Temperature Regulation

Temperature regulation  Body

temp. should be kept within the normal range because any deviation can affect metabolic function & enzymatic activity  Body temp. is controlled by balancing heat production against heat loss

Temperature regulation  Core

temp. Temp. of deep tissues of body. Remains constant( ± 0.6 ºC) , except when person develops febrile illness[ average: 36.6- 37ºC]  Skin temp. in contrast to core temp. rises & falls with the temp. of surrounding.

Estimated range of body (core) temp. in normal people

HEAT PRODUCTION 1.BASAL METABOLIC RATE of all cells in body The BMR can increase by the effect of : - Catecholamines ( adr. & noradr.) - Thyroxine - Growth hormone - Testosterone 2.FOOD INTAKE (Thermogenic effect of food) Extra metabolism needed for digestion , absorption & storage of food 3. Muscle contraction -Exercise (20 x BMR) -Shivering (5 x BMR)

HEAT LOSS  Blood

vessels are distributed profusely beneath the skin.  A high rate of skin blood flow causes heat to be conducted from the core to the skin.  While decreasing blood flow decreases heat conduction  Vasodilatation & vasoconstriction are controlled by sympathetic N.S.

Skin circulation

Effect of changes in environmental temp. on heat conductance from body core to skin surface

HEAT LOSS  Heat

loss from skin surface is by:  1. Conduction : to clothes, chair  2. Convection: to air, water  3. Radiation: Infrared heat rays ( wave length= 5-20 micrometer)  4. Evaporation: for each 1 gm water evaporated 0.58 cal of heat is lost

Heat loss

Water evaporation  Even

if a person is not sweating , water still evaporates insensibly from skin & lungs at a rate of about 600- 700 ml/day.  This accounts for a continual heat loss of 16-19 cal/ hour.  This amount can increase by sweating  Evaporation is a necessary cooling mechanism at very high air temp.

Heat loss  Conduction

& radiation continues as long as skin temp. is higher than surrounding temp.  When surrounding temp. is higher than skin temp. evaporation occur  Clothing decrease heat loss by conduction & convection.

SKIN

HYPOTHALAMUS Preoptic Area W

Warm Receptors

Set W point W

Cold Receptors

Sweating Vasodilation

C

Vasoconstriction Shivering

Temperature regulation  Threshold

for sweating&vasodilatation:37ºC  Threshold for vasoconstriction: 36.8º C  Threshold for nonshivering thermogenesis:36ºC  Threshold for shivering: 35.5º C

Normal thermoregulation  Afferent 

input: cold signal-Aδ fiber warm signal-C fiber

Each contribute 20% of the total thermal input: hypothalamus other parts of brain skin surface spinal cord deep abdominal and thoracic tissues

Normal thermoregulation  Primary

thermoregulatory control center -hypothalamus  Control of autonomic responses is 80% determined by thermal input from core structures  In contrast, behavior response may depend more on skin temperature

Temp.regulating mechanisms  When

temp. regulating areas in hypothalamus are stimulated, reflex & semireflex thermoregulatory responses are activated.  Responses include: autonomic, somatic, endocrine & behavioral changes

Temperature regulation  Reflex

responses activated by cold are controlled from posterior hypothalamus  Those activated by warmth are controlled primarily from anterior hypothalamus.  Serotonin may be released in response to cold  Noradrenaline may be released in central neurons in response to heat

Temperature regulation  Exposure

to heat causes:  1. Increased heat loss  2. Decreased heat production  Exposure to cold causes:  1. Decreased heat loss  2. Increased heat production

Normal thermoregulation 

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Major responses against heat: 1. sweating 2.cutaneous vasodilation Major responses against cold: 1.cutaneous vasoconstriction 2.nonshivering thermogenesis 3.shivering

Normal thermoregulation  Vasoconstriction

occurs in AV shunts located primarily in fingers and toes, mediated by α-adrenergic symp. nerve.  Nonshivering thermogenesis is important in infants,but not in adults (brown fat)  Shivering is an involuntary muscle activity that increase metabolic rate 2-3 times

Effect of changes in int.head temp. on the rate of evaporative heat loss from body. Skin temp determines set point of sweating

Fffect of changes in int.head temp. on the rate of heat production Skin temp. determines set point of shivering

FEVER FEVER = an abnormally high body temperature PYROGEN = a fever producing substance PYROGEN WBC bacterial toxins, leukocytes, viruses, pollen, + monocytes proteins, dust

= Cytokines (Endogenous pyrogen)

act on preoptic area Prostaglandins

Aspirin

RAISES THE “SET POINT”

Effect of changing the set-point of hypothalamic temp. controller

Body temp. under different conditions

LIMITS TO TEMPERATURE REGULATION Heat Exhaustion:

Inadequate water/salt replacement Body temperature may be normal Symptoms: cerebral dysfunction nausea fatigue Vasodilaton causing fatigue or fainting

Heat Stroke:

Temperature regulation lost Symptoms: high body temperature NO sweating dizziness or loss of consciousness Body temperature MUST be lowered!

Hypothermia during general anesthesia 

Inadvertent hypothermia during general anesthesia is by far the most common perioperative thermal disturbance(due to impaired thermoregulation and cold environment)

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Heat transferred from p`t to environment: radiation > convection >>conduction & evaporation

Perioperative hypothermia The human thermoregulatory system usually maintains core body temperature within 0.2℃ of 37℃  Perioperative hypothermia is common because of the inhibition of thermoregulation induced by anesthesia and the patient`s exposure to cool environment  Hypothermia complication: shivering,prolonged drug effect,coagulopathy, surgical wound infection,morbid cardiac event 

Thermoregulation during general anesthesia General anesthesia removes a p`t ability to regulate body temperature through behavior, so that autonomic defenses alone are available to respond to changes in temperature  Anesthetics inhibit thermoregulation in a dosedependent manner and inhibit vasoconstriction and shivering about 2-3 times as they restrict sweating  Interthreshold range is increased from 0.2 to 4℃ (20 times), so anesthetized p`t are poikilothermic with body temperatures determined by the environment 

Thermoregulation during general anesthesia The gain and maximal response intensity of sweating and vasodilation are well preserved when volatile anesthetics is given  However volatile anesthetics reduces the gain of AVshunt vasoconstriction,without altering the maximal response intensity  Nonshivering thermogenesis dosen`t occur in anesthetized adults  General anesthesia decreases the shivering threshold far more than the vasoconstriction threshold 

Patterns of intraoperative hypothermia Phase I: Initial rapid decrease

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Phase II : Slow linear reduction

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Phase III: Thermal plateau

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35 34 33 0 1 2 3 4 5 6

Patterns of intraoperative hypothermia •

Initial rapid decrease

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heat redistribution decreases 0.5-1.5℃ during 1st hr Tonic thermoregulatory vasoconstriction that maintains a temperature gradient between the core and periphery of 2-4℃ is broken The loss of heat from the body to environment is little Heat redistribution decreases core temperature, but mean body temperature and body heat content remain unchanged

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Patterns of intraoperative hypothermia 2. Slow linear reduction decreases in a slow linear fashion for 2-3hrs  Simply because heat loss >metabolic heat production  90% heat loss through skin surface by radiation and convection 

Patterns of intraoperative hypothermia 3. Thermal plateau After 3-5 hrs,core temperature stops decreasing  It may simply reflect a steady state that heat loss=heat production in well-warmed p`t  If a p`t is sufficiently hypothermic,plateau phase means activation of vasoconstriction to reestablish the normal core-to-peripheral temperature gradient  Temperature plateau due to vasoconstriction is not a thermal steady state and body heat content continues to decrease even though temperature remains constant 

Regional Anesthesia  Regional

anesthesia impairs both central and peripheral thermoregulation  Hypothermia is common in patients given spinal or epidural anesthetics

Thermoregulation   

All thermoregulatory responses are neurally mediated Spinal and epidural anesthetics disrupt nerve conduction to more than half the body The peripheral inhibition of thermoregulatory defense is a major cause of hypothermia during RA

Undetected hypothermia  The

core temperature is rarely monitored by medical personnel during spinal and epidural anesthesia  Patients usually do not feel cold

Treating and Preventing Intraoperative Hypothermia Preventing redistribution hypothermia  The initial reduction in core temperature is difficult to treat because it result from redistribution of heat  Prevent by skin-surface warming 

Peripheral heat content ↑ → Temperature gradient ↓ → Redistribution of heat ↓

 Intravenous fluids  1L

of IV fluids at ambient temperature or 1 unit of refrigerated blood decreases the mean body temperature 0.25 ℃  Heating fluids to near 37 ℃ helps prevent hypothermia and is appropriate if large volumes are being given

 Cutaneous Warming  The

skin is the predominant source of heat loss during surgery, mostly by radiation and convection  Evaporation from large surgical incisions may be important  An ambient temp. above 25℃ is frequently required, but this is uncomfortable for gowned surgeons

Conclusions  Temperatures

throughout the body are integrated by a thermoregulatory system  General anesthesia produces marked, dosedependent inhibition of thermoregulation to increase the interthreshold range by roughly 20-fold  Regional anesthesia produce both peripheral and central inhibition

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The combination of anesthetic-induced thermoregulatory impairment and exposure to cold operating rooms makes most surgical patients hypothermic The hypothermia initially results from a redistribution of body heat and then from an excess of heat loss Perioperative hypothermia is associated with adverse outcomes, including cardiac events, coagulopathy, wound infections…… Unless hypothermia is specially indicated, the intraoperative core temperature should be above 36 ℃