Heat transfer and temperature

Question 1

Describe what happens to enzyme reactions as temperature increases?

Answer 1

As temp increases, average kinetic energy of the substances increases causing more enzyme/substrate collisions, increasing the rates of reaction.

When temp becomes too high, the kinetic energy within enzymes causes bonds to vibrate/break and therefore enzymes lose their shape, becoming denatured.

Question 2

What is optimal temperature?

Answer 2

The temperature at which the most enzymatic reactions take place. This is around 37 deg c.

(Much less in the testes)

Question 3

What can giving an anaesthetic do to enzyme activity?

Answer 3

Lowers temp and therefore impairs enzyme reactions.

Question 4

What is energy?

Answer 4

The ability to do work.

It can be mechanical, chemical, electrical or thermal energy.

Question 5

What is heat energy?

Answer 5

The total energy of molecular motion in a substance.

Dependent on the: kinetic energy, mass and physical state.

It cannot be measured directly.

Question 6

What is temperature?

Answer 6

Average kinetic energy of the atoms in a substance.

When measured in Kelvin, this is directly proportional.

Temperature is defined by the likelihood that a substance will transfer heat energy to another. Greater the temp, the more likely the transfer to the other object.

Question 7

Give a comparison between temperature and heat?

Answer 7

An iceberg and a pan of boiling water have completely different temperatures.

But given the mass of the iceberg it contains far more heat energy than the pan of water.

However if they make contact, the hot water will transfer heat to the iceberg and so has a higher temperature.

Question 8

What is the comparison between heat energy and temperature in a body in a theatre?

Answer 8

The theatre has greater total heat energy, however the body has a greater temp and so will transfer heat energy to the theatre.

Question 9

What is specific heat capacity?

Answer 9

The amount of heat energy needed to raise the temp of 1kg by 1 Kelvin.

Question 10

What is the equation for specific heat capacity?

Answer 10

C = delta Q divided by m delta t

C = sHC
Delta q = change in heat in Kj
M = mass (kg)
Delta t = change in temp in K

Question 11

What is the specific heat capacity of water?

(In cals and degrees)

Answer 11

4.81 kj/kg/K

1 kcal = 4.81 kj, therefore = 1 kcal/kg/K

As 1 Kelvin = 1 degree it can also be 1 kcal/kg/deg c

Question 12

What is heat capacity?

Example for 5L of water?

Answer 12

The amount of heat energy required to raise the temperature of an object by 1K

5L of water = 5kg therefore 4.18 x 5 = 20.9 kj/K

Question 13

What is the equation for heat capacity?

Answer 13

Heat capacity = specific heat capacity x weight in Kg

Question 14

If we transfuse 1L of 4 deg c blood (SHC of 3.6) to a 70kgpatient at 37 deg c (SHC = 3.5), what is the final temp of the patient?

(Blood density = 1.125 kg/L)

Answer 14

C= delta Q/ m Delta T or delta Q = c x m x delta T

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

4.05 x (T - 4) = 245 x (37 - T)

4.05T - 16.2 = 9065 - 245T

249.05 T = 9081.2

T = 36.5

Question 15

What is heat transfer?

Answer 15

The passage of heat from a warmer substance to a colder one.

Question 16

What are the three main methods of heat transfer?

Answer 16

Conduction, convection and radiation

(Evaporation is a combination of the above)

Question 17

What is conduction?

Answer 17

Heat transfer due to the collision of molecules of two substances with different temperatures. Higher kinetic energy transfers to lower (conservation of momentum)

Question 18

How does conductive heat loss apply to theatre?

Answer 18

Air is a poor heat conductor.

Patient in contact with table, but we insulate the table and the area of contact between it and the patient is small.

Therefore conduction is minimised and less important.

Question 19

What is convection?

Answer 19

Heat transfer by circulation through a gas or a liquid.

Warm air beside skin, this becomes less dense and rises.

New cooler air is warmed and a convection current is created. This continues until equilibrium.

Question 20

What is radiation?

Answer 20

Does not require for heat transfer and instead uses electromagnetic rays (emitted from all objects above 0K). Higher temp = more radiation and vice versa with absorption).

Question 21

What is the Stefan Boltzmann law?

Answer 21

Electromagnetic waves in the infrared spectrum are felt as heat. The law related the total amount of radiation omitted to its temperature.

E = sT (power of 4)

E = total mount of radiation omitted per m2 of an object
S = a constant
T = objects temp in K

Question 22

What is evaporation?

Answer 22

Heat loss through LHV e.g. sweat, antiseptic or moist body cavities.heat transferred by conduction and taken away by convection.

Question 23

What percentage do the different types of heat loss account for in theatre?

Answer 23

Radiation 40%
Convection 30%
evaporation 20%

Other 10% from respiratory humidification (8%) and warming (2%)

Question 24

What theatre factors increase heat loss?

Answer 24

Theatre colder than ward for staff comfort = increased radiation.

Laminar flow increases convection

Theatre humidity around 50%, altering evaporation.

Question 25

How do anaesthetics cause heat loss?

Answer 25

Increases loss and obtunds normal response to cold.

Drugs cause vasodilation causing radiation and conduction. Convection brings core blood to the surface causing rapid reduction in core temp.

LHV causes loss via respiratory tract and warming inhaled gasses.

Reduced heat production via reduced BMR, NMB reduce respiratory and peripheral muscle tone reducing heat production, no shivering, vasoconstriction or piloerection to cold and no dressing etc.

Question 26

How does surgery effect body temp?

Answer 26

Exposed cavities, exposed body, cold irrigation.

Surgery that promotes third space losses means increased IVF and blood administration.

Question 27

What patients exhibit increased heat loss?

Answer 27

Babies = lower surface to weight ratio. More so through head as larger compared to body.

Elderly = reduced BMR and muscle mass. Reduced adipose and shivering.

Others e.g. hypothyroidism

Question 28

What is the pattern (phases) of temperature loss in anaesthesia?

Answer 28

Initial sharp decline through redistribution from vasodilation moving blood from core to surface.

Then gradual linear loss until a temp is reached which is a new set point for thermoregulation.

Then equilibrium as loss = gains

Question 29

How do we manage temperature in theatre?

Answer 29

1. Minimise loss = avoid exposure, head wraps, avoid muscle relaxants, spontaneous breathing, cover moist body cavities, warm theatre, gases and fluids. Can use space blankets post op, but electrical hazard in theatre.
2. Warming: hot air blankets, overhead heaters and warm mattresses, warmed IV fluids.

Question 30

What is room temperature in Kelvin and Celsius?

Answer 30

20c and 293k