CCP Podcast 090: Mechanical Ventilation with Thomas

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I'm still struggling to get my head around some of the things mechanical ventilators do. I need help. Thomas is here to provide it. Let's go.

Hello, hello, hello, and welcome back to the Critical Care Practitioner podcast. I've got a special one for you today, and hopefully this is one of a series. If you remember back in the past, if you go back to some of my earlier podcasts, we had some about mechanical ventilation. I think it's time we revisited that subject.

So I've got Thomas Pereno with me, who's a respiratory therapist who has kindly come along to talk to us about mechanical ventilation. So we're going to start with some of the basics and I think we're going to start talking about some of the more common modes and how we differentiate between them.

And then hopefully as the series goes on, we can get into a bit more detail. So Thomas, why don't you just introduce yourself first, let the audience know who you are and where you're from, and then we can crack on, I think.

Great, thank you, Jonathan. Yeah, so my name is Thomas Prano. I work at St. Michael's Hospital in Toronto, Ontario, Canada, and my title there is Clinical Specialist for Mechanical Ventilation for the Centre of Excellence in Mechanical Ventilation. It's a long title, but basically clinical specialist for mechanical ventilation. And my role there is actually to work as the bridge between sort of research and clinical practice. So I work with the respiratory therapy department.

as well as with the research department, more specifically in Dr. Laurent Burchard's lab. So if you're reading enough about mechanical ventilation, you've probably come across Dr. Burchard's name. If not, take a quick Google search for Dr. Laurent Burchard and you'll see.

many many publications he's been involved with so I have the pleasure of working with him and his team of researchers that have come from all over the world to practice under him either for a PhD or for postdoctoral fellowship and so yeah so I met you online technically through Twitter yeah and you had sort of put a call out to have someone discuss mechanical ventilation and I like to talk so this is a that's where that's where we end up now

OK, brilliant. All right, Thomas, I don't think we need any further ado. Let's get on with it. If you want to make a start and we'll hopefully get some pearls of wisdom from you and help both myself and my audience understand mechanical ventilation a little better. All right. So.

to first start i mean if someone has stood in front of a few different ventilators they've probably realized that there seem to be a number of modes and in truth there are there are probably over at least 100 named modes But when it comes down to it, many companies name the same mode something different. They try to stand out. They say, well, we have this mode. And if you don't understand how the mode functions, you may not realize that they're actually talking about the same mode.

So if you really want an in-depth look at the taxonomy for mechanical ventilation, I highly recommend a paper by Rob Chatburn that was published in Respiratory Care in 2014 called A Taxonomy for Mechanical Ventilation, 10 Fundamental Maxims. it's a heavy read but it's really if you want to get the in-depth understanding of why modes should be named a certain way and sort of where we've strayed away from this it's a really good overview

but it's probably a bit deep if you're just learning about modes for the first time. So let's start back at learning modes for the first time. So although there's a number of different modes, they're really just different names of the same thing. So I'm going to discuss probably this first time and hopefully we'll be back on and I'll talk about more complex modes. But let's start with what we would consider conventional modes.

Now, if you only learn three modes in your entire career, they're probably going to be the most commonly seen. And that is what is now called volume assist control. And again, different companies may call it something else. They may call it ACPC or ACVC, but it's, and I'll describe each one of these, but volume assist control, pressure assist control, pressure support.

with CPAP and sometimes called CPAP with or without pressure support and then synchronized intermittent mandatory ventilation and I'm going to throw in pressure regulated volume control as probably another conventional mode meaning it's probably more common than some of the

unconventional modes that i'll discuss on another podcast um but really those those five modes are the ones that sort of if you were to just grasp those five you'd be fine but if the top three just volume assist control pressure is control and pressure support if you can grasp those three modes

then you'll probably understand the other two. So really, we're going to start with those first three. And when it comes down to understanding modes, you have to understand the goal of mechanical ventilation. is to either take over the work of breathing for a patient or to assist with the work of breathing for the patient. And when I say breathing for the patient, now I won't get into work of breathing. Maybe that's another episode with you.

because there's the whole work of breathing equation, the PMUS equation. I'm not going to get that complex on this first one. No, let's not. Yeah, no, let's not. I don't want to put you to sleep this morning. The goal of mechanical ventilation, if you want to think about in general terms, is...

ventilation. So it's movement of air and oxygen in and out of the lungs. Okay. So this is typically referred to in clinical practice as minute ventilation, which is just the amount of tidal volume going out per breath per minute. So Respiratory rate multiplied by the tidal volume going in and out is minute ventilation. And this is generally a representation of CO2 removal. So typically, if it stays constant, you're probably maintaining blood pH. So you're removing the same amount of CO2.

However, if there's perfusion changes in the lung, if you start playing with PEEP, which is positive and expiratory pressure, and you start maybe impacting the perfusion to the lung, or improving perfusion to the lung or sorry ventilation to the lung then you could have changes in co2 removal with a constant minute ventilation but we're just going to keep it simple the goal of mechanical ventilation is to maintain or assist with

ventilation and i'm going to refer to minute ventilation as this goal okay so the next thing to understand and this is maybe where i get your input jonathan do you typically refer to compliance or elastance of the lung in your practice.

I think if anything, we're going to probably talk about compliance. Okay, perfect. So when you read papers, typically from Europe, you'll see a lot of them referring to elastins. But in North America, we all refer to compliance. And I think compliance, it's just... it might be a little bit easier to understand. So I'm going to focus on compliance for this one. So the two things you need to understand to understand modes is the goal, which is ventilation.

how many times per minute you're getting uh tidal volume out so minute ventilation and then compliance okay and then i'm going to talk about each mode and the relationship with compliance so compliance is the change in volume over a change or over a change in pressure okay so what this means is that there's a direct relationship between compliance and

So how much volume you can get in will depend on how much compliant it is. But the pressure required to get that volume into the lung is inversely proportional. So what does that mean? I sound like I'm getting complicated, but this is going to get quite simple. If you have a very stiff lung. you're going to need a lot of pressure to get volume into it. Does that make sense? So if you have very good compliance, you're going to need less pressure to get a set volume into the lung.

So getting volume into the lung, we have a target tidal volume that we try to want to achieve. Now, most patients, most mammals actually breathe on average around 6.3 mils per kilo. So what's interesting, there's a push for using lower and lower volumes in sicker and sicker patients, where six mils is actually the mean for mammals.

I'm not making this up. You can look it up in a nature journal. 6.3 mils per kilo is the mean. Now we vary around this, but basically the goal of ventilation is we try to help patients maintain a normal volume. And then how much pressure is required to assist them with depends on how sick the lung is. So how poor the compliance is. So again, a goal of minute ventilation and then considering compliance. The harder it is to get... air into the lung, the less compliant it is. Absolutely.

Right. OK, because I always liken this when I'm trying to teach this to the balloon analogy. So if you've got a brand new balloon and you blow it up, that takes quite a lot of work. So that is a that's not a very compliant lung. Is that fair to say?

Absolutely. And that's typically my analogy. I say to people, how do you blow up a balloon for the first time? Well, people start stretching it. They pull it. And what they're trying to do is make it more compliant. They're trying to sort of break it apart to make it more compliant. They stretch it. And then when they blow into it.

There's also what's what you could argue is an opening pressure right an amount of pressure you have to apply before it starts to open Yeah, and this is actually the analogy for peep that I'm going to give a little bit later Okay, because peep technically is trying to keep the lung at at least

a minimum opening pressure so that when you blow into it with the ventilator it's already compliant and you're already overcoming that sort of opening pressure yeah because if you allow a lung to completely collapse you're going to need a certain amount of pressure to actually open it

But once the airways are open, then you should be able to ventilate more comfortably, assuming there's not a severe disease process going on. Yeah. So if we go way, way back before ventilators allowed patients to do any interaction.

we had what you called controlled mechanical ventilation and it's still available on some ventilators and it's just called cmv and if you had a patient on cmv this was typically used in an operating room We didn't have ICUs before we had operating rooms, essentially. So when controlled mechanical ventilation was happening, patients were typically not interacting with the ventilator. The ventilator controlled every aspect of the breath and the patient could not interact.

So the three things to understand for every mode, just when you break it down, is how does it start? What is controlled? And when does it stop or when does it cycle off? Okay, so there's the start. What's controlled and how does it cycle off? Those three simple things. With controlled mechanical ventilation, when the breaths start would depend on how fast your respiratory rate is.

you set a respiratory rate of 10 you're gonna get a breath that's gonna start every six seconds okay then you're gonna control something now what's interesting is we talked about pressure control and volume control if it's pressure controlled

then you control the pressure the entire time of the breath and it cycles off the cycle point for controlled mechanical ventilation if you're setting pressure is going to be time so it's going to hold that pressure in for a certain amount of time and then it's going to stop

OK, and it's going to go back down, cycle back down to an end expiratory pressure, which is PEEP. So that's the simplest mode out there. There's no patient interaction. If the patient wants to take a breath, they get nothing. Nothing happens. They just are completely.

pulling on the ventilator nothing happens this is controlled mechanical ventilation okay now if you're controlling volume in that scenario same thing it starts based on your respiratory rate the patient cannot interact with it you're controlling now this is where it gets a little

funky with uh with the modes we call it volume control but you're actually controlling the flow okay you're controlling the flow and then it cycles off once the certain volume that you've set as a target is achieved does that make sense

I'll keep referring to you, Jonathan. Sorry, Tom. I'm muting my microphone. Oh, no problem. There's a pause there. Just explain flow then, would you? Yeah. So... flow set on a ventilator this is actually where it's a little counterintuitive because you typically set a flow in liters per minute on a ventilator and if you think of liters per minute if you set let's say 60 liters per minute that sounds really fast which it might be

But you're going to get 60 liters in the lung over one full minute, which we don't give a breath that lasts a minute. So it's not actually that intuitive, but that's how you set it on the ventilator. But 60 liters per minute is one liter per second. Now that makes sense to people.

because one liter in one second means if you're going to deliver 500 mils it's going to be half a second right so to me liters per second actually makes more sense but if you're setting the ventilator it's always it's typically liters per minute yeah so

Well, if you set 60 liters per minute, it's going to be one liter per second. And if you've told the ventilator to cycle off at 500 mils, it's going to deliver the breath for half a second and then stop because you're going to get your target, which is your volume, and then it's going to cycle off.

now these modes you can typically also set a time so you can tell it to deliver the volume but then pause for 0.2 seconds and then release so there is a timing when you look at a ventilator look at a screen and look at modes you can see there's timing

So that's controlled mechanical ventilation. That's the basics. There's no assisting. And this is not a very commonly used mode, especially in intensive care unit where you hope your patients are waking up soon after mechanical ventilation and beginning to interact.

So this is where we get into the next phase. So then when manufacturers of ventilators move forward, they thought, okay, maybe we should allow the patient to actually interact and trigger the ventilator when they want to breath. And this is where modes like volume assist and pressure assist control came in. So volume assist control, again, it starts. So we're going to talk about it again, start control and cycle. Okay. It starts with either time.

or the patient can initiate the breath. Okay, so what does that mean? Again, if I set a respiratory rate of 10 seconds, I like using 10 because it's easy math, of 10 breaths per minute, sorry, then every six seconds it's going to give a breath. If my patient wants to breathe 20 breaths per minute,

then they can pull on the machine 20 breaths per minute. It doesn't matter. It's going to give them a breath every three seconds because that's what they're demanding. But it's going to control. Now, this time it's a volume assist control. We are going to control the flow, right? And then it's going to cycle off once the volume is delivered. And the volume that's delivered is the volume you do set. So this is why people call it volume misses control, because you actually set the flow.

How fast the breath is delivered depends on what you set as a volume. So you are technically setting the volume, but you're really controlling the flow. And this is why it's important to understand that how quickly you set the flow does impact how long the breath is going to last as related to your volume again because it's liters per minute is what you're setting so as you go faster it's going to deliver that volume much much faster if you go higher on the flow

Is that changing of the flow rate, is that something that, and maybe this is getting ahead of ourselves a little bit, but changing of the flow rate, is that something you would do in preference to changing the volume?

That depends. Again, so typically you set a volume based on the physiology of the patient, right? So if it's a normal patient, so most protocols will set volume between, let's say, six to eight mils per kilo for patients that... do not have ards yeah and if they have ards we may be setting a volume that's six mils or lower like four to six

If we're using some form of extracorporeal CO2 removal, which I know you covered a little while ago with ECMO on your podcast, then if you're assisting with some CO2 removal, you may be even to push those boundaries down to something like three mils per kilo. That's been shown to be feasible.

in study so if you're doing full-on ECMO you could deliver no volume if you really wanted to but essentially you set the volume based on the fact that you're trying to be a lung conscious or lung protective yeah I'm going to say lung conscious. I mean, if you give 12 mils per kilo, that's really twice as high as what's physiologically normal. Now, that's what we used to do in the ICU prior to the ARDSnet study. They used to give 12.

mils per kilogram of predicted body weight up to 15 mils per kilogram of predicted body weight and just lower respiratory rates and they they got rid of co2 really well but we were actually causing harm So typically you're setting, if you're going to set volume assist control, you're setting a volume that's specific to the patient's height.

and even their gender. There's actually an equation that takes into account male or female gender, as well as the height of the patient to tell you what your range of tidal volume should be in mils per kilo. typically you try to set that so usually when i refer to mechanical ventilation or i teach it i give examples of six mil per kilo because it's a nice even um even goal for all types of patients

So then when you set the flow again, you're controlling how fast in liters per minute that volume is going to get delivered. So if you go really slow, it's going to take really long to get the breath in. If you go really fast, it's going to be in a shorter time.

in terms of adult patients it's actually this is a very important setting that you have to set 50 to 60 liters per minute is really the target we use this clinically and it ends up giving a short eye time But when you have a patient that begins to interact, you want them to have sufficient flow that exceeds their demand.

and most patients if they are recovering from respiratory failure then they likely have a flow demand that's going to be higher than 40 liters per minute so you should probably try to maintain this for when they start interacting with the ventilator Now depending on the ventilator, what ventilator are you most familiar with? Oh, I'm just trying to think what we're using at the moment. I've gone blank.

That's okay. Hamilton. Okay. Now, do you know in volume control if they set a flow or do they set a time? Because this is where it gets confusing per ventilator. Time I think they said yeah, I think there's a option to set and most ventilators in the When you set up the ventilator, you can tell the company which one you want to set and they can set it as the default. So when you turn on the ventilator, you would set time instead of flow. And this is where... Sorry, Draeger. Draeger, okay.

so dragger um has both options i know with their um with volume control You can set flow and you can set a time. So the reason why there's both is because you may set a flow, but you want the breath to last longer. I mentioned a pause earlier. You can increase the inspiratory time to allow a little bit of a pause at the end of the breath.

and this is actually used to give a real-time measurement of plateau pressure which we can probably get into in another another episode in terms of how to use these modes more specifically in terms of monitoring and how to get values like plateau pressure and resistance etc but some ventilators you only can set the time

for volume control and it seems counterintuitive like i thought i'm supposed to be controlling flow well usually as you adjust the time it tells you how you're impacting the flow so an example of this is we use um We use all kinds of ventilators at our hospital. One of them is the Servo-I, the Servo-U ventilator. And it's set up currently that you adjust inspiratory time. But as you turn inspiratory time right below the time in seconds, it tells you what the liter per minute flow is.

So you would turn that to achieve your liter per minute flow. And then again, it's going to look at what tidal volume you've set and say, okay, you've set 50 liters per minute. This is how much tidal volume you want me to put in. um in this amount of time this is the flow you have to that has to be used to achieve that um so moving on Once you've set this mode again, just to quickly backtrack because we kind of got a little sidetracked there to start.

is the respiratory rate or the patient can pull in this just means they can trigger so now there's a setting on the ventilator called trigger and it's either flow or pressure and allows the patient to demand a breath the machine sees this breath and delivers the breath so it's either time or patient

You're controlling flow or on some ventilators time, but the time is part of flow. Flow is liters per minute. So if you're setting time, then you're affecting part of the equation and it's going to affect your...

how fast it's going in or how slow and then the ultimate cycle is when the volume goes in or if you've set a pause it may be time as well now moving on to pressure assist control now the problem with modes for those listening modes is a very visual uh thing to teach so this is the first time i've had to teach with words and of course i have a nice little diagram that i've created in front of my eyes and perhaps

What I'll do is summarize this in a table, Jonathan, and you can share it either on your website or you can put it as like a show note or a link or something. And then we can actually share this table because it's much easier as a visual guidance. So.

The next one up is pressure assist control. So again, with the start, the control and the cycle, I'm going to go through that again. So pressure assist control, it's the same as volume assist control in terms of we time it with either a respiratory rate or the patient can interact. Okay. Now we're controlling pressure instead of the flow.

Okay, so now we're setting a pressure that is controlled. The ventilator should not exceed that pressure unless the patient coughs or fights the ventilator. The pressure should stay relatively constant and it's limited, it's controlled. And then this breath cycles off based on time. Okay, so this one's a little bit easier to understand. So in terms of respiratory therapists, I've worked with traditionally, and a lot of them like the simplicity of setting pressure control.

Here's where it gets into the different things you have to concern yourself. So now that I've covered pressure control and timing is respiratory rate or patient, you control the pressure and you cycle off based on time.

So how do you know how much volume your patient is going to get? That's actually the one concern or the issue with pressure control is that this minute ventilation that I mentioned earlier is not controlled whereas in volume control you're controlling a minimum respiratory rate the patient can go faster you're also controlling the volume so you know that your minute ventilation cannot be any less than what you've said on the machine

But with pressure assist control, you have to be very conscious that minute ventilation can change. Now, this is where that compliance comes in. Again, compliance is inversely related to pressure. okay so let's say your patient is passive so they're not breathing they're let's say heavily sedated they were just intubated or just came back from the operating room they're not interacting with the ventilator and you've set up your ventilator so you've set a respiratory rate

The patient's not interacting, but you've set the respiratory rate. You've set the pressure and you've adjusted the pressure until your ventilator delivered a tidal volume, let's say around six mils per kilogram of predicted body weight for that patient. So in this example, I'm going to give, let's say the tidal volume is 400 mils. So you've adjusted the pressure. You're getting 400 mils. You've set a time that, let's say, is 0.7 seconds or 0.8 seconds. It delivers the 400 mils.

and then it stops and cycles off into exhalation down to your peep levels the pressure left in the lung and then the breath cycles on again based on your respiratory time it delivers the pressure it holds that pressure for a certain amount of time and then it goes back down again

Now, let's say your compliance, your patient's lung compliance or respiratory system compliance gets worse. What part's being controlled? You're controlling pressure. So compliance goes down, but you've set the pressure. What's going to be impacted? I'll let you answer this one, Jonathan. I presume your volumes are going to drop off. Absolutely.

Absolutely, because you're actually fixing the pressure. The pressure is not going to change. So if the compliance goes down, it's directly related to your volume in this mode. Now, how do you fix that? Well, I need to turn up my pressure to get more volume. OK, so this is the relationship that you have to be aware of. Now, let's say you've set six meals per kilo and your patient's getting better. Again, you fix the pressure. Now the compliance is getting better. So it's going up.

well, now your volume is going to go up and now you have to be conscious of how much volume you're sending into this patient's lungs, right? So this is the problem. I don't want to say problem because if you're very conscious of what's going on, This is what you need to be aware of is that you have fixed the pressure. So any change in compliance will change your volume, which again changes your minute ventilation. And this was why I said the first two things to really understand about modes is.

Minute ventilation is a goal and compliance affects how your ventilator reacts based on what you set. Now to take a step back and go with volume control, if you've fixed the volume and your compliance gets worse, I'll let you answer this, Jonathan. What happens to your pressure? You fixed your volume. Compliance has got worse. So the pressure is going to get higher then. Exactly. So this is.

The thing with volume control you have to be aware of is your plateau pressure. So peak pressure goes up with resistance and maybe we'll save the... lung mechanics for the next episode because i i get really excited with lung mechanics but i don't want to distract from just understanding the basics of the modes um but again with volume control if we just want to talk about compliance if we're Keeping the volume set and the compliance gets worse, your pressure is going to go up.

So the amount of pressure required to get that volume in is going to increase. So you have to set your alarms consciously knowing that, okay, I don't want my pressure to go above a certain level. I need to monitor that pressure.

And the reason why we monitor pressure is we're worried about something called barotrauma. So it's too much pressure to the lung that can cause injury. It doesn't have to be a pneumothorax, which is a hole in the lung. It could just be cellular or tissue damage, but we try to avoid that by limiting pressure. And the other thing we try to limit the amount of stretch the lung occurs by limiting the volume.

So in one mode, you can't control the pressure. It's related to compliance. And if the compliance gets worse, your pressure goes up. And in pressure control, you lock the pressure, but you can't control the volume. So if the patient improves, they may actually be at higher risk because now the volume is too high. So they both have their give and takes. You just have to be conscious of how the mode reacts to patients' change in compliance. And again, the one, I guess...

benefit to volume assist controls. At least you know your minute ventilation won't change if your patient's compliance gets better. It just means your pressure is going to go down, but you don't have to worry about changes in volume. You're not going to all of a sudden be delivering too much tidal volume. It's because it's controlled. I think this is something I think.

uh become more conscious of is that sort of and this may be just whether the centers i've worked in but sort of five six years ago it was pressure controlled ventilation was the thing it seems to be now that volume controlled ventilation is is is the more common mode of ventilation

I don't know whether that's the same for you. Yeah, it's interesting you say that, actually, because a year ago I started at St. Michael's Hospital. And prior to that, I worked eight years at a hospital in Hamilton as a clinical educator.

um indeed uh we were almost exclusively pressure control site at that point um it was one of those things that when i got there that's just what they used and i just you know you choose to pick your battles in terms of changing too much practice so there wasn't really any major evidence for me to change it other than we should make sure that we're very conscious of title volume and in fact on most chart audits that i did while there we were extremely good at

monitoring and maintaining a good level of tidal volume. So it wasn't one of the things that we sort of needed to change drastically. Now, when I came to St. Michael's Hospital, I noticed that almost every patient was on volume control. And I thought,

Well, this is interesting. So I actually asked around about it. And it turns out they used to be an exclusively pressure control site. And indeed, they did change to volume assist control. Now, one of the things that I realized and learned is that they were being taught.

sort of the optimal way of setting volume assist control by none other than dr. Laurent Bouchard so when he showed up he basically gave them some tips and tricks on how to use volume assist control as a monitoring tool and this completely um i don't say opened my eyes but enlightened me to things that i already knew about the mode but hadn't realized and maybe i'll quickly now that we've covered the main

sort of control modes i'll quickly take a moment to explain what i'm talking about because some people might be intrigued by what i just said how can a mode be used as a monitoring tool well we know we can monitor flow and pressure and and volume

uh the screens are basic they're monitors it's another icu monitor but one of the things if you set volume assist control i've mentioned a couple times setting a pause if you set all the ventilator needs is about 0.2 to 0.3 seconds so we typically try to set a 0.3 second pause after the breath is delivered so it delivers a constant flow so it's a flat waveform for your flow it gives the set flow over time and then it pauses for 0.3 seconds where there's zero flow and the pressure in the machine

The initial pressure that goes up is from the resistive forces of the lung. And then when you hold the breath, at the end of the breath, there's zero flow, the pressure drops. And this plateau pressure actually represents the alveolar pressure. So the pressure felt sort of... the distal airways in the alveoli so you've now overcome the resistive forces there's no resistive forces because resistance is again centimeters per water per liter per second

except if you're not delivering any liters per second, you've actually paused, then you're just left with pressure. And this is the compliance related pressure, the elastic pressure of the lungs. So this is your plateau pressure. There's been a huge interest in driving pressure in the last few years I'm sure you've heard the term a lot of people talking about driving pressure and driving pressure is plateau Minus total peep. So in the absence of any auto peep

Your plateau pressure, if you can read it in real time, then plateau minus your set peep level is your driving pressure. Now the problem with pressure control is many people think because the pressure is square, because you're controlling pressure.

The graphic, the scalar, is pressure over time is typically flat. If it's flat, people think, oh, that's my plateau pressure. So I know what my plateau pressure is. I know what my driving pressure is. The problem is you have to be conscious of the flow. in pressure control which is a decelerating flow pattern and again this is more a visual thing but imagine a flow that goes up really fast and then it ramps down if it doesn't ramp down right to zero you actually don't have a true plateau

Now, if it gets really, really close to zero, you could argue that the end of the breath is probably very close to your plateau pressure. But unless you actually have a pause, a moment of zero flow, you don't know the true elastic pressure of the lung. Does that make sense? Yeah, absolutely. So that's one of the reasons people are using volume control.

Sorry, go ahead, Jonathan. No, I was just saying one of the things that I've been doing a lot more is using the function on the Drago ventilator to actually do an inspiratory pause so that you can see the plateau pressure. more clearly that's that's the way i do it now so yep now so what's beautiful about setting a pause is that you don't have to physically do it anymore the the key is doing it for a specific amount of time so at least 0.3 second pause now on the dragger

In volume control, for example, you do have the option to set the flow. So setting it between 50 and 60 liters per minute is typically what we would do. And then when you adjust the time on the dragger ventilator, for example, If you ramp down the time, it will only go as slow as it can based on the flow you've set. So if you set 50 liters per minute, you can't go any shorter in terms of time. You can't go any less than what's...

than 50 liters per minute to get the volume you set but once you hit that stop so you'll see you'll turn the knob and it says sorry i can't go any slower because i need to get the volume in that you've set if you then increase it from that point it actually shows you right on the screen what your t pause is

so it's your pause time and again turning that up until you get about 0.3 seconds you can actually get a real readout of plateau pressure and on every ventilator there's typically a monitored value of plateau What's interesting is on many ventilators, that value is blank until you do a maneuver. Like you actually have to do a hold for plateau to show up. Yeah. But if you have the pause, it actually starts showing up in real time. So then you can just walk in the room and go.

There's my plateau pressure. And if you know your peep, you can say, I know my driving pressure. This is actually key to using that mode as a monitoring tool. The other beauty about that mode as a monitoring tool, again, volume assist control, is that if you set.

60 liters per minute. Well, that's one liter per second. And again, I mentioned resistance before. Resistance is the change in pressure. So it's actually the change in pressure of peak minus plateau. So the problem is if you don't know your plateau pressure.

then and you have a peak you don't know how much resistance you have without doing that pause as i mentioned so peak minus plateau that's the change in pressure that's related to resistance and the liters per second is the other part of resistance so it's centimeters of water per liter per second well if you set 60 liters per minute that's one liter per second so now you're taking a change in pressure and dividing it by one

which means you can just ignore the dividing divided by one. I could walk into a bedside. If I have plateau and peak on my ventilator being monitored in real time, I can say, okay, my peak pressure is 30. My plateau is 22. because I have a pause and I can see that, and my flow is set at 60 liters per minute, which is one liter per second, I can say, oh, 30 minus 22, that's eight. My resistance is eight centimeters of water per liter per second.

So it really becomes a useful tool because this is one of the things that, to be honest, most clinicians aren't calculating resistance. They're not calculating static compliance at the bedside because most of the time you have to go in and do a maneuver to do that.

but you can actually set your ventilator up to basically do the maneuvers for you. And you just can stand there and look at the values and go, that's my resistance and that's my plateau pressure. I know my driving pressure. If you know your tidal volume, tidal volume is just... mils divided by driving pressure. You have all these values that are easily calculated because the numbers are already there on your screen. It's a very cool way to use volume control. Yeah, absolutely.

Onward, your patient starts to interact more. You're like, okay, do I really need to control as many variables? Maybe I should let the patient take over a bit more. Now, what's interesting about pressure support is that we actually control almost as much as we do in pressure control. Now I say pressure control because pressure support, again, as alluded to by the name, you can control pressure. But let's go back again to the start, the control, and the cycle.

The patient has to start and initiate the breath and pressure support because we don't set a rate. There's a backup rate if they stop breathing, but that's not part of the settings in terms of like the real time mode. You just set it up as a backup.

ventilation and you set usually a time if they don't breathe for 20 seconds, the machine kicks in. But ultimately the mode does not have a set respiratory rate. The patient is required to interact and they set their own timing in terms of starting the breath. Now we do control pressure and it's no different than pressure assist control from the control aspect. We set a pressure that we control. It controls that. Now when does it cycle off? Although we don't use time to cycle off the breath.

What's interesting is we can control how long the breath lasts. And this is called a number of things on different ventilators. But the cycle criteria is based on a percentage of the peak flow. So again, with pressure control, you don't set the flow. You don't control the flow like you do in volume assist. The flow depends on how much pressure you set. If you set a really high pressure, the machine has to give a lot of flow to reach that pressure. And then it has to...

still deliver flow to maintain that pressure, but it doesn't have to keep going as fast so it slows down, it decelerates. But the pressure delivered in pressure control is no different than delivery in pressure support. so people often ask what's the difference between pressure control and pressure support and really the only main difference is that you don't control a respiratory rate a minimum respiratory rate

And people say, well, what about time? You don't set an eye time. It's like, well, no, you don't. But you actually can control when the breath cycles off based on the flow. Now, the biggest problem is most ventilators. This setting is actually hidden.

So I'll give you the example, Jonathan, just because you use a dragger ventilator, I'll give you this example. You have to go into, and you've probably seen it, but it's not on the main screen. You have to go into your mode settings. So you bring up your mode settings screen. And then there's an option for trigger and cycle under additional settings. So when you get into trigger and cycle, you'll see your trigger sensitivity, but you'll also see the cycle criteria, which is usually a percentage.

And most ventilators default to around 25%. So what that means is when the machine delivers the breath, it reaches the pressure with a flow. And the flow is very rapid, so it goes up very quick. And then that peak pressure, whatever that peak is, just for easy math, I'm going to say the peak is 100 liters per minute. Usually it doesn't get that fast, but for math, let's use that.

And then if it's set to 25%, that means when the flow deteriorates, it's going to stop delivering the pressure when the flow goes down to 25 liters per minute, if those were your numbers. So it's 25% of your peak flow. Now these machines have computers and they're very fast at calculating this. So it knows what the peak pressure is. It says, okay, when the flow reaches 25%, I'm going to cycle off.

So we. Sorry, let me just Thomas for a minute. Yeah, please. Right. So the breath is being delivered. We're calling it 100 litres a minute. OK, so the breath is being delivered at 100 litres a minute. and it's going to keep going at that flow until it reaches the pressure that you have set it at.

which is usually very very rapid it's usually within seconds that's actually just to quickly stop you for a second another setting in pressure control is called rise time it's not normally tweaked usually when people are tweaking it they probably should be looking at other reasons why the patient is

sort of pulling back on the ventilator like they're having trouble triggering some people will adjust the rise so it goes in really fast but it's in milliseconds so it's like 0.4 seconds it's 0.15 seconds it will Give however much flow is required to reach the pressure in that very very short time So that's why it's very rapid to reach the pressure and then go on

And then so once it's reached the pressure you've set it at, the flow then starts to decline because obviously you don't need so much flow because you've got the pressure you've set it at. And then once it gets to 25%, what happens then? Then the breath cycles off. Right. Yeah. Okay. So that's the cycle off criteria. So remember, there's always a start, a control and a cycle. So in pressure support, the patient starts it. The ventilator controls the pressure.

And it actually cycles off based on a setting. And the problem is most people don't know that this setting exists if they don't look for it. It's not on the front of the screen. There is a ventilator. I believe the... They've changed names a couple of times, but it's the PB840, which is then they were Kvidian. I think now they're Medtronic, but their ventilator has the value on the front of the screen, but it's called eSense. So it's called eSense for expiratory sensitivity. But to me.

The names are one of the problems because the names may not be intuitive Really what it's doing is cycling off the inspiratory breath or it's cycling off pressure support So if it was just called percent

Pressure support cycle off or inspiratory cycle off. I think more people would understand it But the problem is even the modes that actually call it something similar to that like inspiratory cycle off It's a hidden it's like a hidden setting like it's not on the main front of the screen So people don't realize they actually can control the timing of the breath and this setting is

probably one of the main reasons that there's actually there can be a significant amount of asynchrony in patients with that have very high compliance because you mentioned earlier Jonathan you described it very nicely the flow goes

very fast to reach the pressure and then it begins to decline because to maintain the pressure if it kept going that fast the pressure is going to keep going up right so it's not going to maintain a flat pressure it would keep going going higher so it starts to slow down it starts to decline So when that starts to decline, I totally just lost my train of thought there. Don't worry. I had a point there. That's so funny. That's what editing tools are for. Yeah, exactly.

So, oh, let me, oh yes. Okay. COPD. That's where I, that's where I lost my thought. Okay. So you're right. The pressure is delivered. The flow goes up very, very quickly. It reaches that peak flow to maintain that pressure.

It doesn't go any faster. It doesn't maintain the flow because the pressure would keep going up. So it starts to deteriorate. And then as it deteriorates, the pressure is maintained. But the slope of deterioration, how quickly that flow... deteriorates and goes down is actually related to the patient's compliance and or resistance but since we've been talking about compliance I'll focus on that if a patient has really high compliance

So patients with emphysema are classic, right? They have really high compliant lungs. They don't have very elastic lungs. So once you inflate them, they don't want to deflate. These patients have a... Slope of flow deterioration. That's very slow. It's very gradual So what happens is if you let the ventilator Continue to decline the flow until a set percent

Some patients, this may be way too long. Like it completely exceeds the neurological drive of the patient. Like the patient's already checked out. They're like, okay, I want to breathe out. but the machine's like no you know what i haven't reached this criteria of 25 that is set by the ventilator so it ends up delivering more volume

because it delivers more flow for a longer period of time than what the patient actually wants or needs. And then what happens is this increases your inspiratory time beyond the neurological time of the patient. And then by the time it cycles off, you begin exhalation and the patient now wants another breath and then they can't actually trigger the breath because they have too much volume in the lung so they have what's called a missed trigger or an ineffective effort

So this is very important because in those patients, particularly patients with COPD and patients with COPD with emphysema, they're at a very high risk of that timing being set inappropriately. Asynchrony is probably another episode but there are ways of fixing this so one is you can adjust the time But in many patients to get them more synchronous, they actually need less support. So if they're missing efforts, the first thing you should do is try to decrease the pressure support.

And if that doesn't work, you can play with the timing, which in pressure support, the timing is that flow cycle, that percent. So again, the start, control, and the cycle. The start is the patient, control the pressure, and the cycling is based on the flow. Okay, I think. All right. So yeah, maybe we'll stop there. Do you want me to stop there? No, I think stop there, Thomas, for now. I think that's enough for this one. What were you going to go on to do?

Yeah, so the next two I would talk about would be SIMV, which combines those three modes that I just talked about, the last three. So volume assist control, pressure assist control, and pressure support. SIMV uses... either volume or pressure control and it uses pressure support so it's like a it's a combination of the two

Okay, go on to talk about SIMV because I think that would be a logical conclusion, wouldn't it? Okay, yeah, I think so. And then PRVC gets a little bit more complicated, so we'll save that for the next time. Yeah, absolutely. So, yeah, carry on with the SIMV then. That's great.

okay so simv is synchronized intermittent mandatory ventilation so before simv you could just take the s off it was intermittent mandatory ventilation so what does that mean well you would set a volume assist control breath or a pressure assist control breath. So you would, just as I discussed earlier, you would set one of these control modes, you would set that. And then it'd be intermittent. So what does that mean?

Typically, if the patient starts to breathe, they wouldn't actually trigger another control breath. They would get either no support. So if you take away support and you're just left with peep. Typically it's considered CPAP or it's just an expiratory pressure, or you would allow the patient to get a little bit of pressure support as I just described. So SIMV, you really do have to set two modes in one. You have to set the control aspect and then you set the pressure.

pressure uh the the support aspect so the the little less controlled now the problem with imv is that if the patient took a breath And based on the respiratory rate you've set, it's now time to give a breath. So again, I'll use the example of 10 breaths per minute, which is quite, quite slow, but it's easy math. Every six seconds, it would give a mandatory breath.

the problem with imv is that if the patient took a spontaneous breath and then it was now time for the machine to give a mandatory breath it would interrupt them halfway through and deliver the mandatory breath on top of their already started breath so they basically would get a form of breath stacking so they would start and the machine would go nope it's my turn and just throw in the the controlled breath in there so

This was problematic. So they came out with synchronized intermittent mandatory ventilation. All that means is that the machine kind of looks ahead. So let's say the machine is scheduled with a rate of 10 every six seconds to give a breath.

There's like a trigger window where it says if the patient takes a breath just prior to me delivering the control breath that I'm supposed to deliver, the ventilator will actually give it a little bit earlier. It'll say, okay, you know what? I'm going to give it now. so that I don't breath stack you. So what it's called is called a trigger window. It basically, the machine's watching and watching, says, okay, I'm going to deliver a breath in one second.

If anything happens between now and that one second, there's like, again, the trigger windows on some older ventilators, you could tweak how close that trigger window was. But typically, it's not something we tweak now on most modern ventilators. but the machine will pay attention and go, okay, I'm just going to deliver the mandatory breaths now so I don't affect your effort. So the issue with SIMV is that if you set the rate close to the patient's rate,

It's no different than assist control because it's going to look ahead and go, you know what? I don't want to breath stack you, so I'll just give that to you now. So if you have SIMB at a rate of 20 and your patient's breathing 21, they're...

probably going to get every single breath is going to be a mandatory breath. So why was this mode invented? It was invented as a potential weaning mode. So you'd start them off on SINV with a high rate where it would be, again, as I just described, no different than volume assist control or pressure assist control because you've set a control mode. But then you would reduce the set rate.

And then you'd have pressure support as your other option, or again, no pressure support. And then every time the patient breathed above the set rate, they would get the pressure support or no support. So it's like, okay, let's make them work a bit more. And let's reduce the control work so where the machine would take over most or all the work, let's slowly reduce those breaths and let the patient interact more. And the whole idea was you would reduce the rate by more.

Buy more and let the patient take up more of the spontaneous efforts to the point where either you have them completely on pressure support and the machine's not really giving them much of a breath anymore because you've reduced the rate so much. And then you just transition them over to pressure support.

Or if you were using no support, you're like, okay, my patient's breathing most of the time with no support and they're tolerating it. Let's just get them off the ventilator. Let's liberate them. Let's do a spontaneous breathing trial.

But actually this was designed before people were doing spontaneous breathing trials. And when they compared daily spontaneous breathing trials, like literally just saying, okay, you're on AC, let's try you off of AC. That actually was much faster than using SIMV to wean. Any hospitals currently using SIMB as a leaning mode probably missed the papers published in the 90s showing that it was not as effective of just trying to liberate your patient daily.

So this gradual reduction of a rate while the patient takes over more of the machine, take over more of the breathing is actually not an effective weaning mode. There's been argument by many experts as to why do we even still have this on the ventilator? I can tell you one of the more common reasons it's used in an adult ICU that I've just seen through observation over my years of practice is patients will have episodes of apnea, typically overnight, and I'll explain why in a moment.

But when someone is apneic, you get called to the room because the ventilator was alarming apnea. You walk in, the patient's wide awake looking at you. I'm like, okay, well, that's strange. They're wide awake. I don't know how or why they went apneic. And then you leave.

They were on pressure support. And then you come back because they went apneic again. Again, they've been on pressure support. They keep going apneic. What many clinicians will do is they'll use SINV as what I call the, it's like a nuisance mode. where, you know what, if I just set a little rate, then my patient can never go apneic because it'll always kick in and give them a breath every six seconds. If I set a rate of 10, it's going to give a breath, so they'll never go apneic.

the word the problem is and i don't know if you've ever seen this if you work a night shift patients randomly have these central sleep apneas typically the problem is over support Patients can't downregulate properly. You basically go past their CO2 threshold when they sleep by blowing it off too much. And then they actually start having central apnea episodes. So the answer to answer...

apnea, like the answer to correct this apnea overnight is actually to reduce pressure support. But what most people do is they just set a rate to avoid the alarming. So anytime I've seen SINV used clinically in the last 15 years, it's been for that scenario because

It certainly wasn't for weaning because every hospital I've worked at knows it's not an effective weaning mode, but yet you'd come in the morning and see patients on SINV. You're like, why are they on SINV? Oh, they had apnea episodes overnight. You're like, oh, okay.

But really the answer is to try to reduce the amount of support because they're probably being over-supported. Okay. So that's really SINV, although it seems complicated. It's only complicated because you have to set either one of the control modes I mentioned.

plus pressure support or just allowing CPAP. And maybe I'll finish off just by saying that in the pressure support mode, it's the only mode where you can actually turn the support to zero. So in volume assist control, you can't usually set the volume to zero. there's going to be a minimum maybe it's one mil but you can't deliver zero tidal volume and in pressure control you can't deliver zero pressure the minimum is probably going to be like one centimeter of water

But in pressure support, you can actually set the pressure support to zero. And when you do, the mode is basically, it's considered CPAP. It's only giving a constant positive airway pressure, which is whatever your PEEP was set at, that now becomes the...

constant positive airway pressure or continuous positive airway pressure, which is what CPAP stands for. So it's the only mode that you can turn the support level completely to zero. And that's why some ventilators like the dragger that you're using would call the mode CPAP pressure support. or CPAP with pressure support. Yeah. Okay. Brilliant.

Thomas, there's a lot of information in there, which is absolutely fabulous. It's one that I'm going to have to listen to again. I could almost make notes on it as you go because I've learned an awful lot there. The problem with ventilation, certainly in this country, I think...

I think that we almost... have a disadvantage because we don't have respiratory therapists like you do over there who um from what i can hear and see is that you become a bit of a ventilator expert now we don't have them in this country we have physiotherapists physiotherapy are also responsible for the patient's activity rather than just their ventilation so they don't become...

experts as yourself so i think the consequence of that and again this is from a this is a personal point of view and there may be people out there who are shouting at their car radios as i speak personal point of view um i don't feel that i've ever been taught mechanical ventilation particularly well. I've kind of picked it up as I go along and I think there's a lot of ITU nurses that pick it up as they go along and therefore aren't necessarily.

doing it as effectively and as well as they should be which is one of the reasons why I wanted to get you involved with this as well. Just as an aside because I've been thinking this all the way through, if somebody wanted to buy a book

on mechanical ventilation and not spend hundreds of pounds, because you can from what I've seen on Amazon of some of these books that you can buy for mechanical ventilation, which book would you recommend? Yeah, there's a couple out there. There's one that's a little bit smaller. And while I'm chatting, I'm just going to look this up on get the exact title. So Essentials of Mechanical Ventilation, fourth edition.

And it is by two of, I would say probably like icons of respiratory therapy anyways, in North America, but they're also world renowned. So Dean Hess is a PhD RT. Robert Kaczmarek or Bob Kaczmarek is also a PhD RT. They're both from the US and they're both widely known around the world as experts in their field. I consider them personal colleagues. I've met and interacted and actually I've worked with both of them and they are fantastic. And this book, they're the authors of this book.

And it's probably the one that I would choose if you were to choose one. Now, the Canadian price, again, it's hard to find a cheap book. The Canadian price on Amazon right now that I'm looking at is... paperback version 106.95 so it's it's an investment um now there's many articles that describe mechanical ventilation um off the top of my head i can't think of one that gets in

Enough detail that someone could really learn some good basics behind the modes. But if someone is interested in a textbook, this is probably the one I would recommend. I've recommended it to a few residents, and they seem to go for it. and they find it to be sort of in the middle to lower price range compared to some of the other textbooks out there. Yeah, absolutely. Okay, excellent. All right. We'll leave it there then, Thomas. Thank you very, very much for that. Very, very grateful.

And I am planning, well, we are both planning on making this a series. So I won't put you on the spot and ask you what you're going to do next time, because I think probably you might want to think about that a little bit first. But hopefully we can get together.

again sometime in the next month or so and do something similar sounds good yep okay great all right thanks thomas thanks very much thank you very much you've been listening to critical care practitioner if you would like to comment on any of today's topics Find us at criticalcarepractitioner.co.uk Tweet us at ccpractitioner Find us at facebook.com slash criticalcarepractitioner

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