Heat Transfer

Question 1

Regarding heat loss from the body (true or false):

On induction of anaesthesia, the patient undergoes a gradual drop in body temperature

Answer 1

False. Vasodilatation redistributes blood from the central compartment of the body to the peripheral compartment. This causes a rapid reduction in central temperature on induction.

Question 2

Regarding heat loss from the body (true or false):

The anaesthetized patient has impaired vasoconstriction and piloerection in response to hypothermia

Answer 2

True

Question 3

Regarding heat loss from the body (true or false):

Operating theatre humidity should be maintained at no more than 50%

Answer 3

False. Theatre humidity should be at least 50% to reduce evaporative losses from the patient. Previously, when flammable anaesthetic vapours were used, static build-up associated with low humidity could lead to explosions.

Question 4

Regarding heat loss from the body (true or false):

Warmer ambient temperatures are often maintained in paediatric theatres

Answer 4

True. Paediatric patients have an increased body surface area to weight ratio and, therefore, lose heat more quickly.

Question 5

Regarding heat loss from the body (true or false):

Heat loss through the respiratory tract is insignificant

Answer 5

False. Heat losses through the respiratory tract can be significant, because the heat and moisture exchange devices used for the humidification and warming of inspired gases are only about 70% efficient.

Question 6

Which of these does not increase the risk of hypothermia under general anaesthesia?

A. Advanced age
B. The use of controlled ventilation
C. Prolonged duration of surgery
D. Increased body mass index
E. Body cavity surgery

Answer 6

D. Increased body mass index tends to reduce the risk of hypothermia due to the increased layer of insulating adipose tissue.

A. Incorrect. Advanced age increases the risk by virtue of the lower basal metabolic rate, lower muscle mass and less insulating adipose tissue.

B. Incorrect. The use of controlled ventilation reduces the heat generated by respiratory muscle contraction and, therefore, increases the risk of hypothermia.

C. Incorrect. The risk of hypothermia increases with the duration of surgery.

E. Incorrect. Body cavity surgery increases heat losses due to evaporation and therefore increases the risk of hypothermia.

Question 7

The risk of evaporative heat losses from a patient under general anaesthesia is decreased by (true or false):

The use of a heat and moisture exchanger in the breathing circuit

Answer 7

True. A heat and moisture exchanger reduces the evaporative losses from the respiratory tract.

Question 8

The risk of evaporative heat losses from a patient under general anaesthesia is decreased by (true or false):

The use of an intravenous fluid warmer

Answer 8

False. An intravenous fluid warmer tends to ameliorate the heat loss caused by administration of cold fluids, but does not decrease evaporative losses.

Question 9

The risk of evaporative heat losses from a patient under general anaesthesia is decreased by (true or false):

Reducing the humidity of the theatre environment

Answer 9

False. Reducing the humidity of the environment tends to increase evaporative losses.

Question 10

The risk of evaporative heat losses from a patient under general anaesthesia is decreased by (true or false):

The use of laparoscopic as opposed to open surgical techniques

Answer 10

True. The use of laparoscopic techniques avoids body cavity exposure and therefore decreases evaporative losses.

Question 11

The risk of evaporative heat losses from a patient under general anaesthesia is decreased by (true or false):

The use of a reflective space blanket

Answer 11

False. A reflective space blanket tends to reduce radiative heat losses, but not those due to evaporation.