Heat stroke: implications for critical care and anaesthesia

Heat-related illnesses, such as heat stroke, are relatively uncommon in temperate climates. Much of the clinical experience comes from Saudi Arabia (where there is an annual pilgrimage to Mecca) and the military. Case reports and review articles are not commonly seen in anaesthesia-related journals. The intention of this article is to review the available literature on heat stroke and to discuss its relationship with other hyperthermic syndromes, such as malignant hyperthermia. Heat stroke is a medical emergency characterized by a high body temperature, altered mental status and, in classical heat stroke, hot, dry flushed skin.16Duthie DJR. Heat‐related illness.Lancet. 1998; 352: 1329-1330Abstract Full Text Full Text PDF PubMed Scopus (13) Google Scholar It was first recognized by the Romans in 24 BC. But it took until 1946 for it to be shown that heat stroke could lead to multi-organ damage with haemorrhage and necrosis in the lungs, heart, liver, kidneys, brain and gut.8Bouchama A. Heatstroke: a new look at an ancient disease.Intensive Care Med. 1995; 21: 623-625Crossref PubMed Scopus (50) Google Scholar Even in the new millennium, we are not much further on in understanding the mechanisms that take a person from a hyperthermic insult through to multi-organ failure and death. There has been no real decrease in mortality from this disease, which is variably quoted as 10–50%, in the last 50 years.8Bouchama A. Heatstroke: a new look at an ancient disease.Intensive Care Med. 1995; 21: 623-625Crossref PubMed Scopus (50) Google Scholar There are several heat-related illnesses. These may take the form of heat syncope, heat cramps, heat exhaustion and heat stroke, the latter being the most severe.9Bricknell MCM. Heat illness—a review of military experience (Part 1).J R Army Med Corps. 1995; 141: 157-166Crossref PubMed Scopus (20) Google Scholar Heat syncope is fainting due to peripheral vasodilatation secondary to high ambient temperature. Heat cramp refers to muscular cramping occurring during exercise in heat, which is related to salt deficiency and is usually benign. However, there has been a case reported of a young man with post-exercise muscle cramping who subsequently fulfilled the laboratory diagnostic criteria for susceptibility to malignant hyperthermia.32Ogletree JW Antognini JF Gronert GA. Postexercise muscle cramping associated with positive malignant hyperthermia contracture testing.Am J Sports Med. 1996; 24: 49-51Crossref PubMed Scopus (36) Google Scholar Heat exhaustion occurs when the individual becomes dehydrated and weak. Nausea and vomiting occur frequently. Excessive sweating leads to a loss of predominantly water or salt. Salt-depletion heat exhaustion usually occurs when unacclimatized personnel exercise and replace only water losses. Water-depletion heat exhaustion is usually seen in acclimatized personnel who have inadequate water intake during exposure to extreme heat. Whatever the mechanism, the individual collapses from dehydration, salt depletion and hypovolaemia. The core temperature may or may not be raised and tissue damage does not occur.9Bricknell MCM. Heat illness—a review of military experience (Part 1).J R Army Med Corps. 1995; 141: 157-166Crossref PubMed Scopus (20) Google Scholar 14Dickinson JG. Exertional heat stroke‐history and epidemiology: definitions and groups at risk.in: Hopkins PM Ellis FR Hyperthermic and Hypermetabolic Disorders. Cambridge University Press, Cambridge1996: 3-19Google Scholar 35Shahid MS Hatle L Mansour H Mimish L. Echocardiographic and Doppler study of patients with heat stroke and heat exhaustion.Int J Cardiac Imaging. 1999; 15: 279-285Crossref PubMed Scopus (34) Google Scholar Heat stroke occurs when the core body temperature rises against a failing thermoregulatory system.9Bricknell MCM. Heat illness—a review of military experience (Part 1).J R Army Med Corps. 1995; 141: 157-166Crossref PubMed Scopus (20) Google Scholar 16Duthie DJR. Heat‐related illness.Lancet. 1998; 352: 1329-1330Abstract Full Text Full Text PDF PubMed Scopus (13) Google Scholar The core temperature necessary for the condition to be classified as heat stroke varies but is quoted by most authors as a rectal temperature exceeding 40.6°C.35Shahid MS Hatle L Mansour H Mimish L. Echocardiographic and Doppler study of patients with heat stroke and heat exhaustion.Int J Cardiac Imaging. 1999; 15: 279-285Crossref PubMed Scopus (34) Google Scholar As some cooling may take place before reaching hospital, it is probably wise not to adhere to this definition too strictly.1Akhtar MJ Al‐Nozha M Al‐Harthi S Nouh MS. Electrocardiographic abnormalities in patients with heat stroke.Chest. 1993; 104: 411-414Abstract Full Text Full Text PDF PubMed Scopus (68) Google Scholar 14Dickinson JG. Exertional heat stroke‐history and epidemiology: definitions and groups at risk.in: Hopkins PM Ellis FR Hyperthermic and Hypermetabolic Disorders. Cambridge University Press, Cambridge1996: 3-19Google Scholar Heat stroke may be divided into exertional and non-exertional (classical) heat stroke.35Shahid MS Hatle L Mansour H Mimish L. Echocardiographic and Doppler study of patients with heat stroke and heat exhaustion.Int J Cardiac Imaging. 1999; 15: 279-285Crossref PubMed Scopus (34) Google Scholar Exertional heat stroke, as its name suggests, occurs in previously healthy young people exercising, usually in hot and humid climates, probably without being acclimatized. Classical heat stroke occurs during extreme heat waves, the elderly being particularly vulnerable.13Dematte JE O'Mara K Buescher J et al.Near‐fatal heat stroke during the 1995 heat wave in Chicago.Ann Intern Med. 1998; 129: 173-181Crossref PubMed Scopus (345) Google Scholar 35Shahid MS Hatle L Mansour H Mimish L. Echocardiographic and Doppler study of patients with heat stroke and heat exhaustion.Int J Cardiac Imaging. 1999; 15: 279-285Crossref PubMed Scopus (34) Google Scholar Some authors view heat exhaustion and heat stroke as different degrees of severity of a spectrum of disordered thermoregulation.14Dickinson JG. Exertional heat stroke‐history and epidemiology: definitions and groups at risk.in: Hopkins PM Ellis FR Hyperthermic and Hypermetabolic Disorders. Cambridge University Press, Cambridge1996: 3-19Google Scholar Other authors consider these two illnesses as separate entities with different aetiology, biochemistry and predisposing factors.25Howorth PJN. The biochemistry of heat illness.J R Army Med Corps. 1995; 141: 40-41Crossref PubMed Scopus (8) Google Scholar It is important for humans to maintain body temperature within a small range to avoid cellular and enzymatic dysfunction. In humans, this range is usually of the order of 36.5–37.5°C, even in the face of adverse environmental temperatures. This thermoregulation is under the control of the autonomic nervous system, which integrates afferent input and efferent responses. Central control resides in the hypothalamus, where mean body temperature is determined from peripheral and central structures and compared with a 'set point'. The efferent response is both autonomic (sweating and vasodilatation) and behavioural.11Buggy DJ Crossley AW. Thermoregulation.Br J Anaesth. 2000; 84: 615-628Crossref PubMed Scopus (182) Google Scholar There is some distinction made between exertional and classical heat stroke at this point; failure of thermoregulation (lack of sweating) may be more important in classical heat stroke and less so in exertional heat stroke, when there may be persistent sweating.28Knochel JP. Exertional heat stroke‐pathophysiology of heat stroke.in: Hopkins PM Ellis FR Hyperthermic and Hypermetabolic Disorders. Cambridge University Press, Cambridge1996: 42-62Google Scholar If heat gain is to be avoided, heat must be lost through convection, conduction, radiation and evaporation of sweat. In practice, the last is the most important mechanism. The effectiveness of sweating in cooling the body is dependent on both the environmental temperature and the humidity. Low humidity and air movements are important in allowing evaporation of sweat and convection of heat. As air temperature approaches body temperature, the effectiveness of this mechanism is lost.9Bricknell MCM. Heat illness—a review of military experience (Part 1).J R Army Med Corps. 1995; 141: 157-166Crossref PubMed Scopus (20) Google Scholar The military report three measures of environmental temperature load. Dry bulb temperature (DBT) is measured by placing a thermometer in the shade, and this represents true air temperature. A thermometer with its bulb enclosed in a wet wick measures wet bulb temperature (WBT). This can be used in the open air or shielded in a box, and measures the ability of the environment to cool by evaporation. The difference between these two measurements is proportional to the prevailing humidity (WBT