Temperature influences the vapor pressure of volatile anesthetics; higher temperatures increase vapor pressure, enhancing the concentration of anesthetic delivered to the patient.

Saturated Vapour Pressure + Humidity

SVP is an equilibration state that is particular to agent, to system temperature and system pressure.
Hence why the SVP of a volatile anaesthetic agent is given at a temp and (a presumed atmospheric pressure).
Note that it is talking about the partial pressure that the vapour will come to exert as part of the total atmospheric pressure…. ie if we were at 100kpa of pressure, and the gasses SVP was 30kPa, then 30% of the atmosphere you’re now chilling out in is vapour – the pressure remains the same, the mix has changed.

SVP is the partial pressure in a closed system of a volatile agent that has reached steady state – with some agent vapourising off the surface of the liquid and some agent condensing back into the liquid.

Relative humidity

Relative = The amount of water vapour floating around Vs the maximal amount of water vapour the system could theoretically take at this temp.

Absolute humidity

Absolute = the quantity of water vapour currently in the system

Examples

We are walking through Bang Kok – Its 30 degrees and humidity is (relative- RH) 80%..
We know (graphs) air at 30 degrees can maximally hold ~28grams of water per kilogram of air.

Therefore 0.8 of 28 = 22.4 g/kg – this is the absolute humidity at this point in the system.

So in theory we could add more water vapour to Bang Kok and make it more humid but we will only be able to squeeze in another 5.6 g/kg of moisture before we get stuck.

If we wanted to add more moisture, we would have to heat Bang Kok further – hotter air has more space for moisture.

If we returned to our 80% RH Bang Kok world, and somehow dropped the temperature to 15 degrees C, the air would hit 100% humidity, it would only be able to hold ~10g/kg of water and the other 12.2 would have to ‘fall out’ it would condense onto anything it could. In this example we have crossed the dew point for the system.

Thermodynamics inc heat temp and the law

There are four laws in thermodynamics

Zeroth Law

In a multi object system in which thermal equilibrium has occurred between these three objects in contact the thermal energy will be evenly distributed
Enthalpy (ie heat) will be equal across the objects
Neat definition = If two systems (A and B) are in thermal equilbirum with a third system (C) then A and B must be in equilibirum also

First Law of Thermodynamics

In a closed system the amount of energy remains constant
(remembering that energy includes heat, sound, the full electromagnetic spectrum etc)

Second Law of Thermodynamics

In a natural system entropy is always increasing
Entropy is : the amount of disorder in a system, if the energy is all in a mess disappated everywhere then it becomes harder to mobilise that energy for use (ie using energy to create light, movement, heat etc)

Third Law of Thermodynamics

The lower the temperature of the system the less Entropy present.
Think of entropy as the number of differing microscopic states a system of molecules could be arranged in.
As it gets colder, there is less energy and the molecules are not able to jump about quite so far… – they get to their lowest possible energy point when approaching absolute zero.

Vapourisers – Sevo Vs Des

Key points:

The internal workings aim to increase surface area to bolster vapourisation rate
- Doing so with Wicks, Baffles etc
There is a fresh gas flow through the vapouriser
- A portion of this goes via the vapour chamber, becoming maximally saturated
- A portion of this is diverted away from the vapour chamber
The concentration delivered by the vapouriser is governed by a control dial, or in more modern equipment electronic control – ‘Variable bypass vapourisers’
This is called the splitting ratio – which is required for the integral temp compensation mechanism.
The temp compensation is achieved by a bimetallic strip
- This is made of two metals, with known properties, which when exposed to varying temperatures will deform in a predictable manner.
- This known deformation is used to alter the splitting ratio to make the vapouriser leaner or richer
Temperature management is critical in a vapouriser,
- They are very heavy as they contain a wodge of copper
- This is nicely thermally conductive, and has a fairly high specific heat capacity in order to try and slow the cooling that can occur due to latent heat of vapourisation.

Des vs Sevo

Desflurane is an inconvenient volatile in that its boiling point is rather near room temperature.

A cold anaesthetic room with des in a sevo vapouriser would deliver relatively lower amounts of desflurane, compared to a similar situation with a cosy, soporific, warm anaesthetic room which would hose out des in a very spirited manner.

Why this so bad?

MAC 1 of Sevo ~ 2KPa (2% End tidal concentration)
MAc 1 of Des ~ 6KPa (6% ET conc.)

Sevoflurane At 20 degrees 22.7 kPa – boiling at 58.6 degreesC
Desflurane is 88.5KPa at 20degrees – boiling at 22.8 degreesC

So in an uncontrolled situation we might accidentally hand a patient an exorbitant excess of desflurane – and create a significant headache for ourselves.

Or to put it another way – if we poured a load of desflurane into a bucket and left the lid on for a while and let it settle somewhere >22.8 degrees. and then we stuck a pipe in a took a lung full.
we would be inhaling a lot of desflurane.

This is prevented with Volatile Agent bottles being colour coded and having a key system to prevent mixups with filling.

Hence Desflurane vapourisers are :

Heated to 42 degreesC to ensure that the des has ‘boiled’
Pressurises to 2Atmospheres
No need for temp compensation
Deliver a controlled amount of maximally saturated gas into the fresh gas flow to achieve safe concentrations
Intrinsically varies the quantity jetted in if fresh gas flow increases to maintain the same delivered concentration to the system.
Designed with a slope on the top these days so you cant warm your coffee on the vapouriser.

Definition of Kelvin as an SI unit

Kelvin is a unit of temperature – using the same scaling as degrees celsius.

While zero degrees celsius is defined by the freezing point of pure water
Zero Kelvin, aka Absolute Zero refers to a state where molecules have no heat energy whatsoever and are therefore entirely inert.

Before 2019
Zero Kelvin is 1/273.16 of the triple point of water

Triple point of water – where it is in all three common phases at once occurs at 0.01 degrees celsius at 611 pascals.

After 20/5/2019
0K is defined by the Boltzmann Constant

It is now almost as headache inducing as some of the other SI unit Definitions.

1K = (Planck Constant x caesium standard) / Boltzman Constant

caesium standard ΔνCs to do with caesium 133, and its frequency.

Boltzmann Constant relates Thermal Energy in a gas with the Thermal Temperature of the gas.

We would all hope to dodge that one – and it would be a brutal question to get in the exam!

Entonox Vs Nitrous oxide and their critical and psuedo-critical temperatures.

Critical temperature is the temperature level at which no amount of pressure Will convert the gas back into a liquid

And what bears out with a tank of nitrous oxide is that the pressure gauge on this tank naturally measures pressure but as nitrous oxide is stored as a gas liquid mix as you pull gas off you will get evaporation from the liquid into its gases form within the tank and the pressure Will stay the same on the gauge. This means that that until the tank actually empties you cannot determine the content of the tank from its pressure alone. You instead have to rely on weighing the tank and comparing its weight to the TARE weight written on the tank TARE meaning the weight of the tank.
The critical temperature of nitrous oxide is 36 1/2°
And the boiling point of nitrous oxide is -88° so it really would rather be gas at standard temperature and pressure.

Of note if you were to fill one of these tanks completely doing so it may be a 15° industrial setting and then where to transfer it to somewhere nice and warm like Bangkok then a lot more of the contents would desire to be a gas than a liquid and it’s pressure in the tank may become very very high and the tank may fail. This is overcome by not excessively filling the tanks in the UK the full ratio is 0.75 and in tropical regions it’s 0.67

Continuing on the same vain of filling tanks with gases docs which is a 50-50 mix of oxygen and nitrous oxide is stored in tanks and this is used to a pain relief if the temperature of Knox is allowed to go below -5.5 degC (psuedocritical temperature) i.e. if it’s stored in a manifold outside the hospital (or on an ambulance, or in a field providing med cover) then they can undergo liquefaction and separation of the components occurs. This is called the pointing effect and you may find yourself delivering not very analgesic supply of oxygen and that you may find your subsequently just giving nitrous oxide. That’s bad.

The Three Gas Laws and the Unification

Its always worth giving these in their natural order,

Jumping ahead will make the examiner take more interest in your answer than you may have otherwise liked.

Boyles
Constant Temp – Pressure and volume are inversely proportional to one another
P.V = K (Konstant)

Charles
Constant Pressure – Temperature and Volume are in a linear relationship
V/T = K

Gay-Lussac
1Constant Volume – Temperature and Pressure are in a linear relationship.
P/T = K

Combining these concepts into a workable single formula achieves the ideal gas law.

This law only truly reflects reality when Ideal Gases are being used. Unfortunately there is actually no such thing as an ideal gas as you cant compress a gas to nothing so the laws don’t really work in the extremes of the range.

But it approximately works, and thats ok.

P.V=nRT

Pressure is in pascals (so you need to convert that kpa etc)
Volume is in meters ^ 3
n – no of moles of the gas
R = The universal gas constant/ molar gas constant
T = Temp in Kelvin not deg c (ie add 273.15 onto your 100 degree boiling water and you’ll be in kelvin)

BlueSky

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Comparing Volatiles

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Transcript

Vapours and Vapourisation

02:06-03:15

So, Tom, what is a vapour?

A vapour is a state of matter where a gas is formed below its critical temperature and it can exist in equilibrium with its liquid state.

Okay, tell me a bit more about this information.

So, if you imagine a liquid containing a single substance in a fixed container, a certain number of molecules will reach sufficient energy and evaporate from the surface of the liquid and form a gas. And a certain number of molecules per unit time will also form a liquid from the gas itself.

If left to reach a state of equilibrium, for a given substance at a given temperature, you’ll reach a standard pressure above that liquid, which is known as the saturated vapour pressure.

Humidity Concepts

03:31-04:02

Relative humidity, for a given temperature, is the percentage or the decimal fraction of the humidity at the saturated vapour pressure. Absolute humidity is a measure of the water vapour content of a gas by mass per unit volume.

Vapourisers in Clinical Practice

04:02-06:20

So when we use vapours in this container, we are using a vapouriser using an anaesthetic agent. The way it differs is rather than being a closed container, there’s a flow of gas through the vapouriser to allow uptake of the vaporous anaesthetic agents into the anaesthetic breathing system.

The way it achieves this is by having a single limb within the system that passes gas through the vapouriser. This reaches its saturated vapour pressure, which is important, because that gives a known partial pressure of the anaesthetic agent within that mixture. That is then added to the main flow of gas into the anaesthetic system at a particular ratio, which will give you a known concentration of the anaesthetic gas being delivered to the patient.

And what things can affect this concentration and make the composition vary?

So as vapour evaporates, that’s a process that takes up energy known as the latent heat of vapourisation. So as the gas evaporates, it cools the container and the system within which the vapour sits. At lower temperatures, you will have a lower saturated vapour pressure, so this can alter the partial pressure of agent that’s being delivered to the anaesthetic system.

This can be compensated for in different ways, but most typically by a bimetallic strip which alters the splitting ratio, so essentially alters the proportions of gas through the vapouriser and through the non-vapourised part of the anaesthetic machine. I could explain more about how the bimetallic strip works if you’d like to know more about that.

Desflurane Vapouriser

06:26-07:39

Desflurane vapouriser – one of the key differences. So desflurane encounters specific problems because of its low boiling point. So at around room temperature, we’re close to desflurane reaching its critical temperature and therefore not delivering a predictable amount of anaesthetic gas into a vapouriser system such as the one used for sevoflurane.

The way this is overcome is by directly heating the desflurane to give a predictable concentration of desflurane that’s injected directly into the anaesthetic system as a separate jet. It’s known as a TEC 6 vapouriser, which has this specific feature.

Temperature and Heat Transfer

07:52-08:49

So there are two separate concepts that are important to understand for transfer of heat. There is the temperature of a substance and the concept of heat itself. The temperature of the substance is essentially the thermal state of a substance, which tells you about its propensity to give off heat or receive heat to the surrounding environment.

Heat itself is measured in joules, so it’s actually just a measure of energy and reflects the amount of energy contained within a substance as heat. In terms of temperature, that’s measured on a separate scale. So Kelvin is the SI unit for that.

Absolute Zero and Temperature Scales

08:58-09:33

The defining point of the Kelvin system is actually absolute zero, the temperature at which there is no movement of molecules whatsoever, and so they contain no heat energy. And this point is known to be at minus 273 degrees Celsius.

Triple Point of Water

09:33-10:28

So the triple point of water is the temperature at which water can exist in solid, liquid and vapour states. The triple point of water is 0.01 degrees Celsius and 611 kPascals.

Critical Temperature

10:32-11:12

So you mentioned critical temperature earlier. What’s the difference between a substance above and below its critical temperature?

Above its critical temperature a gas can no longer be compressed into a liquid at any pressure. Below its critical temperature there will be a pressure at which that gas can be compressed back into a liquid form.

Nitrous Oxide Storage

11:12-12:17

So if I was storing my nitrous oxide in a cylinder…

So nitrous oxide itself has a critical temperature of around thirty-six degrees Celsius. So, within the room temperature canister, it’s stored below its critical temperature. The practical implication of this means that within the tanks it’s contained, assuming they’re below 36 degrees Celsius, there will be a fluid level, and above that fluid level, nitrous oxide will reach its saturated vapour pressure, and it will do that irrespective of the volume of liquid left within the canister.

This means that pressure readings that we receive from pressure gauges attached to a nitrous oxide canister will always read the saturated vapour pressure until the tank is almost empty.

Entonox and Pseudo-Critical Temperature

12:17-13:36

For Entonox, it has something called a pseudo-critical temperature. And I think the strict definition of a pseudo-critical temperature is the critical temperature of a mixture of gaseous elements.

So this temperature is around minus six degrees Celsius for nitrous oxide. So approaching this temperature: if the canister is allowed to cool, such as in cold environments, or if gas is quickly discharged and the canister cools under the Joule-Thomson effect, then the gases can actually become liquefied and separate.

This means that an oxygen-rich mixture can be administered up to a certain point, and then a hypoxic mixture from the top of the canister could be delivered later on due to this separation. Potentially dangerous and worst case fatal.

Ideal Gas Law

14:09-14:45

So the ideal gas law describes the relationship between a gas’s volume, temperature and its pressure related by the universal gas constant. So there are multiple ways of expressing the universal gas law, but the most common is PV equals NRT, which is pressure times volume is equal to the number of moles of gas times the universal gas constant times the temperature in Kelvin.

Individual Gas Laws

15:00-16:08

So talking about Boyle’s law initially, Boyle’s law tells you that for a constant temperature there is an inverse relationship between a gas’s volume and its pressure. So the smaller the volume, the higher the pressure.

Gay-Lussac’s law tells you for a constant volume there is a direct relationship between a gas’s pressure and its temperature.

Charles’s law tells you that for a constant pressure, there is a direct relationship between temperature and volume.

Visualising Gas Laws

16:15-18:49

Trying to recall the gas laws on the hoof was tricky, but I have ways of doing it, which you saw me hesitating, but I have to think through some ridiculous images in my head to get them the correct way round. Also, we’re doing this in an audio format – they’re very much easier to draw a quick graph, those laws, as long as you label your axes correctly. I think it’s much more clear what you’re talking about.

So Boyle’s law, you’ll see essentially a curve with decreasing gradient from left to right with volume on the y-axis and pressure on the x-axis. So the volume of gas is inversely proportional to the pressure.

The other two are much more simple. They’re both straight line graphs, essentially. They show direct proportionality, and for Gay-Lussac, volume is the thing that remains constant, and so pressure and temperature are directly proportional to one another.

And Charles’s law – it looks exactly the same, but the axes are labelled differently. So, Charles’s law we’ve already spoken about: for a constant pressure, you get direct proportionality between temperature and volume.

Memory Techniques for Gas Laws

18:49-19:34

I always thought that first of all in Boyle’s law, temperature is constant. Charles has pressure constant. I always think that Gay-Lussac sounds a little bit like… well, I imagine someone with a very large scrotum, which helps me remember volume. You asked, James. I said there were some ridiculous pictures in my head.

I just remember those two and then work out the third one from there. Well that’s what I’ve been doing with Charles’s Law, which is why it always takes me a bit longer.

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Article Spring Board

How does temperature affect volatile anesthetic agents?