Now, we're going to dive into the principles of mechanical ventilation. And so, the point of this is just to remind us of kind of the basic and more common modes of mechanical ventilation and kind of why we do some of the things that we think we do. So, we're going to talk about indications briefly for mechanical ventilation. We're actually not. We're going to talk about the physiologic impact of positive pressure ventilation. We have to keep those things in mind for many of our patients. And then we'll touch a little bit about weaning, and then you'll have another lecture about that. So, what I like to teach is when we understand that the ventilator is simply a computer that we are programming to do a couple different things, it becomes less intimidating and it makes a little bit more sense. And so, when we talk about the ventilator in different modes of ventilation, we use terminologies like what's the trigger, what's the cycle, what's the target? And those are very sometimes confusing things, and I think I've found that many people get those terminologies mixed up. But how we're going to talk about different modes of ventilation today is the trigger being how does the machine know to do what you programmed it to do? What triggers it? What tells it that it needs to do what it's going to do? The target is how you're going to deliver the breath. How are you going to govern the machine to deliver flow to achieve your goals? And the cycle is simply how does the machine know to turn off? And if you think about it, mechanical ventilation, we are only controlling inspiration. There's no control of expiration. And so if you tell it when to start, what to do when it starts, and then when to stop, that's trigger, target, and cycle. So typically, traditionally, the trigger is one of two triggers. It is either the patient, so the machine detects that the patient is taking a spontaneous breath, and it goes, oh, let me give the breath that was set to be given. Or the machine is like, OK, I have to give a breath at 12 breaths per minute. The patient didn't trigger. It is now time, based on the respiratory rate, or what we call time, I am going to give the breath. OK? So typically, two triggers for each mode. And then under those triggers, how does the machine know whether or not the patient actually triggered the breath? We have two ways. We can either allow the pressure change as the trigger. So the machine will look for small dips or changes in pressure. And the machine will say, oh, OK, the patient is taking a breath. Let me go ahead and give the breath. Or we can use flow. And so again, because it's a closed circuit, the machine can monitor deflections and flow. And then the machine knows that, OK, the patient took a breath. Now let me give whatever settings the health care provider set. Along those same lines, if it's a breath that was delivered by the machine based on the respiratory rate that was set, that is a controlled or mandatory breath. OK? If it is a breath that was given because the machine noticed the patient was taking a breath, that is what we refer to as assisted breaths. OK? This seems basic, but we're going to cover the basics because I have had experience where people use these terminologies a little bit differently. So I just want to make sure for this morning we're all kind of using the same lingo. So there are three common types of breath that you can have. And they're all based on how gas, oxygen, is delivered by the machine, OK? You can have a situation in which you set the volume. That is what we know as volume control. You can have a situation in which you set the time, OK? Or you can have a situation in which you determine the flow, OK? And we'll talk about these in a little bit more detail. One of the principles to remember is that if you decide to control volume, like volume is very important for you to control because I would say by nature of critical care we're probably all a little bit control issues. If you want volume control, you can control that. You're going to do that with flow. But you're going to take whatever pressures the system gives you, whether or not the lung is compliant or not compliant, et cetera. If you want to be very explicit and say, I don't want the system to get any higher than set pressure, 15 centimeters of water, you can do that as well. But then you're going to have to take whatever volumes the system gives you, OK? So if you want to control volume, you can do that. But you're going to have to take whatever pressures you get, and you'll have to manipulate your volume to get the pressures in a safe range. You want to control your pressures, you absolutely have modes to do that. But you're going to have to manipulate that to also get you the volumes that you need. We'll talk about that a little bit more. So essentially, there are five basic breaths. You have a volume control, you have a volume assist. So either the machine's giving the volume or the patient triggered and then now is getting the volume. You have pressure control assist and pressure control. Pressure control and pressure assist. And you have pressure support, which is a whole different spontaneous breathing mode that we'll talk about a little bit, OK? Because then we like to complicate things, and so then we mix up a couple of things, like SIMV with some pressure support, and then we're like, what are we doing, OK? So we'll sort through that. If you are in a volume control mode, or let's just say if the breath type is volume, and if it's volume control, the trigger is time, based on the respiratory rate, if it's volume control. The target in volume control is flow. So remember, when we're using our volume control modes, we always set a flow. So we're telling the machine, use this flow to get to this volume and stop. So the target, how the breath is being delivered, controlled, is based on flow, OK? And the cycle, when we're using volume control modes, or how the machine is going to know to stop delivering the flow, is what? The actual volume that is reached. So in our volume control modes, we tell the machine, when you hit 12, the time frame, the respiratory cycle for another breath, I want you to give x flow until you register that you've gotten to x volume, and then that breath is over. Does that make sense? In pressure control, the triggers are still the same. They're either patient or time, based on if the patient actually took a breath and the machine detected it, or the trigger, the patient didn't do anything, and it was now time based on the respiratory rate. What changes in pressure control is the target is the amount of, what is a target? Any idea? It's actually the pressure that you want, OK? You tell the machine, I want you to deliver this amount of pressure. That's what we talked about the target being. The cycle is, how does the machine know to stop doing that? So the cycle in pressure control modes is actually time. In all the pressure control modes that we use, we set an inspiratory time, OK? What does that inspiratory time do? It basically says, I want you to give this pressure, 15 centimeters of water, for this amount of time, and then that inspiration is done. So in pressure control modes, the trigger is the patient or time, OK? The target, how the machine does it, is provides pressure for the cycle, which is a given amount of time, which is the inspiratory time, OK? Questions about those? Because then pressure support is a whole different thing, all right? So pressure support is a spontaneous mode of breathing. The only trigger possible in pressure support alone is, in fact, the patient, OK? If your patient is not spontaneously breathing, pressure support is not an option, OK? The target is similar to pressure control, and that is, we say, when the patient takes a breath, please deliver a set amount of pressure, OK? The difference here, and it's the only mode that we're going to talk about where this is a difference, is that flow is actually the cycle. And what that means is that the machine is monitoring flow, and then when the machine detects that the patient is done taking in a breath, and that's usually, the machine notices about a 20% to 30% reduction in peak flow. So if we all take a breath, we feel it fast, and then we feel it starting to slow down. The machine starts to notice that, and the machine is like, oh, you're done taking a breath, so I'm going to be done giving that pressure. So the target for pressure support, not the target, the cycle for pressure support is actually flow, OK? I'm cognizant of time, so I'm going to just skip through a couple things, OK? And I think we've pretty much talked all of it, OK? So then we have some additional modes that we're going to add these concepts to. And we have, I would say, three to four big ones, OK? We have AC. Who uses AC, just by show of hands? AC is assist control. It only, you only got two options. It's an assistive breath or a controlled breath, that's it, OK? And regardless of whether or not it's an assistive breath or a controlled breath, they're going to be pretty identical, because that's all the machine can do. Assisted, OK, I saw you breathe. Here's the breath. Mandatory, you didn't breathe, so here is the same breath, OK? The other breath, the other mode that we use often is SIMV, synchronized intermittent mandatory ventilation. Anybody like SIMV? Oh, you like it, you shout out real fast, all right? They're used, at least at my institute, the two institutions that I've been at, it was split down the middle. The surgeons loved some SIMV, and then the, us medicine folks liked AC, OK? There's no rhyme or reason to it, it's just what would happen, right? And I like to teach that it doesn't really matter of these modes, which mode you use, as long as you know what you're doing and you accomplish your goals, right? So in my training in residency, we only had several events, so we didn't have AC mode, so we had to make the same thing happen using SIMV. So it doesn't matter as long as you understand what your goals are. With that said, I like to tell people that what's happening in SIMV is in the name, OK? SIMV is a little bit of like advancement of IMV. And so now, with SIMV, there's an opportunity for the ventilator to synchronize with the patient's effort, OK? So in SIMV, you essentially have not two breaths like in AC, but you have three potential breaths, OK? It's a very basic explanation, but it seems to work. In SIMV, you can have all mandatory breaths. You paralyze the patient. The patient takes no spontaneous breath. SIMV and AC are going to look the same in paralyzed patients. There's going to be no difference whether it's a volume control, whether it's pressure control. If the patient is not taking spontaneous breaths, those two modes are going to look the same, OK? It is when you have spontaneous breath that SIMV just gets a little bit more complicated, OK? There is some synchrony that happens in the patient who is able to trigger a breath, right? So there are going to be some breaths in which the patient triggers, and the machine goes, OK, I saw that. Let me give you the full volume or the full pressure, OK? Where potentially, like, the patient gets a little bit too sedated after that bolus of fentanyl, now the breaths may all become controlled mandatory breaths, OK? There's the third type of breath that is possible in SIMV, and those are pressure-supported breaths. And the way I like to explain it, because I think a lot of people think that any spontaneous breath or any breath initiated by the patient is pressure-support, that's not true, OK? I like to describe it as a lockout window, where the machine goes, that just happened, and this is supposed to happen, and so all I can do is give you a little bit of pressure support. So the machine noticed that the patient is, like, wanting to do something, but based on the settings, can't give you the full breath, because that just happened, and this is getting ready to happen, so all I can do is provide you pressure support. So that's why we recommend, in patients who you're using SIMV, that you always have pressure support of at least five. Just a little bit about airway pressure release ventilation. Anybody use APRV? Oh, okay. That's a lot. I've never used APRV in my entire career. What did you say? Do you have a drink or a thought? No. We like Puritan Bennett. All right. So the concept behind APRV is that it's kind of like pressure-controlled SIMV, but really long inspiratory times. And then you quickly drop the pressure to allow the CO2 to wash out, and then you go back up for a long inspiratory time. And so it's like providing CPAP, right? So you have this pressure for an extended period of time, and then you got to tank the pressure just for a little bit to get this gas clearance, and you can go back up to your pressure for prolonged periods of time. So I saw something describe it as CPAP with intermittent times of zero pressure, just to allow that gas release. And in that, as you can see here, the patient is still allowed to take some additional breaths to kind of help with some of the gas exchange, et cetera, okay? So really good, potentially good mode of ventilation to help recruit alveoli, okay? Questions? All right. So with that, and your professional experience, let's just look at the tracings and tell which mode it is. That is going to kind of solidify that we understand the different modes, okay? So here's the first tracing. We have pressure at the top, we have flow in the middle, and then we have volume, okay? There are only five options. So which mode of ventilation is this? A. Wonderful. Why is it A? So, that's not A. Yeah, I'm trying, that's not working. Okay. Let's see if I can make the pointer work. I don't know how to make the pointer work. All right. So if you can see, with each breath, the flow is the same. So in volume control modes, we control the flow, okay? If we look at the flow and then we jump down to the volume, each volume is the same. So we are volume controlling, okay? How do we know that it's controlled? What's happening with the pressure baselines at each one? There's no negative deflection. So this patient is not taking spontaneous breaths. Otherwise, in the pressure curve, you'd have the P, then it'll go boop, and then the breath will take off. That's not what we have here. So this is basically volume-assisted AC. I mean, I said that wrong. This is ACVC, but these are all assisted breaths. Okay. What's this? C? PACV. Okay. So what do we see? Let's look at the flows first. What do we see with the flows? Okay. But it's one variable, correct? Yeah. Yeah, so there's some alternate consistency. So we have these two flows that are the same, all right, we have these volumes that look the same, and then the one in the middle, the volumes are different. Is the patient triggering or not? Yeah, the patient's triggering. See that little V at the front of the breath? So each of those breaths, the patient is triggering, okay? What do you notice about the third breath that is different from all the other breaths in the pressure mode, I mean, in the pressure waveform, huh? It's shorter, so it's less time. The pressure is kind of the same, but it's less time, all right? So the answer for this is actually B. This is a synchronized intermittent mandatory ventilation. So when the flows are the same, the volumes are the same, so that's a volume control. That third breath is what type of breath? Pressure support. So it looks different, the flow is different, and the volume is different. So this is SIMV, volume-controlled SIMV. So, is this, are these patient-triggered or not? Patient-triggered. All right. Are all the pressures the same? So, the amount of the pressure is the same, yes. What about the inspiratory time? Inspiratory time is different. Are the volumes the same? Nope, they're a little bit fluctuating here and there. So, we have a mode that's all patient-triggered. The pressure is the same, so we are somehow managing the pressure, but the times of how long the pressure is held is different, resulting in different volumes. So, which mode is this? This is pressure support. It's the same pressure, but the patient is controlling how long for the inspiratory time. And so, that is correlating with different volumes. My kids are in so much trouble walking away from the mic. And that is pressure support ventilation, okay? All right. What do we have here? We have a mixture, okay? What do we have pressure-wise? All the pressures up top look the same, the same amount of time. What is different is that some of them are controlled, the C. There's no negative deflection. And then some of them are spontaneous. There's a negative deflection. But it's the same pressure with the same inspiratory time throughout, okay? So, this is, in fact, which mode? Yes. So, this is ACPC. Okay. Assist or controlled, pressure controlled. I'm using the same pressure, same set inspiratory time. And it doesn't matter if the patient triggered it or if the machine gave the controlled breath. Process of elimination. All right. This is, in fact, SIMV pressure control. So what you see here is those longer, more sustained pressures, those are the synchronized breaths. Why did I say they're synchronized? Because the patient is still triggering, because you have that negative deflection. But it's the same amount of pressure given over the same inspiratory time. But in between, the patient is able to take some pressure-supported breaths. And they look different. The flows look different. The volumes are different. OK? I got you. I'm aware. All right. OK. Hitting the highlights. The other mode that is common. Anybody use PRVC? Yep. People like PRVC a lot. VC Plus? The only thing to remember about VC Plus is that it is actually a pressure-controlled mode. OK? In VC Plus, the reality is the machine is functioning as if it's pressure control. But what the machine does, which is why we're able to set an inspiratory time, but it's a hybrid mode. And so you tell the machine, I want to get 500 cc tidal volume every time. So the machine is like, that's my goal with this inspiratory time. So the machine kind of plays around with what pressure is delivered based on the volume returns. I need 500 cc's. I'm going to monitor the volume returns. If I'm hitting 500 cc's, I'm at a good pressure. If I'm not hitting the 500 cc's, I'm going to increase the, I being the machine, I'm going to increase the pressure until I get there. If I'm overshooting, now I'm going to drop my pressure. So it's an interactive hybrid mode where the machine is actually adjusting the pressures. But it is a pressure-controlled mode. OK? Just so, we're not going to spend a lot of time, but there are some newer modes that I haven't seen, I don't know if anyone else has seen, that is now adding some neural input, trying to synchronize a little bit more with the patients. Some of those are like PAV, Proportional Assist Ventilation, and Neurally Adjusted Ventilatory Assistance. So getting more into the hybrid mode so it's not pure and becomes more comfortable and better synchrony. OK? I'm going to stop there.

mechanical ventilation

ventilator modes

trigger target cycle

volume control

pressure control

neural-assisted modes