The human body can only operate in a narrow range of temperatures. It has several mechanisms to ensure that even in extreme environments the vital organs remain as close to the ideal physiologic temperature of approx 99 degrees Fahrenheit as possible.
Each of these systems can be overwhelmed by prolonged exposure or diminished compensating capacity secondary to extremes of age or other diseases.
Hypothermia: A core body temperature below 94ºF
Hyperthermia: A core body temperature between 101-104ºF
Hyperpyrexia: A core body temperature between 104-107ºF
Radiation: Direct loss of energy through waves (light/heat/radio) into surrounding space
Conduction: Loss of heat through contact with a cooler surface.
Convection: Loss of heat through contact with a cooler gas or fluid.
Evaporation: Loss of heat by its transfer to fluid on the skin, after which said fluid then evaporates into a gas carrying away the heat energy.
Heat exposures are most common in both high temperature and high humidity climates, with the combination of the two leading to the greatest risk.
The high temperature reduces the body's ability to get rid of heat via radiation (the natural process of heat moving from hot to cool areas). The high humidity reduces the body's ability to get rid of heat via evaporation (sweating). The combination of the two can result in illness even in those who are young and in shape.
A person's age, preexisting medical conditions, and medications can all dramatically influence the extent to which heat emergencies affect the body and the tendency for them to occur.
During hyperthermia, the body acts to dissipate heat via vasodilation of the small arteries in the skin to transfer heat from the body core to its surface. However, dehydration or outside temperatures higher than the body's core temperature sabotage this strategy, since convection, conduction, and radiation no longer occur.
Heat Exhaustion (101-104ºF)
Heat exhaustion is the inability to maintain adequate cardiac output during strenuous physical exercise or environmental heat stress.
A core body temp of 101 to 104 degrees in conjunction with tachycardia, weakness, fatigue, and light-headedness are central to the diagnosis.
Other symptoms such as hypotension, syncope, muscle cramps, abdominal pain, nausea, and vomiting are indicative of more severe disease and usually signify significant electrolyte losses (through sweat) and/or dehydration.
Regardless of the symptomatology, the patient must NOT have significant nervous system dysfunction (seizures, altered level of consciousness, respiratory depression) as these symptoms signify heat stroke or another unrelated condition.
Heat Stroke (> 104ºF)
Heat Stroke is a reversible central nervous system dysfunction that is induced by an elevated core body temperature, often above 104 degrees Fahrenheit or 40 degrees Celcius.
Many of the symptoms of heat exhaustion will be present, but the presence of seizure, altered level of consciousness, or focal weakness are signs that point to heat stroke.
Heat stroke is often associated with severe symptoms such as sinus tachycardia, arrhythmia, tachypnea, significant hypotension, and widened pulse pressure.
There are two specific types of heat stroke classic and exertional.
- Classic: usually in the elderly with chronic conditions that compromise thermoregulation, e.g., cardiovascular disease, obesity, disability, drugs (alcohol, cocaine, beta-blockers, diuretics, and anticholinergics.
- Exertional: usually in young, healthy persons during strenuous exercise in high temperature/humid environments, e.g., athletes, soldiers in basic training, etc.
Assessment and Management
SIGNS & SYMPTOMS: Heat emergencies present as a spectrum of symptoms, they begin with sweaty skin, flushing, and a feeling of lightheadedness. They then progress to dry skin, weakness, possible nausea/vomiting, muscle cramping, and a gradually increasing heart rate. Eventually, heatstroke develops, which is defined as the presence of severe neurological symptoms, often a gradual decrease in the level of consciousness.
Signs of a heat emergency begin with an increased capillary refill, mild tachycardia, and a normal or slightly elevated temperature. As the temperature increases the tachycardia will worsen, the skin will feel profusely warm, and hypotension may begin to develop. In severe cases (heat stroke) as mentioned above, neurological symptoms develop.
These signs and symptoms may be minimal in the young and elderly until florid heat stroke develops. These populations are also at increased risk of heat emergencies and should be closely monitored.
COMPLICATIONS: Extremes of temperature reduce the ability of platelets and clotting factors to stop bleeding. These patients may develop external or internal bleeding. The elderly and those on anticoagulant medications are at the greatest risk. In rare cases, hyperthermia can cause significant respiratory distress, watch for changing respiratory rate or significant drops in oxygen saturation.
MANAGEMENT: Temperature is an unreliable sign of hyperthermia, especially in the field, only rectal and esophageal measurements reflect the actual core temperature. Use your clinical judgment about the signs/symptoms and the situation in which you found the patient to make treatment decisions.
The best first action after assessing ABC's is to remove them from the environment that is causing their situation, taking them into an air-conditioned building or the back of the ambulance is an excellent first step. If excess clothing is present remove it and place cool towels or ice packs (wrapped in towels to prevent frostbite) on the neck, in the armpits, and near the groin.
During transport continue to monitor the patient's ABCs and watch for the development of vomiting, placing the patient in the left lateral decubitus position if you feel there is a significant risk of vomiting or aspiration.