasick diver: "Get in the water - you'll feel better"? That may not be good advice. Motion sickness underwater occurs for the same reason as above water. When underwater, spatial disorientation occurs because of the interference with the normal clues. Poor visibility and the visual field restrictions imposed by the mask distort or eliminate visual clues. Neutral buoyancy distorts the clues provided by gravity. Motion from surge which may be encountered during entry causes potent acceleration forces. The brain is unable to reconcile the abnormal sensory input, and motion sickness develops. Anxiety of some degree is inevitable no matter how laid-back the individual, and a panic reaction can easily occur.
Vomiting underwater is not easy. Do you vomit through your regulator or take your regulator out of your mouth? There are valid arguments for both techniques, and I have seen both done successfully. There is no doubt that safety is seriously impaired under either condition.
As an experienced diver, you will be able to recognize clues available during a dive which provide spatial orientation. It is important to enter and exit along a line if visibility is poor and the bottom cannot be seen. Gravitation pull on weight belts provides the "down" sensation, while buoyancy effects will cause the chest to rise. The feet will tend to sink when not swimming. Bubbles, of course, rise. An inexperienced diver may not respond to these clues, especially in a panic situation.
And what about the reverse: sickness on land? It does happen. After you have finished that 10-day "trip of a lifetime" aboard a liveaboard and have stepped onto solid ground, you may suddenly feel funny and maybe even sick. What happened? "Land sickness," or mal de débarquement, occurs when you return to dry land after becoming adapted to an environment in constant motion. Your brain has become accustomed to the new input from increased motion. Suddenly, the motion stops. The abrupt change will promptly produce the same symptoms as originally felt upon going to sea.
Motion is most of the story, but not all. Emotional factors (fear, anxiety, fatigue) act in concert with motion to precipitate an attack. Alcoholic or dietary excesses before or during the trip increase the likelihood of motion sickness. Jet lag, which results from rapid transition of time zones, places you out of synchrony with the local social and time cues, producing fatigue, loss of appetite, gastrointestinal duress and other symptoms. If you feel that way before the dive boat leaves the dock, guess what's going to happen on the way to the dive site!
PREVENTION Now that you know that we don't fully understand the causes of motion sickness, you may not be surprised that we also don't really know how to prevent it. There are literally hundreds of gadgets, procedures, medicines, herbs, foods, etc., all touted as good for motion sickness - that in itself should tell you that none of these choices are completely effective. Perhaps you have already discovered a system that works for you. If so, congratulations. Be sure that your system is safe and stay with it.
MEDICATIONS The use of medications to prevent motion sickness may be helpful, but none of the medications are free of side effects. As most of the side effects affect performance, there are serious questions concerning their use by divers - who must be alert at all times. You must be cautious in their use, and your best plan is to avoid them entirely. If you choose a medication, give it a trial many days before diving in order to determine the response and side effects for you.
Antihistamines The most commonly used medications are antihistamines, available without a prescription, and similar in their side effect profile. The medications include Dramamine® (dimenhydrinate), Bonine® (meclizine), Benadryl® (diphenhydramine) and Marezine® (cyclizine). The common feature of this group is drowsiness, which could seriously impair a diver's ability to perform safely. There are other side effects - you should study all the information which comes with the medication before using it.
Phenergan® (promethazine) is a prescription drug chemically related to the tranquilizers, and it also has antihistamine properties. Drowsiness is a prominent side effect, and it can be used as a sedative-hypnotic. The drug may impair your mental and physical abilities required to perform potentially hazardous tasks. Alcohol and similar drugs accentuate the sedative effects of promethazine. Intramuscular injection of this drug can provide great relief for severely motion-sick individuals. Of course, diving would be out of the question if intramuscular injection is needed.
Other Medications Scopolamine-dextroamphetamine (a combination of 0.4 milligrams oral scopolamine and 5.0 milligrams oral dextroamphetamine) has been studied for use in the space program. These are very potent medications and are useful in situations for individuals performing complex tasks while being closely monitored. A recreational diver will have some difficulty in obtaining these drugs, as dextroamphetamine is a Schedule II controlled substance prescription drug and the combination has not been approved by the Food and Drug Administration (FDA) as indicated for motion sickness. A physician prescribing this combination for motion sickness will be outside the FDA indications.
Trans-Derm SCOP® (scopolamine patch) is used for motion sickness and has been used by many divers who found it beneficial and reported few problems. Trans-Derm Scop does have some unwanted side effects which affect diving adversely.
Dry mouth occurs in about half of the users studied (non-divers) and is probably more prevalent in divers due to the dry air in scuba cylinders. Blurred vision after about 24 hours' use is common and may persist after the patch is removed. Repeated applications will cause visual disturbance to increase. If your finger contacts the medication side of the patch and then your eye, the pupil will dilate. Wash your hands thoroughly after handling the patch.
Trans-Derm SCOP® occasionally causes hallucinations, confusion, agitation or disorientation. These effects are more common in children and the elderly. Therefore, children under 10 should not use the patch. The dose is fixed and cannot be altered by cutting the patch, which also disrupts the rate-limiting membrane delivering the medication. The package insert contains the following precautions: "Since drowsiness, disorientation and confusion may occur with the use of scopolamine, be careful when engaging in activities that require mental alertness, such as driving a motor vehicle or operating dangerous machinery, especially when you first start using the drug system." Studies indicate that the patch is slightly more effective than Dramamine®.
Dilantin® (phenytoin) has been shown to protect against motion sickness in several studies. However, the medication is an antiepileptic drug and has not been approved for use in the treatment of motion sickness. It is a fairly safe drug, but not free of side effects and adverse reactions. There has been one study on divers in chamber tests at 460 kilopascals (approximately 150 feet/ 46 meters of seawater) which did not reveal any change in susceptibility to nitrogen narcosis.
Royal Made Ping an Dan (PAD) is a royal clandestine prescription of the Qing Dynasty Imperial hospital for emperors, empresses, ministers, imperial maids and eunuchs. Experimental study has confirmed that it is effective on motion sickness. There is no information on composition or drug safety for this preparation.
NONPHARMACOLOGICAL INTERVENTION There are many devices, herbs and procedures which are advocated for prevention and treatment of seasickness. The efficacy of both pharmacologic and nonpharmacological agents is difficult to determine in this condition, which has such a powerful emotional component. The placebo effect is very strong here. It is also complicated to determine if the agent used to prevent an event was effective or perhaps the event was not destined to occur. For instance, I keep a charmed shark tooth in my office to prevent shark attack. There hasn't been a shark in my ninth-floor office since I got that tooth - that's 100-percent effectiveness.
There are enthusiastic advocates of ginger for prevention of seasickness, but its efficacy has not been substantiated in controlled laboratory trials.
Seabands® (elasticized wristband devices) are sold as a means of treating seasickness. There is a stud incorporated into the device which applies pressure over the Neiguan point (located within tissue about 3 centimeters above the wrist joint). The Neiguan point is reported as being implicated in the control of nausea and vomiting, although the results of acupuncture applied to this point are contradictory. Seabands have been available for several years, although no controlled trials demonstrating their effectiveness have been published.
FINALLY... So, how do you reduce your risk or susceptibility to motion sickness? First, you should be adequately rested, nourished and hydrated. If you are apprehensive, avoid placing anything at all in your stomach during the two hours more or less before you embark. You will be more comfortable with an empty stomach than with a full one. Adequate rest and hydration means that you have essentially recovered from jet lag, excessive food, or alcohol, and have satisfied your usual requirement for sleep.
After boarding, prepare your gear for diving before the boat reaches open water so that you can avoid working on diving equipment while looking down. Find a place on the boat where the motion is least and stay low. Avoid the bow, flying bridge or upper decks where the motion is intensified. The motion at the stern is not unpleasant, but exhaust fumes may be present. Maintain eye contact with the horizon or slightly above.
If you are using a medication or device to prevent sickness, have faith and it will probably work. Remember that all medications have side effects, and you should have tried the one you will be using long before exposing yourself to motion. The nonpharmacological agents are usually harmless, but you must be certain about your choices.
Adaptation to motion does occur with most individuals, so that motion sickness frequently ceases after a few hours. Motivation and willpower are important, as are sounds, sights and smells; individual tolerance to motion is also a factor. Seasickness is an unpleasant acquaintance, testified to by armadas of past, present and future sufferers.
(c) March/April 1995 Alert Diver. G. Yancey Mebane, M.D., was the DAN Associate Medical Director & Director of EMS Training at the time of this article.