Case 79 - Hypothermia

Question 1

What is hypothermia defined as?

Answer 1

• core body temperature < 35 C
• core body temp = nasopharyngeal, bladder, tympanic membrane, Pulm Art Cath

Question 2

What are the phases of hypothermia that a patient experiences the moment they are induced under anesthesia?

Answer 2

1) Phase 1 - Redistribution

• core temp decreases 0.5-1.5 C during 1st hour 2/2 redistribution of heat from core to peripheray (vasodilation 2/2 anestheia)
  
  • Can Prevent this by increaesing OR temp prior to induction, forced air warming device prior to induction

2) Phase 2 - heat loss in excess of heat production
* general anes decreases O2 consumption, thereby decreases heat production
3) Phase 3 - core temp plateaus 2/2 peripheral vasoconstriction (33-35C)

Question 3

what are the four mechanisms of heat loss?

Answer 3

1) Radiation

• accounts for 60% heat loss
• movement of heat form warm object to colder one that is not in direct contact with patient

2) Evaporation
* body fluid leaving liquid phase to gas phase (resp tract, open abdomen)
3) Convection
* transfer of heat to passing cooler air
4) Conduction
* transfer of heat from patient to object in direct contact with patient, like OR table

Question 4

Are there central mechanisms or diseases that can result in hypothermia?

Answer 4

• loss of central thermoregulation
  
  • stroke
  • head trauma
  • spinal cord injury (unable to vasoconstrict appropriately to retain heat)
• metabolic derangements
  
  • hypothyroid
  • hypglycemia
  • burns
  • sepsis

Question 5

what are the three physiologic responses to hypothermia? Is there a downfall to shivering thermogenesis?

Answer 5

1) vasoconstrictoin

• sympathetic induced peripheral vasoconstriction
• anesthetics reduce vasoconstrictive threshold

2) shivering thermogenesis

• occurs at a lower threshold than vasoconstriction
• method of last resort for the body to conserve heat and produce heat
• anesthestics reduce shivering threshold
• cons - increase O2 consumption significantly –> MI risk

3) nonshivering thermogensis

• neonates/infants
• unable to shiver to produce heat, therefore metabolize brown fat to produce heat

Question 6

What are EKG changes associated with hypothermia?

Answer 6

• sinus brady
• widened PR interval, widened QRS, widenet QT interval
• osborn wave = characteristic for hypothermia
  
  • deflection of J point in same direction as QRS complex. amplitude proportional to severity of hypothermia
    
    • J point = junction between QRS and ST segment

Question 7

What are some physiologic consequences of hypothermia?

Answer 7

Anesthesia related

• decrease MAC
• decrease O2 consumption and CO2 production
• increase O2 and CO2 solubility in blood:
  
  • PaCo2 decreases, PaO2 increases
• plasma catechoalmines increased
  
  • HTN, tachy, hyperglycemia

Cardiopulm

• increase SVR (increase blood viscosity)
• increase PVR

Coagulation

• impaired coagulation factors
• impaired PLT function, sequester PLT in portal circulation

Question 8

What are some deleterious clinical consequences of hypothermia (ie effet on coag, wound healing, immune function, MI)?

Answer 8

1) Impaired coagulation
* higher transfusion requirements
2) increase wound infection

• 2/2 impaired immune function
• sludging of RBC –> decrease blood flow to tissue –> decrease O2 delivery –> impair wound healing

3) increase post-op MI and ventricular arrythmias
4) increase hospital LOS

Question 9

what is current recommendation to maintain temperature?

Answer 9

• greater than 36 C to avoid hypothermia-related adverse outcomes

Question 10

are there any benefits to mild intraop hypothermia?

Answer 10

• hypotheermia applied to patients who survive cardiac arrest but comatose has shown improved neurologic outcomes.
• this has not been shown for intracranial aneursym repair

Question 11

what are the core temp monitoring sites? Why are dual temp sites used in CPB?

Answer 11

core temp

• esophageal, rectal, nasopharyngeal, tympanic membrane, urinary bladder catheter, pulm art cath

CPB

• during cpb, significant graidnets exist betwen core, blood, and peripheral temp
• PAC - blood temp from CPB. nasopharyngeal, tympanic, bladder lag behind

Question 12

how is hypothermia prevented?

Answer 12

KEEP 4 mechanisms of heat loss in mind: radiation, evaporation, conduction, convection

1) radiation

• redistribution and heat loss is primarily due to radiation –> largely preventable mechanism
• heat OR
• radiant heat warmer for infants

2) evaporation

• warm inspired gases
• IV fluid warmer

3) convection

• forced air warming devices - biar hugger
• better to maintain normothermia at beginning of surgery then to rewarm postop:
  
  • rewarming vasoconstricted patients post-op can lead to hypotension 2/2 vasodilation, esp if pt is intravasculary volume depleted

Question 13

How do you treat shivering?

Answer 13

• forced air warming devices
• demerol
• dexmedetomidine

Question 14

is hypothermia prevention warranted for pts recieving central neuraxial blockade?

Answer 14

YES

• neruax anes –> sympathectomy –> loss of sympathetic autoregulation (lose vasoconstriction)
• shivering may be difficult 2/2 motor blockade