Neurogenic fever [相容模式] - ccd.org.tw

1 2015/7/271 Neurogenic fever Body Temperature Regulation The temperature within the deep tissues of the body (core temperature) is normally maintained within a range of C to C Within this range, there are individual differences and diurnal variations: Internal core temperatures reach their highest point in late afternoon and evening and their lowest point in the early morning hours. Body temperature reflects the difference between heat production and heat loss. Body heat is generated in the tissues of the body, transferred to the skin surface by the blood, and then released into the environment surrounding the Temperature Regulation Thethermoregulatory centeris in the hypothalamus and functions tomodify heat production and heat losses as a means of regulatingbody temperature.

2 The thermoregulatory center in the hypothalamusregulates thecore body temperature, not the surface temperature. This center integratesinputfrom cold and warmthermal receptorslocatedthroughoutthebodyandgene ratesoutputresponsesthatlocatedthroughou tthebodyandgeneratesoutputresponsesthatc onserve body heatorincrease its dissipation=thermostatic setpoint. When body temperature begins to rise above the normal range,heat-dissipating behaviors are initiated; When the temperature falls below the normal range, heatproduction is increased; A core temperaturegreater than 41 C or less than 34 Cusuallyindicates that the body s ability tothermoregulate is Temperature Regulation - Heat Gain Mechanism involved in regulation: Vasoconstriction of the superficial blood vessels - confines bloodflow to the inner core of the body.

3 Contraction of the pilomotor muscles that surround the hairs onthe skin - reduces the heat loss surface of the skin; Assumptionofthehuddlepositionwiththeextr emitiesheldclose Assumptionofthehuddlepositionwiththeextr emitiesheldcloseto the body - reduces the area for heat loss; Shivering - increases heat production by the muscles. It isinitiated by impulses from the hypothalamus; Increased production of epinephrine - increases the heatproduction associated with metabolism; Increased production of thyroid hormone - is a long-termmechanism that increases metabolism and heat Temperature Regulation - Heat Loss Mechanism involved in regulation: Dilatation of the superficial blood vessels - delivers bloodcontaining core heat to the periphery where it is dissipatedthrough radiation, conduction, and convection.

4 There arenumerousarteriovenous (AV) shuntsunder the skinsurface that allow blood to move directly from the arterial tothevenoussystemthevenoussystem. When the shunts are open, body heat is freely dissipated tothe skin and surrounding environment; When the shunts are closed, heat is retained in the body. The blood flow in the AV shunts is controlled almostexclusively by thesympathetic nervous systemin response tochanges in core temperature and environmental temperature. Sweating - increases heat loss through Radiation involves the transfer of heat through the air or a vacuum.

5 Heat from the sun is carried by radiation. The human body radiatesheat in all directions. The abilityto dissipate bodyheat byradiationdepends on theypyyptemperature of the environment. Environmental temperature mustbe less than that of the body for heat loss to Conduction involves the direct transfer of heat from one molecule toanother. Blood carries, or conducts,heat from the inner core of the body tothe skin surface. The conduction of heat to the body s surface isinfluenced by bloodvolumevolume. In hot weather, the body compensates by increasing bloodvolume as a means of dissipating heat.

6 Exposure to cold produces a cold diuresis and a reduction inblood volume as a means of controlling the transfer of heat to thebody s fever , orpyrexia,describes an elevation in body temperature that iscaused by acytokine-induced upward displacement of the set pointof the hypothalamic thermoregulatory center. Fevers that are regulated by the hypothalamus usually do not riseabove41 C- safety mechanism. fever can be caused by a number ofmicroorganismsandsubstances that are collectively calledpyrogens(many proteins,breakdown products of proteins, lipopolysaccharide toxins releasedfrom bacterial cell membranes, etc.)

7 Some pyrogens can actdirectlyand immediately on thehypothalamic thermoregulatory center to increase its set point. Other pyrogens, sometimes calledexogenous pyrogens,actindirectly and may require several hours to produce their of fever Central type fever = Neurogenic fever Non central type fever : Infectious disorders Noninfectious disorders: Myocardial infarction Myocardial infarction Pulmonary emboli Neoplasms ( malignant cells in leukemia, Hodgkin s disease produce pyrogens) Trauma SurgeryNeurogenic fever It usually is caused bydamage to the hypothalamuscaused by: central nervous system trauma; intracerebral bleeding.

8 An increase in intracranial pressure drugs(eganesthetics) drugs( ) Neurogenic fevers are characterized by a high temperature that isresistant to antipyretic therapyand isnot associated of fever (1) release of endogenous pyrogen from inflammatory cells; (2) resetting of hypothalamus thermostatic set point to a higher level (prodrome); (3) generation of hypothalamic mediated responses that raise body temperature (chill); (4) development of fever with elevation of body to new thermostatic set point; (5) production of temperature lowering responses (flush and defervescence) and return of body temperature to a lower of fever Exogenous pyrogensinduce host cells, such as blood leukocytesand tissue macrophages, to produce fever -producing mediatorscalledendogenous pyrogens( ,interleukin-1).

9 The endogenous pyrogens mediate a number of other example, interleukin-1 is an inflammatory mediator thatproduces other signs of inflammation, such as leukocytosis,anorexiaandmalaiseanorexia, andmalaise. The phagocytosis of bacteria and breakdown products of bacteriathat are present in the blood lead to the release of endogenouspyrogens into the circulation. The endogenous pyrogens are increase the set point of thehypothalamic thermoregulatory center through the action ofprostaglandin E2.

10 In response to the sudden increase in set point, the hypothalamusinitiates heat production behaviors (shivering and vasoconstriction)that increase the core body temperature to the new set point, andfever is of feverThe effects of fever Itenhance immune function; Increases motility and activity of the white blood cells; Stimulates the interferon production and activation of T cells;I hibitthfibilt Inhibitsgrowthofsome microbialagents: Many of the microbial agents that cause infection grow best atnormal body temperatures, and their growth is inhibited bytemperatures in the fever range (the rhinoviruses responsible forthe common cold are cultured best at 33 C); Dehydration occurs because of effects of fever Metabolic effects: Increased need for oxygen; Increases the heart rate Increases the respiration rate Increaseduseofbodyproteinsasanenergysour ce; Increaseduseofbodyproteinsasanenergysour ce.