eLFH - Gases and Vapours Flashcards
Vapour definition
A gaseous substance below its critical temperature which can be liquified by pressure alone
Evaporation
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
Saturated vapour pressure definition
Pressure exerted by the molecules of the vapour component when a vapour and its liquid are in equilibrium within a confined space
Vapour equilibrium
State when the number of molecules re-entering the liquid phase is equal to the number of molecules entering the vapour phase
What determines how volatile an agent is
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
Volatile agent with higher saturated vapour pressure
Isoflurane
Volatile agent with lower saturated vapour pressure
Sevoflurane
Implication of varied saturated vapour pressures of different volatile anaesthetic agents
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
Relationship between SVP and temperature
Non linear
As temp increases, SVP increases
What happens when SVP is equal to atmospheric pressure
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
Relationship between SVP and ambient atmospheric pressure (eg at altitude)
SVP does not change with change in ambient pressure
But the proportion of total pressure occupied by the given vapour changes with ambient pressure
Effect of changes in ambient pressure on volatile anaesthetic use
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
Latent heat explanation
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
Latent heat of vaporisation definition
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
Latent heat of fusion definition
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
Specific latent heat definition
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
Adiabatic change
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
Isothermal change
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
Purpose of the vaporiser
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
Types of gas flow through vaporisers
Plenum vaporisers
Draw-over vaporisers
Plenum vaporisers
Positive pressure upstream forces carrier gas through vaporiser
Standard type of vaporiser used in the UK currently
Early example of Plenum vaporiser and issues encountered
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
Draw-over vaporisers
Uses negative pressure downstream (achieved by bellows or the patients’ respiratory efforts)
More common in developing world or field anaesthesia
Standard vaporiser chamber used in UK
Tec (temperature compensated) vaporiser chamber
Tec vaporiser chamber mechanism
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
Alternative chamber mechanism to the Tec vaporiser
Copper kettle vaporiser
Copper kettle vaporiser chamber mechanism
Bubbles gas through liquid anaesthetic via a sintered disc
This ensures saturation by producing small bubbles
Splitting ratio
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
Why is temperature regulation required in vaporisers
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
Effect of gas flow rate on vaporiser function
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
Why does flow change the proportion of carrier gas entering the vaporisation chamber
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
Vaporiser temperature compensation mechanisms
Bimetallic strip
Bellows
Metal rod orifice
Metal heat sink
Water bath
Bimetallic strip
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
Bellows
Small, flexible and either expand or contract to open or shut a valve
E.g Ohio bellows
Metal rod orifice
Thermal expansion (and contraction) of a metal rod
Adjusts an orifice to modify flows
Metal heat sink
Large metal heat sinks with great heat capacities to buffer latent heat loss
Water bath
More rarely used now
Water bath controls temperature fluctuations
Water has a great specific heat capacity
