PerioperativeHypothermia

Karim Rafaat, MD

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 environment

Hypothermia complications: Shivering prolonged drug effect, coagulopathy surgical wound infection morbid cardiac event

Normal thermoregulation

Processing of thermoregulatory information:

afferent input central control efferent responses

Normal thermoregulation

Afferent input: cold signal-Aδ fiberwarm signal-C fiber

Each of the following contribute 20% of the total thermal input: Hypothalamusother parts of brainskin surfacespinal corddeep abdominal and thoracic tissues

Normal thermoregulation

Primary thermoregulatory control centerHypothalamus

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 inter-threshold range (core temperatures that do not trigger 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

Normal thermoregulation

Vasoconstriction occurs in AV shunts located primarily in fingers and toes, mediated by α-adrenergic sympathetic nerves

Non-shivering thermogenesis is important in infants, but not in adults (brown fat)

Shivering is an involuntary muscle activity that increases metabolic rate 2-3 times

Thermoregulation during general anesthesia

General anesthesia removes a pt’s 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 more than they restrict sweating

Inter-threshold range is increased from 0.2 to 4 (20 ℃times), so anesthetized pts 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 are

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

Anesthesia Impairs Regulation

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)

Patterns of intraoperative hypothermia

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

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 2-3hrs

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 of heat loss=heat production

If a pt 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

Control

Epidural

RA also impairs the central control of thermoregulationThe 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

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

Disturbs patients and care givers but produces relatively little heat because it is restricted to the small-muscle mass cephalad to the block

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

Temperature Monitoring Core Sites

Pulmonary arteryDistal esophagusNasopharynxTympanic membrane

thermocouple Other generally-reliable sites

MouthAxillaBladder

Sub-optimalForehead skin Infrared “tympanic” Infrared “temporal artery”Rectal Anesth Analg 2008

Potential Benefits of Mild Hypothermia

Improves neurologic outcome after cardiac arrest Bernard, et al. Hypothermia after cardiac arrest study group Now recommended by European and American Heart Associations Number needed to treat: ≈6 Hypothermia recommended by International Liaison Committee

Improves neurologic outcome in asphyxiated neonates Shankaren, et al. Gluckman, et al. Eicher, et al. Number needed to treat: ≈6

No benefit in major human trials Brain trauma in adults (Clifton, et al.) or children (Hutchison, et al.) Anurysm surgery: Todd, et al. Acute myocardial infarction: Dixon, et al

Complications of Mild Hypothermia

Many!

Well documentedProspective randomized trials1-2°C hypothermia

Effects on many different systemsMost patients at risk for at least one complication

Wound infection---the most common serious complication due to

Impaired immune function decreased cutaneous blood flow protein wasting decreased synthesis of collagen

Complications of Mild Hypothermia

Wound Infections: Melling, et al.

Normothermia is more effective than antibiotics!

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

Blood Loss

20% less blood loss per °C

Transfusion Requirement

22% less blood Transfusion per °C

Myocardial Outcomes: Frank, et al.

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. ℃increase the duration of muscle relaxation by 60 percent

Significantly prolongs the postoperative recovery period

Duration of Vecuronium

Recovery Duration

Time (min)

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

Summary: Consequences of Hypothermia

BenefitsImproves neurologic outcomes after cardiac arrestImproves neurologic outcomes after neonatal asphyxia

Major complicationsIncreases morbid myocardial outcomesPromotes bleeding and increases transfusion requirementIncreases risk of wound infections and prolonges

hospitalization

Other complicationsDecreased drug metabolismProlonged recovery durationThermal discomfort

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 ↓

Prewarming Prevents Hypothermia

Airway heating and humidification

Less than 10% of metabolic heat is lost through respiratory route

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

• Fluid Warming

Cooling by intravenous fluids0.25°C per liter crystalloid at ambient temperature0.25°C per unit of blood from refrigerator

Fluid warming does not prevent hypothermia!Most core cooling from redistribution80% of heat loss is from anterior skin surface

Cooling prevented by warming solutionsType of warmer usually unimportant

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

Insulating Covers

More Layers Do Not Help Much

Forced-Air vs. Circulating-Water

Over-body Resistive Warming

Negishi, A&A 2003 Röder, BJA 2011

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 produces 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 ℃