Emergency ascent

Alabama National Guard divers performing a controlled ascent during a training exercise

An emergency ascent is an ascent to the surface by a diver in an emergency. More specifically it refers to any of several procedures for getting to the surface in the event of an out-of-air emergency, generally while scuba diving.

Emergency ascents may be broadly categorised as independent ascents, where the diver is alone and manages the ascent by him/herself, and dependent ascents, where the diver is assisted by another diver, who generally provides breathing gas, but may also provide transportation or other assistance. The extreme case of a dependent ascent is underwater rescue or recovery of an unconscious or unresponsive diver, but this is more usually referred to as diver rescue, and emergency ascent is usually used for cases where the distressed diver is at least partially able to contribute to the management of the ascent.

An emergency ascent usually implies that the diver initiated the ascent voluntarily, and made the choice of the procedure. Ascents that are involuntary or get out of control unintentionally are more accurately classed as accidents.

Terminology for emergency ascents

Independent action

(no assistance required from another diver)

Dependent action

(assistance provided by another diver)

Training policies of various certification agencies

Few issues of diver training have been more controversial than the teaching of emergency ascent procedures. The controversy centers on techniques, psychological and physiological considerations, concern about today's legal climate, and finally the moral issue: is it wise and ethical to train divers in emergency ascent techniques, even though this training may itself be hazardous?
Ronald C. Samson & James W. Miller, 1977[1]

Emergency ascent training policy differs considerably among the certification agencies, and has been the subject of some controversy regarding risk-benefit.

NSTC agreement

In 1977 a formal policy regarding training of emergency ascent procedures was adopted by five major American recreational diver certification agencies: NASDS, NAUI, PADI, SSI and YMCA.[1]

This policy is a general agreement that emergency ascent training is worth the risk on ethical grounds, and recommends those procedures which the agencies consider most appropriate for teaching recreational divers. It does not prescribe training procedures or standards.

This National Scuba Training Committee Ascent Training Agreement recognises that there are a number of options available to the scuba diver in the event of a sudden apparent termination of breathing gas supply at depth, and that the selection of an acceptable response is dependent on several variables, including: depth, visibility, distance from other divers, the nature of the underwater activity, available breath-hold time, training and current competence of the involved divers, stress levels of the divers, obstructions to a direct access to the surface, water movement, equipment, buoyancy, familiarity between divers of procedures and equipment, apparent reasons for air loss and decompression obligations.

Recommendations for training:

Recommendations for choice of procedure:

No other procedures are recommended in this agreement, though the use of a bailout cylinder may be considered effectively equivalent to either octopus assisted ascent, when gas is supplied by a donor, or not actually running out of gas if it is the diver's own bailout set.[1]

Use of a bailout cylinder is the primary source of emergency breathing gas recommended by several codes of practice for scientific and commercial divers.[3]

SSAC

The Scottish Sub-Aqua Club holds that training is primarily to deal with potential emergencies and that it should be practical rather than purely theoretical. This implies that it is better to have some practical experience of ability to cope with a simulated emergency situation as this gives greater insight and confidence, as well as proven ability, provided that the risk in training is appreciably smaller than the risk in not being trained.

The SSAC trains open water free ascent from a maximum depth of 6–7 m, initially using a shot line to control ascent rate, and considers the risk small and the benefit significant in view of their statistics which showed an incidence of roughly 16 free ascents per 10,000 dives.

They (1978) recommend responses to an air supply failure, in order of preference, as:[4]

CMAS

The only reference to emergency ascent training in the CMAS Diver Training Program (CMAS TC Version 9/2002) is in the 1-star course where Controlled buoyancy lift of victim to surface is specified under practical training of rescue skills.

Choice of procedure

When there is no physical or physiological constraint (such as excessive depth, a physical overhead or a decompression obligation) preventing a direct ascent to the surface, an unassisted emergency ascent may be the lowest risk option, as it eliminates the unknowns associated with finding and requesting aid from another diver. These unknowns may be minimised by training, practice, prior agreement, and adherence to suitable protocols regarding equipment, planning, dive procedures and communication.[1]

Scuba procedures

Ascent while breathing from the buoyancy compensator

An alternative emergency breathing air source may be available via the buoyancy compensator. There are two possibilities for this:

  1. If the buoyancy compensator has an inflation gas supply from an independent, dedicated cylinder, this gas can be breathed by the diver by using the inflation valves and the oral inflation mouthpiece. BC inflation cylinders are neither common, nor usually very large, so the amount of air will be small and generally insufficient for staged decompression, but a few breaths on the way up can make a big difference to the stress level of the diver, and may prevent loss of consciousness.
  2. If the buoyancy compensator is supplied from the breathing gas cylinder, the volume available will be extremely limited, but it will expand during ascent, and instead of dumping it to reduce excess buoyancy, it may be breathed by the diver. Anyone who considers this as an option should ensure that the interior of the BC is decontaminated before use, as it is an environment in which pathogens may breed.

Buoyant ascent

Ascent where the diver is propelled towards the surface by positive buoyancy. Generally recommended as a last resort, though a sufficiently skilled diver could control ascent rate by precise dumping from the BC and use this as a low energy alternative to a swimming ascent. In this case weights should not be ditched during the ascent.

Positive buoyancy may be established by inflation of the BC or dry suit, or by ditching weights. Buoyancy from added air can be controlled during ascent by dumping, but the effect of ditched weights is not reversible, and usually increases as the surface is approached, particularly if a thick wetsuit is worn. If weight can be ditched partially, this may be a better option, unless the diver feels that he is about to lose consciousness, in which case a substantial increase in buoyancy may be better.

A method of buoyancy control which will automatically jettison weights if the diver loses consciousness during the ascent is to take them off and hold them in a hand while surfacing. If the diver loses consciousness, the weights will drop and positive buoyancy will take the diver the rest of the way to the surface.

Controlled emergency swimming ascent (CESA)

Ascent during which the diver propels him/herself towards the surface at a safe ascent rate by means of swimming, usually finning, with continuous exhalation at a rate unlikely to cause injury to the diver by lung overexpansion, and remains under control.

This procedure is recommended for ascents where there is no decompression obligation, a free surface with little risk of entanglement, and the diver has sufficient breath hold capacity to easily reach the surface conscious.

Use of the continuous exhalation procedure from moderately (neutrally or relaxed) inflated lungs combines the advantages of lower risk of lung injury compared to either full or empty lungs with improved endurance due to more available oxygen. Keeping the DV in the mouth and attempting to breathe normally or slowly from it may provide additional breaths as the ambient pressure reduces, and helps ensure that the airways remain open.

If the diver is neutrally buoyant at the time that the ascent is initiated, the amount of energy required to reach the surface will be minimised, and frequent controlled venting of the buoyancy compensator can keep the ascent rate under fine control.

Buddy breathing ascent

Ascent during which the diver is provided with breathing gas from the same demand valve (second stage regulator) as the donor, and they breathe alternately.

The out-of air diver must attract the attention of a nearby diver and request to share air. If the chosen donor has sufficient gas, and is competent to share by this method, an emergency ascent may be accomplished safely. Accurate buoyancy control is still required, and the stress of controlling the ascent rate and maintaining the breathing procedure can be more than some divers can handle. There have been occurrences of uncontrolled ascent and panic, in some cases with fatal consequences to both divers.

This procedure is best suited to divers who are well acquainted with each other, well practiced in the procedure, and highly competent in buoyancy control and ascent rate control.

Octopus assisted ascent

(Or just assisted ascent)

Ascent during which the diver is provided with breathing gas by another diver via a demand valve other than the one in use by the donor during the ascent. This may be supplied from the same or a different cylinder, and from the same or a separate 1st stage regulator. The divers' breathing pattern is not constrained by each other, and they may breathe simultaneously.

Lifeline assisted ascent

An ascent where the diver is pulled to the surface by the line tender, either as a response to an emergency signal from the diver, or a failure to respond to signals from the surface. A diver may also be assisted in the ascent by the line tender in a normal ascent, particularly divers in standard dress, where it was often the normal operating procedure.

Surface supplied procedures

Ascent on bailout gas

The diver opens the bailout valve on the helmet, bandmask or harness mounted bailout block. This opens the supply of breathing gas from the bailout cylinder carried by the diver to the demand valve of the breathing apparatus. The bailout gas volume carried by the diver is usually required to be sufficient to return to a place of safety where more gas is available, such as the surface, diving stage or wet or dry bell.

Ascent on pneumo air

Another option for the surface supplied diver is to breathe air supplied through the pneumofathometer hose of the umbilical. The diver inserts the hose into the air space of the helmet of full face mask, and the panel operator opens the supply valve sufficiently to provide enough air to breathe on free flow. Pneumo air can be supplied to another diver by a rescuer in the surface supply equivalent of Octopus air sharing. This procedure would save the bailout gas which would then be available if the situation deteriorates further. Pneumo breathing air supply is not applicable to environmentally sealed suits for contaminated environments.

Hazards

Lung overpressure accidents

The most direct and well publicised hazard is lung overpressure due to either a failure on the part of the diver to allow the expanding air in the lungs to escape harmlessly, or entrapment of air due to circumstances beyond the control of the diver. Lung overpressure can lead to fatal or disabling injury, and can occur during training exercises, even when reasonable precautions have been taken. There is some evidence[5] that a full exhalation at the start of the ascent in the "blow and go" scenario, can lead to partial collapse of some of the smaller air passages, and that these can then trap air during the ascent sufficiently to cause tissue rupture and air embolism. The procedure of slowly letting the air escape during ascent can also be taken too far, and not allow the air to escape fast enough,[5] with similar consequences. attempting to breathe off the empty cylinder is one way of potentially avoiding these problems, as this has the double advantage of keeping the airways open more reliably, and in most cases allowing the diver several more breaths during the ascent as the reduced ambient pressure allows more of the residual cylinder air to pass through the regulator and become available to the diver. A 10-litre cylinder ascending 10 metres will produce an extra 10 litres of free air (reduced to atmospheric pressure). At a tidal volume of about 1 litre this would give several breaths during ascent, with increased effectiveness nearer the surface. Of course this air is not available in some cases, such as a rolled off cylinder valve, burst hose, blown o-ring, or lost second stage, where the failure is not simply breathing all the air down to the pressure where the regulator stops delivering, but if it is possible, the demand valve can be kept in the mouth and the diver can continue to attempt to breathe from it during a free ascent.

Loss of consciousness due to hypoxia

One of the dangers of a free ascent is hypoxia due to using up the available oxygen during the ascent. This can be aggravated if the diver fully exhales at the start of the ascent in the "blow and go" technique, if the diver is so heavy that swimming upwards requires strong exertion, or if the diver is already stressed and short of breath when the air supply is lost. Loss of consciousness during ascent is likely to lead to drowning, particularly if the unconscious diver is negatively buoyant at that point and sinks. On the other hand, a fit diver leaving the bottom with a moderate lungful of air, relatively unstressed, and not overexerted, will usually have sufficient oxygen available to reach the surface conscious by direct swimming ascent with constant exhalation at a reasonable rate of between 9 and 18 metres per minute from recreational diving depths (30 m or less), provided his/her buoyancy is close to neutral at the bottom.

Decompression sickness

The risk of decompression sickness during an emergency ascent is probably no greater than the risk during a normal ascent at the same ascent rate after the same dive profile. In effect, the same ascent rate and decompression profile should be applied in an emergency ascent as in a normal ascent, and if there is a decompression requirement in the planned dive, steps should be taken to mitigate the risk if having to make an ascent without stops. The most straightforward and obviously effective method is for the diver to carry a bailout set sufficient to allow the planned ascent profile if the primary gas supply fails. This makes each diver independent on the availability of air from a buddy, but may cause extra task loading and physical loading of the diver due to the extra equipment needed. This method is extensively used by commercial and scientific divers, solo recreational divers, and some technical and recreational divers who prefer self-reliance. When all else fails, the consequences of missing some decompression time are usually less severe than drowning.

Drowning

Drowning is the most likely consequence of a failure to reach the surface during an independent emergency ascent, and is a significant risk even if the diver reaches the surface if he or she loses consciousness on the way.

Mitigation of hazards

References

  1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Samson RL, Miller JW (eds.) (1979). "Emergency Ascent Training.". 15th Undersea and Hyperbaric Medical Society Workshop. UHMS Publication Number 32WS(EAT)10-31-79. Retrieved 2008-08-07.
  2. ↑ Verdier C and Lee DA. 2008. Motor skills learning and current bailout procedures in recreational rebreather diving. In: Verdier (ed). Nitrox Rebreather Diving. DIRrebreather publishing. ISBN 978-2-9530935-0-6; http://archive.rubicon-foundation.org/7282: retrieved 2012-03-03
  3. 1 2 Department of Labour Diving Advisory Board, South African Department of Labour Code of Practice for Scientific Diving
  4. ↑
  5. 1 2 Brown, Charles V. (1979). Samson RL, Miller JW, eds. "Emergency Ascent Training.". 15th Undersea and Hyperbaric Medical Society Workshop. UHMS Publication Number 32WS(EAT)10-31-79: 42. Retrieved 2008-08-07.
  6. 1 2 Jablonski, J. 2006, Doing it Right: The Fundamentals of Better Diving, Global Underwater Explorers. ISBN 0-9713267-0-3
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