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Hypothermia Risk in Open Water Swimmers

Open Water Swimming presents additional challenges to athletes due to the environment, even for those who are capable long distance swimmers in the pool. The injury risk is much greater than for general swimming and one of the most serious concerns is due to hypothermia. Officials and coaches should be able to recognise the signs and symptoms of hypothermia and be able to access appropriate medical care to ensure athlete safety. Athletes too have a responsibility to ensure that they are adequately prepared and in good health to undertake an Open Water Swimming event.

Swimmers taking part in open water events are vulnerable to the effects of cold water and the weather.The potential for hypothermia, or cold injury, is considered to be one of the greatest risks to the open water swimmer. The factors that influence why some swimmers become cold and end up withdrawing from the event or being rescued during events are not well understood. It is clear that swimmer-related and environmental factors contribute to such adverse outcomes, but the precise nature of these have not been scientifically explored.

Open Water Swimming has been recognised by FINA since 1986. It is defined as any competition that takes place in rivers, lakes, oceans or water channels. In World Championships and FINA competitions the Open Water distances are 5km, 10km and 25km. The Marathon swimming distance is 10km and this was the event used in Beijing when Open Water Swimming premiered at the Olympic Games for first time in 2008. However, some open water competitions, recognised by FINA, can be up to 70km. It is also interesting to note that in the first three Olympiads of the modern era (1896, 1900 & 1904) all the swimming events where held in open water swimming conditions.

At FINA events, a Safety officer and Medical officer are appointed to ensure athlete safety. They have a duty to advise the Referee if environmental conditions are unsuitable for staging the competition, or to make recommendations for the modification of the course or the manner in which the competition is conducted. Sufficient powered craft must be available during the competition so as to provide full safety backup to the escort craft. A suitable number of places to land withdrawn swimmers to allow prompt access to medical care must be identified. As events often take place over a number of hours, it should be recognised that what starts out as a bight calm day on the water may not end up the same way!

The minimum water temperature for FINA events is 16°C. Swimmers are not permitted to use or wear any device which may be an aid to their speed, endurance or buoyancy. Hence wet suits or other costumes that would offer some protection against cold water are not permitted. Goggles, a maximum of two caps, a nose clip and earplugs may be used. Swimmers are allowed to use grease or other such substances applied to the skin providing these are not, in the opinion of the Referee, excessive.

Normal body temperature in humans is 36.5–37.5 °C. Hypothermia is defined as a core body temperature below 35.0 °C. It may be graded as mild (32–35 °C), moderate (28–32 °C) or severe (< 28 °C). Some swimmers may be subjectively cold, demonstrating symptoms of mild hypothermia with a core temperature above 35 °C. There are inherent difficulties in measuring core temperatures as the peripheries of the athlete are invariably colder than the core. Aural and axillary measurements are unreliable and more invasive techniques are impracticable.  At open water swimming events clinical suspicion of hypothermia, by event staff and the medical team, is paramount. An oral temperature measurement, in our experience, appears to be the more reliable non-invasive technique, but should not replace clinical judgment.

Open water swimming typically takes place in cool not cold water. Nonetheless, thermoregulation for the swimmers is often a fine balance between heat generation, due to intense exercise, and rapid heat loss, predominantly by conduction, through contact with the cool water. Effective temperatures for the swimmers depend not only on water temperature but also the ambient air temperature and any additional chilling effect of the wind and water movement. Water temperature may vary by location, season, time of day and due to the effect of tides or currents. Duration of exposure to cool water is obviously important in the risk of developing hypothermia. Open water events typically last for prolonged periods of between one and seven hours. Swimmers are protected against the effects of cold injury by body habitus, due to the insulating layer of subcutaneous fat, and also by cold water acclimatisation.

In mild cases of hypothermia, symptoms may be vague but include:

• maximal shivering

• feeling cold

• low energy

• cold, pale skin

• numbness of extremities

• slowing of thinking and speech

• loss of fine motor coordination

• rapid breathing and pulse rate

The symptoms of moderate hypothermia include:

• shivering that slows down or ceases

• increasing confusion

• loss of gross motor coordination, stumbling

• muscle twitching

• drowsiness and listlessness

• slow, shallow breathing

• a weak pulse and irregular heart beats

In more severe cases, as the body temperature decreases further, shivering may paradoxically stop completely. The level of confusion increases and this may include hallucinations and inappropriate undressing. The heart rate will slow and become more erratic. A person will gradually lose consciousness and ultimately suffer a cardio-respiratory arrest. Severe hypothermia is a life-threatening condition and needs urgent medical treatment in hospital. Athletes need close observation and simple treatment at early stages and to prevent progression to a more serious condition.

Swimmers who are symptomatically cold do not perform well and will often elect to withdraw themselves during competition. Athletes demonstrating confusion may well be hypothermic and may not appreciate the dangers of continuing with the event. Such athletes should be withdrawn from competition by safety officials. Swimmers should only be touched or handled if they clearly display a need, or ask for assistance. Upon leaving the water some swimmers may require assistance as mobility may be poor and uncoordinated. Hypothermia may develop a significant period of time after leaving the water. When blood flow proportionally increases to the legs whilst walking after an event, a phenomenon called “after drop” may occur. Blood is cooled by the legs (which may have a significantly lower temperature than the core) and returned reducing the core temperature and so inducing hypothermia.

Hypothermia is treated by preventing further heat loss and by gentle warming of the athlete. Passive re-warming measures (using the individual’s own heat generating ability to warm up) should be instigated immediately by drying and covering the athlete with insulating layers (blankets, towels or clothes), protecting the head and torso first, and removing them to a warm environment such as a heated room. Foil exposure blankets placed over the top of these will improve efficiency. Gentle rubbing or active movement may improve heat generation. Active re-warming measures (using external methods to supply heat to the individual) may include placing hot water bottles in the armpits, forced warm air blankets and ingestion of hot drinks. Immersion in warm water should be used with caution as cold blood from the peripheries may be directed centrally cooling the athlete further. If used it is recommended that only the trunk and not the limbs are immersed. Hypothermic athletes will have poor temperature perception and it is important that only gentle heating is provided to prevent the risk of unrecognised burns. More aggressive methods of re-warming exist, such as warmed intravenous fluids, but these are usually limited to a hospital environment.

The literature provides no data and only limited information about the particular risks associated with Open Water Swimming. In the United Kingdom, British Swimming has undertaken a prospective epidemiological study looking at injuries and illness at the major domestic Open Water Swimming events during the 2008 & 2009 seasons. The outcome of a total of 542 swims was documented with a cumulative swimming time of 351.2 hours. Females undertook 255 of the swims and males 287. The age distribution of male and female swimmers was similar, ranging from 12 to over 60 years.

Adverse incidents during the events were recorded. Incidents were defined as injuries or illnesses brought to the attention of medical staff, and all withdrawals from the event whether voluntary or enforced.

In total 28 adverse incidents were recorded, representing an incidence of 80 per 1000 swimming hours. Of these, 16 were considered serious, a rate of 46 per 1000 swimming hours. A serious incident was defined as one requiring rescue during the event, evidence of hypothermia or an incident that required medical assessment and treatment by the event medical officer. These included episodes of hyperventilation, water ingestion, chest pain and cramp. Mid-event withdrawals (as opposed to rescues) were not automatically described as serious as there was often an innocuous reason for the withdrawal.

The incidence of illness and injury in this study was 80 per 1000 swimming hours. This is comparable to the level of injuries incurring during rugby union, a collision sport, with a rate of 91 injuries per 1000 player hours during games [Brooks et al 2005].  It is considerably more than football representing 21 injuries per 1000 player hours [Ekstrand et al 2005]. Overall swimming is recognised to be a very safe sport with the documented risks in a general population to be 2 per 1000 swimming hours [Nicholl et al 1995].

11 swimmers required rescue by escort and safety craft, an incidence of 31 per 1000 swimming hours. This was 40% all of all incidents recorded. There is, therefore, a significant burden on safety staff during events and sufficient provision of staff, craft and facilities must be in place. Most frequently, rescues were related to hypothermia but also to shortness of breath, water ingestion, chest pain, fatigue and cramp.

The most common serious incident encountered was clinical hypothermia, with an incidence of 20 per 1000 swimming hours. The distance the hypothermic swimmers managed to cover before rescue or completion of the event seems relevant. Only one swimmer developed hypothermia at a distance less than 2000m and so was relatively uncommon in the shorter distances swum by the younger swimmers. It is also important to be aware that one third of all hypothermic episodes developed in swimmers who had successfully completed their event. Reassuringly all those with clinical hypothermia in our study recovered quickly with passive re-warming measurements, demonstrating an aural or oral temperature over 35 °C within 30 minutes. None required transfer to hospital.

For FINA Open Water Swimming events, the minimum age is 14 years. However, events do take place for younger swimmers and over shorter distances under individual national federation regulations. In England, the minimum age has recently been reduced from 12 to 11 years but swimmers are limited to a distance of 800m.

The potential risk of hypothermia to children on exposure to cold is greater than for adults. Previous work has suggested that hypothermia is more common in those with low subcutaneous body fat, a high volume to surface area ratio and low Body Mass Index (BMI) [Lakeland and Boyd 2008]. These are all consistent with younger swimmers, with the potential for heat loss being greater in those with these physical characteristics. However, limiting exposure by defining the length of swims allowed should make hypothermia unlikely, even in the slimmest of swimmers. It is also important to recognise that previous cold-water exposure is protective against the development of hypothermia [Lakeland and Boyd 2008, O’Brien 2000].

Young swimmers or those new to the sport, should be encouraged only to enter events at distances well within their capabilities in a swimming pool. The additional physiological demands of the transition from pool to open water swimming are not insignificant. The psychological impact of developing hypothermia and/or being rescued during their first attempt at open water swimming should also not be underestimated.


• Swimmer safety in Open Water Swimming events is paramount. Adequate safety and medical provision must be available and the competition should be cancelled, modified or abandoned if necessary.

• Hypothermia is a risk for Open Water swimmers with an incidence of 20 per 1000 swimming hours. Athletes, coaches and officials should be aware of the signs of hypothermia and be able to treat those affected by simple measures.

• The symptoms of hypothermia may progress rapidly and severe hypothermia is potentially life-threatening. Athletes need rapid medical assessment, treatment and transfer to a medical facility as required.

• Athletes should be aware of their own limitations and respect the challenges of the open water environment. Progressive exposure and avoiding competition while suffering from intercurrent illness will be protective against hypothermia, forced withdrawal or rescue from an event.

*Mr. Kevin Boyd is a Consultant Orthopaedic Surgeon and Head of the Sport & Exercise Medicine at the University Hospitals of Leicester in the United Kingdom. He is a Fellow of the Faculty of Sport & Exercise Medicine. He is the current Chairman of the British Swimming and Amateur Swimming Association Medical Committees. He is a former Olympic and World Championship swimmer. He acted as Chief Medical Officer for the 9th FINA World Swimming Championships (25m) and organised the 16th FINA World Sports Medicine Congress in Manchester in 2008.

*Dr. Dominic Lakeland is a General Practitioner and Sports Doctor based in Preston, United Kingdom. He is the team doctor for Preston North End Football Club and is a member of the British Swimming and Amateur Swimming Association Medical Committees. He undertook his Masters of Science thesis at the University of Nottingham on the predictors of hypothermia in Open Water Swimming.

Questions & Answers

Question: My son, aged 14 years, has good endurance as a swimmer. He has expressed an interest in taking part in an open water swimming event but is still quite small and very slim. I think he might suffer the effects of the cold. Is there anything we can do to help prevent this?

Answer: Younger swimmers are more prone to the effects of the cold in open water swimming, particularly if they do not have much subcutaneous fat. Progressive exposure to cold water through training is probably the best thing you can do. When it comes to his first swim, pick an event distance which he can do very comfortably in the pool, ideally less than 30-40 minutes duration.

Question: How would you advise me to get warm again after an open water swimming event?

Answer: It is wise to be prepared! Having a friend or relative to look after you is useful. You should have towels to dry you immediately after leaving the water and lots warm clothing that will be easy for you (or someone else) to put on. Don’t forget a hat. Get yourself away from any bad weather, particularly wind and rain, to somewhere warm and protected either indoors or in a car. Consider bringing a sleeping bag and a flask of hot drink that could be invaluable if you become hypothermic. Finally, if you have any concerns over your own wellbeing or of someone in your care, make yourself known to the event first-aid staff.

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