eLFH - Gases and Vapours Flashcards

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1
Q

Vapour definition

A

A gaseous substance below its critical temperature which can be liquified by pressure alone

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2
Q

Evaporation

A

At the surface of a liquid, some molecules escape as their energy is greater that the Van der Waals’ forces attracating them to other molecules within the liquid

Heat increases this process

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3
Q

Saturated vapour pressure definition

A

Pressure exerted by the molecules of the vapour component when a vapour and its liquid are in equilibrium within a confined space

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4
Q

Vapour equilibrium

A

State when the number of molecules re-entering the liquid phase is equal to the number of molecules entering the vapour phase

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5
Q

What determines how volatile an agent is

A

The higher the saturated vapour pressure, the more volatile a substance and therefore the greater its tendency to vaporise

Therefore the higher the SVP, the higher the concentration of agent in the carrier gas than an agent with a lower SVP when used at the same temperature

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6
Q

Volatile agent with higher saturated vapour pressure

A

Isoflurane

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7
Q

Volatile agent with lower saturated vapour pressure

A

Sevoflurane

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8
Q

Implication of varied saturated vapour pressures of different volatile anaesthetic agents

A

Vaporisers designed differently to account for volatility and therefore the amount of vapour that would be mixed with carrier gas

I.e vaporisers for different agents are not interchangeable

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9
Q

Relationship between SVP and temperature

A

Non linear
As temp increases, SVP increases

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10
Q

What happens when SVP is equal to atmospheric pressure

A

The liquid boils
Vapour concentration at the surface of the liquid is 100%

Therefore at lower ambient pressure, boiling temperature is lower as vapour escapes to atmosphere

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11
Q

Relationship between SVP and ambient atmospheric pressure (eg at altitude)

A

SVP does not change with change in ambient pressure

But the proportion of total pressure occupied by the given vapour changes with ambient pressure

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12
Q

Effect of changes in ambient pressure on volatile anaesthetic use

A

More vapour escapes, so at high altitude with half the atmospheric pressure, giving 1% of volatile anaesthetic agent equates to giving 2%

However action of anaesthetic remains the same as it relies on partial pressure (1% of 1 atm is the same as 2% 0.5 atm)

Therefore no need to change dial percentages or anaesthetic practice at changes in altitude

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13
Q

Latent heat explanation

A

Not all molecules in liquid have the same energy

More vigorous molecules have greatest tendency to escape to gaseous phase

Therefore average energy of those left behind is lower

Energy for molecules left behind to continue to enter gaseous phase at same rate is provided by heat of surrounding / liquid

Therefore lower temperature of molecules left behind

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14
Q

Latent heat of vaporisation definition

A

The heat energy required to convert a given mass of liquid into vapour whilst maintaining the same temperature

E.g When heating water, temp will increase until reaches boiling point. At that point energy is required for individual molecules to convert into gaseous phase so temperature overall does not increase during this stage

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15
Q

Latent heat of fusion definition

A

The heat required to convert a given mass of solid into liquid at the same temperature

Same as vaporisation definition and example but solid to liquid

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16
Q

Specific latent heat definition

A

The heat energy required to convert one kilogram of a substance from one phase to another

Units are J / kg

Must be quoted with reference to a specific temperature as the closer a liquid is to boiling point, the lower the heat required to achieve complete vaporisation

17
Q

Adiabatic change

A

When a gas expands rapidly without heat energy added, temperature falls as energy is required to overcome van der Waal’s forces and the source of energy is from the molecule’s own kinetic energy

Similarly when gas is compressed rapidly, its temperature increases - this is called the Joule-Kelvin principle

18
Q

Isothermal change

A

If compression or expansion of a gas occurs sufficiently slowly, heat can be conducted through the walls of the container

Therefore no change in gas temperature occurs

19
Q

Purpose of the vaporiser

A

Deliver controlled and predictable concentration of anaesthetic agent in a carrier gas at the common gas outlet

Saturated vapour pressure of volatile agents at room temp are much higher that anaesthesia requirements

20
Q

Types of gas flow through vaporisers

A

Plenum vaporisers

Draw-over vaporisers

21
Q

Plenum vaporisers

A

Positive pressure upstream forces carrier gas through vaporiser

Standard type of vaporiser used in the UK currently

22
Q

Early example of Plenum vaporiser and issues encountered

A

Boyle’s bottle

Output from this device was variable - higher flows of carrier gas would not saturate and latent heat would cool the remaining volatile agent

23
Q

Draw-over vaporisers

A

Uses negative pressure downstream (achieved by bellows or the patients’ respiratory efforts)

More common in developing world or field anaesthesia

24
Q

Standard vaporiser chamber used in UK

A

Tec (temperature compensated) vaporiser chamber

25
Q

Tec vaporiser chamber mechanism

A

Network of internal channels and wicks produce high surface area - ensures gas emerging from chamber is fully saturated with anaesthetic vapour

Dial on vaporiser alters amount of anaesthetic-free bypass gas that can mix with the anaesthetic vapour (which is at saturated vapour pressure)

Entrance to vaporiser chamber controlled by a bimetallic strip

26
Q

Alternative chamber mechanism to the Tec vaporiser

A

Copper kettle vaporiser

27
Q

Copper kettle vaporiser chamber mechanism

A

Bubbles gas through liquid anaesthetic via a sintered disc
This ensures saturation by producing small bubbles

28
Q

Splitting ratio

A

Proportion of total gas flow to volatile agent required to achieve desired concentration of anaesthetic agent in carrier gas

Volatile anaesthetic agents with higher SVP are more volatile and require lower splitting ratios to give required concentration

29
Q

Why is temperature regulation required in vaporisers

A

Vaporisation results in remaining volatile anaesthetic agent cooling due to latent heat, which results in lower SVP and therefore lower concentration delivery

Modern vaporisers have temperature compensation mechanisms to either maintain temperature of volatile agent or alter the splitting ratio with changing temperatures

30
Q

Effect of gas flow rate on vaporiser function

A

Flow changes proportion of carrier gas entering the vaporisation chamber

Increased flow rates enhance cooling effects on volatile agent

High flow rates can cause difficulty achieving equilibrium in the vaporisation chamber, altering efficiency of vaporisation

31
Q

Why does flow change the proportion of carrier gas entering the vaporisation chamber

A

Relative resistances of flow between the two gas paths has a greater significance at very low flows

Therefore at higher flow rates higher dial numbers are required to achieve the same carrier gas concentration as lower flows with lower dial settings

32
Q

Vaporiser temperature compensation mechanisms

A

Bimetallic strip

Bellows

Metal rod orifice

Metal heat sink

Water bath

33
Q

Bimetallic strip

A

Bonded strips consisting of two metals with different thermal expansion coefficients

Cause the composite strip to bend and either open or partially close an orifice in response to temperature of volatile agent

34
Q

Bellows

A

Small, flexible and either expand or contract to open or shut a valve

E.g Ohio bellows

35
Q

Metal rod orifice

A

Thermal expansion (and contraction) of a metal rod

Adjusts an orifice to modify flows

36
Q

Metal heat sink

A

Large metal heat sinks with great heat capacities to buffer latent heat loss

37
Q

Water bath

A

More rarely used now

Water bath controls temperature fluctuations
Water has a great specific heat capacity