Reading Time: 4 minutes
- Ian Rogers MMBS FACEM, Professor of Emergency Medicine, St John of God Murdoch Hospital & University of Notre Dame Fremantle
- Jenny Brookes, MBBS FACEM MHPE Emergency Physician
In the outdoors, death or serious illnesses or injuries are relatively uncommon. The frequency of encountering a seriously injured or very ill person in the bush is low, so few outdoor leaders have good hands-on experience in managing medical emergencies. However, conditions arising from extremes of cold and heat are relatively common and are potentially life threatening.
Hence, a knowledge of effective techniques to prevent heat and cold related injuries and conditions, and the correct treatments to apply, can be life-saving. Understanding why heat and cold related conditions develop is key to understanding both their treatment, and more importantly their prevention.
Heat and the body
The body loses and gains heat through: conduction, convection, radiation and evaporation, and generates heat through metabolic activity and muscle movement. These physiologic processes are key to understanding thermal illness
Conduction is the transfer of heat between objects by direct contact. Heat flows from higher temperature to lower temperature. In this way, heat may be lost from the body when a person is placed on the snow with little insulation, or heat may be provided to a hypothermic patient by contact with a warming device.
Contact with clothing that is soaked with water or sweat is another example of heat loss by conduction. The well, alert person usually protects themselves from these heat losses by wearing insulating clothing and footwear and using appropriate equipment. The person who is injured, ill, intoxicated or has an altered conscious state is at risk of significant heat exchange when lying uninsulated on hot or cold ground for prolonged periods.
Of note water conducts heat nearly 30 times more effectively than air – we get much colder in water than air at the same temperature.
Convective heat exchange occurs when heat is transferred between the body and air. Increasing air movement will increase transfer by convection. This is one of the reasons we feel cooler standing in a breeze than in still air.
Radiation is the transfer of heat through space, by electromagnetic radiation, without any intervening medium. Radiation is a major source of heat gain to the body in hot climates. Protection from direct sunlight and the wearing of light coloured clothing reduces absorption of solar radiation.
Evaporative heat loss occurs when water is converted to water vapour on a body surface. This vapourisation requires substantial amounts of chemical heat (which is then effectively lost from the body). Such vapourisation occurs with sweating and with breathing, because the air we breathe out contains water. The scalp, face and upper torso are the most important sites for cooling from sweating.
The capacity for sweat to evaporate varies with humidity, ambient temperature and wind velocity. High ambient temperature, high humidity and low wind velocity minimises evaporative heat loss from the body and results in an increased risk of heat illness. When sweating is minimal, evaporative heat loss accounts for only 15% of total body heat loss. However, with maximal sweating, evaporative heat loss can increase to around 70% of body heat loss.
Evaporative heat loss through breathing can be reduced by breathing warm, moist air.This can be partly achieved by wearing a scarf or mask over the face where it will act as a moisture exchanger. Loose fitting clothing increases both convective and evaporative heat losses.
Maintaining body temperature
The body responds to heat and cold stress by changing blood flow quantities and patterns around the body, modifying sweating, adjusting metabolism, and changing behaviour. Blood flow to the superficial vessels (those just beneath the skin) is reduced in cold conditions to conserve heat and is increased in hot conditions to aid heat loss.
Sweating facilitates heat loss by evaporative cooling and increases in response to exertion and rises in body temperature. It is the body’s most important adaptive method of losing heat. Shivering, which can increase the body’s heat production five-fold, and muscular activity, which can increase heat production ten-fold, are the main means by which the body generates metabolic heat in cold environments.
Shivering and deliberate activity to keep warm have high energy demands and are limited in duration by energy availability. This is why feeding with high sugar foods is important in hypothermia prevention and treatment. People in normal states of consciousness automatically modify their behaviour to cope with heat and cold stress, for example, by seeking shelter or shade, curling up in a ball or lying spread-eagled, depending on the need to gain or lose heat.
The normal body temperature is 37.0°C (range 36–37.5°C). Core temperature refers to the temperature of blood supplying vital organs, such as the brain, heart and lungs and is the most reliable and generally used measure in clinical settings. Core temperature is measured deep within the body (e.g. rectum, bladder) and requires special thermometers.
Whilst portable ear and forehead thermometers are now widely available, they give grossly unreliable readings in hot and cold field settings.
It is generally impractical to measure core temperatures in the field, so heat or cold illness should always be considered when a person displays relevant symptoms and signs in a setting where heat or cold injury could occur.
Alcohol and cold and heat injury
There is the incorrect impression that alcohol has a warming effect on the body. This is partly due to the ‘flushing’, from increased blood flow to the skin and gives a false perception of body warmth, when actually there is increased heat loss.
Alcohol increases the risk of inappropriate behaviour in adverse environments, including failure to take usual precautions to protect against cold or heat, and engaging in risk-taking behaviour.
Alcohol alters the brain’s perception of core temperature, increases blood flow to the skin, impairs body heat production and impairs shivering, all of which are undesirable in the face of cold injury and may result in lowered body temperature.
Alcohol directly impairs the body’s ability to regulate body temperature at both high and low temperatures and should be avoided in situations of heat and cold stress.