Review article 189 Cold water survival – an evidence-based update C House

Abstract

Royal Navy (RN) cold water survival advice was historically based on data collated from immersion incident reports during World War II. This evidence-based review highlights the advances in the knowledge and understanding of the risks associated with cold water immersion and how this has been applied to provide up-to-date advice to maximise the chances of survival for passengers on board RN helicopters ditching into water.

House C. J R Nav Med Serv 2017;103(3):189–193

Introduction The increase in ventilation on immersion is not related to a Royal Navy (RN) cold water survival advice was historically metabolic need, but is due to . Consequently based upon the Molnar curve, which was developed from data there is a reduction of mean arterial tension of carbon diox- collated retrospectively from immersion incident reports of ide, which can lead to tetany and a clouding of consciousness World War II.1 The Molnar curve represents the longest time which is sometimes observed on immersion.2,7 The magnitude that an individual might survive when immersed in cold water, of the response is related to the rate of change of skin temper- and was considered as the only cause of death. ature and the surface area of the body exposed.8 Experimental This review highlights the advances in the knowledge and un- work suggests that the respiratory responses are instigated at derstanding of the risks associated with cold water immersion. water cooler than 25°C,9 and reach a maximum This information has been applied to provide up-to-date ad- in water at 10°C; further reductions in water do vice to maximise the chances of survival for passengers on not increase the magnitude of the response but may extend its board RN helicopters ditching into water. duration.4 There is also an inspiratory shift in end-expiratory lung volume, which can lead to tidal within 1 litre of Cold water immersion total lung capacity, which makes breathing difficult and con- Four stages of immersion in cold water have been identified tributes to the sensations of breathlessness and .9 that are associated with the risk of becoming incapacitated and dying.2 These are detailed in Table 1. Breath-hold time is also reduced on sudden immersion in cold water, and is possibly the most dangerous of the initial cold 1. Initial immersion (0–3 min) due to the cold shock shock responses.10 The mean maximum breath-hold times of ­response 18 normally clothed individuals fell from 45 seconds in ther- 2. Short term immersion (3–15 min) due to neuromus- moneutral air to 9.5 seconds on submersion in water at 5°C; cular dysfunction (caused by cooling of the superfi- the maximum breath-hold time of one of the volunteers was cial nerves and muscles) leading to incapacitation or reduced to 0.2 seconds.11 This is a particular risk for helicop- swim failure ter crew and passengers suddenly ditching in water, who, if 3. Long term immersion (> 30 min) due to hypothermia surviving the initial impact, then struggle to escape from the (falling body temperature) helicopter and get to the surface without inhaling water and 4. During or following rescue due to post rescue . A review of RN helicopter accidents during the pe- ­collapse or secondary drowning riod 1972–1984 included 121 accidents involving 535 crew and passengers, of which 57 (47%) accidents were into wa- 12 Table 1: Stages of immersion in cold water. ter. Thirty-one of the 51 fatalities (60%) were the result of drowning, and all of these occurred in potentially survivable Initial immersion (0–3 min) and the cold shock response accidents. Twenty-seven of the were due to an in- The cold shock response is initiated immediately on immer- ability to escape from an inverted helicopter; none of the fa- sion in cold water. After an initial involuntary gasp of between talities was as a result of hypothermia. Interviews with crash 2 and 3 litres on entering cold water,3,4 ventilation may in- survivors and reviews of relevant reports revealed that vic- crease fourfold with a doubling of respiratory rate and tripling tims were not able to release their seat belts, and that, if they of tidal volume.5 An adult male would die from drowning if he did succeed in releasing them, they were often unable to find inhaled as little as 1.8 litres of sea water (which would repre- the escape hatches because of inrushing water, disorientation, sent about one-third of the volume of his lungs).6 poor visibility and darkness.12 190 Journal of the Royal Naval Medical Service 2017; 103(3)

Brooks et al (2001) determined that escape times for experi- stimulated by breath-holding and face immersion and acts enced instructors and divers in an underwater escape trainer to slow down heart rate and conserve ). The compet- configured to the Super Puma and submerged in warm water ing effects of the two responses have been seen in laboratory ranged from 28 to 92 seconds.13 As it is generally recognised studies18 to cause cardiac arrhythmias, and it is believed that that the time required to make an escape is likely to be beyond such arrhythmias could result in death which would be undi- the time that the escapees can hold their breath, aircrew and agnosable post-mortem.19 passengers are provided with emergency underwater breath- ing apparatus (EUBA). Passengers on RN flights over water In addition to water temperature and clothing, several other are provided with the passenger short term air supply system factors are known to modify the cold shock response. The re- (PSTASS). PSTASS is a small cylinder of compressed air that sponse has been shown to be attenuated in individuals with is designed to provide air for 2 minutes at a depth of 5 metres. superior aerobic fitness,20 and by habituation to cold water im- mersion.21 In contrast, pre-warming (either passively or by ex- However, individuals may struggle to use EUBA once sub- ercise) has been shown to increase the magnitude of the cold merged in cold water. In a laboratory study, three of eight vol- shock response.22 Work involving immersion suits has inves- unteers submerged wearing immersion suits in water at 5 and tigated the cold sensitivity of different areas of the body, and 15°C were unable to use the EUBA, although they had com- shown that cooling the torso results in a greater cold shock pleted training and experimental runs in warm water.14 It is response than other areas of the body.23 important that personnel at risk are trained in underwater heli- copter escape and with EUBA. A review of US Navy helicop- Short term immersion; incapacitation and swim failure ter accidents showed that survival rates without training were The second stage of immersion in cold water associated with 66% and with training were 91.5%.15 Cheung (2010) advised particular risk has been termed “incapacitation or swim fail- that in order to enhance survivability, it is critical to continue ure” and is related to a loss of motor function because of the optimising EUBA design and the training for its use, and to effects of cooling. In water, heat is lost from the body directly also consider other . These might include increasing through conduction and also by convection. The effect is that the speed of escape (by shortening and simplifying the escape outer tissues of the body will cool to temperatures that lead to route), and investigating the possibility of deliberately floating a reduction in nerve and muscle function before general hy- the ditched helicopter on its side to provide an air gap.16 pothermia occurs. Consequently, muscle tone increases and cooling of the peripheral motor and sensory nerves leads to The cold shock response is caused by the rapid reduction in dysfunction similar to peripheral paralysis. skin temperature and is mediated at midbrain level.9 Reducing the magnitude and speed with which skin temperature falls on The effect of the reduction in nerve and muscle function is immersion by wearing protective clothing or by entering the that tasks requiring dexterity or strength, such as opening and water slowly (usually not an option in these circumstances) deploying a flare, climbing into a liferaft or operating the oral can reduce the magnitude of the response. Although clothing inflation valve on a life-jacket become much more difficult. provides some protection against the cold shock response, a Golden and Hervey considered that the risk associated with protective immersion suit provides significantly better pro- short term immersion occurred in the first 3–15 min of im- tection.11 In water at 5°C, volunteers (n=18) undertaking a mersion and described how death at this stage seems to af- mock underwater helicopter escape exercise were able to fect particularly those who try to swim, “in the belief that they breath-hold for a mean (range) of 19.2 (8.9–22.7) seconds can do so in cold water as readily as they can in warmer wa- when wearing a drysuit but only 9.5 (0.2–22.1) seconds when ter”.24 Research showed that swimming time in cold water is wearing a coverall.11 In water at 15°C the breath-hold time much shorter than in warmer water. In an early study, four of individuals wearing swimming costumes was reduced to men were able to swim for only between 2 and 12 minutes in almost half of that in thermoneutral water, but only by 73% in water at 4.7°C.25 A similar study several years later showed water at 20°C,17 which suggests that a substantial gain could that subjects, wearing life-jackets and swimming in open wa- be made to breath-hold time in water at 15°C (but not at 20°C) ter, became hypothermic and cooled much more quickly than by wearing a protective immersion suit. when just floating in the life-jacket.26 This was in contrast to the ability of some swimmers to be able to swim for 12–20 As well as the respiratory responses, the cold shock response hours and swim across the English Channel. Work undertaken also influences the cardiovascular system; there is a rapid in- with cross-channel swimmers showed that those with a higher crease in heart rate, blood , peripheral resistance and body adiposity were better able to maintain their body tem- myocardial oxygen demand on sudden immersion in cold wa- peratures when they increased their rates of heat production ter. These represent only a slight risk to healthy individuals but by swimming, whereas leaner swimmers could not maintain in susceptible individuals may be fatal.10 Recently a further body temperature as cooling rates were higher than heat pro- risk associated with immersion in cold water has been rec- duction.27 More recently this area of work was revisited. Ten ognised relating to cardiac death: it is termed “autonomic male volunteers attempted 90 minutes of self-paced swims conflict”, in which the sympathetically mediated cold shock in water at 25, 18 and 10°C; all 10 completed the swim in response (which increases heart rate) competes with the 25°C water, eight in 18°C and five in 10°C water, with swim parasympathetically mediated diving response (which is failure reported to be due to local limb cooling and muscle Review article 191 incapacitation rather than as an effect of whole body hypother- the data indicate that the upper observed survival time envelope mia.28 It was suggested that the short swim times in the earlier in water temperatures below 15°C is less than that depicted by study were due to the respiratory responses associated with the Molnar curve. However, survival curves and tables are often the cold shock response, whereas in the later study swimming oversimplified, so mathematical models of human thermoreg- began once the response had abated. ulation for incorporation into software tools to predict survival time in cold water have been developed. Hypothermia Hypothermia is the fall in body temperature that occurs on im- The three most significant mathematical models are the mersion in cold water. Death from hypothermia has long been Wissler model, Cold Exposure Survival Model (CESM) and recognised as a risk for victims immersed in cold water. How- the Probability of Survival Decision Aid (PSDA). The Wissler ever, cooling of the deep tissues of the body and hypothermia model requires the input of initial conditions and the environ- will not represent a problem until about 30 minutes following ment with the output of thermal status assessed from central immersion in cold water.29 Normal core temperature is about arterial temperature; 32 this model was used to provide guid- 37°C; a person is considered to have hypothermia when core ance for the selection of protective clothing by the Royal Air temperature falls to 35°C. As an individual’s body temper- . The CESM is used by the Canadian Red Cross to es- ature falls to 35°C they will probably be shivering and their timate maximal survival times for search and rescue purpos- manual dexterity and muscle function will be reduced. The es.33 The model is based on known physiological and physical individual may become confused, disorientated, introverted principles of heat transfer and can make predictions for cold and aggressive. As their temperature falls further, there will air and cold water immersion exposure, but only extends to be a clouding of consciousness (33–30°C), loss of conscious- 36 hours of exposure. The PSDA is the most recently devel- ness (30°C), ventricular fibrillation, and death oped model, and was developed by the US Army Research (28–25°C).29 Institute of Environmental Medicine at the request of the US Coast Guard. The US Coast Guard annually conducts near- Individuals without a means of flotation, even if they have ly 3000 search and rescue missions that involve a victim in been physically able to keep their airway above the water to the water or on an emergency craft, and the PSDA is used that point, will not be able to do so once they begin to lose to allocate resources; hence an accurate estimate of possible consciousness and will succumb to drowning. Life-jackets maximum survival time is required to determine where to best tend to turn the wearer to face the on-coming waves; whilst focus resources and when to stop a search.34 However, the in- conscious, individuals are likely to work to keep their backs to formation attained from mathematical models should be treat- the waves29 which they will be unable to do once they become ed with caution. Golden and Tipton (2002) suggest that the exhausted or lose consciousness. Water splashing over the level of understanding of the thermoregulatory system face may result in drowning. Although life-jackets often in- is insufficient for a definitive model of thermoregulation to clude a splash guard, the benefit of a splash guard is difficult to be produced, and that the prediction of survival time remains predict; the head of an unconscious individual will tend to fall more an art than a science.29 to one side, and it is considered that even a good splash guard might be unable to prevent drowning in these circumstances.30 Cooling rates and hence survival times are also influenced by many other factors, including: sea state; clothing; leakage; Although the Molnar curve is regarded as relatively crude in personal factors; seasickness and mental factors. Individuals providing guidance to the nearest couple of hours, it did iden- immersed in rough sea conditions and waves may struggle tify for the first time that survival time in water below 15°C to maintain their airway clear of the water and may drown was relatively short and suggested that above 15°C survival within minutes of immersion. Individuals not succumbing to time increased reasonably quickly. In the time since Molnar’s drowning are likely to cool more quickly in rough than calm curve was produced, experimental work has been undertaken conditions.35 Leakage of water into immersion suits reduces examining cooling rates in cold water. It has been generally insulation and speeds heat loss,36 and may reduce . accepted that in water at 5°C, the core temperature of an av- For example, when exposed to harsh conditions involving erage young adult male wearing non-specialised clothing will wind and waves, 500mL of water ingress inside the immer- fall by 2°C (i.e. to 35°C) after approximately one hour, in wa- sion suit during immersion in water at 8°C reduced clothing ter at 10°C it will take 2 hours, and in water at 15°C between insulation by 42%.37 Seasickness can increase the rate of heat 3 to 6 hours.29 loss, whereas a positive mental state and determination may prolong survival.29 The most recently produced survival curve is based on data col- lected from the UK National Immersion Incident Survey.31 Data During and following rescue from 1,593 incidents (reported voluntarily by a questionnaire The fourth stage of immersion associated with risk is just be- distributed by HM Coastguard and the Royal National Lifeboat fore, during and after rescue, with approximately 20% of im- Institution, RNLI) indicated that the five most significant vari- mersion-related fatalities occurring in this period.31 Pre-rescue ables for predicting survival time were age; clothing at time of collapse is thought to be due to the sense of relief engendered the accident; whether a life-jacket was worn; water temperature by imminent rescue causing a reduction in catecholamine and water area (inland, coastal, inshore or offshore).31 Notably secretion and withdrawal of its protective effect.38 During 192 Journal of the Royal Naval Medical Service 2017; 103(3) rescue and removal from the water there is loss of hydrostat- 1. Authorised immersion coveralls should be worn on ic squeeze, which in a compromised (from all flights over the sea, in aircraft types for which the effects of prolonged immersion in cold water) can lead to they are approved, when the sea temperature is below circulatory collapse and heart failure.39 Death associated with +15ºC (no change from previous advice). secondary drowning and lung damage is common in immer- 2. Regular helicopter passengers (personnel who fly 5 sion victims who have aspirated water. Even amongst individ- hours or more per year by helicopter) are required uals who work on, or flying over, water, knowledge and appre- to undertake helicopter underwater escape training ciation of the risks associated with immersion in cold water is including training with PSTASS. limited; of n=357 individuals attending sea survival courses in 3. Troops or occasional passengers should undertake Canada, only n=84 (23%) could name one of the stages of im- helicopter underwater escape training (without PS- mersion associated with risk (usually hypothermia) and only TASS) on a 5 yearly basis. n=1 could name all four stages.40 4. Personnel who are required to fly over water in day- light (other than SAR) are not required to undertake Recommendations and advice helicopter underwater escape training undertake or The recommendations following the review are summarised training with PSTASS. in Table 2 and the updated advice in Table 3. 5. A replacement for the HPIS is planned for 2018. 1. The helicopter passenger immersion suit (HPIS) should be worn when flying over water at or below Table 3: Summary of updated advice and actions. 15°C. 2. Helicopter passengers should undertake helicopter Summary underwater escape training including training with Since the production of the Molnar curve, the understand- PSTASS. ing of the risks associated with immersion in cold water 3. “Dress to survive” advice concerning the clothing has improved. It is now recognised that the early responses worn beneath the HPIS should be endorsed; this was to immersion (stages 1 and 2) account for the majority of based upon data developed using the Wissler model open water immersion deaths. However, despite this there with the clothing worn determined by water temper- is a focus on the risk associated with hypothermia and with ature and likely rescue time. providing protection to reduce this risk. Although hypother- 4. A suitable replacement for the HPIS, which was mia is a significant factor, it is important that the risks to found to leak significantly during the development survival from the other stages of immersion are recognised of the “dress to survive” advice, should be procured. to ensure that appropriate survival equipment and training are ­provided. Table 2: Summary of recommendations.

Conflicts of Interest Statement The author has no conflicts of interest.

References

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Author

Mrs Carol House Institute of Naval Medicine Alverstoke Gosport PO12 2DL [email protected]