Essential physics for anaesthesia

Question 1

Define a gas

Define a vapor

Answer 1

Gas
A gas is a compressible fluid phase in which no liquid can be formed at that temperature. The threshold above which a substance exists as a gas is its critical temperature.

Vapor
A vapor is a gaseous phase which is in a state of equilibrium with the same substance in a liquid form. A vapor can exist only below its critical temperature

Question 2

At what pressure does the 20ºC isotherm for N2O go from containing Vapour + Liquid to just liquid?

At what pressure does the 36.5ºC isotherm for N2O go from vapor to liquid

Answer 2

20ºC isotherm for N2O go from containing Vapour + Liquid to just liquid: 52 bar (52 x 100 kPa)

36.5ºC isotherm for N2O go from vapor to liquid: 73 bar (73 x 100 kPa)

Question 3

Describe the names and history of the three ideal gas laws

Answer 3

Boyle’s law - 1662: relating pressure and volume
Charles’s law - 1787: relating volume and temperature
Gay-Lussac’s law - 1808: relating temperature and pressure

Amalgamated to form the Combined Gas Law

Question 4

Define the 1st ideal gas law (Boyle’s law)

Answer 4

Boyle’s law states that at in a closed system at constant temperature the pressure of a fixed mass of gas is inversely proportional to the volume.

P ~ 1/V

PV = K1

Question 5

Define the 2nd gas law (Charles’s law)

Answer 5

Charles’s law states that in a closed system at constant pressure the volume of a fixed mass of gas varies in proportion to the absolute temperature.

V ~ T

V/T = K2

Question 6

Define the 3rd gas law (Gay-Lussac’s law)

Answer 6

Gay-Lussac’s law states that in a closed system at constant volume the pressure of a fixed mass of gas varies proportionally to the absolute temperature.

P~T

P/T = K3

Question 7

What is the difference between the Combined Gas Law and the Ideal Gas Law

Answer 7

PV/T = k (Combined gas law)

PV = nRT (Incorporates the number of molecules present and the universal gas constant R)

Question 8

Why is the pressure a gauge an accurate representation of quantity in an oxygen cylinder but not for a N2O cylinder?

Answer 8

O2 cylinder - O2 is above its critical temperature - therefore with a constant temperature and pressure, according to the ideal gas law P is proportional to the number of moles present (V, T, R all constant).

N2O cylinder - N20 has a critical temperature of 36.5ºC. It is therefore below its critical temperature. As the pressure increases more vapour will be condensed into the liquid phase –> N2O therefore does not conform to the ideal gas law and the cylinder must be weighed to measure its contents.

Question 9

Define Dalton’s law

Answer 9

John Dalton observed that the total pressure is the sum of the pressures of the individual gases, as if they were alone in the same volume.

The pressure exerted by each gas is termed its partial pressure (assumed T is constant and that the gases do not react with each other)

Ptotal = P1 + P2 + P3

From this, you can derive that the fraction of the total pressure exerted by a gas in a mixture is directly proportional to its fractional composition within the mixture

e.g. Air contains 21 % of O2 by volume. Its partial pressure = 0.21 x 101 kPa = 21 kPa

Question 10

Define saturated vapor pressure

Answer 10

As liquid continues to evaporate into a space above it, the pressure of its vapour increases. When a point of equilibrium is reached between the rate of evaporation and the rate of condensation, this vapour pressure is known as the saturated vapour pressure.

Question 11

What single variable changes the SVP of a substance?

Answer 11

The SVP increases progressively with a rise in temperature.

Question 12

What happens when the SVP equals the atmospheric pressure?

Answer 12

The liquid boils. The temperature at which the saturated vapour pressure equals the atmospheric pressure is the boiling point.

Question 13

Define Henry’s law

Answer 13

At a constant temperature, more molecules are dissolved at higher pressure.

The amount of gas dissolved in a liquid is directly proportional to the partial pressure of the gas in equilibrium with the liquid.

Gas molecules dissolved inside the solution reach equilibrium when the partial pressure of the gas in solution equals the partial pressure of the gas above.

Question 14

How does Henry’s law apply to volatile anaesthetic agents?

Answer 14

If the partial pressure of a volatile anaesthetic agent in the alveoli is doubled, once equilibrated the partial pressure and quantity dissolved in the blood will also double

Question 15

How does the quantity of gas dissolved in a liquid vary with different types of gases?

Answer 15

For different substances, the quantity of gas dissolving in a liquid at a given partial pressure can vary hugely and is related to their solubility coefficients.

Question 16

What is a partition co-efficient?

Answer 16

This describes the relative concentration in each of two defined phases when an agent has distributed between them at equilibrium. E.g. Blood:Gas co-efficient

Question 17

How does temperature effect solubility

Answer 17

As the temperature increases, the molecules in the solution have more energy to get out of the liquid and so solubility falls

Conversely, a fall in temperature increases the solubility of a gas in a liquid.

Question 18

What are the important differences between AC and DC current in terms of electrical safety

Answer 18

AC is more efficient for transmission than DC but AC is more hazardous to the body in the event of an electrocution.

Question 19

How is electricity supplied to the hospital: AC or DC current?

Answer 19

AC current

Question 20

Describe the basic arrangement of electricity supply to the hospital

Answer 20

Power station (16kV) to substation (240 V - via transformer) –> delivered to the hospital via 2 wires: live and neutral.

Current flows outward from the substation through the live wire returning to the substation via the Neutral wire which is then connected to the earth.

Question 21

What would happen if a person were to touch a live wire in theatre?

Answer 21

An electric current would pass through the body to the floor and earth - thus completing a circuit back to the sub-station. The effect of the subsequent shock depends on the magnitude of the current

Question 22

What is the unit of measurement of electrical current?

Answer 22

Amperes (A)

Question 23

What does the current in a circuit depend on?

Answer 23

Electrical barrier presented to the Voltage (V).

Question 24

What is the term used to describe the electrical barrier in Direct Current

Answer 24

Resistance (measured in Ohms) R

Question 25

Define Ohms law

Answer 25

The current in an electrical circuit is directly proportional to the applied voltage and inversely proportional to the resistance of the circuit

V = IR

Question 26

What is the term used to describe electrical Resistance with regard to alternating current?

Answer 26

Impedance (Z)

Impedance varies with frequency

As the resistance or impedance falls the current increases.

Question 27

Show the affects to the human body that occur as the elctrical current increases

Answer 27

1 - 5 mA: tingling

10 - 20 mA: Pain - sustained muscle contraction

50 mA: Risk of arrhythmias (ventricular ectopics)

100 - 300mA: Risk of ventricular fibrillation

Question 28

What else influences the risk of VF during a shock

Answer 28

Timing and duration of the shock

IF it occurs during early repolarization of cardiac myocytes (Early t-wave) –> VF more likely

frequency of the Alternating current: The lower the frequency the higher the risk of VF

Question 29

What is protective in the event of electrocution

Answer 29

A high impedance

Question 30

What is the typical impedance value for dry skin?

Answer 30

10 kOhms

With skin as dominant impedance, e.g. 10 kΩ, and mains voltage of 240 V:

= 24 × 10^–3 A

= 24 mA –> pain and muscle spasm

Wet or broken skin –> impedance may fall to 1 kΩ leading to a current of 240 mA and risk of VF

Question 31

Why have static precautions on theatre shoes fallen from use?

Answer 31

Explosive agents are no longer used in theatre

Question 32

What is the effective impedance of the theatre floor and theatre shoes?

Answer 32

240 kΩ

I = V/Z

I = 240V/240 x 10^3 Ω
I = 1 mA

1 mA causes tingling but little harm

Question 33

Give an example of the use of current density in clinical practice

Answer 33

Nerve stimulation is used to locate a nerve or plexus for local anaesthetic block.

The closer the needle is placed to the relevant nerve, the lower the current required to achieve successful motor stimulation –> a current of 0.5 mA or less is usually aimed for, compared to 40 - 80mA for motor stimulation using surface electrodes.

Question 34

What is the risk associated with increased current density?

Answer 34

Increased current density can also be a potentially serious hazard, if an unintended small current is delivered close to the heart. This is known as a microshock.

Question 35

What is a microshock and how does it relate to current density?

Answer 35

Faulty equipment –> aberrant current in a device connected to an intracardiac catheter (CVP line/external pacemaker) There is the risk that the entire current could enter the myocardium through a small area –> thus generating a high density current.

A current as small as 0.1 mA is sufficient to trigger VF = microshock.

Question 36

What is a leakage current?

Answer 36

A small current passing between circuits or to earth because of incomplete insulation and which would normally be harmless via surface contact.

Question 37

What is a floating circuit and why is it used?

Answer 37

A floating circuit, or isolated patient circuit, is usually achieved using a transformer which generates an identical electrical source using elctromagnetic induction, but breaks the continuous circuit between the mains supply and the patient.

Therefore, there is no circuit connection with the substation

Question 38

What are the different British Standards classification for electrical isolation

Answer 38

There are standards for all medical equipment which depends on the type of equipment

Type CF - Direct cardiac connections: Floating circuit with optimal insulation

Type BF- No direct cardiac connection, only indirect (ECG)

Type B - Devices with non-conducting connections with the patient. It is earthed and has no floating circuit.

Question 39

What are the main mechanism to prevent accidental shock

Answer 39

Insulation and cut-out devices

Question 40

How does insulation prevent or minimze electrical current through the body?

Answer 40

Prevents electrical contact between the body and the live electrical source

Used high impedance to minimize the current if an aberrant circuit does occur

Question 41

What are the British Standards specification of the 3 classes of insulation

Answer 41

Class 1 - Low risk equipment - any exposed metal parts (e.g. the case) must be connected to the earth)

Class 2 - All exposed parts have double layer of insulation. Earth connection not required.

Class 3 - Battery powered (run at voltage less than 40 V DC). If AC is used then an isolating transformer is incorporated

Question 42

What are the two types of cut-out devices?

Answer 42

Fuses

Circuit breakers

Question 43

How does a fuse work?

Answer 43

Fuse incorporated into the live supply and the earth wire is connected to the equipment casing.

In the event of a fault that makes the casing live - the earth wire provides a low impedance return pathway to the sub-station - the sudden surge in current melts the fuse, thereby disconnecting the live supply.

the fuse must be appropriately rated so that it does not blow with normal use

Question 44

What is a Current-Operated Earth Leakage Circuit Breaker (COELCB)

Answer 44

Monitors the current flowing in the live and neutral conductors.

A difference in the flowing current indicates that current may be flowing to earth by another means. If such a difference is detected, the circuit breaker rapidly operates a relay on the live conductor, switching off the power.

Question 45

How do floating currents increase safety?

Answer 45

Break the continuous circuit between the mains and the patient by using an isolating transformer