This is why your typical vapouriser is cool, and cools down when you have high flow rates. It transfers heat to the vapour, so appears cold. This has a high specific heat capacity and conducts heat to the vapour well, so minimises changes of temperature in the vapourising chamber. Heat sinks – This is why your vapouriser weighs a tonne! The vapourising chamber is surrounded by a big hunk of metal.This effect is increased the higher the flows as more latent heat of vapourisation is needed. This will lead to a lowered concentration of agent being supplied and your patient waking up or complaining of awareness. Now as we said earlier, this causes a change in SVP of the agent, reducing it in this case. Now the problem with supplying this energy is that it cools the surrounding liquid. This is defined as the energy required to change one kg of liquid to gas. This is the latent heat of vapourisation. When a volatile anaesthetic vapourises, an amount of energy is required to allow the change of phase (e.g. Methods of ensuring FGF saturation with high flow rates. Bubbles – Bubbling the FGF through the anaesthetic agent drastically increases the surface area for uptake making it easier to keep constant concentrations with high flow rates.Baffles – These make the FGF come in repeated contact with the vapourising surface, increasing the uptake of agent and ensuring a constant uptake with high flows.Wicks – These increase the surface area that the gas has to ‘pick up’ vapour from, making it easier to maintain concentration with high flow rates.If we have higher flow rates through the vapouriser, there isn’t much time for the gas flowing to equalibrilate and ‘pick up’ the vapour. Problems with vapourisers High Flow rates So for sevoflurane this would be about 2.2kPa or ~2.2% or ~ 1 MAC. So if we have 10% entering the vapourising chamber, the resultant FGF will have ~10% of the SVP in it. This is what that big knob on the top of the vapouriser does, changes the proportion of the FGF that enters the vapourising chamber. Vapouriser with Variable Bypass – This is basically what is on your anaesthetic machine. Now the pot will have a layer of sevoflurane liquid covered by gas containing the SVP of sevoflurane vapour (eg 22 kPa out of 101.32 kPa atmospheric pressure) Building it up bit by bit allows you to talk through it as you’re drawing, making more efficient usage of your viva time!)įirst, lets get a pot of anaesthetic agent. People get all flustered about vapourisers and overcomplicate it, but if we keep in simple and build it up bit by bit it gets simpler… (Remember in a VIVA situation, keep your diagrams simple to begin with! You can always add things to the diagram if asked about it. There are a few types of vapouriser that you need to know about, We’ll start with the simple Boyle’s/Plenum type vapouriser… Lets Build a vapouriser… Secondly, there would be no way to control the concentration of agent. Now 22kPa out of 101.32kPa atmospheric pressure is a lot! This is 21% sevoflurane concentration, so getting on for 10 MAC, A good way to kill a patient quickly. Why bother with a vapouriser? Well first of all, if we just put a bit of agent in our breathing system it would generate its SVP in concentration, so for sevoflurane for example it would generate 22kPa. This is more than enough understanding to pass the exam! The exact ins and outs are slightly different and more complex. Note: The explanations below are slightly simplified to make them understandable. What you need to know (How it works in practice): Vapourisers Agents with a high SVP (very volatile) need little to no help in boiling. Agents with a low SVP (less volatile!) have a higher boiling point, so need more ‘help’ to boil. Notice also the boiling point of desflurane, that’s important! (see below why…)Īlso notice how the boiling points are almost inverse to their SVP. 22, 33, 88) It makes it nice and easy to remember these for MCQs that way. Notice here that the SVPs of all the commonly used agents today are the same double figures (e.g. All of the agents used in anaesthesia are termed ‘volatile’, not because they can blow up (although some of the old ones did, Cyclopropane being the classic example), but as they readily produce a high vapour pressure. Water produces a vapour above it, but the SVP of water is very low (2.3kPa at 20oC). An increase in temperature will increase the SVP as more energy is available, this becomes important later on… Volatile anaesthetic agents SVP is normally stated at standard temperature and pressure, so it stands to reason that the SVP is altered by the surrounding temperature and pressure. The partial pressure generated by a vapour in equilibrium with its liquid form at standard temperature and pressure (STP) This is the saturated vapour pressure (SVP). That vapour which is produced will exert a small, but measurable pressure in the closed container.
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