Hi, my name is Kalila Gates. I am a critical care physician at Northwestern Medicine in Chicago. And this lecture, we're going to cover the principles of mechanical ventilation, talking about design features and mechanics of commonly used ventilators. So I think we have all experienced the confusion around the terminology when we talk about breath types and mechanical ventilation. And some of the confusion stems from the idea that manufacturers have various trade names and there's no consensus on generic terms. So from machine to machine, manufacturer to manufacturer, the trade names are going to be different and there's no consensus. And so as we move through this lecture, my approach is going to be to describe breaths using three components of the breath. And those breaths are the trigger, the target, and the cycle. So for me, the trigger is going to be what initiates the breath. How does the mechanical ventilator machine know that it is time to deliver a breath? And this trigger could be either a timer or it could be effort. The second terminology is target. How does the machine govern the gas flow to deliver a breath? And this can be either flow or set inspiratory pressure. And then the last characteristic of the breath is the cycle. How does the computer, how does the ventilator know how to stop the breath? And that can be either volume, time, flow, or pressure. And we'll see these components as we discuss the various modes of mechanical ventilation. But the thing to remember is that we have to tell the machine how to start the breath, how to know when it's time to deliver the breath, how to deliver the breath, and then how to stop it. So we'll first go into more detail about the trigger. And reminding you, the trigger is how does the machine know that it is time to deliver a positive pressure breath. And we talk about trigger in the terms of controlled or time triggered. And that is the machine timer. And that is set by our respiratory rate. And we also talk about patient trigger or assisted trigger. And that is determined by patient effort. And with patient effort, we have to set the triggers. It can either be a pressure trigger in which the machine will notice a drop in pressure in the ventilator in the vent circuit and say, OK, the patient is trying to take a breath. Or we can set it as a flow trigger. And that is where the machine notices a change in the continuous flow in the vent circuit and again says, you know what, the patient is trying to take a breath. We can change the sensitivity of both of these triggers depending on the patient and trying to make the patient more comfortable with the ventilator. It's also to be noted that some vents can have both a time trigger and an assisted trigger. And really, which breath is given is really determined by which trigger is detected first. So we talked about the breath trigger. Now we'll talk about the breath target. And remember, target is what governs the gas flow. How does the machine deliver the breath? And in the modes that we will talk about, the target will either be flow or inspiratory pressure. So we will either set flow or we will set inspiratory pressure. And those are our two target options. So once the ventilator is triggered, we then have to talk about how is the gas delivered. And the gas can be delivered two ways. It can be delivered by setting a flow and setting a volume, or it can be delivered by setting a pressure. And as you can see on the left, you see a breath in which gas delivery was set by a determined flow to achieve a certain volume. And the dotted lines actually represent either the dependent variables that are determined by the patient effort or the patient characteristics, like the compliance of the system. On the right, we see a breath that is delivered by using a set pressure. And what we see are the dependent variables in the dotted lines where there's flow and tidal volume. And again, that is determined by patient effort or patient characteristics. So let's look at these graphs a little bit more and the gas delivery a little bit more. On the left, we have a volume-guaranteed delivery. And as you can see, what is guaranteed on the left, what are independent settings, are the flow and the volume. And the volume in this way of ventilating is actually what causes the machine to cycle off. And as you can see, with controlling or the independent variables being flow and volume, the dependent variable is the airway pressure. We can't directly control that. We can manipulate and adjust it, changing our volumes or our flows, but we cannot directly control the airway pressures using this mode. If we go over to the right, we see a pressure mode of gas delivery. And what we see is we can set a pressure and we can set an inspiratory time. And that inspiratory time determines when the gas delivery stops. And that inspiratory time and the effort by the patient will determine when the ventilator cycles off. And it also will determine the volumes based on the mechanics of the patient. And so in this pressure-generated mode, you can guarantee a pressure, but the flow and the volumes are variable. So we talked about the trigger. We talked about the target or how the breath is delivered. And so we'll next talk about the cycle, how the machine knows to stop delivering the breath. And there are three common types of cycle. The machine either stops delivering the breath or stops the flow of gas after a set volume is reached or after a set time that has been programmed, as we saw in the pressure-controlled mode, or after a certain flow reduction. And we'll talk about that a little bit more. So with this terminology of trigger, target, and cycle, again, reminding us of how the breath is started, how it's delivered, and how is it turned off, we can talk about five basic mechanically ventilated breaths. The first breath type we can talk about is volume control. In volume control, it is time trigger determined by our set rate. It is flow targeted, determined by the flow rate that we set, and is volume cycle, which is determined by the set volume that we have placed into the computer. For volume assist breath, it is a patient effort. So that is the trigger. Either using flow or pressure, the machine notes that the patient is attempting to take a breath, and it delivers a set flow, which is our target, and it is cycled again by the set volume that is reached. We switch over to our pressure breaths. The first pressure breath that we'll talk about is pressure control. And in pressure control, it is time trigger, again, because it is controlled and determined by the set rate. It is pressure targeted. We have set the machine to deliver a set pressure over a certain amount of time, which means that time is the cycle. The machine will stop delivering the breath after the set amount of time, which is known as the inspiratory time. In pressure assist breath, it is the patient effort, again, determined by either flow or pressure changes that we've set. The target remains pressure, and the cycle remains time. In pressure support, this is the very different one. Pressure support is patient effort only. There is no time component trigger for pressure support. The target is pressure. And unlike our previous pressure modes, the flow in this situation is cycle. Now, cycle flow means that the machine detects a certain decrease in the flow rate. And that's probably about 20% to 25% change in the flow rate. And that's how the machine knows, you know, the flow is decreasing. The patient has stopped taking a breath. It's time for me to stop delivering the pressure. So we'll start to build on the information that we've talked about so far. So we'll take our five basic breaths, and we'll apply the principles of cycle, trigger, and target. And what we can see is the first two columns, we have our volume breaths. We have volume control breath, in which we've set a set flow, and we have set a set volume. And it is machine triggered. To the right of that, we have our volume assist. And what we can see, if you look closely at the airway pressure, is a negative deflection in airway, signifying that this is a pressure triggered patient generated breath, again, receiving the same set flow and the same set volume. And as we mentioned previously, the solid lines represent the things that we set in this mode. And that is volume and flow. The dotted line represents independent variables that are determined by patient effort or compliance of the system. If we move to the next two columns, these are our pressure modes. And what we can see here is we set a pressure, thus annotated by the solid line. And we set an inspiratory time, where you see that is set TI. We have a pressure control, in which it is a machine triggered breath. And we see that both the flow, as well as the volume, is actually the dotted lines. And they are the dependent variables determined by either patient effort or compliance of the system. We then have the pressure assisted breath. And you can see, again, we set a set pressure. We set a certain inspiratory time. But again, if you look at the airway pressure graph, you see this negative deflection, indicating that it is a patient triggered breath. It is assisted and assisted breath. And again, the dependent variables are flow and volume, again, determined by patient effort and or characteristics. And then the last breath that we'll talk about is a pressure supported breath. And what we can see here in this pressure supported breath is we do set a set pressure. You can also see the negative deflection in the pressure curve, indicating that is a patient triggered breath. And you see, very similar to the pressure controlled breath and the pressure assisted breath, that there is a reduction in flow. And there's a change in volume that can be determined by the patient or characteristics. What is different from pressure support compared to pressure assist is that this cycle in pressure support is a reduction in minimum flow. And so we basically tell the machine to detect a reduction in flow. And that is the way we inform the machine that the patient has stopped the inspiratory effort, which is different than pressure assist, in which the inspiratory time is actually what tells the machine to stop delivering the breath. So we've talked about types of breaths. And now we'll talk about basic modes. And we'll put the two together. So the first mode that we'll talk about is volume assist control. In volume assist control, you have volume controlled breaths, and you have volume assisted breaths. In pressure assist control, you have pressure controlled breaths, and you have pressure assisted breaths. The next modes of ventilation are our SIMV modes, or our synchronized intermittent mandatory ventilation modes, or our modes in which we set the machine up so that it can synchronize with the patient's intrinsic breathing, but there is this kind of lockout window phenomenon in which the patient will only be offered pressure support. In volume SIMV modes, you have volume controlled breaths, and you have volume assisted breaths. In addition to those breaths, you have pressure supported breaths. And those are the breaths that occur in this kind of think of it as a lockout window. In pressure SIMV, you have pressure controlled breaths, you have pressure assisted breaths, and you have that additional pressure supported breath. And then our fifth basic mode of ventilation is the pressure support ventilation, and that is solely a pressure supported breath. It is a patient triggered breath only detected by patient effort, and it is flow cycled. So we've talked about the five basic modes of ventilation, and so I'd like at this point to talk a little bit about APRV, or airway pressure release ventilation. And we've talked about SIMV, and we can think of APRV as pressure SIMV with a long I to E ratio, or a reverse ventilation ratio. And the purpose of APRV is actually to recruit more alveoli. And we can think of APRV similar to our bi-level ventilation. And what happens during APRV is that we provide a set CPAP, or a set continuous pressure, and we set a T high, which is the time in which we will hold that continuous pressure. And then we set a release pressure, or a low pressure, and we set a T low, which is the time for that low pressure, that time for that release pressure. And the interesting thing about APRV is that you can actually, the patient can take spontaneous breaths at either the high or low PEEP. And so this spontaneous breath helps with mixing, which results in reduction of atelectasis. One of the challenges with APRV ventilation is that it can, in fact, produce intrinsic PEEP, which can be problematic. So we definitely have to pay attention to that. So let's see if we understand our various modes. And so we're going to go through a series of tracings, and I want you to decide if you can just look at the tracings and decide what mode of ventilation. So this is our first tracing. So if you chose A, volume-assisted controlled ventilation, you are correct. What we see here is a ventilator tracing in which we look at the pressure curve, which is at the top, and we see a consistent pressure with each breath. And we see no negative deflection at the beginning of the breath, suggesting that this was not a patient-initiated breath. We look at the flow, and the flow is the same with each breath. And we look at the volumes, and each volume is the same as well. So that means that this is volume-assisted controlled ventilation. We see one breath type that is machine-triggered for each of these breaths. The constants are flow and volume, as we can see. The pressures are pretty consistent, but remember, in this mode of ventilation, the pressure is the dependent variable. So here is our next tracing. It's called choose the mode. So I'll give you a few minutes for this. If you chose B, volume-synchronized intermittent mandatory ventilation, you are correct. As you can see here, we have two breath types. They are both patient-triggered. One we can see is constant with a constant flow and a constant volume. And then we see one breath that is variable. And so that suggests that this is volume controlled synchronized intermittent mandatory ventilation. Alrighty so here is our third vent tracing. We'll give you a second or two to take a look and share your thoughts. So if you chose E pressure support ventilation you are correct. What we can see here is that with each breath we have one breath type and that breath type is patient triggered. The thing that appears to be constant with each breath is the pressure. Where we see the pressure at 20 for each breath. What is variable on this tracing is both the flow and the volumes. And so this is pressure support ventilation and let's to remind us the cycle from pressure support ventilation is flow. And so with a certain set reduction in the from the initial flow rate the machine will cycle off and stop the breath. So here's another one we'll give you a few seconds to take a look at the tracing. There's a small hint on here so we'll see and we'll work through this. So the correct answer for this tracing is C pressure assisted control ventilation. In this tracing we see two breath types. We see one breath type that is patient triggered. It has an A at the top of it suggesting that is an assisted breath. And then we see the second breath type that is not assisted. It is actually mandatory or given by the machine controlled and is associated with the C at the top of it. If we look at the pressure tracing for the assisted breaths we can see the negative deflection. Again it's telling us that those are assisted patient triggered breaths. And then for the C's or the controlled breaths we take from right we go right from PEEP no negative deflection up to the set pressure. Pressure is our constant variable and so we can see the same level of pressure for each of these. So if pressure is our constant variable then the dependent variables are flow and volumes as we've talked about before. And to remind us this in this mode time is the cycle. After a set inspiratory time the machine will stop delivering the set pressure and that inspiratory breath will be complete. Alright here's our last one. We'll give you a couple seconds to take a look and tell me what mode do you think that we have here. Alrighty if you the right answer is D pressure synchronized intermittent mandatory ventilation. And so what we have here again are two breath types. We have patient trigger breaths and we also have some pressure supported breaths not the synchronized intermittent mandatory ventilation. So the first breath type that we have here are the pressure synchronized breaths and we can tell that they are synchronized breaths and patient triggered looking at the pressure curve and seeing the negative deflection. The second set of breaths we have in blue and the arrows are showing us is we have the pressure supported breaths. And so again these are patient triggered so we have the negative deflection in the pressure curve and they look different from the synchronized mandatory breaths and they look different from a flow standpoint and what we can see with all of these is that we have variations in the volumes. So in this tracing it's a little bit complex but on the synchronized patient triggered breaths that we have here the constant is pressure and for those the variables are flow and volume and the cycle is time. For the pressure supported breaths which we see in the blue we actually have again the constant variable being pressure that we set. The cycle in that case because it is a pressure supported breath is flow. So now we'll take a minute to talk about some hybrid ventilatory modes and they're hybrids of pressure and flow volume targeted breaths and so this is taking a few concepts and putting them together to maximize the ventilatory strategy. And so we can have hybrids that correct within the breath or we can have hybrids that correct breath to breath. Though the hybrid that corrects within the breath you can think of things like adding a certain volume guarantee to a pressure breath using extra flow at the end of the breath if the volume is inadequate towards the end of the breath. There are also modes in which you can add a pressure guarantee to a flow volume breath if the pressure starts to drop from the patient effort to try to ensure that we reach the pressures that we want and thus the volumes. That happens within the breath. There are other modes that I'm sure many of us are familiar with that actually take into account breath to breath factors and make adjustments and it's either adding or subtracting pressure to subsequent pressure targeted breaths to maintain tidal volumes and sometimes we can maintain other variables as well into a target range. And one of the common forms of this is pressure regulated volume control. We sometimes know it as VC plus. There are some others like smart care and ASV and basically the ventilator basically we set targets of volume and the ventilator interacts and it makes sure that the patient is reaching the targets and if it's not reaching the target there will be adjustments made breath to breath to make sure that we reach those targets. So let's look at several different ways this technology works. So here we can see up top that we have three graphs and we have pressure, flow, and volume and what we can see in the top tracing and we can say that something is obstructing the circuit we can see there's a steady pressure and then there's a change in flow that is detected and because of that change in flow and that's the change in volume we can see the pressures increase to try to make sure that we maintain the flows and the volumes. If you remove the obstruction what you can then see is the computer making adjustments in decreasing the pressures to get us back down to our goal flows and volumes. This is another common mode pressure regulated volume control and again what we notice about this and this is one of the more unique modes is because once we started there's a startup breath to basically try to assess the circuit and then the machine makes some decisions that we are going to apply a certain pressure over a set inspiratory time to achieve a certain volume that is set and from breath to breath the machine evaluates did I reach my goal title volume and if my goal title volume is if the delivered title volume is above my goal title volume then the machine is going to decrease the inspiratory pressure and it'll keep monitoring and if by chance the title volumes fall below my goal title volumes then the machine will increase the pressures to make sure that we reach our goal title volumes on a consistent level. I wanted to introduce two newer modes of ventilation we're not going to spend a lot of time on it but I just wanted to introduce you to and then let you know that it's out there. The first is PAV or proportional assist ventilation and the second is neurally adjusted ventilatory assistance NAVA and the concept of both of these are that they take actual patient interactions into consideration patient effort and they try to convert the patient effort into ventilation and so it's as if the patient is actually controlling more the ventilation. In proportional assist ventilation or PAV the ventilator generates pressures in proportion to the patient's effort so if the patient has a strong effort there will be a proportional pressure that is generated by the machine and vice versa and so this affords some comfortable levels with ventilation and breathing patterns that the patient is pretty much controlling on his or her own. The main advantages of PAV is that it provides automatic synchrony with inspiratory efforts and it is adaptable and it helps assist in changes in ventilatory demand that will happen pretty much instantaneously. The next mode is the neurally adjusted ventilatory assistance or the NAVA and NAVA takes PAV almost to the next level. NAVA provides proportional pressure support based on measurements of the electrical activity of the diaphragm so now we've moved up a notch to inserting probes that are going to measure the electrical activity of the diaphragm and it's going to make adjustments from that. NAVA is not commonly used and neither is PAV but they are available modes of ventilation that we should at least consider in certain clinical scenarios. So now that we've discussed the more common modes of mechanical ventilation and we've addressed somewhat some of the less common modalities I wanted to go back and really talk about the mechanics of mechanical ventilation so that we can start to grapple with and understand the concepts of how to safely ventilate patients and so I first wanted to share a quote that I share with my students and came across as I was preparing lectures for my students and it kind of was my light bulb moment and it says ventilators are simple machines the ventilator is just a slightly more complex leaf blower. If you think about it that's true the ventilator is generating a flow that's going to generate a pressure and all of that is simply determined by the settings in which we put into the computer system. So when we talk about this this leaf blower this air blower that we like to call a ventilator we want to think about the mechanics of it and how does it do that how are we able to blow air into the lung and generate an effective tidal flow and so to do that we have to remember a couple concepts. The first concept is that the ventilator is working against some resistance. The ventilator is working against the airway resistance which is the RAW and for our patients who are ventilated and intubated this circuit the airway resistance consists of two resistances. The first is the resistance of the ET tube and then the second is the resistance of the airways. So airway resistance accounts for both the ET tube as well as the airways. After the airways we can overcome that resistance. The machine then has to generate enough flow and enough pressure to overcome the elastic resistance and that is going to be called here the resistance ERS so the elastic resistance and just like the airway resistance the elastic resistance in this case has two components. It has the elastic resistance of the lungs how easily will those lungs expand but it also accounts for the elastic resistance of the chest wall and so that the ventilator has to generate enough pressure to create flow to overcome these resistances. And so as we talked about to move air into the lungs as you just saw the ventilator must generate a pressure which we will call P applied that is sufficient to overcome the pressure generated by the airway which again is the PRAW and the pressure caused by elastic resistances which is the PERS. Okay so we'll take those concepts and we will apply the equation of motion and as we talked about the equation of motion how do we generate flow and pressure to move the lung to generate a breath and so the equation of motion says that P applied is equal to the airway resistance pressures as we talked about plus the elastic resistance pressures and if we start to break down each of those pressures into their various components we can start to see things that we are very familiar with. So the airway resistance pressures is going to be related to the flow as well as the airway resistance and so we'll say in this case that P applied is going to equal the flow times the airway resistance. For the elastic pressures we know that V is the volume and it's the volume above the FRC times the elastic resistance will equal the pressures generated by the elastic resistance and then we can do a little bit more changing of the equation and say that you know what elastic resistance or ERS is equal to 1 over the compliance of the system which will mean that the pressure ERS would be volume over compliance and so that leaves us with an equation that says the pressure applied by the ventilator to generate flow and thus the breath is going to need to be equal to the flow times the resistance which is our airway resistance pressures plus volume over compliance which is our elastic resistance pressures. If we can set up and control our ventilator to overcome those pressures those resistances then we can generate flow and that's what we do. So as we talk about using flows to generate pressures I just wanted to remind us of some flow pressure relationships that we use on a regular basis. Again remembering that PAO which in this case is the airway opening pressure or is our P applied is equal to flow times the resistance airways which we have no control over plus volume divided by compliance which we have no control over. So the things that we do control to to provide enough pressure to create the breath or the things are flow and volume and so if we take those relationships what we can see is that on the top we have a flow and we have our two common flow patterns. We have a constant flow pattern some of us may call it a square flow waveform and we have decelerating flow okay and what we can see is that we'll take the constant flow pattern okay if you deliver a breath using a constant flow pattern what you will see in the pressure waveform is that from from peep up to peak your resistive pressures remain constant your distending pressures as the breath goes in and the lung expands start to increase so you will get this additive effect at the peak and it'll give our pressure curve a shark fin pattern. That is different from the decelerating flow wave pattern. In the decelerating flow wave pattern what we see is that our resistive pressures will start off high they will go from low to high and then our decelerating flows will start to come down and so our resistive pressures will start to decrease they don't remain constant like in our constant flow and then our distending pressures are gradually increasing because of the extension of the lung and so what we'd see is actually a squaring of our pressure wave our pressure wave curve and so with decelerating flow we see a square pressure waveform. These are very consistent based on the equation of motion. These are very consistent waveform relationships. So we have our equation of motion where again we're just reminding you that pressure applied or the airway opening pressure is equal to flow times the airway resistance plus volume over compliance which is our elastic resistance. We can use that equation and we can solve for various pressures okay. It is important for us to do that because as we are providing mechanical ventilation to patients is important for us to provide it in a safe manner that causes the least amount of injury to the patient during during a mechanical ventilation. And so we have to understand the mechanical effects of the of the pressures that are generated by our settings and we'll talk about those mechanics and the effects of it in terms of transpulmonary pressures and alveolar stretch. We can think of the stress on the system the transpulmonary pressures and we can think of it on the strain or the actual physical stretch that we get from the volume changes. Transpulmonary pressures and we will talk about this in more detail is not always the measured airway alveolar pressure. And the thing to remember is that stress is uniform but strain is regional. So the system is going to feel the same amount of stress but depending on what's going on in different regions of the lung the stretch is going to be different. We have to keep that concept in mind as we're making sure we're having safe ventilatory practices. So let's dive into this concept of stress and strain a little bit more. Stress is the force applied to an object. In the lung the transpulmonary pressure is the stress. Okay the amount of pressure applied to the lung and transmitted across the lung is the stress. The strain is defined as the deformation of an object under stress. So we have this these two lungs that are being stressed and they're going to conform one way or another. And in the lung the volume the static volume is the static strain and a changing volume can be a dynamic strain. So let's first really grapple with this concept of stress and again the stress is the force applied to an object and it is the stress that stress that we will refer to as the transpulmonary pressure. Alrighty so to start to grapple with this concept of transpulmonary pressures we have to remember PPV is positive pressure ventilation. We're looking at a ventilatory circuit and we have the smaller cylinder as the ET2. Okay then we have the airway and then we have the airway going into the lung or the alveoli and then we have the chest wall and all these various components actually contribute to the pressures and the resistances in the system. So as we talked about from the pressure standpoint you have the airway pressure you have the intrinsic pressure you got ET2 pressure then you have the intrinsic airway pressure and then you have the alveolar pressures and then you even have pleural pressures. Okay and then there are some resistances associated with that as well including the airway resistance the intrinsic compliance of the lung and then the chest. Okay so with the identification of the various resistances that are contributing to the various pressures that need to be overcome by positive pressure ventilation let's discuss this concept of transpulmonary pressures. Transpulmonary pressure or TPP is generally considered to be the pressure difference or the pressure drop across the lung that gives rise to pulmonary ventilation. Okay so you'll say to yourself perhaps Kalila why are we talking about transpulmonary pressures we just need to make sure our plateau pressures and our resistance pressures are in our kind of thought about justifiable levels and we'll be okay and what's changing is this understanding that transpulmonary pressures particularly when used at the bedside can help us understand the influence of the chest wall on air pressure airway pressure and it can help us determine pressures that are needed to keep the lung open concepts that we don't necessarily completely address when we just use markers such as plateau pressures and resistance pressures. And so how are we going to define transpulmonary pressures? So transpulmonary pressures can be measured as the airway resistance pressure minus the pleural pressure okay and it's something to remember when we're thinking about this and we're determining safety indices we do this procedure or we do this this calculation or assessment at the end of inspiration as well as at the end of expiration okay so at the end of inspiration where theoretically the breath has been delivered there is a pause and there is no additional flow you have a situation in which your airway pressure is equal to your alveolar pressure which means at the end of inspiration on that pause your airway pressure is equal to your alveolar pressure equal to your distending pressure which will be your plateau pressure. At the end of expiration theoretically there is no flow and so we'll make some calculations about that as well. So when we make adjustments and assessments of our ventilatory pressures to make sure that we are causing no harm we will definitely need to understand the concepts of these are done on inspiration at the end of inspiration as well as the end of expiration. We'll come back to the end of expiration but just think about it again there is no flow at the end of expiration so that pressure at the end of expiration is going to be peak. We'll run through some numbers. So it's always good to plug some numbers into these concepts so we can kind of understand how these tools are helpful and so what we'll do is look at two scenarios and look at the influence of chest wall stiffness on how we ventilate people. So on the left we have a situation in which we have a stiff lung but the chest wall is fine and so the stiff lung is demonstrated by this the large thick black line around the alveolus okay and we take this stiff lung and we apply a pressure in this case of 39 centimeters of water. We want to know if that's a safe ventilatory strategy and we want to do that by calculating the transpulmonary pressure and as we stated on the last slide the transpulmonary pressure is going to be your alveolar pressure in this case your plateau pressure minus your pleural pressure which we get from the insertion of esophageal balloons. In this case we have a alveolar pressure of 39 at the end of inspiration when there's no flow the alveolar pressure is equal to your airway pressure and that's 39. We have a esophageal, a pleural pressure measured by the esophageal monitor of five centimeters of water which would give us a transpulmonary gradient of 34 and we like to keep our transpulmonary pressures a lot lower than that around 20 ish and so in that situation we probably need to back off of the 39 centimeters of water. If we look at the other situation that is on our right in which the chest wall is stiff the chest wall is not moving situations of kyphosis or surgeries with scarring or more commonly experienced obesity we can take that same pressure of 39 centimeters of water but now because the chest wall is so stiff the pleural pressures are different and so with that same 39 centimeters of water the pleural pressure is now 30 as opposed to the 5 when it was not the chest wall and with those same settings we have a transpulmonary pressure of 9 which suggests that we probably are not causing a lot of harm with our settings and that we could potentially even increase somewhat. Now let's take a look at the influence of effort on transpulmonary pressures and so if we have a patient who is let's say heavily sedated paralyzed and this is representation on the right a passive patient if we apply 30 centimeters of water and the esophageal pressure is 5 we're going to get a transpulmonary pressure of about 25 we're going to get some distinction of that low. If we take a patient who is putting forth significant effort generating a significant negative pleural pressure and we apply that same 30 centimeters of water and so the effort is so much that we're dropping our pleural pressures down to minus 10 if we calculate the transpulmonary pressure we get a transpulmonary pressure of 40 which is way too high and way out of our range and could potentially cause more trauma to the patient. So when we talk about TPP transpulmonary pressures on inspiration and we put that in context of effort and chest wall compliance we remember that TPP is alveolar pressure minus the pleural pressure and we then remember that because there's no flow and there's a level of equilibrium the P plats or plateau pressure is going to be our alveolar pressure at the end of inspiration. So in a passive patient who has normal chest wall compliance the plateau is going to be low okay for a given pressure and the plateau pressure is going to be close to the to the transpulmonary pressure so you're not going to have these wide transpulmonary pressures which is a safe mode of ventilating. In a passive patient who has low chest wall compliance and and our example here would be morbid obesity the plateau pressure is going to be high the pleural pressure is going to be high okay and in that setting because the chest wall is stiff the plateau pressure is going to be much higher than the transpulmonary pressure or the pressure across the lung and so we're going to have to calculate the transpulmonary gradient we have to see if we're reaching our goals if we have enough pressure in the system to get our ideal distention and then we also have to remember the impact of of effort. In an active patient a patient who is able to generate a negative pleural pressure the pleural pressure is going to be low and the plateau pressure is going to typically be much lower than the transpulmonary pressure so again we have to make take into account our settings and how those are impacting the pressures that are being seen by the patient's lung. Let's look at expiration so if inspiration our pressures were going to be equal to our plateau pressures on expiration our pressures are going to be equal to our PEEP at the end of expiration and we have to understand how this concept and chest wall stiffness impacts our ventilatory settings as well and our stress and so in this setting we have what we saw at the beginning we have to the left a stiff lung okay and we have to the right a normal lung but the chest wall is stiff and we want to know in that setting do we need more PEEP something we ask ourselves every day when we're managing ARDS so if we go to the the classic ARDS patient with the stiff lung if we if we say we're applying 15 centimeters of water so PEEP of 15 and we have an esophageal balloon in place to measure the esophageal pressures which reflect our pleural pressures we get pleural pressures of 5 okay and that would give us a transpulmonary pressure so the PEEP minus the esophageal pressure that would give us a transpulmonary patient a pressure of 10 so that tells us you know we're in our range and that's the appropriate PEEP and so we don't need to add additional PEEP okay we take the chest wall patient again we use obesity as as as one of the examples of chest wall compliance issues we apply that same 15 centimeters of water but now because the chest wall is tight and it doesn't expand that same PEEP generates an esophageal pressure which reflects the pleural pressure of 20 okay if you calculate the transpulmonary pressure it is a negative 5 centimeters of water negative transpulmonary pressures elimination of transpulmonary pressure causes lung collapse so we actually despite the PEEP of 15 in this situation we are not achieving our goals of PEEP which is actually recruitment of alveoli so what would we do in this situation we would increase the PEEP recalculate a transpulmonary gradient and increase it to our goal so that we can be in a range in which we will ideally recruit the alveoli so we talked about stress the pressure applied to the system and we talked about stress in terms of the transpulmonary pressure so now we're going to talk about strain and remember we define strain as the deformation of an object under stress we apply a stress and how does that object in this case the lung change based on that stress and that concept is is we're going to define a strain it is important to remember that strain, which we're going to define as alveolar stretch through volume expansion, that the strain drives recruitment, ventilation, and injury. And that's what we're trying to prevent, is we want to recruit, we want to ventilate, but we want to prevent additional injury. So along the lines of lung injury, we'll take a look at this study from 1988. And it basically is a study that shows it's not just pressure, but it's also stretch that can cause lung injury. And what the study did was it ventilated rats at different strategies, which we'll talk about, and it measured lung water as a measure of injury in these rats. And what the study showed is that if you ventilated patients with low volume and high pressures, that's the middle column, even if they had low compliance, you had the same degree of injury as your control group. Okay? But if you took a rat that had normal compliance, and you gave them high pressures, but also high volumes, the degree of lung water, and so the degree of lung injury increased. And the same thing happened if you ventilated the lungs of rats with low pressures, but high volumes. So injury is not just a pressure concept. It is also an alveolar stretch or a strain concept as well. So with that concept, we should approach ventilation in the sense of three major mechanisms of causing ventilator-induced lung injury, and three ways in which we can protect the lung from that. The first is collapse-reopen strain. So we fix that by using PEEP. Then there's this tidal stretch dynamic strain. And that's when we ventilate over eight cc's per kilo. So we've decided as a community, we should stick to six to eight cc's per kilo. But now there's this concept as well that we need to remember, that we need to minimize transpluminary pressure. We need to minimize the stretch static strain as well. So we have to take into account the strain of the system, the stress as well. As we talked about earlier, stress is uniformly applied. Okay? The pressure applied to the system is uniform. The strain, and thus the recruitment of the system, the recruitment, the ventilation, and the injury is regional. Depends on what's going on down there. And we'll see this as an example. You have a restricted lung. Again, the compliance is impacted, denoted by the thick black line. You apply, you have a normal chest wall. You apply the same plateau pressure. In one area, let's say ARDS, you have the severely impacted area. Okay? You have a pleural pressure that's generated of five centimeters of water. And so your transpluminary pressures, in both cases, is 34. In one situation, as we see on the left, we may not necessarily have the best recruitment of that area. And our transpluminary pressures are still too high. We wanna keep them at least less than 30, maybe even less than that. And then the other area, what is not as impacted, the chest wall is not an issue, and that area of the lung is not really impacted by the ARDS. You have the same transpluminary pressure of 34, but now we have over-distension, which we wanna avoid as well, because we don't wanna add any additional lung injury. So we have to remember that the stress, the stretch, over-distension of areas of the lung is important to keep in mind as well. So before we go into some examples, I want to just reiterate some key points. The first is that transpluminary pressure and alveolar stretch, one is the stress of the system, the other is the strain. The stress, or the transpluminary pressure, is not always measured solely by the airway or alveolar pressures. So our peaks and our resistances and our plateaus alone aren't always the best gauge of this. And stress is uniform. The whole system sees the stress. But strain, or the physical stretch by the volumes that are generated, the changes in volume from FRC, they too can cause injury, and we have to pay attention to that as well. So we use this concept of transpluminary pressures to deal with that. So let's just take the principles that we've discussed, and let's apply them to a case. So let's look at the effects of positive pressure ventilation in a case. In this case, we have a patient that we are ventilating with an inspiratory flow. It's a constant flow of one liter per second. We have a tidal volume of a liter. We have peak pressures that are ranging about 40, plateau pressures ranging 30. We have a PEEP of zero. And then we inserted an esophageal balloon because we wanted to know the pleural pressures. And the peak esophageal pressure, which would be the pressure on in inspiration is 10, and the base esophageal pressure, which would be the pressure in expiration is zero. So let's first calculate the transpluminary pressure and inspiration. And as I talked to you earlier, we want to calculate the transpluminary pressures in both inspiration and expiration to give us a bigger picture of our ventilatory strategy and if we're reaching our goals. So in this case, the transpluminary pressure in inspiration would be your plateau pressure minus your esophageal pressure during inspiration, which is your peak esophageal pressure, which would be 30 minus 10, which would give you a transpluminary pressure of 20, which is relatively ideal. Now you want to calculate your transpluminary pressure at expiration because we want to know if we have enough people in the system. And so to calculate your transpluminary pressure at expiration, it is your base pressure, which would be your peak minus your base esophageal pressure, which would be zero. And so that means you have a transpluminary pressure of zero and that would become problematic. And if we were to do that math, we would say to ourselves, you know what, I don't want a transpluminary pressure of zero because that sets me up for atelectasis and decreased recruitment. So based on that, I need to increase my peak so that I can get a transpluminary expiratory pressure of about maybe 10, maybe even five, but not zero because zero subjects us to atelectasis. All right, we're almost done. We're going to step away from transpluminary pressures. We're going to go back to our standard pressures, our peaks, our plateaus, et cetera. And the question is, if you look at X, and X is the baseline tracing, and you have a patient who has developed acute pulmonary edema, can you pick out the tracing, A, B, C, or D, that would correlate with the development of acute pulmonary edema? Looking at both the flows, the volumes, the airway pressures, and the esophageal pressures. So if you pick B, you are correct. B represents low compliance, which is what we see in the clinical scenario of acute pulmonary edema, okay? What we see in B is that the flow is unchanged. We see the volumes are unchanged. We see an increase in the plateau pressure, as can be seen by this kind of bump up in the plateau line. That's what it's representing on the airway pressure curve. And we don't see any changes in the esophageal pressure, because nothing changed about the chest wall, the chest cavity. It was a change intrinsic in the lung. If you were to think about which scenarios what we see in A, C, and D, A would represent something that caused increased resistance, like a bronchospasm. The flow still is unchanged. The volume is unchanged. The resistance is higher, because now we have, compared to X, a much higher difference between the peak and the plateau. And again, as kind of a measurement of the chest wall compliance, it hasn't changed, or esophageal pressures don't change. C would actually be a change in chest wall. And what we would see is this plateau pressure bump up, the esophageal pressures now bump up, but the flow and the volumes stay the same. And D would just represent an increase in flow. And what you see, like someone came and changed the flow, you see an increase in the flow, which again gave you an increase in your resistance, as you see in the airway pressure curve. But your esophageal pressures did not change, and your volumes did not change. So let's take our same patient and do some other more traditional calculations. Let's talk about what the driving pressure is, what the compliance is, and what the resistance, the airway resistance pressures are. And just to remind us, the driving pressures we define as assessment of lung strength. Okay, how are we stretching this lung? So if you remember the law of equation, the equation of motion, and you make some algebraic manipulation of the equation, you can solve for resistance. Resistance, as we know, is your peak pressure minus your plateau pressures over your flow. In this situation, our peak pressure is 40, our plateau pressure is 30, and our flow is one liter per second, which gives us a nice resistance of about 10. We wanna see resistance is less than 15. So this ventilatory strategy looks great. All right, what's our driving pressure? What is this global lung strain? We define our driving pressure is our plateau pressure minus our peak. So in this scenario, our plateau pressure is 30, our peak is zero. So we have a driving pressure of 30. We like to keep the driving pressures less than 15. So we would need to make some adjustments so that we can get a more acceptable driving pressure. And the last thing we'll calculate is the compliance. And if we remember, compliance is equal to the tidal volume over the driving pressure, which is equal to the tidal volume over the plateau minus peak, which means that our compliance in this scenario is one liter, which is 1,000 mLs over plateau of 30 minus peak of zero, which gives us a compliance of 33 mLs per centimeter of water. So you can use driving pressures to calculate compliance, but you can also use compliance and tidal volume to calculate our driving pressures. And the ratio of volume tidal volume specifically to compliance, it can be easily calculated and has been shown to be the final mediator of the effects of lowering tidal volume and plateau pressure on mortality. So paying attention to driving pressure has been shown to impact mortality. Okay, we're almost done. So let's talk about fine tuning our settings as it relates to the concepts that we've talked about today. We have a patient with ARDS who is being ventilated with volume assist control. She has extensive infiltrates on chest radiograph, and we want to scale her tidal volume to functional lung size. And she has a markedly reduced remaining functional lung. And we wanna do that rather than the conventional ideal lung size predicted by ideal body weight. So the question is, we don't wanna use body weight. We want to figure out another way to adjust her tidal volumes. What is a practical way to do this? And as we talked about, the practical way to do this would be B, to scale the tidal volume to compliance or use the driving pressure that we talked about that has been shown to impact mortality. So set your goal to a driving pressure of about 10 to 12. And lastly, just wanted to talk about the impact of pressure volume relationships and remembering some safe zones and why we do the things that we do. So if you look at the pressure volume relationship, and this is a inspiratory breath, and we can get this tracing on many of the ventilators. If we wanted to see kind of in real time inspiration and exploration, how we're doing with our set settings, we can pull up this graph. And what we see is that as we increase our pressures, we have a lower inflection point that kind of gives us this complete pop open and we can move nicely up the inspiratory limb and get some good volume recruitment. And then there's an upper inflection point that will start to beak out. And if we see that, that means we've exceeded our upper inflection point. We're actually rounding to hyperinflation and we need to back back, scale back to get closer to the upper inflection point that is gonna prevent hyperinflation. And then we look at the expiratory limb, okay? An expiratory limb shows us that as we're coming back down, we don't wanna go past the lower inflection point because if we do, we're gonna end up in this zone of atelectasis, which is why we like peat, to prevent from moving into the zone of atelectasis. And so if we ever have questions about our ventilatory strategy from a volume pressure relationship, we can look at the curve and make sure that we are ventilating in an appropriate way. And just to conceptualize this a little bit more, we'll take a look at one scenario in which this is a patient that we're ventilating that has poor compliance. And what we see is that the lower inflection point has shifted over. And what that means is that you need a higher pressure to generate the same volume in this patient. So remembering our relationship between volume and pressure and knowing how to look at our pressure volume curve to help us make some decisions about our ventilatory strategies. So some take home points. The first is that TPP or transpulmonary pressure drives the stretch and strain, which drives recruitment, ventilation, and injury. So we really should be moving away from our traditional peak plateau assessments and moving more towards understanding transpulmonary pressures and what that is telling us about the system as a whole. Airway alveolar pressures alone underestimate TPP with effort. And airway alveolar pressures alone overestimate TPP with stiff chest wall, which is kind of what we saw in that situation with the obese patient where we had not provided enough PEEP. And the second take home message is injury caused by excessive maximal stretch, tidal stretch, and collapse and reopening. Those were the three mechanisms of injury, ventilator-induced lung injury we talked about. And we need to keep that in mind because we need to approach that and minimize the injury. So a simple approach is what standardly we do, and that's to keep our plateau pressures less than 30, our tidal volumes less than six to eight cc's per kilo, and use the best PEEP possible. But a newer approach and a little bit more nuanced approach taken into other factors of the system is to think about the stress index and the driving pressures and use those to help guide us. That's the end. Thank you for listening.

mechanical ventilation

ventilators

terminology

breath components

modes of ventilation

volume control

pressure control

pressure support ventilation

transpulmonary pressure

alveolar stretch

ventilator settings