Perioperative Hypothermia

The New England Journal of Medicine June 12, 1997Daniel I.Sessler, M.D.

Senior clerk: 王桓奇 李兆翔 胡名孝

Introduction

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 enviroment

Hypothermia complication: shivering,prolonged drug effect,coagulopathy, surgical wound infection,morbid cardiac event

Normal thermoregulation

Processing of thermoregulatory information: afferent input central control efferent responses Core temperature measurements pulmonary artery tympanic membrane distal esophagus nasopharynx

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

Normal thermoregulation

The interthreshold range(core temperatures not triggering autonomic thermoregulatory responses)is only 0.2℃

Each thermoregulatory response can be characterized by a threshold ,gain,

maximal response intensity Behavior is the most effective response

Normal thermoregulation

Major autonomic defenses against heat:

1. sweating

2.cutaneous vasodilation Major autonomic defenses 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

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 dose-dependent 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 poikilother

mic 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 AV-shunt 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

Thermoregulation during general anesthesia

Inadvertent 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)

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

poration

Patterns of intraoperative hypothermia

Phase I: Initial rapid decrease Phase II : Slow linear reduction Phase III:Thermal plateau

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0 1 2 3 4 5 6

Patterns of intraoperative hypothermia

1. Initial rapid decrease heat redistribution decreases 0.5-1.5 during 1℃ st 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

Patterns of intraoperative hypothermia

2. Slow linear reduction decreases in a slow linear fashion for 2-3h

rs Simply because heat loss >metabolic he

at production 90% heat loss through skin surface by radi

ation 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 mea

ns 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

RA also impairs the central control of thermoregulation

The regulatory system incorrectly judges the skin temperature in blocked areas to be abnormally high

It fools the regulatory system into tolerating core temperatures that are genuinely lower than normal without triggering a response

The thermoregulatory system’s incorrect evaluation of skin temperature in the blocked area

Undetected hypothermia

The core temperature is rarely monitored by medical personnel during spinal and epidural anesthesia

Patients usually do not feel cold

Heat Balance and Shivering

Initial hypothermia (Phase I) Redistribution of heat from core to periphery Primarily caused by peripheral inhibition of

tonic thermoregulatory vasoconstriction Although the vasodilatation of AV shunts is

restricted to the lower body, the mass of the legs is sufficient to produce substantial core hypothermia

Subsequent hypothermia (Phase II)

Loss of heat exceeds production Patients given SA or EA cannot reestablish

core-temperature equilibrium because peripheral vasoconstriction remains impaired

Hypothermia tends to progress throughout surgery

Shivering

Occurs during spinal and epidural anesthesia

Disturb patients and care givers but produced relatively little heat because it is restricted to the small-muscle mass cephaled to the block

Treated by warming surface of skin or administration of clonidine / meperidine

Consequences of Hypothermia

Advantages Provide substantial protection against c

erebral ischemia and hypoxia Slows the triggering of malignant hypert

hermia and reduce its severity Appear to facilitate recovery and reduce

mortality from septic ARDS

Disadvantages Wound infection---the most common s

erious complication, due to Impaired immune function decreased cutaneous blood flow protein wasting decreased synthesis of collagen

Coagulopathy

Hypothermia reduces platelet function and decreases the activation of the coagulation cascade

From in vitro studies, it increased the loss of blood and the need for allogenic transfusion during elective primary hip arthroplasty

Just 1.5 ℃ of core hypothermia triples the incidence of VT and morbid cardiac events

Drug metabolism

Mild hypothermia decreases the metabolism of most drugs

Propofol ---during constant infusion, plasma conc. is 30 percent greater than normal

Atracurium---a 3 reduction in core temp. i℃ncrease the duration of muscle relaxation by 60 percent

Significantly prolongs the postoperative recovery period

Thermal comfort

Patients feel cold in postoperative period, sometimes rating it worse than surgical pain

Shivering occurs in ~40 percent of unwarmed patients who are recovery from GA

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 ↓

Airway heating and humidification

Less than 10% of metabolic heat is lost through respiratory

Two thirds of heat in humidifying inspiratory gases

Passive or active airway heating and humidification contribute little to thermal management

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

Heat loss can be reduced by covering the skin( with surgical draps, blankets, or plastic bags……)

Insulator Forced-air warming Typically, forced-air warming alone or combin

ed with fluid warming is required to maintain normal intraoperative core temp.

The Relative Effects of Warming Methods on Mean Body Temperature.

Conclusions

Temperatures throughout the body are integrated by a thermoregulatory system

General anesthesia produces marked, dose-dependent inhibition of thermoregulation to increase the interthreshold range by roughly 20-fold

Regional anesthesia produce both peripheral and central inhibition

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 ℃