Hypothermia Flashcards
inadvertent hypothermia happens to what percent of patients?
50%
requires a nearly constant
internal body temperature
Homeothermic
organism takes on the
temperature of the surrounding environment
Poikilothermia
[happens during anesthesia]
central body temperature
abdomen, thorax, head
Core Temperature
what controls temp in body?
hypothalamus via ANS
amount of heat necessary to raise the temp of 1 kg of water from 14.5 to 15.5 C?
kilocalorie or 1000 calorie
term for cooling?
thermolysis
what 4 things effect thermolysis ?
conduction
evaporation
radiation
convection
term for bodys ability to generate heat
thermogenesis
4 things that effect thermogenesis
nonshivering
shivering
diet induced
basal metabolic rate
thermoregulation maintains core body temp within what range?
0.2C of normal (37C)
what 8 things can change body temp from normal?
- Circadian rhythm
- Exercise
- Food intake
- Infection
- Thyroid function
- Age
- Anesthesia, sedatives, alcohol
- Obesity
explain thermoregulation differences with regards to infants and elderly?
intact in infants
impaired in elderly
mechanism of action of normal thermoregulation?
T° info from thermally sensitive
cells travels warm (C fibers) & cold (A and D fibers) to
substantia gelatinosa of dorsal horn to the Hypothalamus
the T ° regulating
center
hypothalamus
unmyelinated, small, dull pain
c fibers
thinly myelinated, sharp pain
A and D fibers
can intense temperature be distinguished from pain?
no, they travel along same pathways
First and most consistent response to hypothermia
Vasoconstriction
Vasoconstriction Can decrease heat loss by
25%-50%
why do we not want our patients to shiver?
can cause 2-5 fold increase in whole-body O2 consumption!! BIG problem
can newborns and infants shiver?
Nope! increase metabolic heat production by nonshivering thermogenesis
MOA of Non-shivering thermogenesis
NS fibers (beta receptors) innervate brown fat found in scapula, neck, back, viscera to stimulate lipolysis and heat release
doubles the heat production in infants, but increases it only slightly in adults
Non-shivering thermogenesis
when does bmr peak?
age 2
for each 1 degree F change, what does bmr change by?
7%
most important insulator against heat loss
intact skin
Normothermia
36-37
Mild Hypothermia:
34ºC - 35.9ºC
Moderate Hypothermia
32ºC - 33.9ºC
Severe Hypothermia
< 32ºC
what does anesthesia vasodilation do?
Anesthesia-induced vasodilation allows core heat to flow peripherally.
where is most metabolic heat produced?
core compartment
Mechanisms of Heat Loss
radiation
conduction
convection
evaporation
electromagnetic heat waves emanate from all surfaces; increased rate when temperatures higher than surrounding air
Radiation
Transfer of heat between two adjacent surfaces requires direct contact; molecule to molecule transfer of heat; warmer surface looses heat to cooler surface
Conduction
Loss of heat to air currents i.e. wind chill; normally hair on our body traps air in surface layer on the skin and counters forces
Convection
Loss of heat via water loss from the skin and mucus membranes
Evaporation
list mechanisms of heat loss in order from greatest to least
- radiation
- conduction
- convection
- evaporation
ALL anesthetics produce hypothermia—ALL - Why?
- use of skeletal muscle relaxant (no shivering)
- vasodilation
- decrease BMR (20-40)
- impaired non-shivering thermogenesis
hypothermia occurs in 3 phases, which produces most temp loss?
phase 1
describe phase 1 of temp decrease in OR
steep drop in temp (1-2 degrees) during first hour of procedure due to redistribution of core temp to periphery from vasodilation
gradual decline in next 2 to 3 hours (.5-1 degree/hour) due to continued heat loss to the environment exceeding heat production
phase 2
steady state; metabolic heat production matches heat loss
phase 3
what type of patients at greatest risk of inadvertent periop hypothermia?
pts receiving combined GA and RA [regional anesthes]
do epidural/spinals affect temp greatly?
They affect is but not as much as GA because you are only blocking responses in region of body, not entire body
warm response thresholds increases with anesth meaning
takes longer/higher temp to begin cooling off
cold response thresholds decrease with anesth meaning
has to get colder before techniques kick in
interthreshold range increases
10-20 fold
3 reasons Unwarmed patients become hypothermic with anesthesia
Open body cavities
Cold IV and irrigation fluids
Cold operating room
An operating room greater than ___ is required for
most adult patients to maintain normothermia
24C which is 75F
Not happening
why do infants have higher heat
loss from evaporation
Thinner skin and subcutaneous fat layer
5 Ways to prevent loss of heat in infants
- ambient temp above 24C
- overhead heater, fwa, heating blanket
- kept wrapped and in incubator
- head covered
- warm fluids and skin prep
what 5 things cause elderly to be more prone to hypothermia?
- slowed blood circulation
- thin skin
- decrease bmr
- decrease lean muscle mass
- blunted vasoconstrictor response
what is the treatment for post op shivering?
IV meperidine 25 mg
clonidine 75
and supplemental O2
Consequences of hypothermia?
- increases blood loss
- triples morbid cardiac outcomes
- triples surgical wound infections
- enhances drug effects
cardiac effects of hypothermia?
j wave
dysrhythmias
shift oxyhemaglobin curve to left
increase PVR and blood viscosity
cardiac effects hypothermia in infants?
bradycardia (bad news bears)
common ekg changes in hypothermic pt?
junctional rhythm, prolonged pr interval, altered t wave, j wave
does hypothermia change emergence?
indeed, prolongs
Duration of vecuronium with 2C decrease in temp?
more than doubled
what % does MAC decrease for each 1C?
5%
5 ways to prevent heat loss
- blankets
- forced warm air
- heated inhaled gases
- warming IV fluids
- raise ambient temp
% heat loss from extremities
60%
% heat loss from Thorax/Abdomen
20%
% heat loss from Head and resp tract
10%
3 benefits of hypothermia?
- protection against cerebral and spinal cord ischemia
- malignant hyperthermia harder to trigger
- MS
what types of surgery is hypothermia recommended?
carotid surgery, neurosurgery, TBI, any surgery where tissue ischemia can be anticipated
what 3 things does hypothermia do?
- decreases cerebral met rate 8% and blood flow 7%
- decreases release excitatory enzymes
- o2 consumption decreases 7%
best places to monitor temp?
Core Tº Nasopharyngeal, tympanic, PA catheter, esophageal -in the lower 1/3 of esophagus
