Chapter 017. Fever and Hyperthermia (Part 2) docx

Fever and Hyperthermia Part 2 Hyperthermia Although most patients with elevated body temperature have fever, there are circumstances in which elevated temperature represents not fever

Chapter 017 Fever and Hyperthermia

(Part 2)

Hyperthermia

Although most patients with elevated body temperature have fever, there are circumstances in which elevated temperature represents not fever but hyperthermia (Table 17-1) Hyperthermia is characterized by an uncontrolled increase in body temperature that exceeds the body's ability to lose heat The setting of the hypothalamic thermoregulatory center is unchanged In contrast to fever in infections, hyperthermia does not involve pyrogenic molecules (see

"Pyrogens," below) Exogenous heat exposure and endogenous heat production are two mechanisms by which hyperthermia can result in dangerously high internal temperatures Excessive heat production can easily cause hyperthermia despite physiologic and behavioral control of body temperature For example, work or exercise in hot environments can produce heat faster than peripheral mechanisms can lose it

Table 17-1 Causes of Hyperthermia Syndromes

Heat Stroke

Exertional: Exercise in higher-than-normal heat and/or humidity

Nonexertional: Anticholinergics, including antihistamines; antiparkinsonian drugs; diuretics; phenothiazines

Drug-Induced Hyperthermia

Amphetamines, cocaine, phencyclidine (PCP), methylenedioxymethamphetamine (MDMA; "ecstasy"), lysergic acid diethylamide (LSD), salicylates, lithium, anticholinergics, sympathomimetics

Neuroleptic Malignant Syndrome

Phenothiazines; butyrophenones, including haloperidol and bromperidol; fluoxetine; loxapine; tricyclic dibenzodiazepines; metoclopramide; domperidone; thiothixene; molindone; withdrawal of dopaminergic agents

Serotonin Syndrome

Selective serotonin reuptake inhibitors (SSRIs), monoamine oxidase inhibitors (MAOIs), tricyclic antidepressants

Malignant Hyperthermia

Inhalational anesthetics, succinylcholine

Endocrinopathy

Thyrotoxicosis, pheochromocytoma

Central Nervous System Damage

Cerebral hemorrhage, status epilepticus, hypothalamic injury

Source: After FJ Curley, RS Irwin, JM Rippe et al (eds): Intensive Care

Medicine, 3d ed Boston, Little, Brown, 1996.Heat stroke in association with a warm environment may be categorized as exertional or nonexertional Exertional heat stroke typically occurs in individuals exercising at elevated ambient

temperatures and/or humidities In a dry environment and at maximal efficiency, sweating can dissipate ~600 kcal/h, requiring the production of >1 L of sweat Even in healthy individuals, dehydration or the use of common medications (e.g., over-the-counter antihistamines with anticholinergic side effects) may precipitate

exertional heat stroke Nonexertionalheat stroke typically occurs in either very

young or elderly individuals, particularly during heat waves According to the Centers for Disease Control and Prevention, there were 7000 deaths attributed to heat injury in the United States from 1979 to 1997 The elderly, the bedridden, persons taking anticholinergic or antiparkinsonian drugs or diuretics, and individuals confined to poorly ventilated and non-air-conditioned environments

are most susceptible.Drug-induced hyperthermia has become increasingly

common as a result of the increased use of prescription psychotropic drugs and illicit drugs Drug-induced hyperthermia may be caused by monoamine oxidase inhibitors (MAOIs), tricyclic antidepressants, and amphetamines and by the illicit use of phencyclidine (PCP), lysergic acid diethylamide (LSD),

methylenedioxymethamphetamine (MDMA, "ecstasy"), or cocaine.Malignant hyperthermia occurs in individuals with an inherited abnormality of

skeletal-muscle sarcoplasmic reticulum that causes a rapid increase in intracellular calcium levels in response to halothane and other inhalational anesthetics or to succinylcholine Elevated temperature, increased muscle metabolism, muscle rigidity, rhabdomyolysis, acidosis, and cardiovascular instability develop within

minutes This rare condition is often fatal The neuroleptic malignant syndrome

occurs in the setting of neuroleptic agent use (antipsychotic phenothiazines, haloperidol, prochlorperazine, metoclopramide) or the withdrawal of dopaminergic drugs and is characterized by "lead-pipe" muscle rigidity, extrapyramidal side effects, autonomic dysregulation, and hyperthermia This disorder appears to be caused by the inhibition of central dopamine receptors in the hypothalamus, which results in increased heat generation and decreased heat

dissipation The serotonin syndrome, seen with selective serotonin uptake

inhibitors (SSRIs), MAOIs, and other serotonergic medications, has many overlapping features, including hyperthermia, but may be distinguished by the presence of diarrhea, tremor, and myoclonus rather than the lead-pipe rigidity of the neuroleptic malignant syndrome Thyrotoxicosis and pheochromocytoma can also cause increased thermogenesis

It is important to distinguish between fever and hyperthermia since hyperthermia can be rapidly fatal and characteristically does not respond to antipyretics In an emergency situation, however, making this distinction can be difficult For example, in systemic sepsis, fever (hyperpyrexia) can be rapid in onset, and temperatures can exceed 40.5°C Hyperthermia is often diagnosed on the basis of the events immediately preceding the elevation of core temperature— e.g., heat exposure or treatment with drugs that interfere with thermoregulation In patients with heat stroke syndromes and in those taking drugs that block sweating, the skin is hot but dry, whereas in fever the skin can be cold as a consequence of

vasoconstriction Antipyretics do not reduce the elevated temperature in hyperthermia, whereas in fever—and even in hyperpyrexia—adequate doses of either aspirin or acetaminophen usually result in some decrease in body temperature