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An ascent to the surface by a diver in an emergency From Wikipedia, the free encyclopedia
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 reaching the surface in the event of an out-of-gas emergency, generally while scuba diving.
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Emergency ascents may be broadly categorised as independent ascents, where the diver is alone and manages the ascent by themself, 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.
An emergency ascent may be made for any one of several reasons, including failure or imminent failure of the breathing gas supply.
An emergency ascent implies that the dive plan has been abandoned due to circumstances beyond the control of the diver, though they may have been caused by the diver, as is often the case in out-of gas emergencies in scuba diving.[1] Out of gas emergencies are generally the most urgent contingencies in diving, as the available time to deal with the emergency can be measured in minutes or seconds, while most other non-traumatic emergencies allow more time. Other reasons for emergency ascent may include:
The terminology is diverse, and not always used consistently.
Emergency ascents where no assistance from another diver is given.
Ascent in an emergency with assistance provided by another diver.
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[3]
Emergency ascent training policy differs considerably among the certification agencies, and has been the subject of some controversy regarding risk-benefit.
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.[3]
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.[3]
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.[3]
Recommendations for training:[3]
Recommendations for choice of procedure:[3]
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.[3]
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.[10]
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.[10]
In 1978 the SSAC recommended responses to an air supply failure, in order of preference, were:[10]
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.
Use of a bailout cylinder is the primary source of emergency breathing gas recommended by several codes of practice for scientific and commercial divers. Pneumo gas supplied either from the diver's own pneumofathometer line or from the standby diver's pneumo line in a rescue are also recognised emergency gas sources for surface-supplied divers, and can be used during an emergency ascent.[11][12]
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.[3]
An alternative emergency breathing air source may be available via the buoyancy compensator. There are two possibilities for this:
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 gas requires inflation gas to be available, so may not be possible in an out-of-gas emergency. Buoyancy can be reduced 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 is a technique used by scuba divers as an emergency procedure when a diver has run out of breathing gas in shallow water and must return to the surface.[3] During the ascent, the diver propels themself 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.[5]
The technique involves simply ascending at a controlled pace, typically about 18 metres (60 feet) per minute, while exhaling slowly. As the diver ascends, the air in the lungs expands as surrounding water pressure decreases. Exhaling allows excess volume to escape from the lungs, and by exhaling at a suitable rate the diver can continue exhaling throughout the ascent and still have air in their lungs at the surface. If the diver fails to exhale during the ascent, lung over-expansion injury is likely to occur. If exhalation is limited to relaxing and allowing the expanding gas to escape without effort, there should not be a feeling of running out of breath, as the air inhaled at depth expands during the ascent and the lung volume should remain nearly constant.[5]
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.[5]
Advantages of this method, when applicable, are that no outside assistance or special equipment is required. Disadvantages are that it requires the diver to reach the surface in a limited time, which does not allow for staged decompression, possible delays due to entanglement or snags, or long distances to reach the surface. It also requires the diver to produce propulsive effort, which reduces potential endurance on the single breath or limited gas available.
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. A large cylinder may provide several additional breaths during ascent if the regulator is functioning correctly. On a 30 m ascent, a 12 litre cylinder will provide 36 litres of additional free air, distributed at ambient pressure in proportion to the change in ambient pressure.
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.
While in a practical sense there is little difference between a CESA and a "free ascent" (aka Emergency Swimming Ascent or ESA), the technical difference between the two is that in a CESA the regulator second stage is retained in the mouth and the diver exhales through it (in case gas becomes available due to the drop in ambient pressure) while in free ascent, the regulator is not retained or there is no regulator available, and the diver exhales directly into the water.[6]
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. In most circumstances analysis of the risk would indicate that the divers should have an alternative breathing gas source in preference to relying on buddy breathing. Failure to provide alternative breathing gas without good reason would probably be considered negligent in professional diving.[13][9][14]
Also known as octopus assisted ascent, assisted ascent is an emergency 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 patterns are not constrained by each other, and they may breathe simultaneously. Task loading is reduced in comparison with buddy breathing, and the divers can concentrate on controlling the ascent. If the gas is supplied from an independent cylinder, the cylinder can be handed off to the out-of-gas diver, if there is a good reason to do so and this does not adversely affect buoyancy control and trim of either diver.[9][14]
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.[15]
The controlled buoyant lift is an underwater diver rescue technique used by scuba divers to safely raise an incapacitated diver to the surface from depth. It is the primary technique for rescuing an unconscious diver from the bottom. It can also be used where the distressed diver has lost or damaged their diving mask and cannot safely ascend without help, though in this case the assisted diver would normally be able to control their own buoyancy.[citation needed]
The standard PADI-trained technique is for the rescuer to approach the face-down unconscious diver (victim) from above and kneel with one knee either side of their diving cylinder. Then, with the victim's diving regulator held in place,[16] the tank is gripped firmly between the knees and the rescuer's buoyancy compensator is used to control a slow ascent to the surface. This method may not work with sidemount or twin cylinder sets, and puts both rescuer and victim at increased risk if the rescuer loses grip, as the victim will sink and the rescuer may make an excessively fast uncontrolled ascent.
In the technique taught by BSAC and some other agencies, the rescuer faces the casualty and uses the casualty's buoyancy compensator to provide buoyancy for both divers as the rescuer makes a controlled ascent. If the casualty is not breathing, the ascent will be urgent.[16] If the two divers separate during the ascent, the use of the casualty's buoyancy is intended as a failsafe causing the casualty to continue to the surface where there is air and other rescuers can help. The rescuer will be negative at this point, but this is generally easily compensated by finning and corrected by inflation of the rescuer's BC.
Ascent controlled by a line attached to the diver and to a fixed point at the bottom, with the line paid out by the diver to control depth and rate of ascent when the diver has inadvertently lost full control of buoyancy due to loss of ballast weight, so cannot attain neutral buoyancy at some point during the ascent, and needs to do decompression. CMAS require this skill for their Self-Rescue Diver certification, using a ratchet reel to control the line, though other methods may be feasible. The diver must ensure that gas can be released from the buoyancy compensator and dry suit, if applicable, throughout the ascent, to avoid aggravating the problem by trapped gas expansion. This basically requires the diver to ascend with the feet down and dump valves up, an orientation which can be achieved by hooking a leg around the line. Clipping the reel to the harness should prevent accidentally losing the reel during the ascent. Depending on how the line is attached at the bottom, it may be necessary to cut loose and abandon the line after surfacing.[17]
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.[12]
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.[18]
In the event that a wet bell or stage cannot be recovered from a dive on schedule, it may be necessary for the divers to abandon it and make an autonomous ascent. This may be complicated by decompression obligations or compromised breathing gas supply, and may involve the assistance of a surface standby diver. The procedure depends on whether the divers' breathing gas is supplied directly from the surface (type 1 wet bell) or is supplied from a gas panel in the bell, via the bell umbilical (type 2 wet bell).[19]
To abandon a type 1 wet bell or stage, the divers simply exit the bell on the side that the umbilicals enter, ensuring that they are not looped around anything. This is reliably done by having the surface tender take up slack while returning to the bell and following the umbilical out the other side, after which the tender can simply raise the diver as if there were no bell.[19]
On a type 2 bell, the divers' umbilicals are connected to the gas panel in the bell, and the procedure used should minimise the risk of the umbilical snagging during the ascent and forcing the diver to descend again to free it. If the diver excursion umbilical is not long enough to allow the diver to reach the surface, the standby diver will have to disconnect the bell diver's umbilical, and the rest of the ascent may be done on bailout, pneumo supply from the standby diver, or the standby diver can connect a replacement umbilical.[19]
The only viable form of emergency ascent by a saturation diver is inside a closed and pressurised bell. This can be in the form of an emergency recovery of the original bell, or by through water transfer to another bell at depth. A form of unassisted emergency ascent for a bell with functioning lock and external ballast, is to release the ballast from inside the sealed bell, allowing inherent buoyancy to lift the bell to the surface.[20][21]
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[22] 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,[22] 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 an emergency ascent. If the diver has healthy lungs and the airway remains open throughout the ascent, rate of ascent does not significantly affect risk of lung barotrauma, but it does affect risk of decompression sickness.[4]
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 their buoyancy is close to neutral at the bottom.[citation needed]
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 death by 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.
In freediving the usual emergency ascent involves ditching the diver's weightbelt to increase buoyancy and reduce the effort required. This generally establishes positive buoyancy and gives the diver a chance of not drowning if they lose consciousness before reaching the surface and are assisted by another diver, or are lucky enough to float face upwards and draw a breath.
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