Hyperthermia

Luan E. Lawson, MD
Brody School of Medicine, East Carolina University, Greenville, NC

Edited By: Lorraine Thibodeau
Albany Medical Center, Albany, New York

Objectives

At the completion of this module, the student should be able to:

  • Describe the signs and symptoms consistent with heatstroke
  • Differentiate Classic heatstroke compared to Exertional Heatstroke
  • Describe appropriate methods of cooling heatstroke victims

Heat related illness is a continuum of conditions that results from the body’s inability to deal with heat stress and dissipate excessive heat. It includes minor problems such as heat cramps, heat edema, prickly heat as well as heat syncope and heat exhaustion. Classic heatstroke and exertional heatstroke represent the most severe manifestations of heat related illness and are associated with neurologic dysfunction and multisystem organ damage.

Heat waves claim more lives each year than all other weather-related exposures including hurricanes, tornadoes, floods, and earthquakes in the United States. While heat related illness occurs most frequently during summer heat waves and high environmental temperatures, they can also occur in temperate climates, especially in patients with chronic medical conditions, advanced age, mental illness, occupational exposures to high temperatures, or inadequate acclimatization. Heat related illness most commonly affects the elderly and young children.

Classic Presentation

Presentation of Heat Related Illness

  • Typically temperature exceeds 40C
  • Tachycardia
  • Commonly normotensive, but may be hypotensive
  • Severe CNS dysfunction
  • Pupils may be fixed, dilated, pinpoint or normal
  • Nystagmus or oculogyric episodes may be present
  • Typically hyperdynamic cardiovascular state
  • Hypodynamic state signifies impending CV collapse
  • Tachypnea and hyperventilation
  • Gastrointestinal hemorrhage occurs frequently
  • Jaundice and elecated liver enzymes occurs commonly
  • Fulminant hepatic failure and DIC rarely occur
  • Muscle tenderness and cramping are common
  • Rhabdomyolysis and ARF are common complications of EHS

Heatstroke is defined as hyperthermia usually greater than 40 C associated with severe CNS dysfunction and anhidrosis and has a mortality rate of up to 70%. The temperature is considered a relative criterion since some patients may be cooled prior to the temperature measurement. Some patients may also retain the ability to sweat. Heatstroke is diagnosed clinically by assessing a constellation of symptoms and physical findings, not by an absolute definition. Classic heatstroke and exertional heatstroke have significantly different presentations and clinical scenarios.

Classic heatstroke typically occurs during environmental heat waves as a result of the body’s failure to dissipate heat. It is more common in the very young, the elderly, and the chronically and mentally ill. Patients present with hyperthermia and an altered sensorium that develops suddenly after prolonged exposure to elevations in ambient temperature. Sweating ceases in the majority of these patients. Patients present with a variety of CNS symptoms including irritability, delusions, hallucinations, seizures or coma. Diuretics, antihypertensives, anticholinergics, and neuroleptics impair the patient’s ability to tolerate heat stress. Advanced age, hypotension, coagulopathy, and endotracheal intubation are poor prognostic indicators regardless of successful cooling measures.

In contrast, exertional heatstroke (EHS) usually affects young healthy individuals who are unable to dispel heat due to endogenous heat production. Athletes and military recruits are typically affected and EHS is the third leading cause of death for high school athletes. These patients present with hyperthermia, diaphoresis, and an altered sensorium usually during extreme physical exercise in a hot, humid environment. Abdominal cramping, nausea, vomiting, myalgias, diarrhea, headache, dizziness, dyspnea, weakness, or syncope typically precede exertional heatstroke. Obesity, poor physical fitness, precedent illness, and lack of acclimatization are risk factors that predispose individuals to heat related illness. Exertional heat stroke may also occur as a result of increased motor activity due to drug use such as cocaine or amphetamines or prolonged seizure activity. Since these patients maintain the ability to sweat, they may have significantly lower core temperatures on arrival to the ED. Patients with EHS are much more likely to experience rhadomyolysis, acute renal failure, coagulopathy, lactic acidosis, and hypoglycemia than patients with classic heatstrokes.

Physical Findings and Features

Heat Cramps are brief, intermittent, severe cramps in muscles fatigued by excessive exercise and occur at a time of rest. They are thought to be related to salt deficiency and usually occur in the first days of excessive work in a hot, humid environment.

Heat Edema is characterized by edema of the feet and ankles in unacclimatized individuals exposed to warm tropical or subtropical climates. It is thought to be the result of increased hydrostatic pressure and vasodilation resulting in vascular leak and edema.

Heat Syncope is the temporary loss of consciousness caused by intravascular shunting to the cutaneous circulation, pooling in the lower extremities due to prolonged standing, and volume depletion. The elderly are particularly susceptible and should be warned to move often and avoid prolonged standing.

Prickly Heat is an acute inflammatory skin condition caused by blockage of the sweat glands and a secondary staph infection. It is initially manifestated by pruritic vesicles in the clothed areas that extend producing deeper vesicles that are less pruritic.

Heat Exhaustion is characterized by vague malaise, fatigue, nausea, vomiting, weakness and headache in the setting of heat stress. The core temperature is frequently normal or just mildly elevated. The patient is frequently water depleted from inadequate hydration, but may be salt depleted from rehydration with hypotonic solutions. Patients with true heat exhaustion have normal mental status, but may be tachycardic. Clinically heat exhaustion and heatstroke may be incredibly difficult to differentiate and if the diagnosis is unclear, cooling should be immediately initiated.

Initial Actions and Primary Survey

  • Obtain fasting blood sugar in all patients with altered mental status
  • Rapidly obtain rectal temperature
  • Initiate rapid cooling simultaneously with any basic resuscitative measures

Diagnostic Testing

Diagnostic testing is utilized to exclude other etiologies of hyperthermia and altered sensorium as well as evaluate complications associated with heat related illnesses. No laboratory changes are pathognomonic of heat stroke.

Glucose should be an initial test performed in any patient with altered mental status.

Hypernatremia may be present if the patient has reduced po intake and is dehydrated, but the patient may be hyponatremic if he or she has been using diuretics or drinking free water.

Hypokalemia is common in early heatstroke, but hyperkalemia predominates in the setting of muscle damage.

Hepatic Function tests are frequently elevated as a sign of hepatic failure associated with heatstroke. AST and ALT can rise >10,000 and peak at 48 hours-2weeks.

Muscle Function tests indicate muscle damage and necrosis and CK may rise to 100,000 or more in patients with EHS.

CBC frequently demonstrates an elevation in WBC and thrombocytopenia may be present.

Renal function is critically important to evaluate since patients with muscle necrosis release myoglobin which is nephrotoxic. Without appropriate treatment, permanent renal damage may result.

Urinalysis should be obtained to evaluate for hematuria. Urine dipsticks do not differentiate hemoglobin, blood and myoglobin. A microscopic urinalysis should be obtained to evaluate for the presence of RBC’s and urine myoglobin should be considered. Hematuria on the dip stick in the absence of RBC’s on microscopic exam suggests rhabdomyolysis.

Chest X-ray may demonstrate pulmonary infarction, edema or atelectasis.

Head CT or Lumbar Puncture may be obtained to rule out CNS abnormalities in patients with altered mental status.

EKG typically demonstrates sinus tachycardia with nonspecific ST-T wave changes. Conduction abnormalities may be present (RBBB or prolonged QT).

How do I make the diagnosis?

Heatstroke and heat related illnesses are diagnosed on clinical suspicion alone. Heatstroke is defined as hyperthermia and altered sensorium. Typically the temperature will be elevated greater than 40C, but it is important to remember patients may have been cooled prior to arriving. The presence of anhydosis may suggest the presence of classic heatstroke, but up to 50% of classic heatstroke patients and most exertional heatstroke patients maintain the ability to sweat. A high clinical suspicion must exist to rapidly diagnose these patients. A rectal temperature should be obtained immediately on arrival to the ED in any patient with altered mental status since delay can adversely affect outcome. History should also be obtained from prehospital personnel regarding any methods of cooling enroute.

Treatment

Treatment should be aimed at administration of simultaneous basic resuscitative measures and immediate, aggressive cooling measures. Consider intubation of any patient not able to protect his airway. Two large bore IV’s should be established for resuscitation and the patient should be placed on continuous cardiac monitoring and pulse oximetry. Between 20 and 65% of patients with heatstroke experience acute circulatory failure due to volume loss from dehydration, pooling of blood, distributive shock, or myocardial damage. A thermistor probe should be placed in the rectum or esophagus to continuously monitor core temperature.

Immediate cooling is the cornerstone of management of heat related illness. Rapid reduction of the core body temperature should be instituted immediately upon clinical suspicion of the diagnosis because the duration and extent of hyperthermia is the primary determinant of outcome. The core temperature should be rapidly lowered to 38-39 C. A recent comprehensive review of the literature by Bouchama et al failed to define the optimum treatment of heatstroke. Clothing should be removed. Special cooling blankets are now routinely employed to rapidly lower body temperature. In the absence of these specialized blankets, the patient may be sprayed with water or may be covered with water soaked sheets. Blowing air with fans over the patient aids in evaporative cooling. Ice packs may be placed in the groin and the axilla. Ice water submersion was once a preferred method and is extremely effective, but is now avoided since it may induce shivering which can generate increased heat and leads to vasoconstriction of cutaneous vasculature that reduces heat transfer.

Other cooling methods may be employed including peritoneal, bladder, and gastric lavage with ice water, cold IV fluids, and cool oxygen. Antipyretics should never be given to patients with heat related illness since they have no efficacy in reducing the temperature and may exacerbate the hepatic, renal and coagulopathic abnormalities associated with heatstroke.

Dantrolene is a muscle relaxant that attenuates the amount of calcium released from the sarcoplastic reticulum in the skeletal muscles to the cytosol. Decreased intracellular calcium levels lead to reduced muscle metabolic activity and heat production. It is widely used to treat malignant hyperthermia and neuroleptic malignant syndrome, but its utility and effectiveness in heatstroke is controversial. The limited literature does not suggest utilization of dantrolene as a first line treatment for heatstroke, but it may be considered in patients unresponsive to conventional cooling methods or patients with excessive shivering.

Methods of Cooling

Methods of Cooling

Evaporative Techniques

  • Wetting body surface with continuous fanning

External (Non-invasive) Conduction Techniques

  • Tap water immersion
  • Ice water immersion
  • Application of cold packs
  • Cooling blanket

Internal (Invasive) Conduction Techniques

  • Gastric, peritoneal or bladder lavage
  • Cold IV fluids

Pharmacologic Techniques

  • Dantrolene

Shivering leads to increased heat production and should be controlled with benzodiazepines. They are also the drug of choice for controlling seizures since phenytoin is not effective in this setting. Barbiturates are the second line of therapy.

Management of circulatory failure is an urgent consideration since patients with hypotension have a 33% mortality rate compared to a 10% mortality rate for a normotensive group. Hemodynamic instability requiring vasoactive support is associated with a significantly higher mortality rate and increase in neurologic disability. Fluid resuscitation should be the initial treatment titrated to volume status and hemodynamic parameters. Additional treatments should be based on the individual’s underlying medical conditions, but typically heatstroke mimics a sepsis like distributive shock and similar treatment modalities may be employed.

Rhabdomyolysis is a relatively common complication of exertional heat stroke caused by release of large amounts of myoglobin from damaged muscle that precipitates in the kidneys and can cause acute renal failure. Dark tea colored urine and tender muscles are classic findings. Treatment includes infusion of large amount of IV fluids and alkalinization of the urine with intravenous bicarbonate infusion to prevent myoglobin precipitation in the renal tubules. Hemodialysis may be necessary for patients with acute renal failure.

Poor Prognostic Indicators

  • Initial temp > 41C
  • Persistent elevation in temperature despite cooling
  • GCS < 12
  • Severe pulmonary edema
  • Delayed or prolonged hypotension
  • Lactic acidosis in patients with classic heatstroke
  • ARF and hyperkalemia
  • Aminotransferase levels > 1000 in he first 24 hours
  • Elderly
  • Pre-existing cancer, CAD or treatment with diuretics
  • Transportation to the hospital by ambulance

Pearls and Pitfalls

  • Rectal temperature should be obtained immediately on arrival to the ED
  • High index of clinical suspicion should be maintained and diagnosis based on constellation of symptoms instead of strict criteria.
  • Rapid reduction of core body temperature is the cornerstone of treatment and should be initiated immediately.
  • Degree and duration of hyperthermia correlates with morbidity and mortality.
  • Avoid prescribing antipyretics
  • Evaluate for rhabdomyolysis and initiate therapy immediately if suspected.

Selected References

Hadad E, Rav-Acha M, Heled Y, Epstein Y, Moran DS. Heat stroke : a review of cooling methods. Sports Med.2004;34(8):501-11.

Hadad E, Cohen-Sivan Y, Heled Y, Epstein Y. Clinical review: Treatment of heat stroke: should dantrolene be considered? Crit Care. 2005 Feb;9(1):86-91. Epub 2004 Aug 11.

Bouchama A, Dehbi M, Chaves-Carballo E. Cooling and hemodynamic management in heatstroke: practical recommendations. Crit Care. 2007;11(3):R54.

Hausfater P, Megarbane B, Dautheville S, Patzak A, Andronikof M, Santin A, André S, Korchia L, Terbaoui N,Kierzek G, Doumenc B, Leroy C, Riou B. Prognostic factors in non-exertional heatstroke Intensive Care Med.2010 Feb;36(2):272-80. Epub 2009 Oct 20.