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 hyperventilation. 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 hypothermia 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 temperatures 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 temperature 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 breathing 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 panic.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 drowning. 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 drownings 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 oxygen). 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 solutions. 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.