Hypothermia Flashcards
normal core body temperature is defined as:
37*C
Elderly have set-point ______ than normal
0.5*C Lower
Hypothermia- oral temp =
<36*C
Hyperthermia =
______ am
______ pm
> 37.2*C am
>37.7*C pm
what functions as the “control center” for temperature?
hypothalamus
what thermoregulatory mechanisms does the body exhibit at temps <37*C?
vasoconstriction followed by shivering
what thermoregulatory mechanisms does the body exhibit at temps >37*C
sweating followed by vasodilation
what happens to the threshold temperature for thermoregulatory responses under anesthesia?
widens range. Makes it harder for body to regulate
How are Mild, Moderate, Severe Hypothermia and DHCA defined?
Mild = 34-36 Moderate = 30-40 Severe = <30 DHCA = 18*C
Where can core (trunk & head) body temp be measured?
Pulmonary Artery
Tympanic Membrane
Distal Esophagus
Nasopharynx
Name other places to measure temperature
Bladder
Rectum
Skin/Muscle
When would Bladder and rectal temperatures not be accurate?
Bladder = poor kidney fxn, irrigation nearby Rectal = "impaled on turd"
how do core and peripheral temps differ in their response to external influences?
Core= varies little with environment Peripheral = varies significantly over surface
how is temperature sensed?
afferent thermal sensing:
Cold= delta A fibers
Hot = C fibers
how does the body respond to heat?
sweating and active vasodilation
sweating can dissipate ___X the BMR in a dry, windy environment
10
when heated, what happens to the temperature gradient?
moves toward peripheral tissues
when cooled, what happens to the temperature gradient?
moves towards core
how does the body respond to cold? why can it be harmful?
vasoconstriction- moves circulation to core to preserve vital organs.
This can cut off flow to peripheral tissues and cause necrosis
what is the most effective thermoregulatory response?
behavior
shivering can have what effect on BMR?
increase 2-3x
what is non-shivering thermogenesis?
seen in infants - brown fat metabolism
increased SA to body ratio
the majority of heat loss to the environment is due to?
radiation
radiation is a function of what? This leaves what population particularly susceptible?
BSA
Children
what is the second largest route of heat loss?
convection
How does convection work and what affects its magnitude?
heat exchange from body to surrounding air/fluid
Magnitude effected by temperature gradient
a bair hugger is an example of what form of heat exchange?
convection - per MM and manufacturer website.
where are evaporative heat losses most common?
exposed visceral surfaces to environment and burns
how can regional anesthesia effect thermoregulation?
affects afferent sensing, so hypothalamus isn’t sure what to do
during anesthesia, where is the body heat redistributed?
to the periphery (resultant loss of core temp)
most patients lose _____ *C in first hour
1-1.5*C
what are the phases of cooling?
- Initial - redistribution (core->preiphery)
- core temp decrease in first hour (reduced vasoconstriction threshold)
- linear phase due to heat loss exceeding BMR
- Plateau when heat loss equals production
T or F: Patients usually regain normal temperature by conclusion of surgery
False
What effect does temperature have on wound infections and hospital stays?
Increases wound infection rate 3x
Increases hospital stay by 2 days
thermoregulatory threshold for shivering is decreased _____ by inhalational anesthetics and nitrous-narcotic techniques
34.5*C
thermoregulatory threshold for shivering is decreased _____ by proposal-nitrous techniques
33*C
how do the elder compare to the normal population when looking at cooling and warming rates?
cool faster, warm slower
what are 3 predictive factors of an increased risk for perioperative hypothermia?
old age
short height
low pre-op BP
what two factors have a greater effect on hypothermia than the duration of surgery, ambient room temperature and percent body fat when using regional anesthesia?
high level of blockade
advanced age
What are some possible consequences of hypothermia?
-Poor would healing (Impaired immune fxn, decreased O2 to tissue, collagen production) -increased mortality -arrythmias - ischemia and angina - decreased drug metabolism - reduced platelet fxn - decreased activation of clotting - blood loss -prolonged PACU stay - increased O2 consumption
when is shivering more likely to occur?
when patient is rewarmed faster
what effect does shivering have on O2 consumption?
increases
what are some potential benefits to hypothermia?
- (1-3*) = protection against cerebral ischemia
- more profound = temp-dependent decrease in BMR and CMRO2
- MAY improve outcome in septic ARDS
- decreases MAC 5% per *C
At what temperatures is brain injury likely vs neuroprotection?
37*C may cause brain injury
what effect does temperature have on MAC requirements?
decreases MAC 5% per *C
colder patients require less anesthesia
when would you want to induce deliberate hypothermia and why?
CPB for heart or brain surgery
- improves tissue tolerance for ischemia
- protects against ischemia
in what instance would you not want to rewarm tissue?
Ischemic tissue should not be rewarmed
(ie tourniquet or AAA cross clamp)
-want to keep low BMR in ischemic areas
when sampling temperature at multiple sites, which is the first to cool and rewarm?
esophageal
temperature gradients are related to…..
differences in tissue blood flow
what type of cooling is necessary to decrease O2 consumption for prolonged Circ. Arrest cases?
Uniform cooling
what is used as an indicator of cerebral temperature?
nasopharyngeal temperature
normothermic bypass patients showed an increased incidence of what adverse event?
stroke
what are some methods to prevent hypothermia in the OR?
- warm room, bed, fluids, inspired air
- use warming blankets and keep pt covered
