Australians are well adjusted to the harsh sun, and most stay indoors or under shade on excessively hot days. However, there is a small portion of the population who are required to work through the heat of the day in order to complete time sensitive missions or training, and for these few, heat illness is a real risk. Heat illness is a complex illness and has many different stages of deterioration, all requiring different treatment approaches.
This article will outline the different stages of heat illness, listing the appropriate clinical manifestations and will also provide treatment options based on current evidence-based practice. The health professional will gain an understanding of heat illness aetiology, pathophysiology and prehospital management, including associated drug therapies where applicable.
Introduction to heat illness
Heat-related illness represents a continuum of disorders from minor syndromes such as heat cramps, heat syncope, and heat exhaustion to the severely life-threatening disorder known as heat stroke. Heat illness is a major cause of wilderness-related morbidity and mortality, and is common in the military combat setting (Lugo-Amador et al, 2004).
Aetiology
With the influence of global warming, it is predicted that the incidence of heatstroke cases and fatalities will become more common, however the exact data on occurrence of heat stroke is not precise. Exertional heatstroke (EH) generally occurs in young and fit individuals, usually military personnel and athletes who engage in strenuous physical activity for a prolonged period of time in a hot environment (Dutta and Sahoo, 2009).
A significant number of heat illness cases occur per annum in the ADF. Operational demands—such as lack of acclimatization, wearing heavy/inappropriate clothing and dehydration—are important predisposing factors for exertional heatstroke. Deployment of troops in desert terrain causes an understandable increase in the number of such cases. Other common causes include: increased heat production due to sepsis or increased metabolism; increased muscular activity caused by convulsions or stimulant drugs; a reduction in sweating, and some medications such as antihistamines and anticholinergics.
The clinical changes associated with severe heat illness can be subtle and easy to miss if medical personnel and soldiers do not maintain a high level of awareness and monitor high risk activities closely. Fatigue and exhaustion during exercise occur more rapidly as heat stress increases and are the most common causes of withdrawal from activity in hot conditions (Armstrong et al, 2008).
A case study
It is 2:30 pm on a Monday afternoon when the company medic is called out to a 22-year-old male who has collapsed with heat during a mission rehearsal exercise in Afghanistan. Initial reports indicate that the patient is in an agitated, altered conscious state and has stopped sweating. The medic is five minutes away by road and the temperature is 38°C.
On arrival, the soldier's section commander notifies the medic that they have been training all day, with brief rest periods. The patient had complained of dizziness, finally falling into an ‘apathetic’ state. The medic takes a history and a set of observations:
His vital signs were: pulse, 148 beats per minute (bpm); blood pressure, 80/50 mmHg; respiratory rate, 24 breaths per minute; rectal temperature, 41.1°C; Glasgow Coma Score, 8 (E2–to pain, V2–incomprehensible, M4–withdrawals from pain). The patient's head was atraumatic but his pupils were 5 mm and fixed. His neck showed no jugular vein distension and his lungs were clear.
A three-lead electrocardiograph (ECG) showed sinus tachycardia with no arrhythmias. His abdomen was soft and non-distended. His extremities had no deformities; no cyanosis, erythema, or oedema was evident, and his skin was warm. Rectal examination revealed poor tone with heme-negative brown stool. At this point, the presumptive diagnosis of heat stroke was made.
Pathophysiology
Body temperature regulation is a balance between heat production and heat loss. Voluntary exercise can increase heat production by up to 20 times, with the body's metabolism also being increased by other factors such as hyperthyroidism or ingestion of sympathomimetic drugs.
In the military environment, it is common to see heat factors adding to the heat load. This can interfere with heat dissipation, which is primarily lost by conduction, radiation, convection and evaporation. In extreme conditions however, evaporation of sweat is the dominant mechanism of heat loss. This works by using the principles of converting liquid to gas, or sweat to water vapour, thus using energy and dissipating heat.
Heat loss in a hot environment can impose large metabolic demands on the body, with skin blood flow having the ability to increase from <0.5 to 7–8 litre/minute in a hot environment, with associated sweat loss of 1–2 litre/hour in some cases. These two factors reduce stroke volume, so the heart typically compensates with an increased rate where possible while operating with a lowered blood pressure (Mahadevan and Garmel, 2005).
| Injury | Signs | Symptoms |
|---|---|---|
| Heat exhaustion (moderate) | Elevated body temp | Headache |
| < 40°C | Weakness | |
| Tachycardia | Nausea and vomiting | |
| Hypotension | Altered conscious state | |
| Syncope | Muscle cramps | |
| Diaphoresis | ||
| Heat stroke | Elevated body temp | Nausea and vomiting |
| > 40°C | Agitated state | |
| Tachycardia | Altered consious state | |
| Hypotension | Unconsiousness | |
| Tachypnoea | ||
| Absence of diaphoresis |
Heat syncope is caused by brief peripheral vasodilation combined with dehydration following heat exposure—this will cause a sudden drop in blood pressure, causing syncope.
Heat exhaustion is a clinical syndrome characterized by volume depletion, specifically water and electrolytes that are lost to sweating and poor hydration techniques. In heat stroke, the hyperthermic patient initiates a systemic inflammatory response, leading to a syndrome of multi-organ dysfunction in which encephalopathy dominates and cardiogenic shock is prevalent (Schwartz et al, 2007).
Assessing the patient
When assessing the heat illness patient, the intensive care medic should conduct the following:
Clinical manifestations
Heat illness can be described as a continuum, from mild heat exhaustion to severe heat stroke. However, as initial clinical manifestations are fundamentally the same for both conditions, all conditions will be divided into either heat exhaustion or heat stroke.
Heat exhaustion is the most common heat-related illness, and is characterized by volume depletion. It encompasses all heat-related illnesses that do not involve altered mental status. Patients often complain of fatigue, dizziness, weakness, headache, and nausea.
On physical examination, they may be sweating, have moderate elevations in core temperature (often less than 40 °C), and have a normal mental status. They may be tachycardic and have orthostatic hypotension, aswell as other clinical signs of dehydration (Colwell 2008). As heat exhaustion can range from mild to severe, it incorporates heat cramps and heat syncope.
Heat cramps are characterized by the development of painful muscle spasms in the skeletal muscles, dilated pupils and weak pulse, following exertion in a high temperature. The condition is commonly seen in those who perform heavy labour in intense heat, thus losing too much sodium and water (Ashworth, 1995).
Heat syncope is generally the result of increased venous blood volume in the legs, arms and skin combined with dehydration. If cardiac filling pressure and stroke volume rapidly decline, a fall in cardiac output and blood pressure will occur, resulting in heat syncope (Armstrong, 2003). It is distinguished from heat stroke by the rapid return of a normal level of consciousness and mental status after treatment has been initiated.
Heat stroke
In the context of elevated core temperature, any prolonged alteration in mental status should be considered heat stroke. This life-threatening illness has a mortality rate as high as 50% and is characterized by a rise in core body temperature above 40 °C and central nervous system dysfunction. Patients may complain of fatigue, malaise, dizziness, weakness, headache, and nausea. Vomiting and diarrhoea occur in up to two-thirds of patients.
The critical difference between heat exhaustion and heat stroke is sustained alteration in mental status. Delirium, seizures, or even coma may also develop. Patients are often tachycardic and hypotensive. Tachypnea is generally seen and can result in a respiratory alkalosis with associated metabolic acidosis. The brain is sensitive to heat, especially the cerebellum, so early signs may include an ataxic gait, confusion, and disorientation (Colwell, 2008).
We now understand that the classic clinical features of heat stroke are neurological dysfunction, core temperature above 40 °C and hot dry skin. However, relying on this classic triad to make a diagnosis will result in a number of cases being missed. Loss of consciousness is a constant feature of heat stroke, but by the time medics arrive on scene the patient's conscious state may have improved, although some neurological abnormality will persist. Temperature readings may also be misleadingly low, due to effective cooling by first aiders or incorrect temperature measurements at inappropriate sites, such as the oral cavity or axilla.
Of interest to note, Shapiro and Seidman (1990) conducted a study into field and clinical observations of exertional heat stroke patients, concluding that profuse sweating is in fact a common feature among patients suffering heat stroke. This challenged many theories concerning the clinical presentation, and is an important feature to note when assessing heat illness patients. It is important to take a good history when making a provisional diagnosis. A good history combined with objective and subjective observations will make for an accurate assessment of the patients condition.
Prehospital care management
The following treatment plan should be used when treating heat exhaustion/stroke. It is important to note that, in the military setting, medics should always be aware of any tactical considerations when treating heat illness patients. This means that it may not always be possible to create ideal treating conditions, i.e. ambulance, shade, and it may not always be possible to remove clothing and/or body armour in the middle of an enemy contact. It is for this reason that prevention of heat illness is the best cure on the battlefield, as it allows more soldiers to be actively involved in operations.
Pharmacology
No drugs significantly reduce core temperature in patients with heat illness. In contrast to patients with fever who develop elevated temperatures because of an elevated hypothalamic set point, hyperthermic patients do not benefit from antipyretic therapy (Dilshad and Muhammad, 2007).
Some patients may require glucose replacement drugs, i.e. dextrose 50% when all muscle glycogen stores are depleted. However, the mainstay of therapy involves rapid cooling and rehydration using a crystalloid solution, preferably chilled sodium chloride 0.9% intravenous (IV) solution. Muscle relaxants and neuroleptics have been used in the past to treat complications of heat illness, shivering, and seizure prophylaxis, but clinical evidence are lacking.
Heat illness prevention
Heat illness prevention is the single most important cure. The following tips will help patients avoid heat illness:
As well as mandatory annual heat awareness education, the ADF uses a wet-bulb globe temperature (WBGT) monitor to assess environmental heat stress. The WBGT monitor is a small portable device, which responds to temperature, humidity, sun and wind. It is placed in the same working environment that the soldiers are exposed to and provides real-time accurate weather readings.
The readings are then carefully calibrated, measured and interpreted by trained personnel and the appropriate recommendation is then made on work:rest ratios for soldiers.
As Budd (2008) suggested, the WBGT is not without its limitations. Therefore it is important to remember that, along with the overall weather reading, the medic must ensure that they carefully evaluate the soldier's activity, clothing, and other factors, all of which can introduce large errors into any predictions of adverse effects.
Conclusion
It is important to remember that heat illness presents as a continuum of disorders, and that a patient who experiences heat stroke may have experienced heat cramps and heat exhaustion prior to your arrival. It is for this reason that we as health professionals need to be vigilant of preventative measures and act promptly to stop heat illness from occurring.
This article has addressed many of the issues relating to heat illness, ranging from aetiology and pathophysiology to clinical manifestations, treatment and prevention strategies. As medics within the ADF, it is important to remember that as well as being soldiers, we are medics first, and our utmost duty is to ensure the safety of the men and women within our units. This ensures that the ADF maintains its capabilities, both during wartime and peace operations.