is a critical homeostatic function. Central body temperature is achieved by a feedback system involving the hypothalamus (has "set point") and a variety of target organs.
Infection-induced fever
results from the action of: IL-1, TNF, IL-6, the interferons, and other proteins produced by certain phagocytic cells, on the hypothalamus (post. hypothalamus Ý temp, ant. hypothalamus ß temp). This action can be ameliorated or totally prevented by centrally acting prostaglandin inhibitors (such as aspirin or acetaminophen).
Fever may be beneficial to the host
and its elimination (by antipyretics) may not always be necessary. High body temperature with normal hypothalamic function (hyperthermia) requires a completely different treatment approach.
All the heat needed for maintenance of the high central temperature is derived from metabolic activity.
When body temp is too low or environmental temp threatens to lower it, the hypothalamic instructions result in increased heat production and/or decreased heat loss:
Circulatory system peripheral vasoconstriction (to reduce loss of heat to the environment)
Liver
Ý glucose for rapid metabolism
Muscles shivering
Sweat glands
ß sweat production and thus reducing evaporative heat loss
Brain plays a major role in heat conservation both by initiating certain "instinctive" behaviors (exhaling into hands, changing position to minimize surface area, etc.) and, more complex actions, some of which may be taken in advance of any actual change in body temperature or external conditions (putting on clothes, etc).
These target organs respond in the opposite way when environmental and/or metabolic heat is excessive, and threatens to raise the central body temperature.
Abnormalities of Thermoregulation
(1) Hyperthermia and Fever
: Fever may cause substantial morbidity, including headache, malaise, and fatigue, and may pose an important metabolic stress for the severely ill patient. The importance of accurate diagnosis and treatment of the underlying disease in the febrile patient cannot be overemphasized
4 pathophysiologic pathways can result in elevated body temperature
:
(1) Abnormal "set-point"
Definition: the hypothalamus is trying to maintain a higher-than-normal body temperature (this is true "fever" elevated body temperature brought about by any other mechanism is "hyperthermia"). Pathophysiology: This is the most common cause of abnormally high body temperature. In many infections, autoimmune diseases, and certain malignancies, phagocytic cells, release Interleukin-1 which enters the blood and reaches the hypothalamus. This causes an upward displacement of the set point, probably mediated by metabolites of arachidonic acid, including the prostaglandins. Kids generally develop higher fever temps than do adults, due to an unknown mechanism. (NOTE: Ý set point without IL-1 can occur in cases of primary CNS disease (hypothalamic tumor) and/or exogenous toxins (scorpion venom), but these mechanisms are very rare).
(2) Inadequate physiologic mechanisms for heat loss.
creating false thermometer readings or actually causing self-inflicted fever (ex: self IV injection of pyrogenic agents like urine). This problem of is probably more common than previously suspected.
(2) Hypothermia
: The theoretical causes of hypothermia are similar (but in opposite direction) to those described above for fever/hyperthermia (lowering of "set-point", overactive physiology mechanisms for heat loss, inadequate ability to generate metabolic heat, and excess loss of heat to the environment)
The vast majority of cases of clinically important hypothermia occur as a result of "exposure" to cold with loss of body heat to the environment
Symptomatic Treatment
(1) Symptomatic Treatment of Hyperthermia
Þ Physical reduction of body temperature
The decision to "treat" a symptom involves consideration of at least three possible drawbacks.
(1) Hyperthermia may be beneficial to the patient and thus its elimination or modification may be undesirable.
(2) Elimination of hyperthermia may be difficult to accomplish with minimal risk.
(3) Elimination of hyperthermia may eliminate a means to follow the course of the illness.
Hyperthermia resulting from inadequate ability to lose heat or from excessive metabolic heat production (i.e., set point is normal, but central temperature is too high) is certainly not a beneficial state; lowering of body temperature in these circumstances is clearly desirable. These patients, if conscious, complain of feeling hot and seek a cooler environment. These simple behavioral measures may be sufficient, but if they are not, additional measures such as an "ice mattress" may be needed.
(2) Symptomatic Treatment of Fever:
(1) Reduction of set point, (2) Physical reduction of body temperature
The problem of symptomatic treatment in a patient who is febrile because of disease in which fever is mediated by IL-1 is more complex. Temperatures in excess of 41.1
° C (106° F) are extremely uncommon when fever is mediated by IL-1 . The mechanism for this protection against dangerously high fever-temperatures is not understood, but probably involves feedback inhibition from temperature receptors in the gradually warming skin.
Pts are not at substantial risk from body temperatures of 41.1
° C or below. Therefore, symptomatic treatment of fever itself is rarely a life-and-death matter in otherwise healthy persons.
Fever is probably a part of the immune defense system and lowering of body temp (by treating fever) may compromise these mechanisms, an important consideration when treating resistant organisms; lowering fever may be detrimental.
The situation is complicated by coexistent severe chronic illness where fever may then be a serious stress. Also, fever can alter drug metabolism.
If lowering body temperature in a patient with IL-1induced fever is indicated, the approach will be quite different from that used to treat the hyperthermic patients. Unlike conditions in which the hypothalamic set point is below body temperature, physical removal of heat from the body will be additionally stressful in patients with fever secondary to infection in which the set point is above or equal to body temperature. However, aspirin and acetaminophen act on the hypothalamus and reduce the set point toward normal, presumably by interfering with the metabolism of arachidonic acid, including prostaglandin synthesis. Of course, once the set point has been lowered, removal of heat is theoretically acceptable, but is usually unnecessary because body temperature will fall anyway.
Whether specific antipyretic measures are taken or not, the patient who is febrile from infection should be reassured about the lack of danger from moderately elevated body temperature (below 40.5
° C = 105° F). Unrealistic fears of "brain damage" and death are common and often can be allayed relatively easily.
Assessment of the Relationship of the Set Point to Body Temperature
The relationship of the set point (the temperature the patient's hypothalamus is attempting to attain or maintain) and the actual body temperature is the paramount factor in the selection of symptomatic treatment and diagnosis:
(1) Body temperature is above set point
Þ patient will be more comfortable when heat is physically removed from the body. patient will remove covers and clothing, etc, and will be sweating and inactive.
(2) Body temperature is below set point
Þ patient will be more comfortable when heat is physically added tothe body. patient will add clothing/covers, turn up thermostat, etc. Piloerection, cold extremities and (often) shivering are present.
For children and some adults, an offer of help (e.g, "would you like a blanket?") may yield more information than a pointed diagnostic question (e. g., "are you having chills?" or "do you feel cold?").
Environmental heat overload
: Excessive heat gain from the environment can cause elevated body temperature in otherwise healthy persons. Examples: over-dressed young infant, overuse of a sauna or hot bath, and prolonged exposure to excessive industrial heat. These etiologies are, technically, defects in behavioral thermoregulation.
Heat Stroke
Heat stroke is a condition in which excessive heat gain from the environment and metabolism may be coupled with defective central thermoregulation and inadequate heat loss.
This is an example of hyperthermia resulting from a combination of two basic pathophysiologic mechanisms.
Heat stroke is seen most often in healthy young athletes or infirm elderly persons, and is a medical emergency. Physical removal of heat is urgent.
Hypothermia
Primary lowering of the hypothalamic set point is uncommon.
Patients, if hypothermia is mild or moderate, complain of "feeling hot" and remove clothing and seek a cooler environment even though body temperature may already be below normal. If body temperature is stable and in the mild hypothermic range (e. g., TB above 35
° C) there may be no reason to treat symptomatically.
When the hypothalamic set point is normal and the patient is hypothermic due to excessive loss of heat to the environment (exposure, near-drowning, etc.) or to severe depression of metabolism so that heat production is inadequate to maintain normal temperature, the neurologically intact patient will complain of feeling cold. The hypothalamus will attempt to raise body temperature by increasing metabolic heat and reducing environmental heat loss.
With severe hypothermia, however, metabolism may no longer be able to respond
, even if the cause of the hypothermia has been eliminated. Cardiac abnormalities (especially supraventricular bradycardias) may further limit metabolic heat production. These patients are in great danger.
Management:
Central temperature < 32.3
° C (90F)
Situation can be acutely worsened by external rewarming, since reflex peripheral vasodilation in response to warming of the extremities may result in the sudden return of large quantities of cold blood to the heart
Þ arrhythmias. This "rewarming shock" can be prevented and must be treated, by central rewarming, administered either by peritoneal dialysis with warmed fluids or cardiac bypass.
Central temperature > 32.3
° C (90F)
External rewarming (hot water bottles, etc.) is sufficient