Thermal Regulation

Thermoregulation

Supporting temperature regulation is a fundamental part of caring for a patient in the wilderness. Everybody is at risk of thermoregulatory problems, so much so that hypothermia is one of the ‘three hypos’ (hypothermia, hypoglycaemia and ‘hypohydration’) that we look for in every patient we treat.

Humans, like all mammals, are homeotherms. That is, evolution has adapted our physiology to work optimally around a certain ‘set point’ – in our case, 37°C. Simply put, everything works best around 37°C. Too cold (below 32°C) and cognition is impaired, blood doesn’t clot and eventually the heart doesn’t pump. Too hot (at or above 42°C) and the enzyme processes in the body begin to breakdown.

Core temperature

When we discuss the temperature of a patient, we are interested in the ‘core’ of the body – the internal environment in which the internal organs function: the brain, heart, lungs liver and kidneys. When discussing a patient’s temperature, we need to be clear about the temperature we are really measuring and how that relates to the ‘true’ core temperature of the body. Medical technology gives us several ways of measuring temperature, each has its own pros and cons (see Table 1).

Table 1. Features of different means of temperature measurement

Rectal temperature is a core temperature but it can lag behind core temperature changes. Research has shown that rectal temperature readings are slow to reflect temperature changes elsewhere within the core. For example, when cooling a hyperthermic heat stroke patient, rectal temperature readings will remain high even when the temperature of the vital organs has begun to normalise.

A key part of monitoring a patient’s temperature is understanding that whilst isolated readings can be useful, trends are even more important. A trend will tell you useful information about how your patient is responding to your treatment – far more useful than a single reading. Some methods lend themselves more easily to gathering repeated measurements than others. Be mindful of exposing the patient to cold when taking their temperature.

Experience and trials within WEMSI-International have led us to settle on axillary temperature measurement as a good compromise position. We use commercial ‘indoor-outdoor’ thermometers with a display connected to a probe via a length of wire. This allows live and ongoing temperature readings to be taken with no extra fuss.  Trend data is very helpful.  Essential to getting accurate useful readings are:

• correct placement of the temperature probe high in the patient’s armpit, taped firmly into position

• understanding of the equipment (avoid too many buttons; check batteries and keep warm)

• acknowledgement that it may take some minutes for measurements to stabilise

Normal values

Although humans’ set point is 37°C, there are some individual factors that can subtly alter this. Sex, age, time of day and physiology all play their part (see Table 2).

Table 2. Patient factors that influence the set point of 37°C.

Abnormal Values

If you doubt the core temperature you have obtained from a patient, check your technique, equipment and repeat the reading. Measuring the core temperature of a healthy bystander is one way of checking that your thermometer works.

At a core temperature of 37 ± 2 °C, we work well. Above and below this, we experience increasingly difficulties until the extreme of core temperature kills us. Exact definitions vary according slightly between authorities but Table 3 (below) gives a workable guide. As always, we treat the patient, not the number.

Table 3. Core temperature and patient status.

Heat Loss and Gain

Mankind evolved in a hot environment and we retain the ability to work hard in the heat. Indeed, amongst the primates, we have evolved peculiar bodies that are covered with little hair but many sweat glands. Since we migrated out of Africa, the passage of a few eons of time has done very little to erode our genetic ability to cope with heat. We remain heat specialists.

Healthy and at rest, most heat production is generated as a waste product of metabolic activity in the liver. The brain, because of its relatively high energy consumption, also generates some heat. When muscular work begins, blood flow to the muscles increases and the chemical reactions that allow movement also generate heat. The amount of heat we can generate is quite staggering. At rest we shed as much heat as a 100 W light bulb¹, but this can increase twenty-fold in trained athletes, who can produce the same amount of heat as a 2000 W electric heater.

The body’s challenge is to shed this heat, or risk slowly cooking – and humans are incredibly good at losing heat. Like any other hot object, the body can cool through conduction, convection and radiation as long as our environment is cooler than our skin temperature. These processes certainly help us to cool, but where we really excel is at losing heat through evaporation by sweating.

The way in which we lose heat has been looked at under experimental conditions. The test volunteer sits on a wooden bench in underwear in still air at an ambient temperature of 15°C. Heat exchange between the body and the environment takes place in the following ways:

Conduction

• heat exchange between objects that are touching

• up to 18% of our heat is lost by conduction

• this can be reduced by insulation (particularly from the ground)

• wet clothing increases conduction loss by 5-fold

• immersion in still water increases heat loss 20-fold

Radiation

• heat is lost through the emission of infra red light (or gained by absorbing it)

• accounts for 60% of heat loss under experimental conditions

• infra red light is what a tympanic (ear) thermometer measures

• can be reduced by using insulation and reflective materials

Convection

• heat exchange by the movement of gas or liquid past the body

• at rest in still air about 15% of body heat is lost by convection

• the rate is more in wind, higher in still water and highest in moving water

Evaporation

• the conversion of liquid to vapour

• at rest in still air, about 22% of heat loss occurs through the evaporation

• under normal conditions, we lose of around 700 ml of water each day (300 ml in exhaled air and 400 ml from the skin)

• the dryer the air, the more water we can lose to it, but at 100% humidity sweating no longer works

• at the highest humidities, water vapour may condense onto the skin, increasing heat load

• under extreme conditions, over 2000 ml of sweat an hour may be lost – generating a heat loss of kJ

The Skin

At 10 kg, our skin is our body’s largest organ and is highly specialised to assist in thermoregulation. The skin contains a rich capillary network just below the surface and another, deeper, blood vessel network. When heat loss is required, the superficial capillaries are flooded with blood which facilitates heat loss to the environment. The skin is flushed and warm. As core temperature falls, the connections to the capillary networks are narrowed, less blood flows to the skin surface, the skin is paler, skin temperature falls and core heat is conserved.

Our subcutaneous fat plays an important role in insulating us we wish to conserve heat. It has been calculated that in a person of healthy weight, subcutaneous fat provides as much insulation as an everyday set of clothing. It follows that the absence of fat (for example, when a patient is malnourished or underweight) has important implications for thermoregulation.

Above an ambient temperature of 35 °C, conduction convection and radiation fail us. Above this point, our heat exchange works in reverse and our environment heats us up. Above 35°C, the only thing that stops us cooking is our amazing ability to sweat.

When we sweat, we lose heat to the environment by allowing water to evaporate from our surface. The process of turning water into water vapour takes considerable energy and this is how we ‘use up’ heat. Above 35°C, sweating is the only way that we can lose the heat to the environment. This system works well until whilst we can keep producing sweat, but fails miserably when we become dehydrated.

Heat Production

Heat production is affected by a number of factors that may vary between individuals and over time. These are:

Basal Metabolic Rate (BMR)

• the normal metabolic rate of our bodies work at rest

• affects how much heat we produce naturally

• varies from person to person

• a 1°C rise in core temperature increases BMR by 10%, therefore BMR may be substantially increased during a fever

Muscle Activity (Exercise or Shivering)

• exercise can increase the base rate of heat production 20-fold

• shivering increases heat production by a factor of 5 and can increase core temperature by 1°C/hour if insulated

• exercise and shivering require energy (food) and water to maintain

Nerve System Control

• during exercise or in stressful situations, the adrenal gland releases adrenaline and noradrenaline

• these hormones increase the BMR of body cells

Thermogenic Effect of Food

• digestion, absorption and storage of food can increase the metabolic rate by 10-20%

• effect is greatest after eating a high-protein meal, less so after a meal rich in carbohydrates or fats

Thyroid Hormones

• thyroid hormones are the main regulators of BMR, which increases as the level of thyroid hormones in the blood increases

• thyroid hormone adjustments are slow, taking place over days to week

• other hormones such as testosterone, insulin and growth hormone can increase BMR by around 5-15%

Gender, Climate, Sleep, Malnutrition

BMR is lower in:

• females, except during pregnancy or lactation

• tropical regions

• sleep

• malnourishment

The Hypothalamus

The complicated adjustments involved in thermoregulation are made without conscious effort by signals to and from the hypothalamus in the brain. The regulatory role of the hypothalamus is demonstrated when it is damaged by head injury. In some cases, victims with hypothalamic injury become hyperthermic when their thermoregulatory control fails.

The hypothalamus receives information about body temperature from its own receptors and receptors in the skin and major blood vessels. It integrates these signals and sends electrical messages to the skin (capillary bed control), muscles (shivering), adrenal gland (adrenaline) and thyroid gland (thyroid hormone). The hypothalamus is the ‘conductor of the orchestra’.

As well as head injuries, other factors may influence or upset the way the hypothalamus works:

• fever

• medications (cardiac, psychiatric)

• malnutrition

• thyroid disease

• exhaustion / dehydration

Learned behaviours

Evolution has equipped our physiology remarkably well to enable us to deal with the demands of a hot environment. At the same time, we have developed large brains with great capacity for tool-use, problem-solving and learning. Wherever human populations exist, complex behaviours and cultural practices can be seen which serve to better adapt humans to their environment. These include clothing, social patterns, cultural practices and food-gathering and survival skills.

The best example of these learned behaviours may be seen in the Inuit populations that have adapted to the world’s coldest environments. Modern technology has gone a long way to making the coldest environments more accessible through, for example, the use of highly insulated clothing. But it is the skills and behaviours – rather than just the trappings – that make the difference between surviving and thriving. If our physiology equips us for the heat, then it is our brain that allows us to survive in the cold.

Recommended Reading

Auerbach, P S (2007) Wilderness Medicine (5^th Edition). Mosby ISBN 978-0-323-03228-5.

Forgey, W W (2006) Wilderness Medical Society: Practice Guidelines for Wilderness Emergency Care (5^th Edition). Falcon Guide ISBN 0-7627-4102-3.

Johnson, C et al (2008) Oxford Handbook of Expedition and Wilderness Medicine. Oxford University Press ISBN 978-0-19-929661-3

Stroud, M (2004) Survival of the Fittest. Yellow Jersey Press ISBN 0-224-07507-1