Good afternoon everyone. I've been asked to do a webinar on anaesthetic CRIs and kind of incorporate some calculations as well. As nurses, we often have the responsibility of making these CRIs or calculating them, getting them ready and delivering them to the patients, so comes with lots of challenges and in a busy environment it could be quite.
Distracting to try and calculate those as well. And if you're not used to doing them, that's you know another added thing that you might worry about. So, so this has become highly requested and I hope to be able to deliver it for you so that you can make use of it in practise.
So what we're going to talk about today, we're going to explore the different drug options that are usually used in CRIs. We're gonna understand those drug properties as well. And we're gonna look at how to use them alongside the inhalational anaesthesia.
Look at a little bit about the setup of equipment you might need in case you're worrying that you might not have the appropriate equipment to run these in our practise. And then the biggie is how to calculate those CRIs, especially when it comes to dilutions, percentages, micrograms per minute, all these things that often kind of are a little bit overwhelming at first when you look at them. So the drugs that we often use in CRIs, this isn't all of them, but obviously, we're just gonna focus on the ones that you would commonly come across in practise and what we would commonly use.
So we've got our opioids, we've got our alpha 2 agonists, we've got ketamine, and we've got lidocaine, and we're gonna go through each of these individually. So opioids, we've got methadone, fentanyl, and butterphenol, these can all be run as CRIs and these are the three most common that you might see in practise. There are lots of other ones like remifentanyl, morphine, etc.
Etc. But these are the most common that you're gonna see in your practise and be able to utilise. So methadone and fentanyl have kind of grouped into one because they're what we call they're a full new agonist.
So they produce mild to moderate sedation, depending on obviously the drug that you choose and the dose that you use. And they're also really max burns, so they allow us to decrease our inhalational agents quite a lot. They have a quick onset, especially fentanyl, it's very, very quick onset and it's also got a very short duration of action.
Whereas methadone is again a a fairly quick onset, but it's got that slightly longer duration of action. We sometimes get respiratory depression. Again, this is dose and drug dependent, and obviously we see this when we're given it under anaesthesia as well, more prominently.
So these are the patients that if we need to give small bonuses of these drugs, we might need to help them by giving them IPPV, etc. We sometimes in our conscious patients see panting and this is usually in our non-painful patients, so if you were to give you kind of your bitch phase and and your dog castrations and things like that. Preemptively, so you might see Pantan.
It also, is because it changes the the thermoregulatory set point in the brain. So it kind of tells the, the dog that it's a little bit hot when it's actually not as well. So, but usually we don't see the panting when the patient is actually in pain.
Fentanyl is also a cough suppressant, just in case that is of use for any patients that you might be doing potentially bronchoscopy with, etc. So it's just an added little bonus there, as is bornol though that is also an effective anti-ussive as well, so we're gonna go through that in a bit. Heart rate decreases again, dose dependently, it doesn't usually affect your cardiac output too much, but it, you will see more of a heart rate decrease when your patients are under anaesthesia if you're given bonuses.
And we're talking about CRIs today, so CRIs we usually use lower doses over a longer period of time and therefore we kind of don't don't see these negative effects as much and that's the idea behind them essentially is that we can use them. At lower doses and not see some of these, effects that we don't tend to like to see. We have a decreased gastric emptying time and an increased gut transit time.
Both of those altogether can lead to constipation. And we know in humans, if you ever get sent home with oromorph then you often get given a laxative to go along as well. Again, because we're using the.
Tend to be for our procedures on the day, and we might use a couple of doses over a couple of days if you've got hospitalised patients, but we're not used to seeing this, given very long term, so sometimes we, we might not see the constipation side. We get meiosis in dogs, so the pupil constricts and and my drysis we actually see in cats, so the pupil dilates. I've question marked urinary retention.
It's reported as an effect, but we don't see this really clinically. Again, the doses that we use, and kind of where we, where we use it and what patients we use it and etc. We don't see this clinically, but it, I do have to mention it.
It basically increases the urethral sphincter tone, that's how it causes the urinary retention, but again, don't think we see that too much clinically with these drugs. And I've put there that methadone comes in 10 mg per mL and fentanyl comes in 50 mcg per mL. And also we, we know that this isn't a good analgesic, but it is a drug that we use on a daily basis where I work because we do lots of imaging cases such as MRI.
We do bronchoscopy again, these things are not painful and as I said before, betools and a really good antiitu, so it can be really, really good for these cases. So we do use this on a very regular basis, as I say, a day in day out, but we use it on our non-painful patients. This is actually classified as what we call a mixed agonist antagonist.
So it's an agonist of the cat receptors, so that's what it targets to be like and that's the the effect that it has. And it's an antagonist at the new receptors, so this can actually be used to antagonise any opioids, form your opioids such as methadone, etc. If you're getting any dysphoria, for instance.
So you can get really low doses of this until you stop seeing those effects. So we know that because it's an agonist of the kappa receptors that unless you're a bird, you don't, we don't have very manyAA receptors in our, in our spinal cord, and that's not essentially what a lot of our pain pathways essentially are, we've got not many receptors there. We've got new receptors as as the mammalian species.
So therefore this is why it is a very poor analgesic in mammals. So they say it produces mild to moderate visceral analgesia. Again, this isn't a drug of choice that we would use in any painful patients or patients that we expect pain in, such as our stitch rays, etc.
Durations are variable between 30 and 120 minutes intravenously. It is a really good sensitive, so this is why we use it on a really regular basis because it does produce really good sensitive effects, especially combined with something like an alpha 2 works really, really well, and then we can obviously decrease our inhalation agent then and it can also help us in the recovery periods as well. It is an effective antiitus as I said before, so it can be used with our patients that are essentially coughing, have got a sensitive cough reflex that you might be investigating.
And we do have minimal effects on the cardiovascular and respiratory system, so this is, essentially another really good reason why we use that, so for our heart disease patients that are undergoing imaging, etc. Things that aren't painful, we often use these as well. So we can also see diuresis.
I've just popped in there. It does decrease what we call ADH, ADH decretion going back to our physiology, and the ADH obviously promotes . Essentially, sorry, water reabsorption in the kidney tubules.
So if we've got a lack of that, then you can get diuresis. So it's all I've just mentioned it in case you've got any endocrine patients that you're using it on, you might need to watch your urine output essentially and give them a larger volume of fluids to help them. Because as I say, it can cause diuresis.
So in our volume depleted patients or patients that can't concentrate their urine very well anyway, then this isn't always a great drug to use, but we wouldn't necessarily avoid it, we just help them along with intravenous fluid therapy. Alpha 2 agonist, this is something again we use on a daily basis and can be used as a CRI as well. They cause profound sedation, muscle relaxation, and analgesia, so they're great and they cause significant drug sparing effects, so we can decrease our inhalation agent anywhere up to 70%.
Again, it's all dose dependent and patient dependent dependent. And it works really, really well with other drugs. So there's great synergies with other drugs, as I say, when you combine it with an opioid, for painful procedures like like methadone, or if you're combining it with borphol essentially for sedation, they work really, really well together and you can often get away then with, with less dose of alpha 2 as opposed to if you were to just use it on its own.
You do get a peripheral vasoconstriction. This is something that concerns people because you get the pale mucous membranes. All this does is it basically narrows our blood vessels, and especially in the periphery, and what that does is that actually causes an increase in our arterial blood pressure.
So we're not too concerned about this, as I say, with our cardiovascular stabled patients, we, we don't worry about this too much. And the bradycardia that you get as well is a reflex bradycardia. So in response to the increase in RTO blood pressure, you get a reflex bradycardia, and that is a normal side effects of the drug.
And it's a protective reflex, so we don't want the blood pressure to be really high and the heart rate to be really high as well, because that would actually be more detrimental. You can see Brad's arrhythmias as well, so you can see some ECG changes, you can see the odd BPC, you can see second degree IV block, etc. Again, in, in a cardiac stable patients or a cardiovascular, should I say, stable patients, so not something that is hypovolemic in shock or got severe cardiac disease.
Again, we don't tend to worry as long as we're happy with everything else on our patients. It does decrease cardiac output, but again, we don't worry too much in our in a cardiovascular stable patients, and they've shown that oxygen delivery to the tissues in these and as I say, the normal stable patients isn't affected. We can get an increase in urine output again, similarly to the renal with the ADH, lack of secretion, and then we can also have increased glucose levels from a decrease in our insulin secretion as well.
So again just to be aware of that, obviously again you can promote diuresis with these and obviously the glucose can increase. You can get a decreased hepatic blood flow again just from the slight vaso constriction there but again if we've got any hepatic disease patients, we sometimes avoid them or we just use slightly lower doses. And they say it's neuroprotective because it causes that vasoconstriction with essentially decreasing slightly the blood flow to the brain so we can decrease our intracranial pressure if there is a problem there.
However, we know that alpha 2s, especially when given as a pre-med or on its own, can potentially, and more so in cats, produce vomiting, and that would increase our intracranial pressure. So sometimes we give really low doses to our intracranial pressure patients as a pre-med, or sometimes we anaesthetize them. And then give them a small bolus of of alpha 2 just to help decrease our inhalation agents that's your body.
It does depress the thermoregulation centre, but because we get that vaseo constriction, especially the peripheral vaso constriction, it does actually help preserve heat loss, so. It's actually better at maintaining temperature than, say, using something like aromazine. So there are all of those, drugs and the effects, but as I say, when we run our CRIs, we're usually running them at much lower doses, and we often can get away with some of these effects.
Ketamine, so this usually comes at 100 mg per mL and it has a rapid onset, a dose dependent alteration in the CNS activity and a rapid recovery as well. It's an NMDA receptor agonist, sorry, and the receptor antagonist, that should be, that's my mistake. So this is part of our pain pathway and so it essentially causes profound analgesia, so it's great for analgesia as well.
And there are also reports that it has some opioid receptor action as well. It's significantly max bearing again dose dependently, so the higher dose we use, the more max bearing it will be. But with our CRIs we're usually using subclinical doses, but again it's still very max bearing because it is producing really good analgesia and as I say, it is essentially a disassociative disassociative anaesthetic so we can often get away with much lower doses of our inhalational agent.
It's a sympathomimetic, which is a big posh word for basically stimulating the sympathetic nervous system, so it'll increase your heart rate and your blood pressure. So this is something that we want to avoid in our cats with HCM, so hypertrophic cardiomyopathy, just because that heart's already working really hard as it is, so we don't want to ask it to work any harder because we can essentially push that patient into cardiac failure or a severe life threatening arrhythmia. Ketamine is a bronchodilator and it maintains laryngeal muscle tone, so it can be really useful for certain patients.
You might be thinking off the top of your head, like something like our BAS patients, so it can help. However, it does increase saliva production. So again, trying to just weigh that all up.
It's not a reason not to use it in our patients, but sometimes if we're. Increasing our salivary production, we can get more mucus, forming and obviously that can affect the airway just as much as obviously everything else that's going on with them. Does dilate your pupil and centralises your eyes, so unfortunately that kind of reflex that you might look at with monitoring your anaesthesia.
You're no longer able to kind of use your eye position and and and pupil position and size to monitor your depth. Goes on a little bit this one ketamine has lots of effects, so increases your internal pressure se risk, we tend to avoid these in our, in our brain cases. It does increase intraocular pressure for any of you that do eyes, but not when it's combined with other drugs.
So it's only if you were to use it solely on its own, which we would never do. We tend to combine it with an alpha 2 or etc. Just be careful with repeat dosing and CRIs over time again.
Usually we don't see any effects in that 24 hour window, but as we continue on past 24 hours, the metabolite norketamine is active and, and occasionally you start to see some dysphoria sort of 24 to 48 hours later if they are on a CRI maybe if it's an inpatient, that's quite painful. It can have negative inotropic effects, so, decrease the contractility of the heart, and you can see ventricular arrhythms, but this is at really high doses. We don't see this.
We tend to use really subclinical doses and as I say, CRIs are even essentially more subclinical, so we, we tend to only see these, these are at the really high doses that you probably won't be using, but again, it's just worth mentioning. You might see what we call anapionistic breathing pattern, so, and respiratory depression, respiratory depression you'll probably definitely see with small boluss, especially during anaesthesia. I mean they might need just a little bit of IPPV or mechanical ventilation to help with that.
The Anicu breathing pattern you might have seen with your cat cat neuters, so if you're giving them a triple combination, for instance. And what you'll see is they kind of take a few breaths and then hold the inspiration for a few seconds and then do it again. You might kind of realise that, oh yeah, I do remember seeing that and that's just, again, it's an effect of ketamine, but again, this is usually that kind of triple combination that you give cats it's usually got a quite a high dose of ketamine in there.
So again, we don't tend to see this with our CRIs. Does cross the placenta, so we don't use this in our caesarean sections because it's also associated with what we call the worst APA scores. So this is just a scoring system that they use in puppies to assess the reflexes and essentially all those kind of these scores essentially lead to whether the mortality rate, it's linked to the mortality rates basically.
So ketamine is actually one of the worst drugs that we can give in our caesarean sections, so we don't want to do that. And it does cause hypercisibility and sensitivity and recovery, again, this is again something that we see with those slightly higher doses in those maybe triple combinations. We don't tend to see this as much with our CRIs because we're using quite low doses.
Lidocaine, you can give us a CRI as well, so we usually comes in in 1% or 2% vials that we have. I must say dogs only, so there's a, it's not that it absolutely can't be given to cats, but there's such a small safety margin that it's, it's not worth it, essentially, so there's only very, very, very small occasions that we would give a small bow as a lidocaine to a cat. If it was in a severe ventricular rhythm, that we couldn't give another drug so it wasn't responding to another drug, but stay well away for our CRIs, especially for analgesic purposes for cats.
It works by blocking sodium channels, and this is how it works for all of the uses that we that we use it for, so an antiarrhythmics, so it blocks the sodium channels in the heart. It also is a free radical scavenger, so it mops up some free radicals, so it can be great for like GDBs and things and again GDBs have often have arrhythmias, so it's kind of doubly good for those. We use it for local anaesthesia and obviously we're gonna talk about the intravenous route for our CRIs, so.
It's really good basically for, especially for neuropathic pain, that's the main one that's been studied. It is significantly max-bearing, so we can decrease our inhalation agent, which is what we want to do because that is the baddie. Our inhalation agents will decrease contractility and cause vasodilation and both of which will drop our blood pressure and it's not gonna help us to perfuse our organs very well.
So this is essentially why we use CRIs, so that we can reduce our inhalational agents. I sometimes get anorexia and nausea. This is sometimes heavily debated between clinicians, etc.
That they don't really want to see any anorexia or nausea in their patients, which usually at the higher end of the dosing and it's rarely quickly. Eliminated from the body, so the effects, so you're looking at 20 to 30 minutes once you've stopped the CRI that the anorexia and the nausea would stop. So it's not usually a reason to, to not give a CRI of lidocaine if they've, especially if we've tried the opioids and maybe we've tried ketamine, etc.
And they're not really responding, then there's no reason to not try it and lidocaine CRI and then we can always lower the dose or stop it if it, if this nausea and anorexia is really severe. So midazolam, midazolam we can use as a CRI just as I've mentioned, as well, something that you might not think about and the other one is Bopol you might not have thought about that to give that as a CRI. The midazolam causes sedation and greats anti-anxiety medication as well and causes muscle relaxation quite profoundly.
It's got no analgesic effects, unless it's associated with the muscle spasm. And one thing just to touch on, oh actually I have mentioned it a little further down, I thought I hadn't, don't worry. So there is some max bearing effects only of around 20%, but again, something is better than nothing.
We've got minimal effects on the cardiovascular system, which is great because that's these are the drugs that we want to potentially use in our, in our heart disease patients, etc. We know that there's no analgesia, so that's the only thing I'd say is that we know that this is an an analgesic therapy, but we're not just talking about analgesia, CRIs here, it's all anaesthesia CRIs, so I just sort of pop this in. All the other ones do have analgesia properties other than, as I say, bornol which we mentioned, but these can be really good for our cardiovascular unstable patients.
You will get some respiratory depression and for that reason, which is what I was gonna touch on, is we want to avoid these and severe respiratory disease patients or any neuropathy patients, so something like myasthenia gravis, for example. They've already got muscle weakness and they are struggling sometimes, well with severe myasthenia gravis is struggling as it is to ventilate because obviously it requires your intercostal muscles to work, etc. So this can, this can sort of make that animal decompensate a little bit, so just be really careful with respiratory patients and neuropathy patients.
It's an anti-convulsant, so that's great if we've got a seizure risk patients. And again, it's controversial in liver disease patients and especially portosystemic shunts, we tend to avoid it in portosystemic shunts. So continuous rate infusions your eyes.
So the reason why we use them is because we're we're using them over a period of time to ensure the medication achieves and sustains therapeutic concentrations whilst avoiding those peaks and troughs, essentially, so those high peak concentrations and those side effects that we talked about. So we're trying to avoid this and essentially just ensuring that there's therapeutic concentrations within the plasma. And essentially that allows us to decrease our inhalation agent rate and it provides analgesia to that patient or makes their anaesthetic more stable, then this is great.
That is what we use them for. So it allows our inhalational agent to be decreased, it's additional analgesia we're coming from a multimodal approach, so we're targeting different pain pathways. You get a more stable plane of anaesthesia throughout, you definitely get that.
And it does allow titration, so you can titrate up and down. You've got dose ranges, which I'm not gonna give you because, well, one, I am a nurse, but also even the anaesthetists that I work with don't tend to like to give set doses out as such. However, I have had to go through a lot of calculations and I have used drug dosages that we might commonly use in those .
In those CRIs in order for us to, to help you to work out, work them out. So I do, I have used drug dosages in this lecture, but it's, it's, as I say, it's for examples for calculations. But you can titrate them up and down the dose range and you can give small bolus.
Because we should be providing, if there is no Cception, which is a posh word for pain, we shouldn't be increasing our inhalation agent, we should be using. A, a pain medication, an analgesic, medication to treat that, not increase our inhalation agents. We know that, inhalation agents don't provide any analgesia, and all it's going to do is blunt that response.
So yes, you will see the patient essentially go a bit deeper and just your rate, decrease and heart rate decrease, etc. But usually that's because of the negative effects of that drug, so that we know that isofluorine and fluorine, they are still drugs. And we are given essentially more when as soon as we increase our levels we're given more of that drug and therefore we're gonna see more of the side effects.
And they are, they do have some of the worst side effects in our anaesthesia so. We should be using analgesic therapies to respond to the patient's painful, and when you're using CRIs, you can do this. You can give small bolus, you can have them drawn up separately if you want to, or you can give them via your syringe driver or your infusion pump.
It does help you in recovery as well, and then all I'd say is when you are using them, they are essentially another drug that you are given in an anaesthetic, so you need to keep a really close eye on your depth and adjust it accordingly, because often you can, as I say, decrease your inhalation ageing quite significantly, maybe something lower to what you're usually used to, so it can be a little take a little bit of getting used to if you're not using CRIs already in practise. The equipment that you need, you don't need posh equipment to do this as I said earlier, this can be done with just a normal given set. A normal bag of Harman's, you don't even have to have I've put in this picture a 100 mL bag of saline, which I'm sure you have in practise, most of you anyway, but we're gonna go through a few examples where you don't need a 100 mL of saline, you can use a 500 mL bag of Hos if you need to.
But you do need an infusion pump. This isn't something we should be working out in drops per minute anymore. We should be using an infusion pump minimum.
We need a given set. We need the crystalline solution that you're going to use, calculator and your drug, and then a needle and syringe to draw up your drug accurately. If you're using a syringe driver, again, it's pretty much the same, except you're gonna need an extension line as well and the needle and syringe that you're gonna pop into the syringe driver along with your needle and syringe that you need for your drug.
And then you will need saline for dilution as well. So, the maths bit, I apologise in advance, this is quite a bit of maths, but, hopefully you, you will get a lot out of it, and we're gonna go through it together, so get your phones ready with your calculator on if you've not got one to hand. So Matt, just a quick recap, 1 gramme is equal to 1000 milligrammes.
1 milligramme is equal to 1000 mcg. And then examples of this is 0.1 milligrammes is 100 mcg and how we work that out is we do 0.1 times 1000.
And that's how we come to our 100 mcg. And then, for instance, 10 mcg is equivalent to 0.01 mg.
So if you ever see 10 mcg, for instance, and you're like, oh, I'm not really sure what that is in milligrammes for definite, just divide it by 1000 and it will give you the correct milligrammes. Percentages I just wanted to touch on because as I said, we've mentioned lidocaine for an example and it's something and you might be using glucose, etc. Etc.
Percentages mean out of 100, and it's agents with a dose listed as a percentage represents that represents grammes per 100 mL. So a 5% solution has 5 grammes per 100 mL, which is the equivalent to 5000 milligrammes in 100 mL. So when we want to work out how much it is in migs per mL, we do 5000 divided by 100 and that will give us 50 migs per mL.
But just for ease, just move the decimal place or times it by 10. So 10% is 100 mix per mL. 2% is 20 mg per mL, and if you get something like pivacaine which is 0.5% or 0.25%, that's 2.5 mg per mL.
So, Another quick slide is to how to work out what Mgpamil is your new solution. So say you've put a certain amount of drug in a certain amount of saline, and you think, I don't know now what this Mgpamil is of this solution. All you need to do is the volume of drug that you've put in.
You divide that by the volume of what you've put it into, so 0.1 mLs into 10 mLs for instance, times that by the original mg per mL that it was, and that will give you your new Migs per mL. So an example is Dexammethotomidine is 0.5 mg per mL.
So if I was to put 0.3 mLs of Dexedotomidine into 10 mLs of saline, so if you do this on your calculator now. And I'll do it with you.
We put, we do 0.3. Divided by 10.
You should get 0.03, and we're gonna time it by 0.5 because that is what dexamotamidine is.
So your new solution is 0.015 Migs for nil. So that's how you work out what your new mygamil is.
Now, when we make up the solution, we always remove the volume of drug, to be added from the total volume of the solution. So for instance, in this example, it would be 9.7 mLs of saline, but when we're working out the actual what it is, we always use the volume of which the total volume will be, if that makes sense.
So we wouldn't. We wouldn't do 0.3 mLs of dexamed divided by 9.7 mLs of saline, if that makes sense, but when we're actually making it up, you need to make it so the, the whole volume is, is just 10 mLs, not 10.3.
So, dilution, so it looks a little bit more complicated, but hopefully you're still with me on this, so I'm giving you a bit of a formula to take away this will work with every CRI that you need to do to work out and we're just gonna use a few examples to go through this, so. You might want to take a quick picture as well. So the CRI rate at the top, so if your vet gives you a CRI rate, it, you need to have it in kg per hour, so maybe they're going to give you it in microgram per kilogramme per hour or microgram per kilogramme per minute.
You need to convert it into mg per kg per hour first in order to be able to use this formula. You've got your CRI rate of gig per hour, and then what you're gonna do is you divide it by the fluid rate at what you want to deliver to that patient. So you might want to give it a maintenance rate of 2 mL per kg per hour, and this is where you can use a 500 mL of heart means and you therefore can think, right, OK, I'm gonna run a ketamine CRI and I'm gonna.
Deliver a surgical fluid rate of 4 mg per hour throughout its anaesthetic at the same time as it's ketamine CRI. So you can, you can do that just all in the one bag of hormones, OK. So you pick your fluid rate in mL per kg per hour.
And then whatever dilute and volume you want to use, so maybe you're gonna use a 100 mL bag of saline if you want to just do a CRI alongside your fluids, or maybe, as I say, you want to give a 500 mL bag of Hartmans, and use this as a total volume all in one, your surgical fluids and, and your CRI all in one. That will give you the mix that you need to add to that solution that you've picked, so 50 mil of Hetmans, for instance, and all you need to do is divide it by the mg per mL of of the drug. So, and then that will give you the amount of mils that you need to pop in.
Might sound complicated, but we're gonna go through it now again, get your phones or your calculator ready. So the 12 kilogramme dog needs a ketamine CRI and it needs it at 0.6 MB per kg per hour.
And we would like to run it at 4 mL per kg per hour for our surgical fluids, so we're getting both in one. So we're gonna run it in a 50 like of Hartman's. So in order to do this, so we're gonna do, you do this on your calculator now, 0.6 times 12, we know it's 7.2, OK.
And if we do, so remember 7.2, because we need to do this separately. So 4 times 12 is 48.
So we do 7.2 divided by 48, and that should give us, you should have 0.15 on your calculator.
I didn't want to put this on because it was too many numbers. Times it by 500. Because that's the amount of hormones, that's the size bag we want to use, and that should give you 75, that's 75 milligrammes of ketamine.
We know ketamine is 100 megs per mL, so 75 divided by 100. So it needs 0.75 mLs into 500 mLs basically.
So into that solution. So take 0.75 mLs out of 500 mil by Hartman's put 0.75 mL of ketamine.
Run it at 48 ms an hour and you will deliver 0.6 MK per hour to that patient. The couple of things that I wanted to mention is obviously if you do that, that's absolutely fine.
You just can't give bolus of, you know, if you need to give a bolus because they've got low blood pressure or they're bleeding, etc. You might just need to pop, you'll need to pop another bag on that patients and give the boss with just normal hormones. But yeah, so I just thought I'd mentioned that.
But now you're probably thinking, well, that's fine, but I just, I need to know now what mg per mL that solution is. So if we go back to what I said earlier, if you want to know what mg per mil this new new solution is, we, we've put 0.75 mLs and we've put it into 500, so 0.75 divided by 500.
Should give you 0.0015 on your calculator, and the original Mgfamil solution of ketamine was 100, so times it by 100. So your new Mpamil solution, so that 500 mL bag of Hartman's is now 0.15 mg per mL of ketamine.
So. If you're, you're used to obviously calculating, which we all are as nurses, make per kg and then we divide it by MG per mL. If you want to triple check your calculation now using just this Migs per mL, if we so if we do 0.6, because we want to run this CRI at 0.6 times it by 12, so we know it's 7.2, and then we have to, if we divide this just by our 0.15.
Makes for milk because that's the new solution, you should see that it's 48, so 48 m an hour, so it's just to kind of do it the opposite way just to check your calculation. Hopefully that makes sense. We're now gonna go through a few examples.
Ketamine, you know, ketamine's 100 mg per mL, and we're gonna have a 20 kilogramme dog. It needs 0.5 mg per kg, bowless first, and then we're gonna follow it by 10 mcg per kilogramme per minute.
We're just gonna ignore the half of the kick bolus because I know that you're all quite capable of doing that. So we just ignore that, we're gonna have that drawn up separately, but we're just gonna focus on the CRI of 10 mcg per kilogramme per minute. So, 10 mcg.
OK, so we're already not able to use, as I say, that formula from before and you don't have to use it. That is just, as I say, one of those, formulas that you can pop in, whatever you want to deliver, essentially, and it will do it all for you. But in order to use that formula, or to be honest, just to, as I say, understand, a little bit better, if we push this into Migs per hour, it will be much easier because we've got ketamine and migs per mL.
So you can't work out micrograms and then divide it by milligrammes when you can use your calculations you need it all to be in the same units. So in order to do this, we're just going to do 10 times 60, so 10 mcg times 60 minutes. This patient needs 600 mcg per kilogramme per hour, so we've got our hourly rate.
However, I want to know what it makes is. So we do 600 and we can divide that by 1000, and that should give you 0.6 migs an hour.
So this patient needs 0.6 mg per kilogramme per hour, and that is exactly the same as 10 mcg per kilogramme per minute. So now we've got that, we can do 0.6 times 20, and you should get 12.
And then we're gonna divide it by 100 because we know ketamines 100. Now that equals 0.12 mLs an hour, which is a very small volume, especially given that this is a 20 kilogramme dog.
So you can imagine 10 kilogrammes or 5 kilogramme cat, etc. Gonna need a very, very small volume and even the best syringe drivers, it would be quite unreliable to give this over an hour period. So what we tend to do just to make it easier, because ketamine is an easy solution, 100, we just dilute ours in a 100 mL bag of saline, so we remove a mL of saline, put 1 mL of ketamine in.
So we're essentially putting 100 mg of ketamine into 100 mLs of saline. Using that little formula that I gave you before, we'll do we'll put in 1 mL of saline, so 11 mL of ketamine, so we're putting 1 divided by 100 because we're putting it into 100 mLs of saline and the times and by the original mix of the mL it is, so times 100, gives you 1. I'm not gonna go through the whole calculation again, we know if we just skipped it, it needs 12 Migs an hour.
12 divided by 1, because now that's our new solution, 12 mLs an hour. So that's what we tend to use, we tend to make it up to a 1 M. Per meal solution and it is just a lot easier to run, especially with those smaller patients.
You can, as I say, put that into that formula I gave you as well and it will work out exactly the same, but you just have to change the micrograms into migs and the minutes into hours, so. Dexametotamazine is a little bit more tricky because it's usually a very small small bolus and then followed by a very low dose CRI and obviously it's also only 0.5 per mL, so sometimes this can come up again really low volumes and we need to dilute.
So our dexamedatominine CRI then, so sorry, just to go back, . 10 kilogramme dog, we're gonna ignore the bolus like we did before and we're just gonna focus on the CRI so it needs 1 mcg per kilogramme per hour. So this is in micrograms and we want to get it to milligrammes, so we do 1 divided by 1000 if you've got your calculators, and that should be 0.001, Ms per kilogramme per hour.
We then know that this dog is 10 kg, so we times that by 10, and this dog needs 0.01 Migs an hour. We then divide this by the solution that Dexedotomidine is, which is 0.5 mg per mL, and that is 0.02 mLs an hour, which is a really, really small amount, and it's, it's not, it's not doable.
So we need to dilute this. So we're going to use that formula it's making a reappearance, like we did before. So our CRI rate, so we've got it now in the milligramme per kilogramme per hour.
And the fluid rate, I've just chosen as an example that we want to run this at 5 mil an hour, so we're gonna run this not in our fluid bag again, our foundrymil Harman's I've chosen to show you one that's now gonna run alongside, if you like. So just using a 100 mil bag of sealing. So I've just chosen, I want to run this a 5 mil an hour.
Now the reason why I've just plucked a number out of thin air almost is because. Don't forget when you're running lots of CRIs all at once, or maybe you're just running one and then fluids, just bearing in mind the total volume that's going into that patient per hour, so you don't want to volume overload that patient. So I've chosen 5 mLs an hour, this is a 10 kilogramme dog.
So it's like a quarter maintenance. So it's a really safe amount to give and it's, but it's also a sensible amount. So you can just pick the mils an hour.
You don't have to pick a certain mL per kg per hour. You can just pick, oh, I want to run this at 2 mil an hour for the cat or 3 mil an hour, whatever. So I've chosen 5 mil an hour for this patient.
So. 0.01 because it needs 0.01 makes an hour.
I'm not, I've just skipped out the make the cake we've already done that, so we don't need to do that again. And I've picked 5 mL an hour, so we divide it by 5. Hopefully on your calculators you will get 0.002.
And we're going to times that by 100 because I want to put it in a 100 mL bag of saline. And we need 0.2 migs of dexamedotomidine.
Now we know dexamethasomidine is 0.5 mg per mL, so we need to divide that by 0.5, and it's gonna give us 0.4 mils to add.
So take 0.4 mils out of your saline bag, put 0.4 mLs of dexamettatoidine in your saline, and run it at 5 mil an hour, and therefore you will deliver 1 mcg per kilogramme per hour.
I've also chosen that 5 mL an amount, one because it's a sensible dose, but two, if I need to increase this to 2 mcg per kilogramme per hour, which again is within that dose range. Then again, moving up to 10 ml an hour, which would be the total dose, again, is only going to half maintenance. So again, it's another, it's another safe volume that we can give.
So I've not got to worry. Oh gosh, you know, I've got to lower my fluids, etc. Etc.
So again, that is the other reason why I just picked that as well. That's all the maths for now. I think you've got hopefully enough examples to go away with and kind of you can play with that formula, go away and do do some calculations, examples, etc.
Until you're comfortable. Some general rules, or CRIs require a loading dose before starting, so we need to get the plasma concentrations up to a certain level, to a therapeutic level, in order to be able to just maintain that. And then it can, you can do this via a pre-med or an induction, so sometimes we induce with ketamine and propofol together.
Or we might give a dexamine pre-meds and then follow it on with the CRI. Or sometimes if we've not maybe expected that we need a CRI, maybe our local block hasn't worked and the patient is responding with pain during our anaesthetic, then we might need to start off with a bolus in theatre. So we give a small bolus and then we start the CRI.
Utilise those dose ranges, I'd say to look in an anaesthesia textbook or the papers, anaesthesia papers because they're often a lot lower than what's in the formulary. If you need to increase the CRI, then you, you often say, for instance, that 1 mcg per kilogramme per hour dexamedotomidine wasn't quite enough for that patient, or, the fentanyl CRI that you're using, fentanyl's got a really wide therapeutic range that you can use. If you need to increase it because maybe the patient is responding.
With a painful response, then you often do need to give a little small bolus to increase the plasma concentrations because if you were to say, say you were using fentanyl at a 5 mcg. Per kilogramme per hour, and you thought, right, OK, we need to go up, we might need to go up to 7 or 8, then it's such a small, volume over that hour that it won't really make that much difference to the patients whilst it's reacting. So you often need to give a small bolus and then increase your CRIs just to let you know that as well.
And finally, just the top tips, ensure that you're using the same units in the calculation, so start converting minutes to hours and and micrograms to milligrammes just to help with that formula really, and to help you. Fentanyl, don't forget, is 50 mcg per mL, so either convey the fentanyl. Intermix to help you divide it by your mix the milk or do all your calculation throughout that in micrograms, but keep the units the same.
Ensure no one disturbs you, so you should have no one disturbing you while you're doing calculations, and this is just no one's doing it on purpose, but in human hospitals, they often walk around with a red, red like tabard thing on, just to say that I'm giving medication out and don't disturb me. I know that's maybe going. A little bit far for our, for our patients, it could be something that you definitely use but try and sort of have it as a general rule that if you can see someone with a calculator, don't disturb them.
Have a calculation check policy in place, so even with us if I calculate a drug I get someone else to check. It doesn't matter if it's a vet or a nurse and our anaesthetist will ask me to check this, etc. It's just a double checking system we just signed to say we've checked the calculation and it's all correct because everyone makes mistakes.
Work out what volume a bolus would be. So what, you can either have it drawn up separately if you do need to give a bolus, if you're using maybe a slightly older fashioned infusion pump, you're not really confident about giving a bolus from that, or so you can have it drawn up separately or work it out as a volume just on your, on your anaesthetic record before you start off if you do need to give that small bolus because it is reacting a little bit, then you know exactly how much and you're not trying to do that as the patient's reacting. And you might make a mistake, so have that already either drawn up and calculated and written down.
And you can if you You know, if you, if you're struggling to kind of Get these into your practise maybe if there's a cost issue or etc. Etc. You can use these kind of, as I say, 100 mL bags of saline with potentially ketamine in you can use that, it's not amazing practise, but you can use it for more than one patient providing you as you've got good aseptic technique handling.
And you've got, you change a line so you can get like little teapot connections between each patient, essentially, so there's something always going in between the patient and that set that is fresh and clean and sterile, and you can use that for about 24 hours and then it starts to salt out and dilute and essentially lose its potency. So I'd say definitely for only 24 hours or just for that day of operating. So that is an idea that you can get in if you're struggling to kind of argue your case to use these.
And thank you very much. I hope that has helped you with some maths and I am, again, apologies for it being not as exciting and there's a, there's a lot of a lot of stuff to take in, but hopefully you've come away with something. Thank you.
