Thermoregulation and hypothermia

Question 1

What is the definition of hypothermia

Answer 1

< 36 deg C but occurs commonly in anaesthesia and surgery

Question 2

What are the risk factors for hypothermia

Answer 2

1. Cold operating room temp
2. Extremes of age
3. Abdominal surgery
4. Prolonged procedures

Question 3

What are the deleterious effects of hypothermia

Answer 3

A
Breathing - left shifted OHDC
Circulation - Cardiac dysrhythmia and ischaemia
- Increased SVR (afterload)
Disability - Altered mental state
E
F
G
H - Reversible coagulopathy (platelet dysfunction)
I - Infection risk | Impaired wound healing
K - Kidney: impaired renal function
L - Liver: Delayed drug metabolism (Muscle Relaxants)

Question 4

Describe the different phases of temperature change during general anaesthesia

Answer 4

Phase 1

• GA/RA –> Peripheral vasodilation
• Redistribution of heat from warm ‘central’ compartment to cooler ‘peripheral’ compartment causes dilution of heat and a relatively fast decrease in core temperature
• Decrease of ± 1 - 2 degrees C in 1st hour

Phase 2

• Continuous heat loss to environment
• Relatively more gradual heat loss
• Decrease ± 1 deg C over 4 hours

Phase 3

• Steady state
• Heat loss = metabolic heat production

Question 5

How do anaesthetics agents affect the hypothalamus

Answer 5

NORMAL - narrow inter-threshold range for thermoregulation

• Slight increase core Temp –> sweating and VD
• Slight decrease core Temp –> shivering and VC

GA
- Increases inter-threshold range so compensatory mechanisms significantly delayed

RA

• Hypothalamus does not sense temp changes leading to delayed compensatory mechanisms
• Increasing inter-threshold range

Question 6

What causes shivering

Answer 6

1. Actual hypothermia
2. Volatile agents: Neurological effects of GA agents
3. Neuraxial anaesthesia lowers the shivering threshold
4. Sepsis
5. Drug allergy
6. Transfusion reaction

Question 7

What are the possible physiological consequences of shivering

Answer 7

1. Increased CO2 production –> hypercapnoea
2. Increased VO2 –> hypoxia
3. Acidosis –> lactic acidosis
4. Hyperthermia

Question 8

To what extent can post operative shivering increase oxygen consumption

Answer 8

5 fold –> increased risk of myocardial ischaemia

Question 9

What is the treatment of shivering

Answer 9

1. Warm the patient
2. Pethidine 12.5mg - 25 mg
3. Mechanically ventilated patients: sedation/paralysis until hypothermia is resolved.

Question 10

To what temperature should IV fluids be warmed to

Answer 10

39 degrees