Regulation of Body Temperature
Internal body temperature is tightly regulated to maintain vital organ function, particularly the brain. Temperature deviation of more than 4 C above or below normal can produce life-threatening cellular dysfunction. Internal temperature is regulated be the hypothalamus, which maintains a temperature set point. The autonomic nervous system maintains body temperature by regulating blood flow, conducting heat from the internal organs to the skin, and innervating sweat glands. Increasing flow and dilating cutaneous capillaries radiate heat away by conductive loss whereas sweat increases evaporative heat loss. Behavioral adaptations are also important; in hot conditions, people become less active and seek shade or a cooler environment. Decreased body temperature is countered by shivering (increasing heat generation in muscles) and by behavioral adaptations such as putting on clothes and seeking warmer environs. Deviations of body temperature indicate changes in the set point, increased heat production, decreased heat dissipation, failure of regulatory system, or any combination of these.
- ANS (blood flow [from internal to superficial; skin blood flow], sweat glands, and shivering)
- Behavioral adaptations
Normal Body Temperature
Internal body temperature is maintained within a narrow range, +- 6 C, in each individual. However, the population range of this set point varies from 36.0 C to 37.5 C making it impossible to know an individual's normal temperature without an established baseline. Without a baseline it is reasonable to regard an oral temperature above 37.5 C and a rectal temperature over 38.0 C as fever. The minimum normal temperature is more difficult to define; the oral temperature often dips to 35.0 C during sleep.
Diurnal variation of body temperature. Daytime workers, who sleep at night, register their minimum temperature at 3 to 4 AM, whence it rises slowly to a maximum between 8 and 10 PM. This pattern is reversed in night-shift workers. The transition from one pattern to the other requires several days.
Simultaneous temperatures in various regions. Heat is produced by the chemical reactions of cellular metabolism, so a temperature gradient extends from a maximum in the liver to a minimum on the skin surface. Customarily, the body temperature is measured in the rectum, the mouth, the ear, the axilla, or the groin. Among these sites, the rectal temperature is approximately 0.3 C higher than that of the oral or groin reading; the axillary temperature is approximately 0.5 C less than the oral value.
Increased body temperature results from excessive heat production or interference with heat dissipation. Each of these mechanisms may be physiologic (i.e., occurring as a normal response to a physiologic challenge) or pathologic (i.e., temperature elevation as a result of damage to the normal thermoregulatory pathways). Physiologic elevation of temperature results from an elevation of the hypothalamic physiologic point for body temperature, a feve. Pathologic elevations of body temperature, hyperthermia, result from unregulated heat generation and/or impairment of the normal mechanisms of heat exchange with the environment.
Physiologic Elevated Temperature – Fever
Release of endogenous pyrogens, particular interleukin (IL-1), triggered by tissue necrosis, infection, inflammation, and some tumors, elevates the hypothalamic set point leading to increased body temperature. Onset of fever may be marked by a chill with shivering and cutaneous vasoconstriction as the body begins generating increased heat and decreasing heat loss; particularly severe chills are called rigors. When the new set point is reached, the skin is usually warm, moist, and flushed; but absence of these signs does not exclude fever. Occasionally, the skin temperature may be subnormal or normal, while the core temperature is markedly elevated. Tachycardia usually accompanies fever, the increase in pulse rate being proportionate to the temperature elevation. During the fever, the patient usually feels more comfortable in a warm environment. The new set point and the pattern of the fever reflect the dynamics of particular pathophysiologic process. Return of the set point to normal, either temporarily or permanently, is marked by sweat and flushing as the body dissipates the accumulated heat. Night sweats occur in many chronic infections, inflammatory diseases, and some malignancies, particularly lymphomas. They represent an exaggeration of the normal diurnal variation in temperature, the sweat marking the decline of the temperature at night.
- Continuous fever. The diurnal temperature fluctuation is 0.5 C to 1.0 C.
- Remittent fever. The diurnal temperature fluctuation is more than 1.1 C without any normal readings.
- Intermittent fever. Episodes of fever are separated by days of normal temperature.
- Relapsing fever. Fevers occur every 5 to 7 days in borreliosis (Lyme disease) and Colorado tick fever.
- Episodic fever. Fever lasts for days or longer following by remission of fever and clinical illness for at least 2 weeks.
- Pel-Epstein fever. Several days of continuous or remittent fever are followed by afebrile remissions lasting an irregular number of days. This is characteristic of Hodgkin disease.
- Congenital: familial Mediterranean fever, other familial periodic fevers, porphyrias
- Endocrine: hyperthyroidism, pheochromocytoma
- Infection: bacterial, viral, rickettsial, fungal, and parasitic infections either localized, or systemic
- Inflammatory/Immune: systemic lupus erythematosus (SLE), acute rheumatic fever, Still disease, vasculitis, serum sickness, any severe local or systemic inflammatory process
- Mechanical/Traumatic: tissue necrosis, exercise
- Metabolic/Toxic: drug reactions, gout
- Neoplastic: leukemia, lymphomas, and solid tumors
- Neurologic: seizures
- Psychosocial: factitious
- Vascular: thrombophlebitis, tissue ischemia, and infarction, vasculitis, subarachnoid hemmorrhage
Fever of Unknown Origin/FUO
Fever of Unknown Origin. Three conditions define a fever of unknown origin (FUO): 1.the illness has lasted >3 weeks; 2.the temperature is repeatedly >38.3 C; and 3. >= three outpatient visits or >=3 days in the hospital have not yielded a diagnosis.
- Nonifectious inflammatory diseases: still disease, SLE, sarcoidosis, Crohn disease, polymyalgia rheumatica, vasculitis (giant cell arteritis, Wegener disease, polyarteritis nodosa)
- Infections: Endocarditis, tuberculosis, urinary tract infection, cytomegalovirus, Epstein-Barr virus, HIV, subphrenic abcess, cholangitis and cholecystitis
- Neoplasms: Non-Hodgkin lymphoma, Hodgkin disease, leukemia, adenocarcinoma
- Miscellaneous: habitual hyperthermia, subacute thyroiditis, Addison disease, drug fever
Pathologic Overproduction and Impaired Dissipation of Heat
Hyperthermia. Unregulated heat production or damage to the heat dissipation systems leads to rapid and severe uncompensated temperature elevations.
- Impaired heat loss: high environmental temperature and humidity, moderately hot weather for a person with congenital absence of sweat glands, congestive heart failure, heat stroke, a ticholinergic drugs and toxins; poverty, homelessness, and psychosis all of which inhibit the ability to adapt to environmental challenges
- Increased heat generation: malignant hyperthermia, neuroleptic malignant syndrome, heavy exertion in hot and humid environment.
Lowered Body Temperature
Decreased hypothalamic set point, insufficient heat generation, and excessive heat loss due to behaviors and environmental conditions all lead to a sustained decline in core temperature. Low body temperature impairs cellular metabolism and brain function, particularly judgement, and the combination prevents protection from continued exposure leading to fatal hypothermia. Hypothermia also protects the tissue from ischemic injury, so complete recovery is possible from rapid and sustained cooling even when the patient appears clinically dead. Relative or absolute hypothermia in situations where fever would be expected is a poor prognostic sign.
Idiopathic: Advanced age
Mechanical/Traumatic: Exposure and immersion, hypothalamic injury from trauma or hemorrhage, burns
Metabolic/Toxic: Antipyretics, hypoglycemia, drug overdoses
Neoplastic: Brain tumors
Psychosocial: Poverty, homelessness, and psychosis