Updated 13/11/23

HEAT ILLNESS

Human physiological and anatomical systems are ‘designed’ for life in moderate and warm environments. In healthy adults, the normal core body temperature when measured internally is about 37°C.  This fluctuates by about 0.5 °C throughout the day as the body’s needs and activities change.

The term ‘hyperthermia’ is simply a rise in body temperature, and may not be a problem. Although there is some variation, you should consider a temperature >37°C as abnormal until proven otherwise. Professor George Havenith, who spoke at the 2015 MREW Medical Conference, explained that exercise will cause an increase in body temperature, with temperatures around 38°C typical for moderate work and even higher values (occasionally up to 40°C) for heavy exercise (e.g. marathon). Provided that they are transient and controlled, and the person remains well, increases such as this are seldom a problem to the body and should be considered a normal phenomenon in thermoregulation.

‘Heat illness’ is not just simple hyperthermia. It includes a temperature rise with a number of clinical conditions, ranging from muscle cramps which are benign, to heat stroke, which is a life-threatening emergency. As such, absolute temperature thresholds alone should not be routinely used to define heat illness in individuals who are otherwise well.

Heat can be lost by several routes, as shown in the diagram below. A rise in blood temperature of less than 1°C triggers compensatory mechanisms to start.

Heat is generated within the body by metabolic processes, and is absorbed from the environment when the ambient temperature is higher than skin temperature. A rise in blood temperature by less than 1°C triggers thermoregulation reflexes to increase blood flow to the skin by up to 8 L/min, with sixfold blood flow increases to the forearms alone. As blood is shunted to the skin, less flow will go to the internal organs such as the kidneys. An increase in body heat only becomes a problem (“illness”) when the various body mechanisms to dissipate the heat are unable to cope. There are a number of predisposing factors t As o heat illness:

• When the environmental temperature is greater than 35°C, particularly in the presence of high humidity (80%). This is because as the environmental temperature increases, the body will actually gain heat from the environment, so that evaporation of sweat is the only cooling mechanism.

• Non-heat-acclimatised individuals

• History of previous heat illness especially heat stroke

• Acute sunburn over a large area of the body is linked to impaired sweating, and this effect can last for a week.

• Muscular exertion (in temperatures above about 21°C), especially in physically unfit individuals, people who are not heat acclimatised, or if heavy clothing is worn (obligatory for e.g. fire fighters).

• Dehydration, especially during exercise, results in reduced blood flow to the skin which impairs production and evaporation of sweat

• Excessive clothing

• Extremes of age

• Lack of sleep (reduces sweating and blood flow to the skin)

• Obesity

• Excessive alcohol consumption

• Chronic illness e.g. cardiovascular disease, diabetes

• Some drugs (prescription and drugs of abuse) increase heat production (e.g. amphetamines such as Ecstasy) or reduce the body’s ability to get rid of excess heat e.g. by impairing sweating (e.g. antihistamines)

Heat Acclimatisation – A summary

Acclimatisation is the body’s ability to improve its response and tolerance to heat stress over time, and it is the most important factor that determines how well an individual withstands extreme heat. This is an adaptive response by the body to repeated heat stress. It results in:

• Improved heat loss by dilation of blood vessels in the skin

• Improved heat loss by increased sweating that starts at a lower body temperature

• Reduced sodium loss in sweat

For practical purposes, assume that all casualties we are called to that have any form of heat illness are not heat-acclimatised. This means that they will be hot, dehydrated and low on sodium. Their heart will also be under considerable stress.

Heat illnesses relevant to Mountain Rescue

The term ‘heat illness‘ incorporates a range of conditions from minor to severe. The most important ones for MR are:

• Heat cramps

• Heat syncope

• Heat exhaustion

• Heat stroke

Use symptoms, signs and temperature to help differentiate between these. Always check BM with any altered mental state.

General points for assessment

Typical symptoms of heat illness are:

• Thirst

• Throbbing headache

• Dizziness

• Nausea ± vomiting

• Feeling ill, tired, weak or faint

• Muscle cramps

• Prickly heat (heat rash)

Ask about fluid intake, the level of exercise and look at the clothing that the person is wearing. Most importantly, assess their mental (i.e. alert, drowsy, confused, etc. – see below) and neurological states (e.g. poor coordination, etc. – see below)

General principles of management in Heat Illness

REMOVE FROM THE HEAT AND IMMEDIATELY INSTITUTE RAPID COOLING MEASURES, BECAUSE THE LONG TERM OUTCOME IS RELATED TO THE DEGREE AND DURATION OF THE HYPERTHERMIA.

• Stop exercise

• Shade from the sun

• Remove excessive clothing

• Active cooling

• Fluids

Heat cramps

Muscle cramps are associated with exertion in hot environments. These are painful, involuntary muscle contractions during or immediately after exercise. Dehydration and electrolyte disturbances have been associated with these, and rehydration with electrolyte-containing fluids has been found to restore normal function.

Heat Syncope

Heat syncope (fainting) is transient loss of consciousness in the context of heat exposure with a relatively rapid return to normal function. It is particularly likely after prolonged standing in a hot environment or after rapidly standing up from a lying or sitting position. It is considered a benign clinical condition that should resolve with rest and possibly re-hydration at comfortable ambient temperatures.

Management

• Check for injuries sustained because of the fall

• Lie the casualty down. Feet up if possible.

• Cooling

• Fluids when they are conscious. Dilute carbohydrate-containing drinks may be absorbed faster than water. NB concentrated sugar drinks are very slowly absorbed.

Patients with heat syncope usually recover rapidly with the correct treatment. If they don’t, consider other causes of loss of consciousness.

Heat Exhaustion

Heat exhaustion may develop slowly over several hours, typically with prolonged low to moderate intensity exertion in warm hot weather; or more quickly from exposure to high ambient temperature or strenuous exertion. It has symptoms that can range from uncomfortable to debilitating, and may limit continuation of exercise in the heat. Heat exhaustion can progress to heat stroke if left untreated or unrecognised in a hot environment, although it is important to appreciate that heat stoke can occur on its own without preceding heat exhaustion.

Heat exhaustion is hyperthermia + dehydration + NORMAL mental state

Symptoms & signs

• Normal mental state

• Lack of coordination

• Sweating is present

• Nonspecific symptoms (weakness, fatigue, thirst, headache, weakness, nausea, dizziness, loss of appetite and muscle aches)

• Vomiting may occur

• Drop in blood pressure on standing (this can lead to feeling faint)

• Fast heart rate, low BP

• Core body temperature >37° but less than 40°C. Occasionally, it may be normal by the time the team arrives because the casualty has stopped exercising.

• Thirst/Dehydration

Management

• Stop all exertion

• Shade from the sun, if possible

• Lie the casualty on their back and raise their legs if they feel faint

• Remove excessive or restrictive clothing and rucksacks

• Cool the patient by a combination of any methods available e.g. place wet packs on the neck, chest wall, arm pits, and groin; fan the patient, while spraying with tepid water.

• Administer fluids (ideally containing some salt). A salty snack is appropriate. Hydration tablets and peanuts are available from the equipment room.

• Do NOT give aspirin or paracetamol (as you would to someone who has a raised temperature due to illness such as the flu)

• You can give buccastem to control nausea and/or vomiting if this is excessive and persistent, but do not use it prior to doing all the above as it can interfere with thermoregulation. Check with a doctor first.

Generally patients recover fairly rapidly, and hospitalisation is not necessary. Do not restart any exercise until they have cooled and settled. It may then be possible to escort the casualty from the mountain.  

Heat Stroke

This is hyperthermia (>40°C) + ALTERED mental state, seizures or coma

Heat stroke can occur on its own, or it can be the end stage of heat exhaustion when compensatory mechanisms for dissipating heat have failed. The transition from heat exhaustion to heat stroke is indicated when the victim begins to show abnormal mental status (e.g. confusion) and/or neurological function (e.g. poor coordination). Mental state changes in an individual who is performing exertion in the heat are the defining characteristic of heat stroke. Sweating is still likely to be present in the early stages of heat stroke.

In the general population, vulnerable groups such as children under two, the elderly, and people with certain chronic diseases e.g. heart or circulatory problems can develop heat stroke even without physical exertion, so long as the conditions are right.  For example, the August 2003 heat wave in France resulted in many thousands of excess deaths from heat stroke, particularly in elderly people. This is referred to as ‘classical heat stroke’.

In mountain rescue,  hyperthermia is usually only seen in people who have engaged in sustained physical exercise during periods of high ambient temperature, particularly if the humidity is high and/or they are dehydrated. Heat stroke developing in these people is called ‘exertional heat stroke’. It is also seen in competitive sport, the military (e.g. two fatalities in July 2013 on a military training exercise in Wales) and firefighters. The rest of this section will focus on this type of heat stroke because that is what we see in MR.

Note – cases of exertional heat stroke can occur at mild ambient temperatures (21°C) if the level of exercise is sufficient, coupled with inappropriate clothing and inadequate hydration.

Symptoms & signs

• Altered mental state (confusion, disorientation, bizarre behaviour, poor coordination, seizures, unconsciousness). Loss of coordination is one of the earliest manifestations.

• Core body temperature usually greater than 40°C

• Fast heart rate (may be greater than 130/min)

• Low blood pressure

• Rapid, shallow breathing

• Sweating may be present or absent (sweat rates reduce when about 5% body weight has been lost i.e. about 3.5 litres of fluid)

• Unexpectedly, can be shivering (which occurs due to disturbances of the thermoregulation centre in the brain)

• Dehydration

The importance of altered neurological state cannot be overemphasised. Solitary walkers who develop heat stroke will not be able to call for help when they collapse.

Why people die from heat stroke

Published evidence indicates that there are four main reasons:

• Inaccurate temperature assessment

• No care, or treatment delayed

• Inefficient cooling techniques

• Delay in transport to hospital

These factors result in severe damage to the internal organs of the body and it is this that proves fatal.

Management

• If the casualty presents with a seizure and the diagnosis of heat stroke isn’t immediately apparent, check the temperature and BM for hypoglycaemia.

• IMMEDIATE RAPID WHOLE-BODY COOLING IS ESSENTIAL. Any body of water can be used e.g. pond, stream, river, etc. Research shows that ideally, this should be done within 30 minutes of the onset of the condition. The gold standard for rapid cooling is cold water immersion. because this provides a high thermal gradient between the cold water and the skin. In the initial stages, the colder, the better (ice water is described in the literature). The casualty should never be left alone due to risk of aspiration and drowning in a person who has impaired conscious level.

• If immersion or conductive cooling is unavailable, evaporative cooling measures should be initiated by loosening or removing clothing and dousing the patient with cold water to maximise the water-vapour skin interface. Cold bottles of water or wet pads can be placed in the groin and under the arm pits.

• The aim is to get the body temperature down to <39°C.

• Shade from the sun with good ventilation, and remove tight/heavy clothing and rucksacks.

• Protect the airway.

• Re-hydrate with IV fluids. This reduces strain on the heart and improves heat dissipation from the body. If possible, give 1 L (2 L maximum) saline. Be very cautious about giving oral fluids. A casualty with impaired conscious level is at risk of vomiting and inhaling the vomit, particularly if they go on to have a seizure.

• Treat seizures with midazolam

• Do NOT give aspirin or paracetamol

• Monitor the temperature every 30 minutes if possible during cooling.

• Immediate evacuation by air (if possible) to hospital but only after the casualty has been cooled to below 40°C (ideally nearer 38°C).
  
  

Hyponatraemia

Low sodium in the blood is called hyponatraemia (pronounced ‘hyponatreemeeah’). The physiology underlying this is complex, but the most important contributing factor is over-drinking of water or sports drinks, in excess of sweat, urine and respiratory losses, before or during activity. Such a practice has not been shown to have a significant effect on heat tolerance, but can create a problem by diluting the sodium in the blood. An aggravating factor affecting water retention is non-steroidal anti-inflammatory drugs, such as ibuprofen, which can cause water retention by the kidneys.

Current expert advice is that drinking to thirst is the primary strategy for preventing exercise-associated hyponatraemia. Importantly, sodium supplementation during exercise in an attempt to offset over-hydration, has not been shown to prevent the development of hyponatraemia during physical activity lasting less than 18 h. In addition, getting an imbalance between taking extra sodium and fluid intake can actually lead to the opposite problem i.e. a high sodium in the blood, if the sodium intake is greater than the water intake.

These facts are important for MR teams, because it means that we don’t have to worry about drinking or taking in extra salt when on rescues during the summer. A normal diet and drinking to thirst are sufficient. Sodium and/or salty snacks are fine along with the appropriate fluid intake, particularly in long, hot events in non-heat acclimatised individuals, but all excesses should be avoided.

Symptoms, signs and management
Hyponatraemia is a critical illness that can only be accurately confirmed with serum sodium measurement, but this isn’t possible in remote locations. It is therefore necessary to look at the clinical features. A history of excessive fluid intake in excess of likely losses is important. The clinical features of hyponatraemia are superficially similar to heat illness (headache; nausea; vomiting; thirst; dizziness when standing up; dry mouth; etc.), but the presence of altered mental status, seizures or coma, without another explanation, such as hypoglycemia or trauma, and the absence of significant hyperthermia suggests sodium imbalance.

The key differences between heat illness and hyponatraemia are the fluid intake and the body temperature.

If hyponatraemia is suspected, stop further intake of dilute fluids. Oral sodium in foods with high sodium content (salty snacks) may increase serum sodium levels and enhance symptom relief more than fluid restriction alone if the hyponatraemia is mild. However, if the casualty is really ill, with significant neurological manifestations, they should be given intravenous saline on the hill. All casualties who are believed to have a problem with sodium balance should be transferred to hospital. Alerted the receiving agencies of the potential diagnosis of hyponatraemia so that they don’t inadvertently give the casualty dilute fluids, which will lower the sodium levels further.

Rescuer Safety

Hyperthermia can affect rescuers too. Make sure you are heat-acclimatised as possible before going out on a rescue. With the requirement to engage in significant physical exertion and carry heavy loads on a rescue, reduce the risk of running into trouble by taking frequent breaks and being careful to ensure that you are adequately hydrated before starting out.

Because most cases of hyperthemia will occur during periods of high ambient temperatures ± humidity, rescuers must be especially aware of the dangers when responding to this type of incident. If someone else has been affected, you could be too.

Heat acclimatisation in more detail

Acclimatisation to a hot environment is an important adaptive response. It usually requires 1-2 hours of heat-exposed exertion over 10-14 days to reach maximum benefit. Once established, these adaptations will last for a month. During initial exposure to a hot environment, workouts should be moderate in intensity and duration. A gradual increase in the time and intensity of physical exertion over 10-14 days should allow for optimal acclimatisation. Individuals with high levels of cardiopulmonary fitness tolerate more activity in heat-strained conditions and acclimatise to heat more rapidly because they have increased sweat volumes and higher subjective tolerance for activity when hyperthermic. The heart has to work very hard to support the adjustments to the circulation that are needed to transfer heat from the core and working muscles to the body surface. It may have to pump 4-5 times as much blood per minute as it would at rest in normothermia. This explains why people with poor cardiac function are particularly vulnerable to heat illness.

Evidence suggests that a bout of heat stroke may acutely reset these physiological adaptations and cause elevated risk for subsequent heat injury for months after the initial event.

Heat acclimatisation is specific to the climate and activity level. If individuals will be working in a hot, humid climate, heat acclimatisation should be conducted under similar conditions. Importantly, there are limits to the degree of protection that acclimatisation provides from heat stress. Given a sufficiently hot and humid environment, no one is immune to heat injury. Furthermore, evidence suggests that a bout of heat stroke may actually reset these thermoregulatory adaptations and cause elevated risk for subsequent heat injury for months after the initial event..

Acclimatisation and sweat
The evaporation of sweat is the body’s most effective heat loss mechanism. Sweat-related fluid loss can easily be >1-2 L/h. This will result in dehydration if not replenished by adequate fluid intake during activity. Once heat acclimatisation is achieved, skin vasodilation and sweating are initiated at a lower core temperature (e.g. 37.5°C) than in a unacclimatised person (e.g. 38.5°C), and higher sweat rates can be sustained without the sweat glands becoming “fatigued.”

Although there is inter-individual variation in the sodium content of sweat, as a general rule, an unacclimatised individual will secrete sweat containing a lot of sodium (concentration approximately 40% of the level in the blood), whereas the concentration of sodium secreted from the sweat glands of an acclimatised individual is significantly lower (approximately 5% that in blood).

For information only, but to give an idea of actual amounts, the graphs below are taken from a study looking at sweat rate and sodium concentration in athletes.

Maximum sweat rates are reached when core temperature has risen to 39°C. The sweat rate reduces when about 5% body weight has been lost (i.e. about 3.5 litres of fluid). Provided that fluids are not restricted during physical activities, heat-acclimatised individuals will be better able to maintain hydration during exercise.

Hydration and heat illness
The most readily modifiable physiologic risk factor for heat illness is hydration status. Although endurance athletes may comfortably tolerate weight losses of 3 to 4% during events, fluid losses that result in a 2 to 3% decrease in body weight (i.e. 1.5-2 litres fluid loss) correlate with greater core temperatures at a given work load in the heat. Dehydration increases physiologic stress, decreases sweat rates, increases perceived exertion, and leads to increased core temperatures. People who lose 6-7% of their body weight in fluid (i.e. around 4.5 litres) will collapse and a 10% loss may be fatal. The modern advice is to “drink to thirst”, as this will prevent >2% loss of body weight.

Activity considerations
The accumulation of heat by the body may be tempered, in some circumstances, by an activity that will enhance heat transfer to the environment e.g. a swimmer, or wind passing over a cyclist. But for most people this isn’t the case, so it may may be necessary to take breaks during intense exercise. This is relevant to MR teams undertaking rescues on hot days.

Additional Resources

Lipman G, et al. Wilderness Medical Society Practice Guidelines for the Prevention and Treatment of Heat Illness: 2019 Update. Wild Environ Med 2019 DOI: https://doi.org/10.1016/j.wem.2018.10.004

Bennett BL, et al. Wilderness Medical Society Practice Guidelines for the Management of Exercise-induced hyponatremia: 2019 Update. Wild Environ Med 2020;31:50-62

Auerbach PS. Wilderness medicine (7th Ed). 2017. Elsevier.

Truhlar A, et al. European Resuscitation Council Guidelines for Resuscitation 2015. Resuscitation 2015; 95: 148-201