Alistair Simpson / Specialty Trainee in Anaesthesia / South-East Scotland
Alistair Simpson, former Medical Officer with the British Antarctic Survey tells Adventure Medic what we need to know about hypothermia. Alistair has lived for more than 16 months in Antarctica. He has also served on a number of expeditions, including an unsupported ski crossing of the world’s second largest icecap and two high altitude research expeditions in the Bolivian Andes.
Hypothermia
The expedition doctor faces many challenges, in particular the environment to which his charges are exposed. With increasing ease of access to high altitude and polar regions, cold exposure is a genuine concern.
Humans are poorly adapted to cold. Since physiological homeostasis mandates a core temperature of 37°C, insulation from the environment is required in temperate or cold climates. However, when insulation is inadequate or the environment becomes more severe, homeostasis can fail resulting in cold injury.
Cold injury can be categorised into hypothermia, non-freezing injury and frostbite. This article will focus on the first of these injuries. Hypothermia is classically defined as a core body temperature below 35°C. (1,2)
Causes
Body temperature is maintained by a balance of heat gain and heat loss. Regulation is under hypothalamic control, with peripheral temperature receptors sending signals to the hypothalamus which are then relayed via the sympathetic nervous system to effectors in the peripheral vasculature, piloerector muscles and sweat glands, as well as the brainstem (shivering) and higher centres. (3)
Following exposure to cold, this process leads to peripheral vasoconstriction, redistribution of blood flow centrally, cooling of the extremities and shivering. Under normal conditions, body temperature can be maintained by wearing clothes, seeking shelter, shivering and exercise. Adult humans do not exhibit significant non-shivering thermogenesis.
Exercise is very effective at maintaining body temperature and can increase heat production from 100 to 1200 Watts.(4) Clothing can insulate the body to the extent that it creates a warm microclimate next to the skin.5 Sufficiently warm shelter can obviate cold environmental conditions entirely. Shivering can increase heat production to 500 W, increasing energy consumption by the equivalent of five times resting metabolic rate. (4,6)
Heat homeostasis is the result of a balance between heat production and heat loss.7 Therefore if heat production is impaired or heat loss increases, a fall in core temperature will result.
Wet skin or clothing increases thermal conductance and increases heat loss by convection and evaporation.8 Water immersion can produce a dramatic and rapid reduction in body temperature and causes 100 000 deaths worldwide per annum.4 Windchill can dramatically reduce relative temperatures due to convective losses: at an ambient temperature of 0°C, a wind speed of 30 knots will produce a relative temperature of -20°C. (9)
Although exercise will increase heat production, if fatigued, body glycogen stores are depleted resulting in a decreased capacity for shivering and further exercise. A low blood sugar can also impair hypothalamic responses to cold.4,8 Body habitus can predispose to hypothermia. While obese people are well insulated by subcutaneous fat, slim individuals will lose heat more rapidly and will rely more on other methods of insulation and heat production. (4,8)
Physical fitness per se does not confer improved tolerance of cold, although it will allow exercise to be maintained for longer, which may help due to increased thermogenesis. (8) Alcohol ingestion can predispose to hypothermia by inhibiting vasoconstriction and impairing shivering. (10)
It is worth noting that whilst peripheral vasoconstriction and cooling reduces core body temperature loss, it also reduces dexterity significantly and this can have risks in its own right for the individual in a challenging environment.
Prevention of hypothermia
- Ensure adequate preparation and risk assessment, especially as regards kit selection
- Take adequate clothing
- Have a means of providing rewarming, for example sleeping bag, warm bottle or stove
- Avoid saturation of clothing with water or sweat – especially if windy
- Change wet clothing as soon as able
- Take adequate food supplies
- Keep moving where appropriate
- Seek appropriate shelter when conditions deteriorate
- Use a buddy system
- Be aware of the possibility of hypothermia
Symptoms and Signs
Symptoms of hypothermia are generally related to core body temperature and are progressively more severe as temperature falls. With mild hypothermia, shivering begins and mental status is altered – individuals may be lethargic or mildly confused.
As temperature falls, shivering becomes maximal, walking is impaired and speech becomes slurred. Confusion and drowsiness will become apparent and behaviour becomes irrational, including paradoxical undressing.
With severe hypothermia, shivering stops, further exacerbating temperature loss. Pupils may become fixed and dilated and the affected individual comatose. The appearance of the individual can mimic death. Cardiovascular instability is also a feature, particularly ventricular fibrillation (which may be provoked by rough handling or sudden changes in posture and cardiac arrest. (7)
Once core temperature reaches 24°C, survival is unlikely. However, survival has been recorded following a core temperature as low as 13.7°C.11 Note that there can be significant inter-individual variability in symptoms for a given core temperature. (12)
Management
Accurate core temperature measurement will assist in management of a hypothermic patient. Oral, tympanic and axillary temperature recordings can be inaccurate; oesophageal, urinary bladder or rectal measurement is preferable. (4,7)
First aid methods should be employed, including seeking shelter (especially from wind and rain), insulating from the ground, removing wet clothing and providing external heat, such as via a stove or by contact with another individual. Warm drinks may also help.
Of note, metallic ‘space blankets’ are of no proven benefit.4 Wrapping the victim in dry sleeping bags and insulating using a roll mat and bag will help.
Chemical heat packs or bottles filled with hot water may also be used with caution so as to avoid burns; wrap bottles first before applying to skin. (7) There are differing opinions as to whether warm baths should be used; core temperature can fall due to blood flow to cold peripheries (afterdrop) and the vasodilatation may also cause profound hypotension and provoke dysrhythmias. (4,7) If used, great care should be taken and the patient monitored closely.
Such active external warming techniques are likely to be most appropriate for mild rather than severe hypothermia. (10) Invasive warming techniques include warm intravenous or peritoneal fluids and cardiac bypass. If hypothermia was of slow onset or prolonged, rewarming should be gradual and undertaken with care.
A cold diuresis can result in fluid loss and hydrostatic effects can exacerbate this in immersion victims. These patients should be extracted and nursed recumbent to avoid causing severe postural hypotension. Cell membrane instability can result in significant ion flux, especially of potassium. Pancreatitis and rhabdomyolysis are also recognised complications. (4)
Pronouncement of death is difficult due to the appearance of the severely hypothermic patient. (9) Patients should therefore be rewarmed before death is declared (the victim is ‘not dead until they are warm and dead’). Very prolonged resuscitation may be required. (11)
Conclusion
Cold exposure can result in a number of injuries. However, with appropriate planning preventative measures can be taken to reduce the impact of low environmental temperatures. If hypothermia develops, it should be recognised and treated as soon as possible. Familiarity with management principles can reduce morbidity and mortality.
Take home messages
- Cold exposure can result in a number of injuries, including hypothermia, non-freezing injury and frostbite
- When heat loss is greater than production, hypothermia will ensue
- Hypothermia must be recognised early
- Suspect hypothermia in individuals displaying any signs, including slow mentation, withdrawal or confusion
- Instigate early appropriate management: isolate the victim from the environment and rewarm as appropriate to the degree of hypothermia
- Pre-expedition planning and risk assessment are essential and may reduce the risk of hypothermia developing
- Consider selecting equipment for managing hypothermia, including thermometers and rewarming equipment
- Declaration of death is difficult in the severely hypothermic victim; rewarm patients before death is declared
Getting cold up high
Avinash Aujayeb / SpR in Respiratory Medicine.
Three hours before summiting Spantik, 7031m in the Karakorum I turned around. I just couldn’t get warm.
That morning an early start had meant the snow was hard underfoot and made for fast climbing. The alpenglow had mesmerised us at sunrise and then the sun made its appearance. Now, though, on the summit ridge, we were being hampered by soft, waist deep snow and were being forced to dig a trench up the mountain to ascend. A slight wind was blowing and at nearly 7000m, this chilled us to the core.
Lagging behind the front two climbers, I was slowly making my way up, wrapped in my own psychological and physical battle against the elements. Any exposed flesh was literally freezing. My balaclava and buff kept the wind off my face, but were suffocating me, so I had to keep taking them off to breathe. My down jacket felt like no more than a cotton t-shirt and I had to alternate hands to wield my ice axe as its cold penetrated my gloves, even though I had taped the metal parts up. We had stopped earlier, to sun ourselves but nothing remained of that warmth. This was my first big mountain but between them, my two friends had both had forays up eight 8000m peaks and were far more experienced than me and more accustomed to the cold and the hardship.
Somewhere further up the slope, I stopped to wriggle my now partially numb toes and to rotate my arms. I looked up and saw a foot being warmed in an armpit! Snow had been filling up the boot of the trailbreaker due to a loose boot. I waded up to them, rested and when we all felt better and the feet rewarmed, continued.
At that point, I had been going for about nine hours and had reached a personal high of about 6710m, a mere 300m below the summit. However, despite my efforts and whatever I was doing, I could not get warm. My toes had gone, my hands were cold and every time I stopped, I would be shivering. I had no more layers to put on. Our leader, Brad, had graciously offered me another down jacket but now he was himself too cold to be without it.
I knew then I would not make the summit. I was exhausted and hypothermic. After a brief conversation with Brad and an exchange of encouragement, I started descending.
The decision to turn round had been a pragmatic one, without any emotion on my behalf. I might have been able to get up to the top, but probably not down again. I was hypothermic and didn’t want to get frostbite. Once I had descended the final slope back to camp, I had warmed up considerably and I sat down in the snow just above the tent. It was only then I welled up, as I realised what had just happened in all its intensity. It had taken six months of planning and training to get here and it was almost too much for me. However after just a few hours, warm and rehydrated, I was able to share the joy of my friends’ summit success and had already started thinking of next time.
References
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- Haman F. Shivering in the cold: from mechanisms of fuel selection to survival. J Appl Physiol. 2006 May;100(5):1702-8.
- Davis PR, Byers M. Accidental hypothermia. J R Army Med Corps. 2005 Dec;151(4):223-33.
- Castellani JW, Young AJ, Ducharme MB, Giesbrecht GG, Glickman E, Sallis RE. American College of Sports Medicine position stand: prevention of cold injuries during exercise. Med Sci Sports Exerc. 2006 Nov;38(11):2012-29.
- BASMU. Kurafid – The British Antarctic Survey Medical Handbook. 6 ed. Grant IC, Cosgrove H, Thomson L, Guly H, editors. Plymouth: British Antarctic Survey; 2005.
- Keim SM, Guisto JA, Sullivan JB, Jr. Environmental thermal stress. Ann Agric Environ Med. 2002;9(1):1-15.
- Gilbert M, Busund R, Skagseth A, Nilsen PA, Solbo JP. Resuscitation from accidental hypothermia of 13.7 degrees C with circulatory arrest. Lancet. 2000 Jan 29;355(9201):375-6.
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