Hi, and welcome to this webinar on low flow anaesthesia. My name's Sarah Gibson. I'm an anesthes at Davies's Vets.
I've worked there for 16 years, and, low flow anaesthesia is something that we've worked on quite hard over the last few years. So I'd quite like to share some tips and practicalities with you. So here's an overview of the webinar.
Firstly, we'll go over the definition of what low flow anaesthesia actually is. Then the main advantages, which are humidification and warming of inspired gases, reduction in our use of volatile agents, saving money and also environmental pollution. And then there are a few disadvantages and potential areas where things could go wrong with low flow anaesthesia, so we'll talk about these.
And then I'll go over some practical aspects of low flow anaesthesia, some of the potential problems to watch out for and some ideas of flow rates to use in our patients. So there are different definitions of what low flow anaesthesia actually is. Fresh gas flow is the total of our flow rates of the carrier gases we use in the anaesthetic machine.
So these are oxygen, sometimes with air or nitrous oxide. So some people define low flow anaesthesia as a fresh gas flow significantly lower than the minute volume of the patient. A minute volume is the volume of gas that's inhaled or exhaled over one minute.
An easy rule of thumb is 200 mL per kilo. So for a 300, sorry, a 30 kilogramme dog, the minute volume would be 30 times 200, which is 6 litres. So we would aim for significantly less than this.
Another definition is that 50% or more of gases are rebreathed after carbon dioxide absorption. I like this classification of flows suggested by Baxter, which defines medium flow as 1 to 2 litres per minute, low flow as half to 1 litre per minute, and then they also classify minimal flow as 250 to 500 mL per minute and metabolic flow less than 250 mL per minute. So we're going to be concentrating on low flow, which is 500 to 1000 millilitres per minute.
So just to, to point out, these flow rates are the flow rates used during the maintenance of anaesthesia. So there'll be much higher flow rates at the initial phase of anaesthesia. So here are the advantages of low flow anaesthesia.
Firstly, we can better maintain temperature and humidity in our patients' airways, which we will see in the next few slides is highly beneficial. There's less oxygen, volatile anaesthetic agent usage, which in a time where we may not have access to oxygen as easily as normal, is very important. And we've also seen shortages in isofluorine in the past year.
Using less gases and anaesthetic agents reduces the financial costs and low flow anaesthesia importantly reduces workplace and environmental pollution. So let's look at how low flow is beneficial to maintain temperature and humidity. Oxygen and air are stored and delivered from cylinders, so the gases are cold and dry.
Usually, air is humidified by the nose and mouth, but when a patient is intubated under anaesthesia, these normal mechanisms are bypassed. So if we do nothing, cold, dry gases are inhaled. The problem with this is that water will move out of the mucus lining of the airways to try and humidify the air.
This results in more viscous mucus, which can obstruct bronchi and the Chia, causing a ectasis and infection. Normally, the mucociliary escalator works to move mucus and debris out of the lower airways to the pharynx. This relies on cilia, which line the airways, moving in a coordinated way.
Dry air also destroys cilia, so there's a loss of function and mucus and debris are not removed as normal. Studies in people have shown that using a fresh gas flow rate of 0.5 litre per minute maintains temperature and humidity in the airways.
So here's a study from 2011, which looked at the effects of low or high flow anaesthesia in 50 people. They were split into two groups. One had a fresh gas flow of 1 litre a minute, and the other received a fresh gas float of 3 litres per minute.
Hemodynamic parameters, changes in humidity and temperature of the inspired gases were measured. Mucociliary clearance was also measured. So in the lower flow rate group, humidity and temperature was higher.
Respiratory function was better. A mucociliary clearance was higher. So using 1 litre per minute compared to 3 litres per minute was better for the trachea and bronchus to function properly.
The next advantage of low flow is to reduce our use of oxygen and volatile anaesthetic agents. There's been a big increase in oxygen use by the NHS due to COVID-19 pandemic. Patients presenting with COVID needing ICU are needing extended periods of ventilation.
So this requires a huge amount of oxygen. There have been warnings that hospitals could run out of oxygen in hours rather than days at an association of anaesthetists webinar discussing the crisis. But have guidance on their websites, and I've put a link to the updates that they give at the bottom of this page.
So you can keep up to date with their most current advice. They've increased their stocks of gases and they've asked the customers with medical oxygen ensure the empty cylinders are available promptly for collection. Only empty cylinders will be exchanged for new ones.
I'll just put a reminder of how much each oxygen cylinder contains, this might be useful later. So an E cylinder contains 680 litres of oxygen and a J cylinder, 6800 litres. So we need to be prepared for reduced supplies and modify our case management where possible to reduce our use of oxygen and conserve supplies.
So later on at the end, we'll go over some other ways other than local anaesthesia to reduce our use of oxygen. Yes. So in this we will use less volatiles of low-flow anaesthesia.
These are the results from a study in people where they were divided into three groups, with fresh gas blows of 1.5, 3, and 6 litres per minute. As you might expect, using increasing the fresh gas flow increases see fluorine consumption.
You can see on this grass, the graph on the X axis, we have the groups of 1.5, 3, and 6 litres in the different colours and the see fluorine consumption on the Y axis. What's surprising is the see of fluorine consumption, the increase is not linear.
So when you double the fresh gas flow rate, you more than double the use of see fluorine. So going from 3 to 6 litres a minute is a fresh gas flow, the see fluorine increases from 0.16 to 0.41 millilitres per minute.
In a different study, they implemented a low flow, a low fresh gas flow policy throughout a hospital department, which meant a maximum of 1 litre per minute was allowed for maintenance of anaesthesia. When a change in volatile concentration was needed, the fresh gas flow was increased to 2 to 3 litres per minute and then reduced back down to 1 or less. They calculated the number of anaesthesia hours they could get out of each bottle of Sivaflurane to see how the use was changing.
After 5, 10, and 15 weeks, they did calculations to work out the number of hours of anaesthesia. After 5 weeks, this has increased by 17 hours per bottle of Sivaflurane. So if you look at the price, there's these are a little bit out of date, but are similar.
The Siva fluorine is 187 pounds plus VAT and isofluorine 28 to 45 pounds per bottle. So getting 17 hours extra for a bottle of Siva flura can lead to big cost savings. Yes.
There are some theoretical disadvantages to low fly anaesthesia, which you just need to be aware of when starting out, and most of these can be avoided quite easily. Firstly, because we're adding much lower quantities of fresh gases to our breathing systems, and these, this is what contains the oxygen and the anaesthetic agent. These will be diluted by any gases already in the patient's respiratory tract and the and the gases already in the breathing system.
So this means there's a potential for oxygen to be diluted and we could be delivering less than we think, and this could result in a hypoxic gas mixture. So this is a consideration at the start of anaesthesia and at the end if using nitrous oxide. Also, because of the gas dilution, less anaesthetic gas will be could be delivered than we think.
And so it can be more challenging to keep the patients under a stable plane of anaesthesia. And when we need to change depth of anaesthesia, this takes more time because the fresh gas flow is low. So it's a matter of learning how to use the breathing systems in this way.
A circle breathing system is used for low flow anaesthesia, and there are some disadvantages associated with this that we'll discuss later. We need to consider the patient's size. Not all patients will be suitable to use with a circle breathing system.
There's a higher chance of leaks, and there's also more airway resistance because there are more components in a circle breathing system. Although low fly anaesthesia can potentially save a huge amount of money, there are equipment costs during the setup for the breathing systems, gas monitoring, and also making sure the anaesthetic machines are capable of delivering low flows accurately. It might be useful to calculate how much isofluorine or see the fluorine you're using now, and then you see how much you could reduce your use by using low flow.
So I've included this calculation just so you can work out how much you're using. So you can work out the volume of anaesthetic agent if you know the average or the mean fresh gas flow rate and the mean anaesthetic concentration and the time that the animals anaesthetized for. These, these are plugged into the equation and at the bottom, you just need to know the anaesthetic saturated gas volume, which is the volume of vapour you get from 1 mL of the agent.
So for isofluorine, it's 195 mLs and Siva fluorine 184 mLs. So here's just an example. If you wanted to work it out for a 30 kilogramme dog using a circle.
So for the 1st 10 minutes, we've got the oxygen flow rate at 4 litres per minute and the eyes are fluorine at 2%. Then for 90 minutes, the oxygen at 2 litres per minute and the eyes at 1.8.
So you can plug these into the equation. The mean fresh gas flows, if you're calculating that, you've got 4 times 10 plus 2 times 90 divided by 100 minutes. So the mean fresh gas flow is 2200 mL.
And then you've got the mean isofluorine calculate percentage, which is 2 times 10 plus 1.8 times 90 minutes, divided by 100 minutes overall. So the mean isofluorine is 1.82.
So these are the two numbers you need. And then if you put it into the equation from before, the isofluorine volume comes out at 20.5 mLs.
So that's how much you would have used for that anaesthetic using those flow rates. And this table just shows some different scenarios for the same dog depending on what anaesthetic agent you use, and it might be useful for reference. In purple is isofluorine and in yellow is safluorine.
So we start off with each scenario with your fresh gas flow rate of 2, going to a fresh gas flow rate of 1. And you can see for isofluorine, if you do that, the cost is £1.75.
If you double those flow rates, the cost goes up to 3 pounds 50, and if you use a LAC, which requires a much higher flow rate of 6 litres per minute, the cost is £8. You can see for sevoflurane, those costs are much more expensive. Using a lack with Siva fluorine is 72 pounds.
So this makes Sivaflurane quite prohibitive, and it's, this is why low anaesthesia makes Sivaflurane a much more cost effective option. We'll talk about the environmental effects later, but you can see in the carbon dioxide column second to the right. How much better for the environment Siva fluorine is.
It produces much less carbon dioxide, even at the high flow rates of 6 litres per minute, where we've used, we've used 22.7 kilogrammes an hour of carbon dioxide we've produced, and whereas see fluorine, we've only produced 9.6.
And then the final column on the right, just look at the oxygen usage. If you were to use a lack in 100 minutes in a 30 kg dog using 6 litres per minute, you would use 600 litres of oxygen, which is a significant amount. An easier way to manually calculating the use of volatiles is to download this app from the Apple or Play stores.
It's the anaesthetic impact calculator by the Association of anaesthetists. You first need to tap on the settings and you can enter the prices of your anaesthetic agents that you pay for, and then tick tap on the tech plain icon to use the app. So when you you go on settings, you can put your costs in here per bottle of what exactly what you pay, so it'd be accurate when it gives you the numbers later on.
And then when you click on the tech planum part, you come up with this screen. Then you can adjust the oxygen or nitrous or air, whatever you're using, and then you can change the Sifluorine and isofluorine concentrations. So this will tell you cost per hour and also the environmental impact in carbon dioxide equivalents of kilogrammes per hour.
And you can move the bobbins up and down or turn the vaporizer dials. So if we put the fresh gas flow rate at 6, the eyes of fluorine at 1.6% and the see fluorine at 2.5%, they're kind of similar, I think.
And I've circled the cost per hour. So you can see with this fresh gas flow rate, at these concentrations, is a fluorine will be costing £4 an hour or receive the fluorine nearly 43 pounds an hour. And if you look at the carbon dioxide production, the isofluorine is 22.7, but sevofluorine is much lower, less than half at 9.6.
If we just reduce our fresh gas flow to 1 with the same settings, the cost of isofluorine comes down dramatically to 0.67 and see fluorine to 7.16.
So if we just go back, it was 4 pounds an hour and 43. Just by reducing our fresh gas flow rate down to 1, we've dropped it down to less than 1 pound for of fluorine and only 7 pounds for see fluorine. So let's talk a bit about the environmental impact of our anaesthetic gases.
Anaesthetic gases are greenhouse gases, and they're bad for the environment and they're bad for our carbon footprint. But I thought we could just go over the basics of greenhouse gases. So what exactly are they?
So greenhouse gases any gas that absorbs heat, keep keeping the atmosphere warmer than it should be. The main greenhouse gases in the atmosphere are water vapour, carbon dioxide, methane, nitrous oxide, and ozone. The Kyoto Treaty is an international agreement to control the levels of about 7 greenhouse gases, and nitrous oxide is one of the gases on the list.
So anaesthetic gases are potent greenhouse gases, and in addition, nitrous oxide also causes destruction of the ozone layer. So this diagram is taken from an article in BJ Education and the references at the back. At the back of the webinar and also down here.
The diagram shows how solar radiation enters the Earth's sur Earth's atmosphere, and this is essential for life, so that's the yellow arrow coming down. It then gets emitted back out from the earth's surface as thermal radiation. So greenhouse gases are a problem because they last in the atmosphere for a long time and they absorb infrared radiation that should be leaving the atmosphere and instead they reflect it back down to Earth again.
You can see that red arrow going up and coming back down again. Normally infrared radiation, between 8 to 14 micrometres can escape from the atmosphere, but anaesthetic gases are a problem because they absorb wavelengths around 10 micrometres, which is right in the middle of this spectrum, and this causes something called positive radiation radiative forcing, and that causes warming of the planet. So the effects of the different greenhouse gases on warming are not the same.
It depends partly on how long they last in the atmosphere and how much they absorb and reflect infrared radiation back to the earth. To compare the impact of different gases, GWP or global warming potential is used. So this gives an idea of the amount of warming a gas would produce over 100 years.
For this scale, carbon dioxide is assigned a GWP of 1. So looking at global warming warming potential, these are some of the Kyoto gases. You can see from the table how much more of an impact nitrous oxide has than carbon dioxide at 298.
So moving on to our anaesthetic gases, these are the GWPs for those. I've highlighted the ones most commonly used in veterinary practise. So you can see how much worse for the environment isofluorine is than Siva fluorine.
Isofluorine has a GWP of 510, whereas Siva fluorine is 130. This makes the point that although we don't produce relatively much volume from the anaesthetic gases, they are still really potent greenhouse gases because the wavelength of infrared radiation that they absorb. Nitrous oxide also hangs around in the atmosphere for 110 years, and because of this, we've stopped using it in our practise and recommend not using it in veterinary anaesthesia.
The Sustainable Development Unit has a report available online called the Carbon Footprint from anaesthetic gas use. So this is from human hospitals and they've calculated that the environmental impact of gases in the healthcare system in the UK. And they've identified the anaesthetic gases are the potent greenhouse gases, as we've discussed.
And there are some quite surprising findings, 5% of the carbon footprint of what they call acute organisations. So those doing surgery comes from anaesthetic gases, and that's a big figure considering how big the NHS and private healthcare is in the UK. It's the equivalent to half of the emissions for heating all the hospital buildings and water.
Nitrous oxide use in the healthcare system in the UK makes up 1.3% of total total nitrous oxide emissions in the UK. The report states that desfluorine and nitrous oxide are the worst gases and see fluorine has the least effect.
They advise consideration of IV anaesthesia and the need to measure, monitor and report use. For us in veterinary practises, volatile anaesthetic agents will make up a large part of our carbon footprint, and this is something we can easily reduce straight away. These graphs come from the report.
The left hand graph shows the global warming potential for some of the anaesthetic gases, with des fluorine, which is most commonly used in, which is commonly used in people, having the biggest effect in the atmosphere, and Siva fluorine has the least effect. The right hand graph shows the estimated total emissions from the NHS in England. You can see that nitrous oxide emissions are over 5 times that of any other gas.
So less than 5% of inhalation agents are metabolised. Most of the volatiles and anaesthetic agents we use are vented straight into the atmosphere by scavenging systems. So the less we use and waste, the better.
Ways we can reduce our waste are to use low for anaesthesia, which we'll discuss a lot more. We should avoid volatiles with a high environmental impact, as we saw in the earlier slide, desfluorine and nitrous oxide are particularly bad for the atmosphere. Siva fluorine is better than isofluorine, although more expensive, but this cost impact can be minimised by using less of the agent by low flow anaesthesia.
Using other drugs such induction or infusions during anaesthesia can also reduce how much volatile anaesthetic is needed to keep the patients anaesthetized. And Seedling, which is being developed in the UK by SageTech. The idea behind this is to literally collect and capture exhaledever fluorine and process it so it can be used again.
We are hoping this will be rolled out to the veterinary market in the future. An important factor in this is looking at how the volatile is transported to and from the treatment plant, and they are aiming to use. Heat pollution.
So now we move on to how to use local anaesthesia in our patients. I appreciate every practise is different with equipment available, so we need to be using circle breathing systems, either more traditional styles or the Humphrey ADE. Ideally, Caography is used with low flow to help guide monitor using the vaporizer settings, but also for safety to ensure there is always sufficient oxygen and not hypoxic mixture.
So for monitoring, we'll discuss later, we need to have a pulse oximeter and it would be nice to have catography and agent monitoring, but of course this isn't always possible. I'll briefly di mention the pharmacology of the volatiles and then the technique. So just to go over a few useful definitions when talking about local anaesthesia, rebreathing is when expired gases are rebreathed, and the tidal volume is the volume of gas exhaled in one breath.
So it's 10 to 15 mL per kilogramme. The minute volume is the volume of gas exhaled in 1 minute, so that's usually about 200 mL per kilogramme. And the functional residual capacity of the lungs is the volume of gas that's left in the lungs at the end of expiration.
So to be able to use a fresh gas flow rate of less than 1 litre per minute, the anaesthetic machine needs to have a scale lower than than 1 litre per minute. In this photograph, you can see this particular machine has a separate glass tube to the left with a scale going from 0.1 to 1 litre per minute.
If your machine doesn't have a scale lower than one, it doesn't matter, but I just wouldn't go below 1 litre per minute, so you can be sure of what you're delivering. So remember to read bobbins from the top and balls from the middle in the flow metres as accuracy is important with low flow. So it's important to make sure that enough oxygen is delivered to the animal and there's not hypoxic mixture.
So using a pulse oximeter to monitor oxygen saturation of haemoglobin is essential. An alarm for low inspired oxygen concentration would also be ideal. Capnography measures the inspired and expired carbon dioxide concentrations, so it gives us information about whether the animal is hypoventilating or whether the soda lime is expired.
For this slide, just so you know, FI denotes inspired and ET stands for end tidal. Anaesthetic agent monitoring is expensive and it's not widely used in veterinary anaesthesia, but if you have it, it's really useful and it would be very essential for very low flows. This is because of low flows, what is dialled on the vaporizer is often different to all the concentration the animal's actually breathing in and out.
It's important to ensure there are no leaks in the breathing system or the endotracheal tube. So the patient will need a cuff under your tube. It may be possible with the laryngeal mask airway.
So I want to just show you how gases get diluted and how we need to allow for this. So when the fresh gas flow rate is less than the minute volume for the patient, the concentration delivered by the anaesthetic machine will be different to that inspired by the patient. So remember, the minute volume is a volume inhaled or exhaled in a minute.
So for 30 kilogramme dog, that's 6 litres per minute. So in this example, the total fresh gas flow is 2 litres per minute. We've got 1 litre a minute of oxygen and 1 litre a minute of air, and the isofluorine set at 1.5%.
However, on the monitoring, we can see that the inspired isofluorine FIIO is only 1.1 and the Nidal Io is 0.85.
So this is lower than is dialled, and that's what happens when the fresh gas flow rate is less than the minute volume. So we just need to learn to account for that. So how much oxygen do we actually need?
There are two things to consider. First, the oxygen requirement of the body, which is 2 to 3 mL per kilo minute. So a 30 kilogramme dog actually only needs 90 mL per minute, and a 100 kilogramme dog needs 300 mL per minute.
Secondly, the minimal flow that we need for the breathing system to stop prevent or stop or prevent rebreathing. So this doesn't apply to circle breathing systems, just non rebreathing anaesthetic systems. And where we have to have a fresh gas flow rate high enough to avoid rebreathing.
So for a lack, which is considered efficient for a non rebreathing system, there's a circuit factor of one. So the fresh gas flow needs to be one times the minute volume to prevent rebreathing. With circles, we don't need to prevent rebreathing as there's soda line to absorb the carbon dioxide, and that is why the flow rates can be so much lower.
We do need to allow for a catography sampling line if you're using capnography, and this can take up to 250 mL per minute from the breathing system. So for safety, I would suggest not going below 1 litre per minute if there's no capnography and gas monitoring. With experience, capnography and gas monitoring, and an anaesthetic machines capable of lower flows, going to 0.5 litre a minute is possible.
Just to go over breathing systems, so their function is to deliver oxygen, to deliver the inhalation or anaesthetic agent, and to remove carbon dioxide. And there are classifications of all the different kinds of breathing systems. I've just highlighted and read the ones that we would most commonly come across.
So we use the the semi closed breathing systems where the fresh gas flows not diluted by room air, and they're either non rebreathing, such as the lack vein or TPs or rebreathing with carbon dioxide absorption, and the circle was the most commonly used one. You can also use circles in a closed mode, but as we'll mention later, we don't do this in veterinary anaesthesia as it requires very careful monitoring and it can lead to a much higher risk of Awareness and hypoxic mixtures. We'll just do a couple of slides on the pharmacology of the volatile agents.
At a stable plane of anaesthetic, you would eventually reach a steady state where the vaporizer setting of concentration is in equilibrium with the alveoli and also in equilibrium with the blood concentration. This is rarely reached. It would take hours of anaesthesia.
So what's important to us is the uptake of the anaesthetic agent, and that will determine how quickly the patients go to sleep and how quickly the depth of anaesthesia can change if the animal becomes lighter or deeper, and we want to alter that. And it does depend on many factors. The partial pressure gradient across the alveolus, the alveolar ventilation, how well the animal is breathing, the blood gas solubility of the anaesthetic agent we're using, the kayak output, and also the solubility in the central nervous system.
So the amount of volatile added to the breeding system is small compared to the volume of the breathing system. And as a result of this, any change in depth of anaesthesia can be slow. One thing that will influence how quickly the depth of anaesthesia can change is the blood gas solubility.
A volatile anaesthetic with a low blood gas solubility will be much faster to recuriate. Sevoflurane has a lower blood gas solubility than isofluorine. And we'll see in this graph.
So this graph shows the uptake of the inhalational agent sevofluorine in yellow and isofluorine in purple. The X-axis is time and the Y axis is the alveolar concentration to inspired concentration ratio. So the steeper the curve rises, the lower the blood gas solubility coefficient.
So teva fluorine, you can see has a steeper curve and it reaches 1 on the Y axis faster than isofluorine. And that, and when you reach 1, that is when you're reaching the steady state. And that makeste fluorine more suited for using local anaesthesia, as it allows faster changes in the depth of anaesthesia.
So now we're going to talk about the equipment we would use. So there are two types of circle breathing systems that you might commonly see, . There'll be there are semi-disposable circles, for example, this one on the left from Burton's and there will be other manufacturers and other types of circles.
And then there's the Humphrey ADE which is quite commonly used in veterinary anaesthesia on the right. So the circle breathing system has the following components. There's a canister of sodalime with two ports.
You can see the sodali arrow where it's labelled in this picture. And the teapots the tubing attaches. And then behind these, there are two unidirectional valves, one inspiratory and one expiratory, so that the gas can only flow in one direction around the circuit.
So the tubing can be different widths. In this photograph, it's 22 millimetres in diameter. The Humphrey ADE uses 15 millimetres.
And what's good about the Humphrey ADE tubing is that it's smooth inside, and that produces less resistance and reduces the work of breathing for the animal. So there's also a reservoir bag, which should be at least twice the tidal volume. And a white piece.
The Humphrey AD is very similar. The APL valve is shown with the arrow at the top there. And the expiratory and inspiratory valves are also clearly labelled on the breathing system.
So one of the requirements to use local anaesthesia is to ensure there are no leaks. There are a few common places where leaks occur, so I just wanted to highlight those to you. The endotracheal tube cuff, not inflated or leaking or or just changing during the anaesthesia is a common cause of a leak in a breathing system.
So it's important to check that the cuffs are OK before you anaesthetize the animal. Another commonplace is the Y piece. You can get cracks, where it attaches to the patient.
You can also get, if you're using a catnograph, where I've circled on the attached to the heat moisture exchange and also to the machine, those Capnograph lines can split. Often at the junction there. And also the anaesthetic bags, they can become friable and perish around the top.
Another common places where the solar lime canister attaches. I find that particularly in the Humphrey ADE, especially if the solar lime is overfilled. So it's important to make sure that not too much soda lime is put in.
They can leak at the bottom where there are 4 little . Silver rings that attached. Also, where the circle, or when you use the circle breathing system with the canister and the Humphrey ADE, there are 4 connections to put the solar line canister into the breathing system, and each one of those is a potential place for leaks.
So checking the breathing system is really important, and I'll just recap the main steps. So firstly, you should check that the breathing system is assembled correctly and there are no obvious holes or cracks or obstructions. Next, attach the system to the anaesthetic machine and check this is secure.
And then you need to check for leaks. So close the APL valve and occlude the patient end with your finger or a bung. And so the system should be filled with oxygen to a pressure of 30 to 40 centimetres of water or until the bag looks descended.
Check the bag doesn't deflate, then switch off the oxygen and check the pressure is maintained for at least 10 seconds, or that a flow of no more than 200 mL per minute is needed. You can check the unidirectional valves by including the patient end, closing the APL valve, and squeezing the bag a few times, you can see them move. Whenever you check your breathing system, just ensure that the APL valve is open after the checks.
Just want to mention soda lime. The function of soda lime is to absorb carbon dioxide, and that's why we use it in the circle breathing system. There are different kinds of soda lime, and the constituents of soda lime were mainly calcium hydroxide, along with strong alkalis to catalyse the reactions.
So they've been, these have been associated with the formation of carbon monoxide and compound A and so have been removed or reduced in modern soda line formulations. Changes also included adding zeolite, which maintains pH at a high level for longer and retains moisture, so improving carbon dioxide absorption. It also has a lower risk for carbon dioxide, carbon monoxide and compound A formation.
Absorb is a different sodalime that contains no strong alkali and has anti-drying agents, so there's no risk of carbon monoxide or compound A. So just, be aware of which soli you're using. Usually in the UK there are two kinds with two different indicator dyes.
Either they go pink or red to white or they turn white to purple. Just to mention these unwanted substances as these are a potential disadvantage of using circle breathing systems. So they're formed by reactions with soda lime.
Compound A is formed by the reaction of seva fluorine, and moist soda lime at low fresh gas flows, and it has been found to cause nephrotoxicity in rats. But there's a minimal risk in humans. They Some countries limit exposure, but in the UK that doesn't apply.
Carbon monoxide is another unwanted substance, and it's formed when dry soda lime is mixed with isofluorine and flu or deser. And this was found after a weekend of not using an anaesthetic machine when there was a minimal flow going through all weekend, and they were finding carbon monoxide on the Monday morning. So as long as you switch off the anaesthetic machine and switch off the oxygen when you're not using it, that shouldn't be a problem.
So other disadvantages of circles are, first of all, the cost, they are more expensive than the disposable non rebreathing systems. Cleaning can be more tricky. So we use heat moisture exchanges, HMEs, and there's a picture of three different sizes in the top right hand corner, and there's a photograph at the bottom of a heat moisture exchanger attached to an endoki tube with a cat graph line coming off it.
There's more resistance to breathing because there's valves, there's a risk of dust, but if you use a heat heat moisture exchanger that has a philtre in it as well, that will reduce any risks of dust affecting the airways. Dead space, can increase in a patient if there's channelling through the soda line. So if it's not filled properly, you can get lines where the gases go down certain channels, not throughout the whole solar line, and that makes it less efficient.
If the valves aren't working as well, this can increase dead space. And also, as you've mentioned, the unwanted substances. So going back to our disadvantages of low for low anaesthesia, we've covered those associated with the circle breathing system and the setup costs.
And now we're going to talk about the practicalities of low for anaesthesia, and this should cover the problems that you may have initially when starting out with low for anaesthesia. So these are inadequate depth of anaesthesia, a delay in changing the depth of anaesthesia, and the risk of hypo hypoxic gas mixtures. And these are all avoidable by following some of the principles that we'll discuss.
So using a circle in practise can be divided into four phases. There's the start of anaesthesia after induction with an injectable anaesthetic agent such as propofol or faxolone. And then there's the maintenance phase for the majority of the anaesthetic.
And then during this, we may need to change the depth of anaesthesia, and then finally the washout phase at the end of anaesthesia. So at the start, after induction of anaesthesia, when the patient is first connected to the circle, the flow rate needs to be a little higher for a few minutes to replace all the air in the breathing system and the functional residual capacity of the patient. So that's the gases, which will be air that's in the patient's lungs at the end of expiration.
So the volume of the breathing system and the tubing will affect how long this takes and the volume of the patient's lungs. So if you have wide, very long tubing, as we do, for example, in our MRI scanner, we're a long way, we're a long way from the patient. We need a higher flow rate for longer.
For this process to happen. So this table shows the max of the two anaesthetic agents, isofluorine and see fluorine. That stands for the minimum alveolar concentration of the anaesthetic inhalation agents, and it refers to the end tidal concentration of the volatiles that we would expect to prevent movement to a surgical incision in half of unpremedicated patients and is useful to bear in mind.
So I would suggest using for the start of anaesthesia. 2 litres per minute, with a vaporizer setting of 1 and about 1.5 MC for 5 to 10 minutes, so around 2% isofluorine.
If you have monitoring, an anaesthetic agent oxygen monitoring, this will help you to be more accurate because you can measure the FIO2, so you can check that the inspired oxygen concentration is acceptable. And you can also measure the Nidal anaesthetic agent and see when you're getting close to what you want the mach of your agent. So in a dog with isofluorine, you're aiming for around 1.3.
So here, the fresh gas flow is 2 litres per minute of oxygen with an isoflurane setting of about 1.8. So if you look on the monitor here, I've circled first of all, the oxygen, the FIO2 is 98, so that's great.
The animal is breathing nearly 100% oxygen. And the isofluorine and tidal at the top is 1.4.
So that's the one that's similar, that's the one we look at for the Mac. So the Mac is just a little bit higher than the Mac of 1.3, and you can see the inspired isoflurane is 1.1.
So we can think about switching to the next phase of anaesthesia. We've had our high flow, the anaesthetic agents have become high enough and that we can then switch to maintenance. So during the maintenance phase, we want to aim to have a fresh gas flow of 0.5 to 1 litre per minute.
As a guideline, I've put, if you have no catography or agent monitoring, perhaps it's safest to go to a minimum oxygen of 1 litre per minute. If you have an air mixture, make sure there's at least 0.5 litre per minute of oxygen.
So you can use agent monitoring to guide the volatile setting if you have it. If you, in the volatile setting will need to be higher than on the vaporizer then you would use normally in high flow anaesthesia. So at a low fresh gas flow, you'll need a higher volatile percentage on your vaporizer than you would at a higher fresh gas flow.
So in people they end they aim for an end tideland setting agent of 0.7 to 1.3 times max.
So I would suggest having your vaporizer setting at about 1 to 1.5 times max with a fresh gas flow of 0.5 to 1 litres per minute.
So here I've reduced the fresh gas flow of oxygen to half a litre per minute with a vaporizer setting of about 2.3. This is resulting in an inspired isofluorine of 1.5 and then then tidal of 1.2.
So changing depth of anaesthesia is another thing we may need to do during local anaesthesia. If a patient becomes particularly light and we need to quickly deepen the depth deepen the depth of anaesthesia. So there are some options.
You can basically give an inhalation or bolus of anaesthetic agent. By increasing the vaporizer setting and the fresh gas flow for a few minutes, so up to 1.5 to 2 times max on the isoflurane or superflurane on the vaporizer.
So I would increase the fresh grass flow to 2 litres per minute. If it's a bigger dog, you may need to put it a little bit higher. You can dump the reservoir bag contents if needed, and that might help to change the concentration of anaesthetic agents in the whole breathing system faster.
Some people advocate increasing the fresh gas flow to 100 mL per kg per minute, which would be 3 litres per minute for a 30 kilogramme dog. Just to give you a rough idea. So I would just adjust it depending on the patient's size as the minute volume of the dog will influence how quickly the inhaled volatile concentration will change.
And then at the end of anaesthesia, there's a washout phase. So we could use a 100 change back to 100% oxygen if you were using air or nitrous oxide. If you're using nitrous oxide, we should do this for 10 minutes.
100% oxygen at 2 litres per minute for for around 5 minutes. So I just want to give you and show you an example of how the gases get diluted. So here we have a 20 kilogramme dog, and the minute volume of this dog is 200 mL per kg per minute, which comes to 4 litres per minute.
So at the washing phase, the fresh gas flows 2 litres per minute, and the maintenance phase we're going down to 0.5 litre per minute. So just to show the dilution effect, if you put as a ratio the minute volume to the fresh gas flow rate in the washing phase, the minute volume is double the fresh gas flow, so it's 2:1.
When we go down to the maintenance phase, the minute volume is much higher. There's an 8 to 1 ratio. So this is why I change, we can really dilute the gases and our change in anaesthetic depths will be much slower unless we increase the fresh gas flow rate.
So just to mention the option of using a closed circle system. So this is something that we don't do in veterinary practise, but just so you're aware, this would be supplying oxygen at the requirement of 2 to 3 mL per kg per minute. So it's really the minimum that's needed.
And anaesthetic agent and oxygen monitoring is absolutely essential for this. So the fresh gas flow is low, so that the inspired concentrations are very different to the vaporir settings. So gas leaks, agent uptake into tissues, it takes hours to reach the steady state.
And you'd need to include the catnograph volume of 200 mL per kg per minute. It's not performed in veterinary practise. And just to note about nitrous oxide, we need to be aware about hypoxic mixtures if you're using nitrous oxide.
So oxygen is taken up into the body, so with time, the oxygen concentration in the circle will decrease, but nitrous oxide is not taken up into the body. So the relative concentration of nitrous oxide is going to increase. So monitoring of inspired oxygen would be ideal here.
And when there's no monitoring, don't go below a 50/50 mix of nitrous and oxygen. So always make sure there's 50% oxygen. And make sure the flow rate of oxygen is over 20 mL per kg a minute.
But as I said before, we would advise avoiding nitrous oxide due to the environmental effects. So I just wanted to mention about the different patient sizes for low flow. We've seen many different benefits of low flow, but what about cats and small dogs?
So starting with cats, the American Association of Feline Practitioners has published feline anaesthesia guidelines. Those are referenced at the bottom and at the end of the webinar. They recommend the use of non rebreathing systems in all cats, but modern circle systems with lightweight plastic rebreathing valves and minimal dead space can be used safely in cats over 3 kilogrammes.
They highlight the importance of minimising dead space and using paediatric tubing. So I think it's sensible to consider the following when deciding what to use in your cat's experience, the equipment you have. So is the circle low resistance with smooth internal board tubing?
The monitoring you have, so we'll see in a minute. Capnography is very useful to monitor hypoventilation. So a circle could be quite a lot of resistance and increase the work of breathing for a small cat.
And Capnography can help us determine if the animal is coping or not. So the individual patient, the weight, the condition score, any underlying disease would also help decide and the expected length of anaesthesia. So hyperventilation may worsen as anaesthetic time and hypothermia progresses.
So I'd consult the manufacturer's instructions for the circle, so for the circle breathing system that's being used and intend for the intended use that they have designed this for. So Humphrey ADEs are designed to be used without the canister in cats as a lack, but the flow rates are low at 70 to 100 mL per kg per minute. So, personally, you, I would consider using a Humphrey ADE with a sodali canister in cats around maybe over 4 kilogrammes, but I would be monitoring catography and be there to ventilate or take the, can of sodali canister out if it was too much for the cat.
I wouldn't use, the regular burst and semi-disposable circles and cats. So small dogs also are a little bit tricky sometimes to decide what to use. So the Humphrey ADE is designed for less than 7 cats, they say to use without the sodali canister.
Over 7 kilogrammes, they say to use with a soda lime canister. The Burton semi-disposable circle, which we have in the previous slides before the photograph, that's designed for dogs that are 10 kilogrammes or more. And the Burton cyclo flow, there's a photograph here in the top right-hand corner, that's actually designed for animals 7 to 150 kilogrammes.
It has much lower resistance and it's more suited for smaller animals as well. And I've got two pictures here of two different dogs. The one on the left is overweight and, probably around 8 kilogrammes, and the one on the right is much leaner and probably weighs more like 5 kilogrammes.
. And I would be more happy using the circle and the dog on the right because of the condition score. She's not overweight. And I think she probably would cope better with the circle breathing system than the one on the left.
So look at the dog, the weight, the condition score, and the confirmation. All those Things together might help decide for these borderline animals, what's best. If you have capnography, then that's great because it will tell you if the animal is coping or not.
And if you don't, you probably needs to err on the side of caution and use the lower resistance breathing system. So just to quickly mention the catography, so canography gives us a trace of the carbon dioxide that the animal is breathing out. So you get a trace like this, you can see where inspiration and expiration are on the trace, and inspired carbon dioxide is measured at the lowest level of the curve.
And in this particular case, it's 2 and then the end tidal carbon dioxide is the highest peak of the graph. In this case it's 42. So normal entidal carbon dioxide is 35 to 45 millimetres of mercury, and it can go higher than this when the animal is hypoventilating.
So that's particularly, something we want to watch for low low anaesthesia. If the end of carbon dioxide is going high into the 50s, then we'll, we need to be aware that this animal is hyperventilating and we either need to ventilate or look at see if they're borderline. Perhaps the circle breathing system isn't the best, breeding system for them.
Other causes are increased cardiac output and metabolism. And then there's also causes of low and tidal carbon dioxide. So hyperventilation, so the animal is breathing, faster, perhaps panting, is a cause.
But also other causes of decreased cardiac output or metabolism, dilution with a high fresh gas flow in the te piece. If there's a leak, which is a very common cause of low end hydrocarbon dioxide. So if you ever see that, then just check that the endoschial tube cuff hasn't deflated.
And a sudden drop in entidal carbon dioxide is always a worry. It could be disconnection so that the breathing system has come apart from the endochial tube, but other causes a cardiac arrest or a pulmonary thromboembolism. So the cat graph is also useful to help us see when our soli is exhausted.
So the inspired carbon dioxide levels in this case. So we normally don't want the inspired carbon dioxide to be above maybe 4 or 5. It can, it can go up if there's increased dead space in the breathing system.
So, any distance between the incisors of the animal and where the fresh gas flow and, splits from the exhaled gas. So the Y piece, for example, in the circle, that's all dead space. So soda lime has a dye which changes colour.
So this one is white soda lime that's changed purple. You can see in the picture here. So you'd need to be, it's important to be familiar with what the colour changes for you particularly so soda line.
And you can see here on the left hand trace, the FI, which is the inspired carbon dioxide, went up to 10. You can see it's not going down to the baseline. And so we changed the soda line canister and then you can see on the right-hand side, the FICO2 has gone down to 3.
So that's a sign that the car, the solar line was exhausted. If you don't have catography, it's sensible to change the so line when about 2/3 of the canister colour has changed. The problem also with exhausted solar line is that once it, isn't used after it's been used on a patient and it's exhausted, the colour will revert back to the original colour, and you might think that the soda lime is OK.
So it's important to change it as soon as practical after the case if you've seen that it's changing colour. So I just got a slide here about how we could reduce oxygen use generally in practise, not just low-flow anaesthesia. .
First of all, we want to minimise the time under anaesthesia. So just have a think about is the anaesthetic really needed and could we do any part of the procedure under sedation? So for example, if the animal needed some X-rays, is it safe to do the X-rays the day before or under sedation, before the anaesthetic?
And look at efficiency in the workflow. So make sure everything's ready for your case. Make sure every people are ready and available so that we can minimise the time the animals under anaesthesia.
It's beneficial in terms of the physiology of the patient, but also saves a lot of money, . For the owner and the practise. And then secondly, use lowerlates flow rates of oxygen.
So firstly, think about pre-oxygenation. The best and most effective way, if you need to pre-oxygenate an animal is to use a face mask for 3 minutes. Flow by oxygen, may make a small difference, maybe up to an inspired oxygen concentration of 30%.
But it also can be quite wasteful. So, just, consider, does the patient need pre-oxygenation? And if so, if the animal will tolerate it, a face mask is the most efficient way to do that.
Some anaesthetic machines have an on-off switch for the oxygen, so only make sure that's switched off after the patient, so that there's no residual oxygen being vented into the environment and wasted, wasted. Make sure you switch the oxygen on only when the animal is connected to the patient. Other drugs such as alpha 2 agonists, infusions, and regional anaesthesia can reduce how much volatile antstic agent is needed, and that can also help to reduce the oxygen use because there may be less times the animal will become light under anaesthesia.
You could include air in the fresh gases if you have access to that. If you use ventilators, lots of these are powered by oxygen, so reducing your use of ventilators, if possible, could reduce your oxygen use. And then as we've been talking about low fly anaesthesia can make a big difference in reducing oxygen use.
So in conclusion, we've gone through some benefits of low fly anaesthesia, and overall these outweigh the risks and the potential problems. A lot of using low fly anaesthesia is just getting used to it and experience, and after, maybe after a few days, weeks, you'll probably be a lot more comfortable using low fly anaesthesia. There is an urgency to minimise environmental pollution, and we've seen that se fluorine is the least harmful volatile for the environment.
And using low flow anaesthesia makes Cfluorine a lot more cost effective option. I would advise advise, avoiding nitrous oxide. There's not enough benefit and the, and the environmental risks, outweigh any potential benefit.
So you need, we will need some investment in equipment and training initially. But hopefully the long term benefits will in cost and the environment will outweigh this. So just consider that some supplies may be limited during the COVID-19 crisis, so consider all workflow methods to reduce oxygen use as well.
So here's two slides of the references of the studies and things that we've talked about in the slides. And if you have any questions, I'd be happy to answer them. This is my email address.