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How Deep is Too Deep?

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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:

  1. the resistance to breathing caused by breathing denser gas through a regulator and against a higher ambient pressure;
  2. reduced ventilation efficiency due to the denser breathing gas; and
  3. 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.