Thermoregulation

Question 1

What is Normal Body Temp (Celcius)
Hypothermia?
Hyperthermia?

Answer 1

37C
36C
38C

Question 2

What provides primary control of body temperature and how?

Answer 2

Hypothalamus

Through Afferent input, Efferent response to cold, Efferent response to Heat

Question 3

What types of Afferent Input does the Hypothalamus use for Thermoregulation?

Answer 3

C-fibers – Heat and warmth receptors
A-delta fibers – cold receptors
Travels via spinal thalamic tracts in anterior spinal cord

Question 4

What type of Efferent response to cold does the Hypothalamus use for Thermoregulation?

Answer 4

Increase metabolism to increase heat production…or try to alter heat loss. All regulated by hypothalamus.
Vasoconstriction (shunt blood away from skin) and shivering (increases metabolic demand by approx 600)

Question 5

What type of Efferent response to heat does the Hypothalamus use for Thermoregulation?

Answer 5

Cutaneous Vasodilatation

Sweating

Question 6

How is Sweating used in Thermoregulation?

Answer 6

Mediated by postganglionic ACh mediated nerves

Only mechanism to transfer heat to the environment when ambient temperature is greater than body temperature

Question 7

How is Vasodilation used in Thermoregulation?

Answer 7

Diverts blood flow to periphery environment transfer of heat

Question 8

How is Vasoconstriction used in Thermoregulation?

Answer 8

Alpha-1 mediated (primarily) but also some alpha 2.

Reduces transfer of heat to environment

Question 9

How is Shivering used in Thermoregulation?

Answer 9

Increases metabolic heat production increased oxygen consumption
Infants- brown fat thermogenesis (nonshibering thermogenesis…by beta3 adrenergic receptor stimulation) – mediated by Beta-3 stimulation
Measure metabolic heat by CO2??

Question 10

How does General Anesthesia impact Thermal Regulation?

Answer 10

Core Temp drops 1.0C (0.5-1.50 C) in first 30 min after induction

Followed by 0.3C drop each hour for first 4-5 hours (plateau) during GA

Question 11

What factors result in heat loss after induction of General Anesthesia?

Answer 11

Prepping
Exposure
Loss of vasoconstriction response
Depression of central temperature regulation

Question 12

How is plateau temperature affected by Regional Anesthesia?

Answer 12

Plateau is not reached, it keeps dropping.

Epidural/spinal anesthesia cause neural-blockade induced vasodilation

Additional decrease of 1.0 0C with general/regional combined techniques

Question 13

What is the Order of Heat Loss during Surgery?

Answer 13

1. Radiation
2. Convection (moving air or moving fluids, magnitude of heat loss depends on air speed…prevent convention heat loss through blankets)
3. Conduction (occurs from direct contact of skin on table. Heat loss proportional to …)
4. Evaporation (from skin respiratory tract, open surgical wounds….cold wet towels or drapes in contact with patients body.)

Question 14

What are the Mechanisms of Radiation Heat Loss?

Answer 14

Infrared heat loss
Major source of heat loss
Proportional to surface area exposed
Increased in infants high surface area/body mass ratio

Question 15

What are the Mechanisms of Convection Heat Loss?

Answer 15

Heat transfer through air currents
Warm air rises and cooler air falls
Cycle of body transferring heat to cooler air
Similar to convection oven- uses warmed air currents to cook

Question 16

What are the Mechanisms of Conduction Heat Loss?

Answer 16

Heat exchange loss
Physical contact allows heat transfer
Transfer from warmer object to cooler object
Examples- warming blankets, cold OR table, cold irrigation solutions

Question 17

What are the Mechanisms of Evaporation Heat Loss?

Answer 17

Heat loss from changing liquid to gas phase
Heat loss occurs from surface of evaporation
Surgical preps
Exhaled water vapor during ventilation

Question 18

What are the effects of Hypothermia on the body?

Answer 18

• Alters platelet function and impaired coagulation factors
• Increased risk of wound infections
• Impaired immune function
• Increased catecholamine levels (Elevated BP and HR)
• Increased oxygen consumption (135% to 468% increase oxygen requirements)
• Increased risk in cardiac disease
• Reduced drug metabolism
  
  • Increased DOA for NMB agents (Vecuronium; DOA doubled with change in 2 degrees C)
  • Propofol- increased plasma concentrations by 30%

Question 19

What intraoperative devices are used for warming?

Answer 19

• Forced air warming blankets via convection (or air to surface warming)
• IV fluid warmers
• Warming pads
• Warm cotton blankets
• Hydrogel thermal pads
• Radiant heat lamps
• HME- artificial nose

Question 20

What are the two Temperature Monitoring Standards?

Answer 20

ASA- monitor temperature when changes are “anticipated, intended or suspected”

Cases greater than 30 minutes…

AANA- monitor “all pediatric patients receiving general anesthesia and when indicated on all other patients”

Question 21

What are the Temperature Monitoring Modalities?

Answer 21

• Mercury thermometers- not typically used in surgical patients
• Thermistor/thermocouples
  
  • Wire devices generate current from junction of 2 metals
  • Accurate and reliable
• Liquid crystal
  
  • Skin temperature monitoring
  • Reduced accuracy
  • Impacted by vasodilation of skin

Question 22

What Anatomic Sites can be used for Temperature Monitoring?

Answer 22

• Bladder: correlates well with core temperature
• Pulmonary artery: “Gold standard”
• Esophageal (distal esophagus – 1/3 to 1/4): easily accessible during anesthesia; distal 1/3 of esophagus
• Nasopharynx: similar blood supply as hypothalamus
• Tympanic: close to carotid artery; reflects brain temperature
• Rectal: does not reflect core temp
• Axillary: more reliable in infants/children
• Skin (forehead): reflects peripheral perfusion; requires 20 C compensation

Question 23

What monitoring sites are the most reflective of thermal status?

Answer 23

Core temperature monitoring:

Bladder, PA, and esophageal are most reliable