Heat Transfer and Temperature

Question 1

Describe the relationship between temperature and enzymatic reactions?

Answer 1

As temperature increases kinetic energy of molecules increase.

Increasing the likelihood of an enzyme substrate collision (and reaction).

Therefore temperature increases enzymatic activity up to a certain point. Beyond this enzyme structure denatures.

The optimal temperature in the human body for enzyme reactions in 37degrees C.

Question 2

Define energy?

Answer 2

The ability to do work it can be mechanical, chemical, electrical or thermal (heat) energy

Question 3

Define heat energy?

Answer 3

A measurement of the total energy of molecular motion in a substance (which cannot be measured directly) and is dependent on the:

+Kinetic energy of the molecules in the substance
+Physical state of the substance
+Mass of the substance

Question 4

Define temperature?

Answer 4

A measure of the average kinetic energy of the atoms which make up a substance.

When measured in Kelvin, it is directly proportional to the average kinetic energy of the molecules of that substance.

Question 5

Comparing an iceberg and a pan of boiling water, which has more heat energy?

Answer 5

The iceberg has more heat energy as heat energy is dependant on mass, as well as state of the substance and kinetic energy of the molecules.

The pan of boiling water however has a greater temperature therefore if you were to pour the boiling water on the iceberg heat energy would be transferred from the boiling water to the iceberg.

Question 6

Define specific heat capacity, and what is the formula to calculate specific heat capacity?

Answer 6

Specific heat capacity is the amount of heat energy required to raise the temperature of a mass of 1kg by 1 Kelvin

c = ∆Q/m∆T

c=specific heat capacity

∆Q= the amount of heat energy transferred in kilojoules (kJ)

m= mass of the substance being heated in Kg

∆T= the change of temperature in kelvin

Question 7

What is the specific heat capacity of water?

Answer 7

4.18kJ/Kg/K

1 kcal= 4.18kJ

1 kelvin temp rise is the same as a 1 celsius temp rise

Therefore it can also be expressed as 1kcal/kg/C

Question 8

Define heat capacity?

Answer 8

It is the amount of heat energy required to raise the temperature of an object by 1 kelvin.

Heat capcity = specific heat kapacity x mass of object

Question 9

You transfuse 1L 4 C blood (specific heat capacity of 3.6 kJ/kg/C) to a 70kg patient at 37C (specific heat capacity of 3.5 kJ/kg/C).

What is the final temperature of the patient?

Note: Density of blood is approximately 1.125kg/L.

Answer 9

Take the formula for specific heat capacity.

c = ∆Q/m∆T

Rearrange this to:

∆Q= m∆T x c

The ∆Q is the amount of heat energy transferred. I.e amount of heat transferred from the body will equal the amount of heat received from the blood.

You can therefore create a new formula:

(body) m∆T x c = (blood) m∆T x c

70 x (37-T) x 3.5 = 1.125 x (T-4) x 3.6

Once this is rearranged to get T the answer is 36.5

Question 10

What are the 4 methods of heat transfer?

Answer 10

1. Conduction
2. Convection
3. Radiation
4. Evaporation (which is a combo of the above)

Question 11

How does conduction work?

Answer 11

Heat transfer due to the collision of molecules with different temperatures.

Molecules with a higher temperature (which have higher kinetic energy) collide with molecules of a lower temperature (which have lower kinetic energy).

Conduction is a minor cause of heat loss in theatres as, air is a poor conductor and the operating table is well insulated.

Question 12

How does convection work?

Answer 12

Heat transfer by circulation through a gas or liquid.

In theatre, the surrounding air around the patient is warmed made less
dense, it rises, and new cooler air moves closer to the patient.

This air is subsequently warmed and a convection current is created.

This is continuous until an equilibrium of surrounding temperature is reached and accounts for 30% of heat loss in theatre.

Question 13

How does radiation work?

Answer 13

All substances greater than 0K omit radiation as electromagnetic waves.

Electromagnetic waves falling in the infrared spectrum are felt as heat.

Radiation accounts for 40% of the heat loss in theatre.

Question 14

What is Stefan-Boltzmann law related to and what is the formula?

Answer 14

Stefan-Boltzmann law
relates the total amount of radiation emitted as a function of its temperature:

E = s x T(power 4)

E = the total amount of radiation emitted per m2 of an object
s = a constant
T = the temperature in Kelvin of the object

Question 15

How does evaporative heat loss work?

Answer 15

This loses heat through the process of latent heat of vaporisation. (As a liquid vaporises its temperature remains static but heat energy is required from the surroundings for the liquid to vaporise)

Evaporative heat loss may occur when sweat or antiseptic solution evaporates from the skin or fluid evaporates from exposed moist internal body cavities. Heat may be transferred to fluid on the exposed surface through conduction and taken away by convection.

Accounts for 20% of heat loss.

Question 16

How much heat is lost by each method of heat transfer in theatre as a percentage?

Answer 16

Radiation 40%
Convection 30%
Evaporation 20%

Humidification (8%) and warming (2%) of gases in respiratory tract.

Question 17

What factors affect heat loss during an operation?

Answer 17

Environmental factors:
+Ambient temperature of the theatre (radiation)
+Laminar flow (increase convection currents)
+Humidity (evaporative losses)

Anaesthetic factors:
+Heat loss:
-Anaesthetics causing vasodilation (increase radiation and conductive loss)
-Latent heat of vaporisation from respiratory tract in invasively ventilated patients
-Use of non warmed fluids or blood

+Reduced heat production:
-Reduced BMR
-Not spontaneously ventilating
-Reduced muscle tone
-Absent behavioural mechanism (putting on more clothes)
-Blunts the normal physiological response to
hypothermia. An anaesthetised patient is unable to generate heat by moving or shivering and has impaired vasoconstriction and
piloerection in response to hypothermia.

Surgical factors:
+Moist body cavities exposed (evaporative)
+Cold irrigation intra-op

Patient factors:
+ Extremes of age; children have a greater surface area to mass ration, elderly have lower metabolic rate
+Hypothyroidism

Question 18

Describe the temperature loss pattern which occurs during anaesthesia?

Answer 18

There is an initial sharp decline on induction of
anaesthesia due to vasodilation and redistribution of core blood to periphery.

There is subsequent gradual linear loss of heat until a temperature it reached which is the new set point for
thermoregulation.

At this point equilibrium is reached where loss to
environment equals that generated by the patient.

Answer 19

Minimising Heat Losses
+Avoid unnecessary exposure of the patient
+Use head wraps to combat losses through the head
+Consider avoiding the use of muscle relaxants and allowing spontaneous ventilation
+Minimize the exposure of moist body cavities – i.e. plastic covering over exposed bowel.
+The theatre environment should be kept warm
+Inhaled gases should be warmed and humidified
+Warm irrigative fluids to body temperature before use

Warming Methods
+Hot air blankets aka bear huggers
+Warmer ambient temperatures with use of overhead radiant heaters and warm mattresses –
more common in paediatric theatre.
+Warming of IV fluids and blood products