Understanding CPAP and PEEP in Ventilation

Welcome to another MedCram lecture. We're going to continue in our series here. We're going to talk about additional modes of ventilation, specifically now CPAP or PEEP. CPAP is typically the term that is used if this is non-invasive ventilation. In other words, if instead of putting a tube down into someone's throat, we instead did a mask. to fit over their face, like a BiPAP mask or a CPAP mask, in fact, is what it's called. So that would be more CPAP. If you have a ventilator on, and it's invasively, then it's kind of known as PEEP. So what is this? What is this mode of ventilation? This mode of ventilation is simply, you're not dialing in a vent setting. You're not dialing in a specific volume. In fact, what you are dialing in, however, is a pressure. But this pressure is continuous. In other words, it doesn't matter whether the patient is breathing in or whether the patient is breathing out. They're always getting the same amount of pressure. And so you can set CPAP anywhere from 5 centimeters of water pressure all the way up to 20. PEEP is usually measured in millimeters of mercury and that can be anywhere from 5 to 20 as well. But these pressures that we see here are pressures that are there on inspiration. and specifically on exhalation. And that's where this PEEP comes from. PEEP stands for positive and expiratory pressure. That positive and expiratory pressure is there even at the end of exhalation so that when you breathe out, instead of having zero pressure in your lungs, there's actually still pressure in there. So let's say if it's set to five, there's still pressure of five in your airways. Now, what's the purpose of this? So basically you're breathing in and out with the same pressure. This could have a lot of good impact in the non-invasive mode if you've got obstructive sleep apnea because that pressure in your airway is going to keep your tongue forward. It's going to keep your airway open so that you don't have obstructive sleep apnea. In the invasive mode of ventilation here, remember these airways come down to small little tiny airways where they have little grape-like clusters. That PEEP or that positive inexpiratory pressure is going to keep your tongue forward. is really good at keeping those alveoli open and what we call recruited. So if you'd like to keep those alveoli open, you can increase the PEEP on the ventilator. And usually it's its own switch here, or basically its own dial, where you can increase the positive and expiratory pressure. The next mode of ventilation is called pressure support, or just PS. Now, this is kind of like PEEP and CPAP, except it only occurs in the same room. on inhalation. So this is kind of like AC except that instead of delivering a specific volume, this patient is going to get a certain amount of pressure. So it delivers a specific pressure support for each breath. Now the patient initiates all the breaths. So patient initiates each breath. This is a little different from pressure control where in pressure control you can actually set the rate. So higher pressure supports give bigger breaths. The bigger the pressure, the bigger the breath is going to be. This is a very popular weeding mode of ventilation as well as CPAP from what we talked about. So we've already talked about four different modes. We've talked about AC where the patient triggers the ventilator and the patient gets a specific volume of breath. We've talked about... pressure control where the patient or the ventilator can both cause the ventilator to give a specific pressure and then come back down to specific pressure. And we've talked about CPAP where the patient is on a continuous pressure regardless of whether or not they're taking a breath in or out. And then we just talked about pressure support ventilation where on each breath that the patient triggers the ventilator on, they get a specific amount of pressure. That could be anywhere from 5 to 15. Okay. Now we're going to go into these modes a little bit more carefully and kind of dissect them out a little bit more for your benefit. Now there's four things that you should know about writing the orders for vent. The first thing you need to know is you need to write a mode. In this case, we're going to do AC. Remember, that's assist control, continuous mandatory ventilation. This means that whenever the patient triggers the ventilator, he's going to get a certain amount of volume, but you're going to set up a backup rate. So that's the other thing that you put with this is the backup rate. So if we put 16, that means the patient is going to receive at least 16 breaths per minute if he doesn't breathe. The ventilator is going to give that to him anyway. The next thing you're going to put in, since this is AC, is you need to put in a tidal volume. And let's just say it's going to be 550 milliliters. The next thing you put in is the FiO2. So what is that? The FiO2 is the fractional inspiration of oxygen. And let's just say we're going to set it to about 50%. That means 50% of the volume that the patient's breathing is going to be oxygen. And then finally the last thing that we set is the PEEP. Let's say it's 5. So these are the four things that generally are set in AC mode ventilation which is the most common mode of ventilation that you'll see in a medical intensive care unit. So what we're saying here is we're using assist control. That means that the patient gets a certain volume. We're writing in what that volume is. We are putting in how much FiO2 the patient's going to be receiving, how much oxygen, and then how much pressure is going to be left in the circuit at the end of exhalation. Remember, this pressure is there to recruit alveoli. And so what you'll notice here is that the first two parameters are going to affect carbon dioxide. How fast you're breathing and how much breath you're taking with each breath is going to affect your minute ventilation. And the last two is going to affect your oxygenation. Obviously, the amount of oxygen you put in is going to affect how much oxygen you read on your saturation. And as it turns out, the higher the PEEP, the more alveoli you can recruit, and therefore, the better the oxygenation is going to be. And of course, we can manipulate these values to get the effect that we want with the blood gas. By the way, if there's any questions about blood gases, please refer to our acid base. lectures on interpretation of acid base and blood gases. So let's take a look and see what a pressure volume flow graph would look for AC mode of ventilation. Okay so what we have here recall is AC 16, tidal volume 550, Fi O2 of 50% and a PEEP of 5. So the first thing you want to notice, let's look at the pressure diagram. Remember there's a PEEP of 5 and so there will always be a certain amount of pressure in the circuit until the patient takes a breath. Now when the patient takes a breath in, that pressure is going to go down to a negative pressure. And at that point it's going to trigger the ventilator that it's time to give a certain amount of breath. And so the volume that gets delivered is going to go up. to a certain preset tidal volume. Okay? And of course, as that volume starts to enter into the lung, the pressure in the lung is going to go up until it reaches the same point at a maximum. Now, of course, flow into that lung is going to start right at that time, and it's going to immediately go up. and it's going to be a constant flow that you can preset until it reaches that point. So this here, you can actually set, by the way, it's not here in the four different settings, but you can actually set how fast that flow is going to go in, and that's important when we talk about ventilating patients in COPD. Now, when you have that preset tidal volume that you've set into your ventilator, this is when the ventilator is going to stop giving flow. And what you'll see is flow will not only stop, but flow will start to come out of the patient and start to go back to normal. And of course when that happens, the pressure in the lung will start to fall back down, but it will not go to zero because remember we have a PEEP of five. That means there's always a pressure of five left in there. And of course when that occurs, volume will come back out of the lung again and come back. to zero. And so what we have now is the status quo where we have a pressure of five left in the lung, flow is back to zero, and volume is back to zero. And the same thing will happen again. If the patient decides to have another breath, the pressure will go back to zero and go beyond it, which will trigger the ventilator to do the same thing again. And so you'll see that the flow rate will go up and then back down again. And here, right when the ventilator is triggered, you will see volume go back into the lungs and then come back out again. Here, of course, as soon as the trigger is set, you will see flow go up at a certain constant rate until the target volume is reached, then flow will come back out again and it will go back to zero. Now if the patient decides not to get a breath, okay, so we're talking about a patient let's say who is overly sedated, but you've set a backup rate and because you've set a backup rate, the ventilator is not going to allow the patient to go long without a breath and that will look a little bit different. Because the patient will not have triggered a breath, you will just see instead of a negative deflection, you will just see it go up. And at that time that it decides to go up, everything else on the ventilator will look the same. Flow up, flow across, flow down. Notice that in a patient triggered breath, you will see a negative deflection. in the pressure circuit, but in a ventilator given breath, it will be missing. Now, this is a good time to again talk about compliance of the system. Notice that when we're talking about the AC mode of ventilation, there is a preset tidal volume that we are entering into in this ventilator. If for some reason the compliance of the system goes down, the ventilator is still going to give the same volume. But what you would see is you would see a higher pressure. And that higher pressure is a result of decreased compliance in the lung. And of course the thing to know there is you can actually set a pressure alarm here so that if the compliance of the lung does go down, and what I mean when the compliance of the lung goes down is let's say it becomes with pulmonary edema or there's a pneumothorax or there's something that prevents the lung from expanding. as easily as it would have normally been. If this pressure exceeds the set pressure alarm, there will be a bell that goes off and the respiratory therapist or the nurse will be drawn to the bedside because there's a problem. So remember, in AC mode you dial in the volume, the pressure is variable depending on the compliance. But as most things in medicine, it's not always as simple as you may think. there's actually two types of pressures that you've got to be concerned about. One is a peak pressure and the other is a plateau pressure. And we're going to talk about that in the next lecture.

In other words, if instead of putting a tube down into someone's throat, we instead did a mask. to fit over their face, like a BiPAP mask or a CPAP mask, in fact, is what it's called. So that would be more CPAP.

If you have a ventilator on, and it's invasively, then it's kind of known as PEEP. So what is this? What is this mode of ventilation?

This mode of ventilation is simply, you're not dialing in a vent setting. You're not dialing in a specific volume. In fact, what you are dialing in, however, is a pressure.

But this pressure is continuous. In other words, it doesn't matter whether the patient is breathing in or whether the patient is breathing out. They're always getting the same amount of pressure.

And so you can set CPAP anywhere from 5 centimeters of water pressure all the way up to 20. PEEP is usually measured in millimeters of mercury and that can be anywhere from 5 to 20 as well. But these pressures that we see here are pressures that are there on inspiration. and specifically on exhalation.

And that's where this PEEP comes from. PEEP stands for positive and expiratory pressure. That positive and expiratory pressure is there even at the end of exhalation so that when you breathe out, instead of having zero pressure in your lungs, there's actually still pressure in there.

So let's say if it's set to five, there's still pressure of five in your airways. Now, what's the purpose of this? So basically you're breathing in and out with the same pressure.

This could have a lot of good impact in the non-invasive mode if you've got obstructive sleep apnea because that pressure in your airway is going to keep your tongue forward. It's going to keep your airway open so that you don't have obstructive sleep apnea. In the invasive mode of ventilation here, remember these airways come down to small little tiny airways where they have little grape-like clusters. That PEEP or that positive inexpiratory pressure is going to keep your tongue forward.

is really good at keeping those alveoli open and what we call recruited. So if you'd like to keep those alveoli open, you can increase the PEEP on the ventilator. And usually it's its own switch here, or basically its own dial, where you can increase the positive and expiratory pressure.

The next mode of ventilation is called pressure support, or just PS. Now, this is kind of like PEEP and CPAP, except it only occurs in the same room. on inhalation. So this is kind of like AC except that instead of delivering a specific volume, this patient is going to get a certain amount of pressure.

So it delivers a specific pressure support for each breath. Now the patient initiates all the breaths. So patient initiates each breath. This is a little different from pressure control where in pressure control you can actually set the rate. So higher pressure supports give bigger breaths.

The bigger the pressure, the bigger the breath is going to be. This is a very popular weeding mode of ventilation as well as CPAP from what we talked about. So we've already talked about four different modes.

We've talked about AC where the patient triggers the ventilator and the patient gets a specific volume of breath. We've talked about... pressure control where the patient or the ventilator can both cause the ventilator to give a specific pressure and then come back down to specific pressure.

And we've talked about CPAP where the patient is on a continuous pressure regardless of whether or not they're taking a breath in or out. And then we just talked about pressure support ventilation where on each breath that the patient triggers the ventilator on, they get a specific amount of pressure. That could be anywhere from 5 to 15. Okay.

Now we're going to go into these modes a little bit more carefully and kind of dissect them out a little bit more for your benefit. Now there's four things that you should know about writing the orders for vent. The first thing you need to know is you need to write a mode. In this case, we're going to do AC. Remember, that's assist control, continuous mandatory ventilation.

This means that whenever the patient triggers the ventilator, he's going to get a certain amount of volume, but you're going to set up a backup rate. So that's the other thing that you put with this is the backup rate. So if we put 16, that means the patient is going to receive at least 16 breaths per minute if he doesn't breathe. The ventilator is going to give that to him anyway.

The next thing you're going to put in, since this is AC, is you need to put in a tidal volume. And let's just say it's going to be 550 milliliters. The next thing you put in is the FiO2. So what is that?

The FiO2 is the fractional inspiration of oxygen. And let's just say we're going to set it to about 50%. That means 50% of the volume that the patient's breathing is going to be oxygen. And then finally the last thing that we set is the PEEP. Let's say it's 5. So these are the four things that generally are set in AC mode ventilation which is the most common mode of ventilation that you'll see in a medical intensive care unit.

So what we're saying here is we're using assist control. That means that the patient gets a certain volume. We're writing in what that volume is. We are putting in how much FiO2 the patient's going to be receiving, how much oxygen, and then how much pressure is going to be left in the circuit at the end of exhalation. Remember, this pressure is there to recruit alveoli.

And so what you'll notice here is that the first two parameters are going to affect carbon dioxide. How fast you're breathing and how much breath you're taking with each breath is going to affect your minute ventilation. And the last two is going to affect your oxygenation. Obviously, the amount of oxygen you put in is going to affect how much oxygen you read on your saturation. And as it turns out, the higher the PEEP, the more alveoli you can recruit, and therefore, the better the oxygenation is going to be.

And of course, we can manipulate these values to get the effect that we want with the blood gas. By the way, if there's any questions about blood gases, please refer to our acid base. lectures on interpretation of acid base and blood gases.

So let's take a look and see what a pressure volume flow graph would look for AC mode of ventilation. Okay so what we have here recall is AC 16, tidal volume 550, Fi O2 of 50% and a PEEP of 5. So the first thing you want to notice, let's look at the pressure diagram. Remember there's a PEEP of 5 and so there will always be a certain amount of pressure in the circuit until the patient takes a breath. Now when the patient takes a breath in, that pressure is going to go down to a negative pressure.

And at that point it's going to trigger the ventilator that it's time to give a certain amount of breath. And so the volume that gets delivered is going to go up. to a certain preset tidal volume.

Okay? And of course, as that volume starts to enter into the lung, the pressure in the lung is going to go up until it reaches the same point at a maximum. Now, of course, flow into that lung is going to start right at that time, and it's going to immediately go up.

and it's going to be a constant flow that you can preset until it reaches that point. So this here, you can actually set, by the way, it's not here in the four different settings, but you can actually set how fast that flow is going to go in, and that's important when we talk about ventilating patients in COPD. Now, when you have that preset tidal volume that you've set into your ventilator, this is when the ventilator is going to stop giving flow. And what you'll see is flow will not only stop, but flow will start to come out of the patient and start to go back to normal.

And of course when that happens, the pressure in the lung will start to fall back down, but it will not go to zero because remember we have a PEEP of five. That means there's always a pressure of five left in there. And of course when that occurs, volume will come back out of the lung again and come back. to zero. And so what we have now is the status quo where we have a pressure of five left in the lung, flow is back to zero, and volume is back to zero.

And the same thing will happen again. If the patient decides to have another breath, the pressure will go back to zero and go beyond it, which will trigger the ventilator to do the same thing again. And so you'll see that the flow rate will go up and then back down again.

And here, right when the ventilator is triggered, you will see volume go back into the lungs and then come back out again. Here, of course, as soon as the trigger is set, you will see flow go up at a certain constant rate until the target volume is reached, then flow will come back out again and it will go back to zero. Now if the patient decides not to get a breath, okay, so we're talking about a patient let's say who is overly sedated, but you've set a backup rate and because you've set a backup rate, the ventilator is not going to allow the patient to go long without a breath and that will look a little bit different. Because the patient will not have triggered a breath, you will just see instead of a negative deflection, you will just see it go up.

And at that time that it decides to go up, everything else on the ventilator will look the same. Flow up, flow across, flow down. Notice that in a patient triggered breath, you will see a negative deflection.

in the pressure circuit, but in a ventilator given breath, it will be missing. Now, this is a good time to again talk about compliance of the system. Notice that when we're talking about the AC mode of ventilation, there is a preset tidal volume that we are entering into in this ventilator.

If for some reason the compliance of the system goes down, the ventilator is still going to give the same volume. But what you would see is you would see a higher pressure. And that higher pressure is a result of decreased compliance in the lung.

And of course the thing to know there is you can actually set a pressure alarm here so that if the compliance of the lung does go down, and what I mean when the compliance of the lung goes down is let's say it becomes with pulmonary edema or there's a pneumothorax or there's something that prevents the lung from expanding. as easily as it would have normally been. If this pressure exceeds the set pressure alarm, there will be a bell that goes off and the respiratory therapist or the nurse will be drawn to the bedside because there's a problem.

So remember, in AC mode you dial in the volume, the pressure is variable depending on the compliance. But as most things in medicine, it's not always as simple as you may think. there's actually two types of pressures that you've got to be concerned about.

One is a peak pressure and the other is a plateau pressure. And we're going to talk about that in the next lecture.

Transcript for:Understanding CPAP and PEEP in Ventilation

Transcript for:
Understanding CPAP and PEEP in Ventilation