Desflurane for the FRCA and Vapourisers

© GasGasGas – The FRCA Primary Anaesthetic Sciences Podcast 2025

---

Desflurane Vs Polar Bears

Greenhouse Gas Emissions of Common Inhaled Anesthetic Agents. (Adapted from Ryan, et al.⁷²)

MAC inhaled agent	Atmospheric lifetime (years)	100-year Global Warming Potential (GWP)71 (per kg, in comparison with 1 kg CO2, where GWP CO2 = 1)	Equivalent auto miles* driven MAC-hour of anesthetic use at 1 L/min
Isoflurane 1.2%	3.6	539	8
Sevoflurane 2.2%	1.9	144	4
Desflurane 6.7%	14	2,540	190
60% Nitrous Oxide (0.6 MAC)	114	273	49

This table is directly pilfered from the American anaesthetic association website found here it is widely known knowledge, and if they take umbrage at my directly referenced use, please send an email.

The crux of release these gases into the atmosphere is based on either how potent they are in terms of global warming potential, but also how long they last in the atmosphere. Because if something’s a little bit potent but stays for a thousand years, it may have an outsized impact compared to something quite potent that disappears quickly.

Desflurane Physico-Chemical properties

Name	Desflurane
Class	A Halogenated Ether, particularly a Fluorinated ethyl methyl ether
Chemical Make Up	C3 H2 F6 O Structurally similar isoflurane with a swap out of the isofluranes chloride for another fluorine.
History	First synthesised in 1970’s quite challenging to synthesise, was always quite pricey.
Isomer Status	It comes as a racemic mixture! R and S (rectus and sinister)
Colour/Appearance	Clear colourles s liquid – Non-flammable
Stability	Probably the most stable of the anaesthetic agents, with no apparent hepatic/renal toxicity, there may have been the odd case of liver problems. Can produce CO in a Hot and Dry Soda-Lime/Bara-Lyme environment (like other volatiles can)
Molecular weight	168g/mol

---

Desflurane for the FRCA primary

Desflurane Boiling Point	22.8 °C (Peck says 23.5 °C)
Desflurane Saturated Vapour Pressure (@20°C)	88.5 KPa
Mac of Desflurane	6.6%
Blood:Gas Solubility Coefficient Desflurane	0.45 (Peck says 0.42)
Oil:Gas Solubility Coefficient Desflurane	26

---

Desflurane Pharmacodynamics

Mechanism of Action	Volatiles seem to disrupt synaptic transmission – especially ventrobasal thalamus Might make GABAa and glycine receptors more jazzed up May antagonise NMDA receptors (N-methyl D-aspartate) ‘Meyer-Overton’ – expansion of hydrophobic regions in the neuronal membrane either within the lipid phase or within the hydrophobic sites of cell membrane proteins ‘Gets around the CNS and causes mischief that renders people hypnotised, amnesic and generally less mobile’
Chief Effect / Actions	Hypnotic
Dose	See End Tidal Mac for Dosing
	Side Effect Profile
Cardio-Vascular Side Effects	Desflurane has classic volatile CVS effects, but this is overlaid by sympathetic nervous system stimulation at concentrations of around and above 1 MAC Chronotropy : Dose dependent increase in HR once around 1MAC+ Inotropy : Decreased Dromotropy : Lussitropy Bathmotropy : Coronaries: Vascular Resistance: Desflurane causes peripheral vasodilation and low end doses like all volatile agents (lowers BP) Rapid desflurane rises cause a great SNS activation type response, and MAC>1.25 demonstrates this SNS response. The mechanism is unclear, but may be centrally drive (remember the SNS originates in hypothalamus) noting that it is sometimes a transient SNS response. Caution with ischaemic heart disease given this SNS stimulus, and also consider is your hypertensive/ tachycardic patient aware? or just being tickled by Desflurane….. (although 1MAC arguably is more than enough!) Joyfully, there are fewer intra op hypotension events, but you could fix that in other ways doctor!
Respiratory Side Effects	Rate – Increased Depth – Decreased Parenchymal effects: > Hypoxic pulmonary vasoconstriction inhibited > Bronchodilating > Irritant = Coughing++ (Most irritant out of Sevoflurane, Desflurane and Isoflurane)
Central Nervous System Side Effects	ICP : Can increase ICP in SOL patients Seizure Threshold : Not lowered Cerebral Vasculature: Vasodilating with auto regulation abolished >1.5MAC CMRo2 depressed Nausea and vomiting
Gastro-Intestinal Side Effects	nil
Metabolic/MSK: Side Effects	Potentiates NMB drugs
Obstetric	Uterine hypotony
Toxicity Signs	Malignant Hyperthermia Trigger End point of volatile toxicity: CVS collapse, in spont ventilating patients, apnoeas will slow the rate of increasing toxicity and likely prompt an alteration in drug delivery Note in a toxic collapsed patient, your ability to get the vapours back out of the human is severely impaired…. as they require cardiac output to circulate and excrete agent via the lungs. The difference between anaesthetic dose and CVS collapse dose is wide…. and equipment and monitoring should stop you over doing it. But abroad under a bush with marauding rifle carrying individuals creates a different situation!

---

Desflurane Pharmacokinetics

Absorption	1st Pass metabolism, Oral ingestion is likely bad for your health, stick to swigging petrol instead, as long as you consume less than the recommended daily allowance.
Distribution	Extensively, favouring areas receiving higher blood flows
Metabolism	Liver Phase I : 0.02% metabolised into triflouroacetic acid (oxidative) Phase II: nil Remember: Phase I : [oxidation, reduction, hydrolysis] (more cytochrome action here, more O2 required) Phase II : [conjugation, glucoronidation, acetylation, sulphylation]
	Renal : Active Metabolites: Other Routes:
Elimination	99.98% excreted via Lungs
Other Notable Information	The vapouriser needs to be heated with electricity in order to avoid unpredictable dosing as the boil point is so near to room temperature.

Vapourisers

Historic Vapourisation Devices – Anaesthetists were vaping before it was cool

Bits of bushel mask fun were had – dropping volatiles onto a cloth suspended in a frame

Early vapourisers like the Boyles bottle did work but had there issues (i.e the got cold due to physics, there was less evaporation and thus anaesthetic concentrations diminished. You could deliver a number of flammable lagents with it, but if you were warming the tank you ran the risk of accidental explosions!

The Anatomy of a Modern (tec5) Vapouriser

These are variable bypass devices, that are temperature compensated, must not be ‘in-line’ with your circuit as the resistance is too high and a patient cant exert force to breath through them, they are heavy and fortunately do not require electricity.

They contain:

• A vapourising chamber
  • Wicks (increase surface area)
  • Baffles (force flow to circulate broadly in the chamber) – ie, less channelling (like making an espresso, requires flow throughout all the ground beans, not preferentially through the easiest, poorly compacted route)
  • Liquid phase anaesthetic
  • Vapour phase anaesthetic.
• Control Dial
• Bypass route for FGF
• Route for FGF to enter Vapourising Chamber
• Temp compensating bi-metallic strip
• Safety features
  • Contain a mechanism to block more than one vapouriser being on at once (back bar inter-lock)
  • Have back flow valves so if turned upside down doesn’t slosh volatile into the bypass FGF
  • Non-return valves/high flow resistance so that circuit gases can’t be driven back though the vapourising chamber
  • Keyed system to prevent the wrong bottle filling the chamber

Gas Flow in a Tec5 Vapouriser:

1. Fresh gas flow (FGF) enters the vapouriser
2. This FGF is split into that which bypasses exposure to anaesthetic agent, and that which goes through the chamber containing agent.
3. The chamber FGF mixes with the anaesthetic agent in its vapour phase and it exists the chamber fully saturated.
4. It subsequently mixes with the bypassed FGF and exists the vapouriser
5. The amount that is split, the so called splitting ratio defines the fraction of volatile concentration leaving the vapouriser.
6. A Temp compensating bi-metallic strip, ever so slightly modifies the split ratio, depending on how warm the system is, it will vary an orifice, altering flow rates so the exiting fraction is accurate (a cold vapourising chamber provides less volatile, needing more gas to run through it)

Why do we need temp compensation? Because of the Latent Heat of Vapourisation! As volatile shifts into vapour phase from its liquid, the liquid itself becomes cooler, enabling less volatile to escape, unchecked this would lead to diminished SVP and thus less volatile for your patient.

Vapourisers handle this by being huge blocks of material, (often copper) that stores quite a bit of heat energy, this attenuates the cooling issue and also by having FGF bypass fraction compensated for temperature within the system. The hypothetical extreme of this, if you ran antarctic air through your vapouriser, eventually it will struggle as the copper cools.

Of note the safe operating temperature of a Tec5 vapouriser is 16-35°C, so using it under a bush, or in a tent in a chilly field hospital may not work favourably. Note, that if you’ve the luxury of gas analysis you are still measuring Inspired and Expired volatile fractions, and you could crank up the vapouriser and might get enough out of it for sufficient anaesthetic depth, tight alarms are always your friend when bush whacking or in challenging clinical situations.

Desflurane Vapouriser Differences

Are injector vapourisers that are electrically heated with increased pressure contained within the vapour chamber side of the device. FGF does not enter the vapourising chamber.

Why do we need Tec 6 vapourisers?

Because the boiling point of desflurane is low, i. e. 23 degrees. In a standard vapouriser it would be incredibly unpredictable as to the concentration being delivered. Has it all boiled off and all become a vapour? this owl deliver supra anaesthetic concentrations because its saturated vapour pressure is quite high. Therefore, you need a device that can reliably control the vapour and the concentration delivered, the best way to do that is to warm up the desflurane so that you have a predictable vapour concentration which is injected into the circuit.

Tec 6 – Desflurane Vapouriser anatomy

• Vapourising chamber
  • Heating element
  • Desflurane in liquid and vapour phases
• Differential pressure transducer
• Concentration Dial
• Vapour Flow micro controller
• Adjustable orifice valve
• Safety Features
  • Contain a mechanism to block more than one vapouriser being on at once (back bar inter-lock)
  • Have back flow valves so if turned upside down doesn’t slosh volatile into the bypass FGF
  • Non-return valves/high flow resistance so that circuit gases can’t be driven back though the vapourising chamber
  • Keyed system to prevent the wrong bottle filling the chamber
  • Pressure and Temperature Alarms in some models

The vapour chamber is heated to 42°C, and the chamber becomes pressurised to two atmospheres (remember 1 ATM is 101.325KPa, so 2 ATM is 202.65 KPa)

As the system knows the temperature is it at (42°C, with a thermostat to control this) other temperature compensation mechanism are less important.

if you were running your fresh gas flow rate at a litre per minute. with vapouriser dialled at six. The vapouriser needs to have an understanding of what the flow rate is across itself in order to inject the right amount of desflurane to achieve the appropriate target concentration, in this case 6%.Because in situations where you increase that flow rate to 10 litres, for example, you would reduce your concentration factor by 10. So therefore, your desflurane vapuriser must contain a device that detects flow rate.

This is the differential pressure transducer within the system. Now as you can imagine, the tube through which fresh gas flows through the vapouriser is fixed.

therefore if you increase the flow rate, the pressure must go up. Therefore pressure and flow are directly related and you can infer flow rate from a differential pressure inducer, which is comparing the pressure within the vapourising chamber to the fresh gas flow rate. Does some clever maths using a microprocessor and subsequently compensates for changes in flow rate.

May well be in an exam that they want you to compare and contrast these two vapourising devices with their advantages and disadvantages.

The advantages of a desflurane vapouriser is it’s unaffected by ambient temperatures because it’s heated. As long as the heating element can achieve enough warmth against the heat loss of the system. So if it was outside in minus 80, it might be that the heating element simply can’t provide enough heat to compensate and achieve a stable 42 degrees C.

Because it’s electric. It has some integrated alarms for being under temperature. or if it lacks pressure in the tank because you’ve run out of desperate. Disadvantages though, it requires power, so if you’re under that bush, desflurane is probably not your best choice. And it takes time to warm up, so you have to turn it on and plug it in and wait.

So it’s quite a bit different compared to a TEC 5 vapouriser. And all the vapourisers that came before it Like the Boyles bottle

Vapourisers at altitude?

The ultimate migraine inducer.

Fact One:A Tec5 Vapouriser dial set at 2% will provide 2KPa of Volatile at sea level (101.325KPa)

Fact Two: A Tec5 Vapouriser dial set at 2% will provide 2KPa of Volatile on the top of Mount Everest (33.7KPa)

Fact Three: The difference between Facts One and Two, is the altered atmospheric pressure, a lower overall pressure is present atop Everest, but the amount of drug delivered remains as 2KPa worth of volatile.

Fact Four: Saturated Vapour pressure does not vary with Ambient pressure (only temp)

Fact Five: As it is the KPa of the agent that influences anaesthesia, you can use this device as you otherwise would.

Fact Six:This is crude, as it assumes we have kept the vapouriser warm, the gases delivered warm, and our anaesthetic machine resistance is unchanged despite there being lower pressure (depending on machine used)

N.B. therefore it makes better sense to think of those %’s on a dial as KPa Delivered. as 2kpa at 50KPA atmospheric is a 4% concentration!

Baboozly bafflement over?!

A Tec 6 Vapouriserdoes not cope at altitude like this and you must increase the fraction delivered using the dial (they do go up to 18%!)

References

1. JAKOBSSON, J. (2012), Desflurane: a clinical update. Acta Anaesthesiol Scand, 56: 420-432. https://doi.org/10.1111/j.1399-6576.2011.02600.x
2. Kapoor, Mukul Chandra; Vakamudi, Mahesh¹. Desflurane – Revisited. Journal of Anaesthesiology Clinical Pharmacology 28(1):p 92-100, Jan–Mar 2012. | DOI: 10.4103/0970-9185.92455
3. Patel, S.S., Goa, K.L. Desflurane. Drugs 50, 742–767 (1995). https://doi.org/10.2165/00003495-199550040-00010
4. Yasuda, Nobuhiko M.D., Ph.D.^*; Lockhart, Stephen H. Ph.D., M.D.^†; Eger, Edmond I. M.D.^‡; Weiskopf, Richard B. M D.^§; Johnson, Brynte H. M S.^¶; Frelre, Beth A. B. S.^**; Fassoulakl, Argyro M.D.^††. Kinetics of Desflurane, Isoflurane, and Halothane in Humans. The Journal of the American Society of Anesthesiologists 74(3):p 489-498, March 1, 1991. | DOI: 10.1097/00000542-199103000-00017
5. Hendrickx JFA, Nielsen OJ, De Hert S, De Wolf AM. The science behind banning desflurane: A narrative review. Eur J Anaesthesiol. 2022 Oct 1;39(10):818-824. doi: 10.1097/EJA.0000000000001739. Epub 2022 Aug 29. PMID: 36036420.

---

“Thanks for listening guys… Every day you are getting better at this. Take it day by day, don’t overcook yourself, don’t freak out, and keep studying!”

Podcast Information

Transcript – Desflurane and Vaporisers

Welcome and Introduction

0:00-1:30

Summary: Welcome to Gas Gas Gas, the anaesthetic science podcast for FRCA Primary exam preparation. This episode covers desflurane pharmacology and vaporiser technology.

Hello, Team Anaesthesia. Welcome to Gas Gas Gas. This is the best anaesthetic science podcast for the FRCA Primary exam. Our goal is to fill your brain with all this highly useful information. Now you might be in the gym right now, commuting, or ironing your scrubs, and there’s no judgement here. Gas, Gas, Gas will prime your brain for the monsoon of knowledge you need to imbibe. But regardless, the revision is eventually going to end. But for now, expect facts, concepts, model answers and the odd tangent.

Now remember to check out the website, that’s gasgasgas.uk. There are show notes there with all the detail plus links to foundational reference papers and anything else useful I find for you guys. Anyway, buckle up, get ready for your mind to be bent into a new shape, and let’s get on with the show.

Environmental Impact of Desflurane

1:30-4:15

Summary: Desflurane has significant global warming potential. NHS guidance supports decommissioning desflurane, backed by AAGBI and Royal College. Environmental comparison shows desflurane use equivalent to driving 190 miles per MAC hour versus 8 miles for isoflurane.

Hello everyone and welcome to another episode of Gas Gas Gas. Now we’re going to talk about the volatile anaesthetic that shall not be named. Now I’ve checked the house, I’ve looked under the rug, and in the garage, and I cannot find a single polar bear, so I think we should be fine to talk about this drug. I jest. But desflurane has a significant global warming potential, and it is rather hard to justify its use, at least in the eye of most UK anaesthetists.

Now the NHS has gone so far in agreeing guidance for the decommissioning of desflurane in clinical use, and this is actually supported by some pretty major players in UK anaesthetic provision, including the AAGBI and the Royal College, etc. Now, I imagine in some places this is still a QI project waiting to happen. And if you are out there and you are holding on to your blue vaporiser, please just put it in a cupboard and use something else.

So we’re going to talk about desflurane, its pharmacokinetics, its pharmacodynamics, but we’re also going to introduce vaporisers, including their baffling array of internal components. They’re large bimetallic strips, that may be compensating for something, and an obstructive interrelatedness that really upsets the wicks of the neighbouring vaporisers.

Now, as we all know, this is an episode part of the Volatile Anaesthetic Agents series. So if you’ve just popped in to listen to this episode, maybe think about going to listen to the sevoflurane, isoflurane and the Introduction to Volatiles episode.

Now before we get into the long grass, may I just further robustly defend my position that desflurane kills polar bears. There’s quite a lot of information on the internet. But there’s an adaptation with perhaps some slightly liberal mathematics, because it’s hard to really prove these things, that compares how many car miles, auto miles, because this is from America, that carries an equivalence for a MAC hour of anaesthetic use at one litre per minute.

So that would turn around to saying you’re keeping someone at a MAC of one with isoflurane, sevoflurane, or desflurane, and the equivalent of keeping them there for an hour and how far that is to drive. So isoflurane for every hour of use in that theoretical system, you will have driven eight miles in an American average car. Who knows what that is? Sevoflurane, four miles. Desflurane, you will have driven 190 miles.

So that’s the equivalent of like me doing a slightly lazy desflurane anaesthetic for an hour, maybe for, I don’t know, a particularly protracted inguinal hernia in a significantly large person, which takes surgically a long time. That’s like driving to Scotland when I could have driven to the shops and back. And that’s why you should point folks to that decommissioning statement, and you should just take the desflurane vaporisers off of the anaesthetic machines and put them somewhere. Because if it’s not there, most of the time people will just be like I’ll just use sevoflurane and you’ll have saved a polar bear, maybe. Now, I’ll eventually do an episode on this, but that’s enough to wet your teeth, and maybe inspire that QI project.

Chemical Structure and Physicochemical Properties

4:15-9:30

Summary: Desflurane is a fluorinated ethyl methyl ether with six fluorines (hence the name). Structurally similar to isoflurane but with fluorine replacing chlorine. It is a racemic mixture (R and S enantiomers), clear, colourless, and non-flammable. Molecular weight is 168 g/mol.

So now that we’ve painted it in such a terrible light, the volatile who shall not be named, desflurane, we need to still understand it though, because it could be in the exam. So it is a halogenated ether, much like its friends sevoflurane, isoflurane, and halothane, and in particular to desflurane, it is a fluorinated ethyl methyl ether. I’d be devastated if they asked you that in the exam.

It is structurally quite similar to isoflurane, however, where that chloride atom is on an isoflurane molecule, you will find a fluorine. Desflurane. Des-six. Six fluorines on desflurane. Desflurane was initially synthesised in the 1970s, marketed as something that has a rapid onset time and offset with no nephrotoxicity or hepatic mischief. However, it was quite expensive to produce because the challenges posed with synthesis, which meant that it was quite pricey. The prices have obviously come down, but its global warming potential was always there.

It is a racemic mixture of R and S. Remember R rectus, S sinister. It is clear and it is colourless and it is non-flammable like the other halogenated ethers. That’s great, non-flammable, nice and safe. Now on paper, desflurane is the most stable of the volatile anaesthetic agents we generally get our hands on. No apparent hepatic or renal toxicity. Generally does not misbehave and try and dissolve things like sevoflurane when it’s dry could. But naturally, like all the other volatile anaesthetic agents, if you expose them to a dry and warm soda lime or baralyme environment, you can end up producing carbon monoxide, but as long as you’ve got a nice steamy looking tank of soda lime, it isn’t a problem.

Its molecular weight is, you’ve guessed it, 168 grams per mole, of course. Very important to know that. Very, very, very.

Key Physicochemical Properties:

What about its physicochemical properties? These are the ones we’re really interested in. You’ve probably got tables on the back of your door in your bathroom to try and memorise these things. Aren’t you lucky? You just take a moment and just remember how joyful it is to have to learn all this stuff. Yeah, I bet there’s rainbows coming out of your ears.

• Boiling point of desflurane is 22.8°C, although Peck and Hill likes to say 23.5°C. Yes, you’ve thought it, and you probably already know it. That’s quite normal. Like maybe some lunatic might heat their house to 22.8°C, and hospitals are practically Saharan in temperature, especially in the summer and the winter. So therefore desflurane will boil at room temperature, and by that virtue almost all of it will be a vapour and none of it will be a liquid on the floor. This leads to some temperamental behaviour that we will discuss later when it comes to how you deliver desflurane to a patient and the differences in vaporisers.
• The saturated vapour pressure of desflurane at 20°C is 88.5 kilopascals. This is significantly different to the 20°C SVP of sevoflurane, 22.7 kilopascals. So almost four times saturated vapour pressure in desflurane at that temperature compared to sevoflurane.
• What is the MAC of desflurane? This really confused me when I first started anaesthesia. What’s a MAC of something? It doesn’t make sense because MAC is one. But how much desflurane do you have to measure exhaled from a patient as end-tidal for them to have the equivalent of one MAC is 6.6%. So they’ve got to be at 6.6% exhaled desflurane to have them at one MAC, which is loads. Because remember, isoflurane needs an end-tidal of 1.2%, sevoflurane 1.8-2.2% depending on who you quote, halothane 0.75%.

Solubility Coefficients and Clinical Implications

9:30-11:45

Summary: Desflurane has very low blood:gas solubility (0.45) enabling rapid onset and offset. Oil:gas solubility is 26 (lower than sevoflurane’s 47), explaining why higher concentrations are needed for anaesthesia. Low blood:gas partition means quick equilibration; low oil:gas partition means lower potency.

So you need quite a lot of desflurane. And as I said earlier, it’s not cheap, but it does come with vaporisers that are blue, so that’s nice. Not yellow or orange or purple. Blue, nice blue colour. Anyway.

The other important feature that is often pressed on is the blood:gas solubility coefficient of desflurane, and this is 0.45. So it really does not like being in blood. Noting sevoflurane’s blood:gas solubility coefficient is 0.69, so less than. This means that onset is sharper and offset is sharper.

But its oil:gas solubility is 26, and the oil:gas solubility of sevoflurane is 47, so almost twice. We know the oil:gas solubility is the thing that dictates the potency of the drug. So the more it likes oily things, it appears the less drug you need to achieve the same level of sleepiness. Although I’m sure there’s more factors than just that. It’s a useful way of differentiating things.

So from that information we can infer that you need quite a lot of desflurane to keep someone asleep, but it’s quick to get the desflurane in and out of them. But one of the challenges with desflurane is the fact that it wants to boil at room temperature, and therefore you will have an unpredictable amount of desflurane if you just tip it into your sevoflurane vaporiser, because have you got it cold, have you got it warm? Where are you? So we know that it’s not perfect.

Mechanism of Action – Pharmacodynamics

11:45-13:15

Summary: Like other volatile agents, desflurane’s mechanism involves GABAₐ receptors, glycine receptors, and NMDA receptors. The historical Meyer-Overton hypothesis (lipid expansion) has been superseded by receptor-based understanding, though the nature of consciousness remains unclear.

So, mechanism of action – when we’re talking about pharmacodynamics, we should probably discuss that, is the same for desflurane as it is for halothane, isoflurane, sevoflurane. It alters how your brain works. No, you can’t say that, surely. If you say that in exam, it won’t go very well.

There was the historical hypothesis from Meyer and Overton which described expansion of the hydrophobic regions in the neuronal membrane. You know, those lipid bilayers we all learn about. And therefore it disrupts the function of the nerve and impairs transmission. Old hat.

New hat: we have discovered that there are things on cells which when you tickle them changes things that go on in the cells. These things are receptors. And they can allosterically change and modify the intracellular environment through either secondary messenger systems or transfer of extracellular or intracellular ions to their opposing environment, in or out. The receptors that volatile anaesthetic agents may work on include GABAₐ receptors, glycine receptors, and NMDA receptors. That would be the exam answer.

How does that mean that you can’t form memories anymore and you can let a surgeon rummage around inside you? Well we don’t know the nature of consciousness, so we can’t answer that question truly.

Side Effects – Cardiovascular System

13:15-16:30

Summary: Desflurane causes peripheral vasodilation (decreasing SVR), negative inotropy and chronotropy, but cardiac output is generally preserved. Unique to desflurane: sympathetic nervous system activation at MAC >1.25 or with rapid increases, causing transient tachycardia and hypertension. This is particularly important in ischaemic heart disease patients.

Side effects. So when talking pharmacodynamics you need to do side effects and it’s certainly very sensible to talk this in an organ system style and you can subdivide it into individual elements of that organ system, especially if it’s a complicated thing.

So desflurane, much like its other volatile anaesthetic pals, has the classical volatile cardiovascular system effects, whereby it causes peripheral vasodilation, dropping your systemic vascular resistance and therefore lowering blood pressure, impairing inotropy and impairing chronotropy, i.e., slowing them down, reducing contractility. But the drop in peripheral vascular resistance means that generally your cardiac output is preserved in an acceptable range.

And you’re probably saying, hold on, no, I’ve heard something different about desflurane. And you’re right, you have. In high doses, interestingly, and I’m gonna say high doses like a MAC of greater than 1.25, those poor polar bears, you can notice a degree of sympathetic nervous system activation whereby you get a bit of a tachycardia, an increase in blood pressure. This is transitory normally, it doesn’t last. And you can also trigger it by turning up the desflurane quite quickly to start with. If you have someone on 3% and then you bosh them up to 12%, you will see that SNS activation. But then if you lower them down to 6% and cruise for a bit and then bosh them back up to 12% again, you will see less of a response the second time round.

It’s important to remember this, because it might come up in an SBA, for example, but it’s also important to consider in patients with ischaemic heart disease where you’re trying to take off and have a nice smooth flight, not too much of a high blood pressure, not too much of a low blood pressure. Keep that myocardial oxygen demand on the down low, and then you tickle them with too much desflurane because you’re getting impatient and just turn up the gas. You will potentially put unnecessary strain on that patient, especially if they’ve ischaemic heart disease.

Now also you might end up getting transient questions in your mind coming through if you’ve accidentally done this and not thought about it. Oh, is the patient – well their blood pressure’s gone up, their heart rate’s gone up – are they in pain? You might turn up the remifentanil if you’re somehow using remifentanil and desflurane because you’re flush with cash, or give them a dose of maybe fentanyl. So that’s just important to remember, that too much desflurane tickles your sympathetic nervous system.

How does it do this? They used to think there were receptors in the lungs, or receptors in your vasculature. The current thinking is it probably does something to the central circuits that drive your sympathetic nervous system and they live in your hypothalamus.

Side Effects – Respiratory, CNS, and Musculoskeletal Systems

16:30-19:45

Respiratory System:

Summary: Increases respiratory rate but decreases tidal volume. Inhibits hypoxic pulmonary vasoconstriction (can worsen V/Q mismatch). Desflurane is bronchodilating but highly irritant – most irritant of all volatiles – causing coughing. Not suitable for gas induction or asthma management.

You’ll be happy to know that that’s cardiovascular system over. Respiratory system: so it makes you breathe quicker, but it makes you breathe less deeply. And the same parenchymal effects as you see with sevoflurane. Hypoxic pulmonary vasoconstriction is inhibited. That means you will improve perfusion to the whole lung parenchyma. Sounds great, but if Mrs Miggins, who’s got a BMI of four million, doesn’t terribly have excellent bases in their lungs, you are now perfusing an area that is not ventilated. And that leads to shunting of deoxygenated blood across the lungs, to the left side, off to the body, you have yourself a shunt, and therefore lower saturations. So it’s not as great as it sounds.

Desflurane is bronchodilating, but if you’re reaching for it in someone who’s having a severe asthma attack, you won’t look terribly smart. Because it is very irritant. It causes a lot of coughing, and it is the most irritant out of sevoflurane, isoflurane, and desflurane. Desflurane is the winner. The volatile who shall not be named. Another mark against his name.

Central Nervous System:

Summary: Provides anaesthesia but increases intracranial pressure (vasodilator effect). Autoregulation abolished at MAC >1.5. Decreases cerebral metabolic rate (CMRₒ₂). Causes postoperative nausea and vomiting.

CNS effects, well, we know that it anaesthetises, yeah, but it does increase intracranial pressure because it’s a vasodilator. And I can tell you that the autoregulation of your brain’s vasculature is abolished at MAC greater than 1.5 of desflurane. God knows why you’d be up there. It depresses your cerebral oxygen consumption rate, your CMRₒ₂. And as with other volatiles, it makes you sicky-vomity afterwards.

Musculoskeletal System:

Summary: Causes muscle relaxation and potentiates neuromuscular blocking drugs. Causes uterine hypotonia (not ideal for postpartum haemorrhage). Consider transition to propofol for prolonged obstetric procedures if volatile use becomes problematic.

I’m sure you can tell me what it does to the musculoskeletal system. But just in case you haven’t been listening, it causes a general degree of muscle relaxation and potentiates neuromuscular blocking drugs and if you were to waft it at a uterus it would become hypotonic. So not ideal if managing a PPH. Volatile is not your friend.

But volatiles are quick and easy to administer. And when managing a PPH, are you really going to get a propofol pump and start sorting it out immediately? No. But if it’s a long one and there’s lots of bleeding and they’re thinking, oh gosh, we’re gonna have to do a hysterectomy, it would make sense to be thinking about transitioning to propofol. You will reduce the bleeding, but you need to be able to do that safely without having a period where the patient might be aware. Not something to be doing by yourself at two in the morning. Phone a friend if there’s a hysterectomy happening, obviously. That friend being the consultant on call, of course.

Toxicity:

Toxicity-wise, it’s gonna trigger your malignant hyperthermia, like all the volatile anaesthetic agents will do.

Sponsor Message – Teach Me Anaesthetics

19:45-22:00

Summary: Teach Me Anaesthetics offers 1,100+ purpose-built SBA questions for FRCA Primary, with explanations and flexible study modes. Questions are specifically written for anaesthetic science (not rehashed MRCP questions). Affiliate links available through gasgasgas.uk.

Anyhow, time for a brief mention from the sponsors of Gas Gas Gas. These guys are behind an excellent single best answer question resource. Now, firstly, I took the joyful, challenging, and intermittent bashing through the 1,100+ questions they have written for the FRCA Primary exam. Now I secretly did love it, and there are plenty of questions to test your knowledge.

Now when I was studying for the Primary, it didn’t exist, but there were other packages online. These cost a lot more and they didn’t really seem to reflect the content of the exams. And whilst doing these I came across rehashed MRCP questions, and I wasn’t terribly inspired, and really I was a bit cheesed off. Whereas these single best answer questions have been built from the ground up and they have not sniffed an MCQ in a past life. They are based on the anaesthetic science you need to know. And there are explainers with all the questions, so it builds your knowledge as you work through them.

And there are also multiple different ways you can study this. You could choose to do a random battle with 1,100+ questions, or you can split it by subject area, you can go back and redo the questions you didn’t get right. Very malleable. Now if you’ve been thinking about which question resource you might want to get your hands on for the exam, you shouldn’t really look any further than Teach Me Anaesthetics.

If you reckon it’s for you, all the links to Teach Me Anaesthetics from gasgasgas.uk, they are affiliate links. If you’re signing up to Teach Me Anaesthetics through one of those, you’re helping to support Gas Gas Gas. So if you think you’re gonna go for it, click a link with me. You know you want to. You’ll not only be supporting your exam preparations, but you’ll also be supporting your 100% favourite tell-your-nan-over-Sunday-lunch podcast, which is of course Gas Gas Gas.

Pharmacokinetics

22:00-25:00

Summary: Absorption is via inhalation. Distribution favours highly perfused organs (brain, heart, kidneys, liver) before adipose tissue. Metabolism is minimal (0.02% – the least metabolised volatile) via hepatic phase 1 oxidation to trifluoroacetic acid. Elimination is 99.98% via the lungs.

Desflurane’s pharmacokinetic properties. And remember, in the exam, break pharmacokinetics down into absorption, distribution, metabolism, and elimination. It’ll make it clearer in your mind and in the mind of the examiner when they’re looking at their list of absorption, distribution, metabolism, and elimination on their marksheet.

So if anyone asks you about the absorption of desflurane, don’t be facetious and say well if you splash it on your neck eventually it’ll get in and it has a nice bouquet of coughing – won’t go well. Equally oral ingestion is probably bad for your health. I would suggest swigging petrol is safer as long as you consume less than the recommended daily allowance. Which in the eyes of the government toxicology website, linked in the show notes for those interested, 7.5 kilos of petrol is perhaps your limit depending on your body mass. I wouldn’t recommend it though. Tastes awful.

Distribution: volatiles are distributed extensively, and they do favour areas receiving higher blood flows, i.e. likes to go to your brain, myocardium, kidneys, liver, etc. before poodling off into adipose tissue.

The metabolism, so it’s sensible to break this down by organ who is doing metabolising. Often, remember folks, it’s the liver, sometimes it’s your kidneys, and sometimes it’s your lungs, and we will get there one day. Desflurane: chiefly phase 1 metabolism into our friend trifluoroacetic acid. This is an oxidation reaction. But you’ll be delighted to know that only a teeny tiny amount of desflurane is metabolised, 0.02% – a smidgen, the least metabolised. So it’s very resistant to those marauding liver enzymes. They try their best and they do not succeed.

And by that virtue, you can tell the examiner from an elimination perspective, 99.98% is excreted via the lungs, i.e. almost all of it. I’ve tried my best to make desflurane’s pharmacokinetics as interesting as possible. Let me know if I did.

Introduction to Vaporisers

25:00-28:30

Summary: Vaporisers deliver controlled volatile concentrations safely. Historical context: from rags soaked in volatile, to Boyle’s bottles (requiring water baths), to modern tech 5 vaporisers. Key challenge: latent heat of vaporisation causes cooling, reducing vapour output. Modern vaporisers use temperature compensation and copper heat sinks.

But now we need to talk about how to get these vapours. We’re inhaling vapours now. We’re in Victorian England and someone’s had an attack of the vapours and maybe is having other vapours whiffed under their nose to remove said attack of the vapours. I don’t know if vapours were actually ghosts. Maybe they had ghosts in them. Anyway, how do we get volatile anaesthetic agents into a person in a safe and controlled manner at the right dose without wasting too much of it? And the answer here is we need a specialised device, a vaporiser.

Once upon a time, we would slosh a bit of volatile on a rag, and that would increase our surface area and allow us to vaporise some agent and administer it. Then we had a mask, which was a rag in a frame, a Schimmelbusch mask, where you would drip a certain amount of ether on it using a device that meant that you weren’t going to accidentally slosh all the ether everywhere, noting that it is flammable, and you wouldn’t want everyone else to start feeling woozy.

But then with circuits, ventilators, masks, seals, devices were developed to deliver a certain degree of volatile to that system, and they came across a few issues which created technical challenges which have been solved. So one of the biggest sticklers is when you have an agent that is volatile. Remember, volatile means it has a propensity to become a vapour, i.e. molecules of that compound escaping off and floating away into the atmosphere.

The molecules that escape and float off generally have higher energy, they are the warmer or hotter molecules in that system. When those warm and hot molecules escape off away into the atmosphere, the remaining liquid, its net energy goes down, it is perceptibly cooler. As it gets generally cooler, less and less and less of those molecules will be able to escape. And therefore you can see the problem here. And this is latent heat of vaporisation. That’s the technical term. If your liquid’s getting cold, you’re not going to be able to get as much volatile vapour off of it. That means you won’t be delivering as much to the patient.

So once upon a time they had these things called Boyle’s bottles, named after a chap called Boyle, I think, and these were a glass bottle with volatile in the bottom, they would bubble the fresh gas that eventually was going to go to the patient through it and lift off volatile as required to keep the patient asleep. But they would see condensation form on the glass and notice that their patient might be a bit more wriggly than when they were half an hour ago. They would bathe these Boyle’s bottles, not like a baby, but like, you know, put it in a water bath that is warm consistently, to maintain a temperature of that volatile compound so that they would then get a reliable rate of vaporisation from it and thus reliable anaesthetic.

But you don’t really want a water bath sloshing around in theatre, the urologists are spilling enough of that everywhere. So eventually came a procession of vaporising technology that leads us to today where we have Tech 5 vaporisers. There are Tech 6 and Tech 7 vaporisers, but Tech 5 is the classical one that’s probably going to be discussed in the exam.

Tech 5 Vaporiser – Anatomy and Function

28:30-35:45

Summary: Tech 5 vaporisers are variable bypass, temperature-compensated devices. Not in-line due to high internal resistance. Components include vaporising chamber with wicks and baffles, control dial, temperature-compensating bimetallic strip, and safety features (colour coding, keyed filling system, back bar interlock, backflow valves).

So, these are variable bypass devices. They are temperature compensated. They are not in line with your circle circuit, unlike some other vaporisers of the past, and this is because they have quite a high internal resistance, and if you were trying to breathe through one, it would be too hard. They do not require electricity, but they are rather heavy, as I’m sure you’ve noticed if you’ve tried to put one onto the back bar of an anaesthetic machine.

Anatomy of a Tech 5 Vaporiser:

What is the anatomy of one of these vaporisers? So they contain a vaporising chamber. That makes sense, doesn’t it? But if you literally just had a space inside your vaporiser full of volatile, that surface area on the top of the volatile, which is one of the critical elements to getting vapour out of liquid phase into a vapour phase, that surface is quite low.

So inside you have wicks, much like the wick of a candle, or if you’ve seen those liquid fuel flame devices or citronella candles, it draws liquid up that, increasing its surface area, therefore more surface area, more vaporisation. There are baffles within this chamber. These force the flow of fresh gas to circulate more broadly in the chamber, and preventing channelling, if you just had a hole at the top on one end and an exit at the top on the other end, most of that airflow is just gonna wanna cross the top of the chamber, isn’t it? Quite useless.

Preventing this channelling means that you’re going to pick up and have a more consistently saturated gas flow. Remember channelling, it’s like when you’re making an espresso. You require water to flow across all of those ground beans and not have water preferentially eking through the easiest and least compacted route because you just get over-extraction on one part and no extraction elsewhere and have an unpleasant espresso. In that vaporising chamber you will also find a liquid phase and a vapour phase of your anaesthetic agent.

A vaporiser has a control dial, you need to ask it, tell it, kindly request in triplicate the concentration it’s going to be delivering. There’s one entry point for fresh gas flow, but then this is split and there’s a bypass route for fresh gas flow and then a route for fresh gas to enter that vaporising chamber. You will also find a temperature-compensating bimetallic strip. We’re going to talk about that in more detail in a minute. And it has a number of safety features.

Safety Features:

They could just say, tell me about the safety features of a vaporiser. And there’s a few elements:

• Arguably it is colour coded, so you should be able to match the bottle of volatile to the vaporiser. But it goes further in that it is a keyed system that means that one bottle of volatile, say sevoflurane, will not actually plug into an isoflurane vaporiser because it won’t fit into the device that allows you to fill the vaporiser. It’s keyed to a container.
• There’s a mechanism to block more than one vaporiser being on at once and this is called a back bar interlock.
• They have backflow valves, so if you turn a vaporiser upside down, you don’t slosh volatile out into your fresh gas flow. That would significantly alter the vapour pressure of the end result gas coming out of the vaporiser.
• Like I said earlier, it’s quite hard to breathe through a vaporiser, although I’ve not tried. Maybe I will. And this is why you can’t just have it in line in your circuit because then the patient will be trying to breathe through it. But this is also a feature in that that high resistance reduces the chance of back pressure in your system from driving your vaporiser gas mix that’s coming out at your dialled temperature, preventing that to be driven back across the chamber where it could pick up even more volatile and therefore start delivering a muddled mixture to your patient.

So there’s quite a few features designed to make these things safe. Probably derived from complications historically, leading to people thinking, hey, you know what, let’s redesign this.

Gas Flow Through Vaporisers and Temperature Compensation

35:45-42:30

Summary: Fresh gas flow splits – majority bypasses the vaporising chamber, minority enters chamber and exits fully saturated. The bypass gas dilutes the saturated gas to achieve desired concentration. Bimetallic strip adjusts the splitting ratio based on temperature to maintain consistent output. Copper heat sink provides thermal stability.

How Gas Flows Through a Vaporiser:

So how does gas flow get across a vaporiser? So you have fresh gas flow from your back bar, you know you’ve got your dialled oxygen, your dialled air, and obviously you’re not using nitrous oxide anymore. And this enters your vaporiser. Now that fresh gas flow is split. Quite a large portion actually bypasses the vaporising chamber and traverses your vaporiser to the exit end. And then there is a portion – the other half of that split or third or quarter of that split – enters the vaporising chamber and exits that chamber fully saturated.

i.e., when we’re using sevoflurane, it’ll come out of that vaporising chamber with a 22.7 kilopascal wedge of sevoflurane in that mix of gas. Now if we were to be inhaling 22.7 kilopascals of sevoflurane, that would probably get us quite toxic quite quickly. And that is why you have fresh gas flow that avoids the vaporising chamber, because then you add that back to your non-volatile gas and get the concentration you wish to deliver to the patient.

Temperature Compensation Mechanisms:

But I’m sure you’re now remembering, hey, Dr Gas, you said there was a temperature-compensating bimetallic strip. Well, why? So we’re saying that sevoflurane produces at 20°C 22.7 kilopascals as its saturated vapour pressure. But we know that hospitals can be warmer and colder than 20°C. So we need to be able to compensate for that altered vapour pressure exiting the vaporiser, and therefore it would alter our splitting ratio, the split between the bypassed gas and the gas that goes through the vaporising chamber. Otherwise we would end up with different mixes if our vaporiser was cold or if our vaporiser was warm.

So this bimetallic strip, bimetallic – two metals. That means that one metal has a proclivity to expand at a different rate depending on the temperature than the other, and these will shift. That shift alters an orifice. That orifice means that there’s more or less flow in the bypassed element of the circuit. Therefore, changing the fraction that comes out of the vaporiser based on the temperature in the system. And obviously this bimetallic strip is calibrated to the engineering of the whole vaporiser.

Copper Heat Sink:

There’s another way that a vaporiser avoids this problem, or at least attenuates this problem. And that’s because I’m sure you’ve picked one up and thought, bloody hell this is heavy. And it’s because it’s got a hecking fat chunk of copper in it. Most likely. Might be some other metals as well these days, depending on the expenses of copper. But copper stores a reasonable amount of heat energy and it is quite conductive. This attenuates the drop in temperature of the system that invariably happens when you pass fresh gas flow through it.

I imagine if you could measure its temperature when you’re howling 18 litres through your vaporiser, eventually you would cool that vaporiser down to the air temperature that you’re delivering to it, but then it will keep getting cooler as you vaporise anaesthetic agent out of it. But in normal usage, that heatsink of copper that’s within it, that bimetallic strip, compensates for reasonably significant temperature changes within the operating temperature of a vaporiser, delivering safe volatile anaesthetic concentration.

Operating Temperature Range and Clinical Considerations

42:30-47:15

Summary: Tech 5 vaporisers operate safely between 16-35°C. Outside this range (e.g., field hospital, bush medicine), output becomes unreliable. Gas analysis is essential – if available, can compensate by adjusting dial setting. Clinical tip: tighten alarm limits once at ‘cruising altitude’ to reduce cognitive load and ensure meaningful alerts.

And now you’re thinking, well yeah, but what is the safe operating temperature of a Tech 5 vaporiser? Won’t be in the exam, so just ignore me. But it’s 16-35°C. So if you’re using it under a bush or you’re in a tent in a chilly field hospital, then a sevoflurane vaporiser may not work favourably.

Is this a problem? It is a problem if you don’t have gas analysis in your circuit. And remember we analyse for CO₂, but also we analyse for oxygen fraction and volatile anaesthetic agent fraction inspired and expired. If you have the luxury of this technology and you’ve got a really cold vaporiser, you can still probably deliver an anaesthetic as long as you turn the vaporiser up more. So you are in theory asking it to alter its splitting ratio to pull more volatile out of that chamber, but it’s cold, so you’re getting less volatile. But as long as you can measure it, you could deliver an anaesthetic.

Remember, tight alarms in this situation and other challenging clinical situations or bushwhacking, tight alarms are always your friend. It might be worth tomorrow, later today, Monday, having a look at the automatic alarms set on your monitoring, and really having a look, because you’ll find that the hypoxia alarm might be set at 90%. The oxygen fraction being low alarm might be set at 18% or something ridiculous. And that actually you might be tempted to alter these.

Would you really want to only be finding out you’re accidentally delivering 18% oxygen when you’re there? Or would you like to notice when you’re only delivering 24% oxygen and trying to figure out why. Just as an experiment, go have a look. And if you’re ever tired, fatigued, or there’s a lot going on, taking 30 seconds to tighten up your alarms once you’re at cruising altitude and you know what your end-tidal volatile concentration wants to be and where your oxygenation wants to be, etc., etc., and blood pressure.

Setting those alarms will take away some of the cognitive load so that you can focus on the patient more. Because you’ll know an alarm is useful to you.

SBA Question from Teach Me Anaesthetics

47:15-49:30

Oh, I didn’t realise I could talk for so long about desflurane and vaporisers.

Tech 6 Vaporiser – Desflurane Specific Design

49:30-54:00

Summary: Desflurane requires a Tech 6 vaporiser due to its low boiling point (22.8°C). The vaporiser heats desflurane to 42°C in a pressurised chamber (2 atmospheres = 202.65 kPa). Uses injection system with differential pressure transducer to measure fresh gas flow and microcontroller to adjust orifice valve, injecting precise amounts into fresh gas flow. Includes electrical safety features: pressure/temperature alarms and warming indicators.

So finally, the last bit I’m going to talk about for this episode is the difference between the desflurane vaporiser and a Tech 5 vaporiser. Desflurane is a Tech 6 vaporiser. It has to heat the desflurane up so that you can consistently get a saturated vapour pressure from it. It does this with a vaporising chamber with a heating element inside it. And the vaporising chamber is a fixed space. It doesn’t just let the desflurane escape, so the pressure in that vaporising chamber is also raised.

Components of a Tech 6 Vaporiser:

The other anatomical elements of a Tech 6 vaporiser includes the concentration dial, a differential pressure transducer, and I’ll talk about this in a second, a vapour flow microcontroller and an adjustable orifice valve. It contains the same safety mechanisms of that interlock in the back bar, backflow valves that stop mischief, non-return valves, the keyed system to prevent wrong bottle in chamber, but because it’s an electrical piece of equipment, it also has some pressure and temperature alarms and a few lights and bits and pieces to let you know that it’s warmed up enough.

The vaporiser itself heats the desflurane up to 42°C, and the chamber is pressurised to two atmospheres. Remembering one atmosphere is 101.325 kilopascals, so two atmospheres is 202.65 kilopascals. So not bad. That’s the equivalent of diving down ten metres.

How the Tech 6 Injection System Works:

A desflurane vaporiser works differently in that it injects desflurane into a fresh gas flow because obviously it can’t just have an equal pressure in the system because all the desflurane will constantly escape. So there’s a microcontroller that adjusts the size of an orifice that releases desflurane into the fresh gas flow.

But I’m sure you’ve rightly guessed, how on earth does a vaporiser know how much fresh gas flow is going through it in order to inject the right amount of desflurane in? And you’re right, and the engineers thought about this one too. So there is a differential pressure transducer that compares the pressure on the chamber side and the fresh gas flow side of the vaporiser, running it through a microcontroller to infer flow rate and therefore the required amount to be injected into the fresh gas flow to achieve your target desflurane concentration.

Now there’s more about this in several books and also in the show notes. There are also a few facts about how vaporisers behave at altitude. When we do a full vaporisers episode, we will go into that in greater detail, but ultimately it just causes – it causes me, causes everyone a headache.

Closing Remarks

54:00-55:00

Ahoy Team Anaesthesia, you’ve survived yet another episode of Gas Gas Gas. Now, if you’ve found it useful or harrowingly awful, please like and subscribe, drop us a star or twelve, and follow with whichever podcast platform you find yourself using. Please leave a comment or ping off an email if you think I need to square something away.

Now there are a bunch of ways to support the costs of Gas Gas Gas. From buying me a coffee to venturing forth via an affiliate link to the hoard of joyful SBA questions from Teach Me Anaesthetics. Those links are on the website and in the show notes. Speaking of website, definitely check out gasgasgas.uk for the show notes, diagrams, details, the references.

Now we all know guys that this is a bucket of content to consume, and it is like drinking from a fire hose. So I want to finish by saying, take it day by day. Don’t overcook yourself, don’t freak out and keep studying.