1. Fever Chapter 3 Dr. Gary Mumaugh and Dr. Bruce Simat
University of Northwestern - St. Paul

2. Fever
Fever (Pyrexia)- the elevation of the body’s temperature over its normal range (98.6°F, 37°C)
Febrile- feverish patient
Often accompanies inflammation
Closely regulated because cellular functions change significantly at different temperatures
Normally, core temp. doesn’t vary more than 0.5°C (1°F)

3. Maintenance of body temp:
Balance heat production against heat loss
All cells produce heat from normal metabolism
The blood disperses heat throughout the body
Heat is lost primarily by heat from blood escaping through skin into environment

4. Mechanisms of Heat Production
Cellular metabolism
Shivering- rapidly alternating skeletal muscle contractions that produce no skeletal motion- stimulated by ANS
Chemical energy converted into mechanical energy
Inefficient, but increases temperature
Voluntary muscle movement
ANS- autonomic nervous system

5. Mechanisms of Heat Loss
Heat lost through skin (90%) accomplished by 4 mechanisms:
Radiation- Energy moves directly away from warm skin surface
Evaporation- Water “takes” heat from the skin as it vaporizes
Main mechanism in hot environments or when exercising
Conduction- Heat loss via direct physical contact with colder objects
Convection- Air currents move heat from body
Main mechanism if windy

6. Regulation of Heat Loss
Thyroid activation- hormone increases MR and heat production (In mild hypothermia)
Reflex shivering
Behavioral responses- add insulation, reduce surface area, escape cold environments
Blood flow can be diverted to or from the skin’s surface depending on heat requirements
Sweat production- sweating can cause heat to leave the skin more rapidly
Insensible loss- loss of heat by evaporation from skin or lungs

7. Negative Feedback Control of Body Temp
The body has a set point temperature
Thermoreceptors- sensors in the body that can detect changes in temperature
Thermoregulation Center- portion of the hypothalamus that gathers information from the thermoreceptors
Hypothalamus may respond by inducing changes in sweat glands, dermal arterioles, shivering, etc.
Negative Feedback Control- Works to oppose changes in temperature
Positive feedback for fever occurs only when failure of homeostasis exists. Ex. Hypothermia- low body temp leads to drop in BMR, which lowers heat production

8. Normal Thermoregulation (pg. 54)

9. Hyperthermia Not mediated by pyrogens
No resetting of the hypothalamic set point
41o C (105.8o F): nerve damage produces convulsions
43o C (109.4o F): death results
Forms
Heat cramps, heat exhaustion, heatstroke

10. Hyperthermia Body temp. above 40.5 °C (105 °F)
Factors- Excess heat production, impaired heat dissipation, environment, primary hypothalamic dysfunction
Classic Heatstroke- body cannot cope with heat
Dehydration leads to anhydrosis- absence of sweating
Usually very young or old, debilitated, or in hot environments without sufficient water, shade, or food

11. Exertional heatstroke- intense activity in hot environ
Exertional heatstroke- intense activity in hot environ. generates heat faster than can be cleared
Usually young competitive male athletes
Heat exhaustion- mild form of hyperthermia
Malignant hyperthermia of anesthesia- genetic predisposed to react to some anesthetic agents/muscle relaxants- muscle rigidity and severe hyperthermia

12. Hyperthermia Characteristics
Classic heatstroke- Skin feels hot and dry
Exertional heatstroke- skin is hot and clammy
Most are confused or delirious
May be in coma or seizures
Tachycardia
Dehydration- stains heart and kidneys
Treatment- physical cooling of body/skin, careful rehydration, antipyretic medications

13. Heat Cramps Severe spasmodic cramps in the abdomen and extremities
Following prolonged sweating and associated sodium loss
Common in individuals not accustomed to heat or those performing strenuous work in warm climates
Fever, rapid pulse, and increased blood pressure often accompany the cramps

14. Heat Exhaustion
Collapse as a result of prolonged high core or environmental temperatures
Prolonged vasodilation and profuse sweating
Dehydration, depressed plasma volumes, hypotension, decreased cardiac output, tachycardia
Manifestations
Dizziness, weakness, nausea, confusion, and syncope

15. Heatstroke
Potentially lethal result of an overstressed thermoregulatory center
Brain cannot tolerate temperatures >40.5o C (104.9o F)
Temperature maintained by blood flow through the veins in the head and face
Cardiovascular and thermoregulatory centers may cease functioning with higher temperatures

16. Heatstroke Manifestations
Cerebral edema, degeneration of the CNS, swollen dendrites, renal tubular necrosis, and death
Rapid peripheral cooling will cause peripheral vasoconstriction and limit core cooling

17. Heatstroke Children are more susceptible
Produce more metabolic heat when exercising
Greater surface area to mass ratio
Sweating capacity is less than in adults

18. Malignant Hyperthermia
Precipitated by inhaled anesthetics and neuromuscular blocking agents
Increased calcium release or decreased calcium uptake with muscle contraction
Causes sustained muscle contractions
Increased oxygen consumption and lactic acid production
Symptoms include absent reflexes, fixed pupils, apnea, flat ECG

19. Mechanisms Underlying Fever
Many factors can stimulate the same febrile response (a rise in temperature)
Pyrogen- A substance that causes fever
Endogenous Pyrogen- Internally produced by the body itself which lead to readjustment of temp set point (many interleukins, TNFs, etc.)
Produced in inflammatory response by mononuclear phagocytes (monocytes and macrophages)
Target cells- hypothalamus
Exogenous Pyrogen- Not made by the body itself (viruses, products of microbes, drugs, etc.)
Endotoxin- prototype exogenous agent
Endotoxin- lipopolysaccharide (LPS) component of cell wall of Gram-neg. bacteria

20. Role and Effects of Fever
Role of Fever Higher temperatures may enhance many aspects of the immune system and phagocytosis
Many pathogens reproduce much more slowly in higher temperatures
Over 41°C (106°F) requires emergency medical care

21. Fever Resetting of the hypothalamic thermostat
Activate heat production and conservation measures to a new “set point”
Exogenous pyrogens
Endogenous pyrogens

22. Benefits of Fever Kills many microorganisms
Decreases serum levels of iron, zinc, and copper
Promotes lysosomal breakdown and autodestruction of cells
Increases lymphocytic transformation and phagocyte motility
Augments antiviral interferon production

23. Harmful Effects of Fever
1°C rise in temperature = 13% increase in oxygen consumption
Increase cardiac workload
Damage to brain- formation of extracellular fluids
May damage body’s cell structure
May harm fetus during pregnancy

24. Treatment
Fever of Unknown Origin (FUO)- Fever of 38.3°C (101°F) persists 3+ wks without diagnosed cause
Hard to treat
Possible causes- local/systemic infections, tumors, autoimmune disease, collagen-vascular diseases

25. Antipyretic Therapy- Designed to lower “inappropriate” fever in a patient
Both aspirin (acetylsalicylic acid) and acetaminophen can lower fevers
Inhibit prostaglandin formation in the hypothalamus- blocks set-point elevation
In serious cases, may involve physical methods (cooling blankets, ice baths, or cold IV fluids)

27. Hypothermia Body temperature less than 35o C Produces:
Vasoconstriction, alterations in the microcirculation, coagulation, and ischemic tissue damage
Ice crystals, which form inside the cells, causing them to rupture and die

28. Hypothermia Factors- Prolonged or extreme exposure to cold
Mild hypothermia- core temp between 35-32°C
Intense shivering, muscle cramping
Warm environment, warm fluids, physical activity
Severe hypothermia- Core body temp below 35°C (90°F)
Shivering less effective/stops, BP drops, impaired judgment, altered consciousness, sense of euphoria
Warm environment, active re-warming
Body temp may continue to drop after rescue
In water- heat loss 30x faster (convection)
Dr. Gentry (Winter eco. Prof) says that conduction is for fluids and solids, but book says convection is air or liquid and conduction only for solids…confusing

29. Hypothermia Tissue hypothermia Slows the rate of cellular metabolism
Increases blood viscosity and slows blood through the microcirculation
Facilitates blood coagulation and stimulates vasoconstriction

30. Hypothermia Accidental hypothermia
Commonly the result of sudden immersion in cold water or prolonged exposure to cold
Therapeutic hypothermia
Used to slow metabolism and preserve ischemic tissue during surgery or limb reimplantation
May lead to ventricular fibrillation and cardiac arrest

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