vapourisers Flashcards
what is a vapouriser?
device that delivers a controlled specific amount of anaesthetic agent to the FGF to achieve safe predictable anaesthesia.
how can vapourisers be classified?
in circuit - draw over
out of circuit
variable bypass
* plenum - Tec series
* draw over - oxford minature, goldman
measured flow
* desflurane - Tec 6
* can also get direct injecion volatile anaesthetic (DIVA) for any agent
what is the difference between variable bypass and measured flow vapourisers?
variable bypass - fraction of FGF is diverted into a vapourising chamber. becomes saturated with vapour. then regoins the remainder of FGF to give controlled % of volatile
measured flow - anaesthetic agent is heated so it is all gaseous state and an amount is injected into FGF. used for desflurane where the boiling point is low and SVP is high so variable bypass would be less predictable.
what does Tec mean?
temperature compensated
standard vapouriser series in UK - improvements from 3-7
Tec 6 - for des specifically
what are the features of an ideal vapouriser?
efficiency
* fully saturates at range of FGF rates 0.5-15L/min
* temp compensation
* not effected by ambient pressure/ temp changes
* low resistance to flow i.e. doesnt effect work of breathing
safety
* antispil mechanism
* downsteam one wave valve preventing pumping effect
* selectactec system
practical
* small, portable
* long life - resistant to corrosion
* minimal pollution i.e. no leaks
how does a variable bypass vapouriser work?
proportion of FGF enters chamber
becomes fully saturated with anaesthetic agent
rejoins FGF
if this proportioning did not occur - SVP of agents is too high, much higher than MAC e.g. SVP of sevo 22 hence 22%. MAC is only 2.2. therefore only need a 10th of this.
for it to be accurate, the proportion that enters chamber must fully saturate - wicks and baffles
wicks - material soaked in liquid - increases S.A for vapourisation
baffles - channels that prolong path and increase time for saturation
dial - will have a calibrated scale with % of anaesthetic agent that comes out at the end. this % will correspond to a specific splitting ratio.
why are plenum vapourisers agent specific ?
each agent has a specific saturated vapour pressure e.g. sevo 22, isoflurane 32
therefore to get 2.2% of sevo a splitting ratio of 1:9 could be used i.e. 10% would be fully saturated.
however if this splitting ratio was used for iso it would give 3.2%
ALSO the SVP of an agent changes with temperature differently i.e. not proportionally. depends on boiling point etc. One vapouriser will have temp compensation method e.g. bimetallic valve specific to its relationship.
theoretically agents with same SVP could be used in same vapouriser e.g. sevo and enflurane. however would need to dial different % as MAC is different for each agent.
what drives plenum vapourisers to work?
out of circuit
not by ventilator/ breathing
driven by FGF
colours of vapourisers
sevo = yellow
des = blue
halothane = red
iso = purple
en = orange
describe the issues with vapourisers and methods of improving the efficiency of a plenum vapouriser?
2 main issues
- vapour not being fully saturated - would lead to inaccurate % at the end. -solved with wicks, baffles, bubbles
- latent heat of vapourisation - as vapour evapourates it uses energy from surroundings as latent heat of vapourisation. this cools the surroudings and changes SVP and hence reduces output. solved by heat sink and temp compensation e.g. bimetallic valve or constricting bellows
how does the heat sink work?
high conductivity
high specific heat capacity
e.g. copper
therefore allows transfer of heat energy from surrounding to keep vapourising chamber at equilbrium with srurrounding
how does the bimetallic strip work?
2 metals of different coefficients of thermal expansion
therefore as temp changes they will bend
this will alter the splitting ratio of gas entering the chamber
e.g. as it gets colder, more gas enters chamber to compensate for the lower SVP
how do bellows work as a method of temp compensation?
bellows constrict as temp drops and increases the amount of FGF allowed to pass thrrough chamber
how has the Tec series improved?
Tec 2 - bimetallic strip on inside - preservatives of vapour caused corrosion and worsening of function
Tec 3 - put on outside of vapourising chamber to prevent corrosion
tek 4 - antispill
tek 5 - electronic controls
what are the problems/ safety issues of plenum vapourisers and how are they overcome?
pumping effect - back pressure from ventilator causes FGF to go back into the vapourising chamber increases amount of anaesthetic agent then leaving vapouriser. solved by one way valves at the outlet of the vapouriser. also have high internal resistance to resist backwards flow. also very long outlet so prevented from reaching bypass channel
damage by excessive pressures - pressure relief valves present
overfilling / spillage of anaesthetic agent - if agent gets into the bypass chamber can result in very high % i.e. the SVP being delivered. now there are anti spill mechanisms preventing this. also avoided by purging vapourisers and low fillinf ports with transparent lines
incorrect agent introduced - vapourisers are agent specific so this will give inaccurate amounts. overcome by colour coding and geometric coding of filling port
pollution - when filling the vapoursier can spill and vapour can escape. antipollution cap and filling mechanisms help this.
selectactec system means can only use one at a time and only when attached properly
agent depletion - transparent level visible and alarms
what is a draw over vapouriser
in circuit vapouriser
whereby FGF is drawn over the vapouriser by patients respiratory efforts or the ventilator
increases resistance to breathing so must have lower resistance than out of circuit ones
e.g. goldman and oxford minature
pros and cons of draw over?
portable and small
good for field medicine
dont require additional FGF , can use patients resp efforts
cheaper
however
increases resistance and work of breathing
less accurate and efficient - cant use wicks and baffles/ temp compensation (although some have ethylene glycol sink)
less safe - no anti-spil mechanism, liquid could get into breathing ciruit.
what vapourisers use continous vs non continous flow?
continous - plenum and tec 6
non-continous - draw over
why does desflurane need its own vapouriser?
low boiling point - 23.5 degrees so at room temp could all be vapour - hence SVP would be unreliable
even if below this - very high SVP and small changes in temp will have big chnages in SVP - so less accurate/ predictable
so uses its own = tec 6
how does a tec 6 vapouriser work?
desflurane containing in a chamber
heating element that heats it to 39 degrees
pressurised to 2atm
small amount is injected into FGF
this depends on…
dialled amount - user chooses a % which will change the amount entering
presssure transducer - controls a valve depending on pressure difference in FGF compared to desflurane injectate. hence this accounts for changes to FGF rate. (otherwise high flow rates would dilute it)
both of these controls work via altering the resistance to flow from desflurane output into FGF
usually a microprocessor takes input from dial and pressure transducer and controls a solenoid valve
electronically powered and controlled
what ranges of FGF are Tec vapourisers accurate?
0.5L/min - 15L/min
disadvantages of Tec vapourisers?
bulky - not good for transport
more expensive
more parts that could go wrong
specifically Tec 6
requires electrics
takes few minutes to heat up
what is the aladin casette vapouriser
these are plenum vapourisers with electronic controls
hybrid between variable bypass and measured flow
still uses a splitting ratio
however electronic controls - measure temp and calculates SVP and alters splitting ratio based on this.
plus electronic dial
advantages of this are that it is more accurate, continually monitoring conc of agent and making adjustments in real time.
can the carrier gas composition effect the functioning of a plenum vapouriser?
composition effects viscosity which effects flow so technically yes
although minimal significance clinically
which factors affect how much inhalation agent is delivered?
SVP
FGF
splitting ratio
what considerations are taken into account when building a vapouriser?
SVP - splitting ratios
boiling point - is it suitable for plenum/ measured flow
MAC - dial range
how are plenum vapourisers effected by altitude?
in short - plenum vapourisers are not affected by altitude because it is the partial pressure of an agent that exerts its effects, not the %.
e.g. consider pressure drops from 100kpa to 50kpa.
and imagine there is no splitting for now. sevoflurane SVP = 22.5
so at 100kpa = 22.5kpa is delivered = 22%
at 50 kpa = SVP doesnt change 22.5 is delivered = 44%
now imagine splitting ratio of 1:9 is used. therefor 2.2% and 4.4%
2.2% of 100 = 2.2 kpa = 1 mac
4.4 % of 50 = 2.2 kpa = 1 mac
this is all because the SVP doesnt change with pressure and it is the partial pressure that has the effect.
in reality temp drops with altitude which would effect SVP.
(daltons law: PP = total pressure x % conc)
how is the tec 6 vapouriser affected by altitude
this one is affected by altitude
a set concentration of des is injected which remains the same at sea level and altitude
therefore 6% of des at sea level = 6kpa
at half the pressure 50Kpa = 3kpa
how does the selectactec system work?
metal rods between vapourisers
when one is on the other is inactivated
cant turn both on at once.
also cant be turned on unless correctly attached.
what is SVP?
Saturated vapor pressure (SVP) is the pressure exerted by a vapor in equilibrium with its liquid (or solid) at a given temperature.
as temp increases, more energy for liquid molecules to vapouriser, therefore SVP increases
can draw a diagram to show evapouration and condensation and equilibrium
SVP of different volatiles…
sevo = 22
iso = 33
halothane =32
en = 23
des = 89
what happens to the amount of anaesthetic agent delivered by a draw over vapouriser if minute ventilation is increased?
not enough time for evapouration
reduced concentration
