What we’re going to talk about today is ventilator pearls. So, you know the basics about when a patient goes on a ventilator, and what it means in terms of endotracheal tube and inflating the cuff. There are some subtleties, however, in terms of positive pressure ventilation and what happens to the lung and the heart. What is it that a ventilator does to a patient and what are the pitfalls and how can you avoid some of those pitfalls, so that’s what we’re going to talk about in this lecture.
So the first thing you should know about is the major differences between spontaneous breathing and positive pressure ventilation. That is the pressure, so what we’re going to do is we’re going to take a look at the lungs in terms of their anatomy and how this makes actually a difference.
You can see here that we’ve got the trachea, and we’ve got the right mainstem bronchus and the left mainstem bronchus, and those go to the lungs, respectively. Now those lungs are in separate areas, and they are enveloped by something called the pleural. Each pleural is separated from the other pleural, and those lungs sit in the thoracic cavity, but it’s interesting to note that the heart also sits in that thoracic cavity.
So there is a muscle here called the diaphragm, which is split into two hemidiaphragms, and when you go to take a breath in, that diaphragm contracts, which causes it to fall. Now when the diaphragm falls, that causes a negative pressure to form in the intrapleural space. That negative pressure causes the lungs to expand, and as a result of that, air moves into that negative pressure. So that’s called negative pressure ventilation, and that’s exactly what happens in spontaneous breathing.
So spontaneous breathing just like you’re breathing right now. We actually have a negative pressure, and air from the outside is sucked into the lungs, (The key word there is ‘sucked into the lungs’) so that negative pressure is also transmitted to the heart, specifically to the right side of the heart, and that causes more blood to come to the right side of the heart. We’ll talk about that later.
So on inspiration, the right side of your heart actually gets more blood. So that’s what happens in the spontaneous mode of ventilation. This would occur if you’re not on a ventilator or if you’re not on positive pressure ventilation, you are just breathing normally.
If we were to graph this Pressure versus Time and we were to start at zero, what we would see is that you would start off at your functional residual capacity (FRC), as you were to take a breath in, the pressure would go down, and then when you were to relax your lungs and let the air come back out again, the pressure would come back up to a regular baseline.
Now in the case of a ventilator, we have positive pressure ventilation where an endotracheal tube is going down into your lungs, and the endotracheal tube is also hooked up to a ventilator. That causes air to be pushed into the lungs. So we’re not having a negative intrapleural pressure. We’re having positive pressure and this could happen if you’re either on a ventilator or you’re on a BiPAP or a CPAP machine. In both of these cases, you’re going to see a deflection that looks like this, and then back again.
So here is the key that you must understand between negative pressure and positive pressure ventilation: the intrapleural space goes through a completely different type of pressure change depending on whether you’re on negative pressure ventilation or positive pressure ventilation, and that has significant consequences, which we’ll talk about.
So here are some of the consequences that can happen. Here we have our heart, and of course, we know that all venous blood comes back to the right atrium. As we said, this heart sits in this cavity called the intrathoracic cavity, which is generally negative, and that helps out because as you recall all blood goes back to the right atrium.
So here we have again Pressure, and pressure is starting out at around 15 and coming up at around zero to 5, which is right about here. So here is the barometric pressure, or here’s the pressure in the right atrium, which is about 0 to 5. It’s sitting in the negative intrathoracic space and the venous return is quite nicely sliding down the slide.
Now when you go from negative pressure ventilation, or spontaneous ventilation, to going on a ventilator, which is positive pressure ventilation when you intubate somebody, or put them on BiPAP or put them on CPAP, you’re going from this box over to this box. Then since this is now positive pressure ventilation, it’s going to be transmitted. That positive pressure is now going to be transmitted to the right atrium. So instead of the right atrium being at zero to five now, maybe it’s going to be up to 10, and as a result of that, this venous flow, or this venous return, is now going to be coming back to a chamber that is higher in pressure. As a result of that, there is less flow, so venous return is reduced when you go from negative pressure to positive pressure ventilation.
Since the heart can only pump what it’s given, this decrease in venous return is going to also drop the cardiac output, which is going to lead to a drop in blood pressure. That’s the key there that I want to emphasize to you: when you take somebody who is already maybe a little volume-depleted, or is in septic shock and is doing worse and you’re going to intubate, you’re going to take them from negative pressure ventilation to positive pressure ventilation, and you’ve got to be careful with a drop in blood pressure. That’s very very important.
In terms of a real-world example of this, how many times do we see patients that come in with septic shock and their venous return is already a little bit on the limited side. So it’s going to be less. So that’s going to also decrease this slope of getting fluid back to the right atrium. They are sick, their lactic acid level is high. They look ill-looking and you’re looking at them, you’re saying you’ve got to intubate, and already let’s say their blood pressures like 90 over 60 or something low.
You know in the septic shock patients, these are the ones who are already volume-depleted. And if you go ahead and intubate them and put them on positive pressure ventilation, you know that their blood pressure is just going to krump, you need to make sure they’ve got good IV access.
Okay need to have good IV access; think about even doing a central venous line and give them volume because what that volume will do is it will push the pressure up and compensate for the fact that this is already been pushed up as well when you’re about to intubate them.
So I would rather have the venous pressure up before I increased intrathoracic pressure because that way I’m not going to get as much of a drop in venous return, which will correlate with a drop in cardiac output, which will correlate with a drop in blood pressure.
Also, the nice thing about getting a central line in these patients is that remember with a CVL (central venous line) you can measure the central venous pressure. Getting that central venous pressure up in the eight, nine, ten, eleven, twelve region is going to be key especially in your septic shock patients. That way you can actually measure what this right atrial pressure is going to be. That’s important to get a central line in these patients where you have some time. Sometimes you don’t have time, you just have to intubate, but just be aware that if you are and you’ve got IV access, which usually does that, you need to give lots of volume.
Let’s just take a step back just a bit. We talked about going from negative or spontaneous ventilation to positive pressure ventilation. The next step that you can go in this direction is actually adding on PEEP. What’s Peep? Peep stands for positive end-expiratory pressure; the key there is end-expiratory. So what does that mean? You now know that we are putting into the lungs a certain amount of volume, and at the end of that exhalation, volume is going to go in. Okay, and then volume is going to come back out, and at the end of that exhalation, if we decide to leave a certain amount of volume of air in the lungs, we can do that, and by doing that there’s a certain amount of pressure that’s left in the lungs.
Now that pressure is usually there to make sure to keep the alveoli open. So there are these air sacs at the very terminal end of the lungs, and by applying pressure to it, it’s going to keep these alveoli open with the help of surfactant.
So keeping the alveoli open is very important in terms of oxygenation. Sometimes you’ll see that especially in patients with, for instance, congestive heart failure, exacerbations, or ARDS, that you’ll see that the PEEP is elevated in these patients, and the rationale for that is: on the end of exhalation, don’t let all of the air out because that will cause collapse of these alveoli, which have been recruited to give you good gas exchange.
People set PEEP usually like to 5, but you can set PEEP to 10 12 13. Sometimes you have to go pretty high to get oxygenation up. While that PEEP is going to be transmitted to the bronchioles, it’s gonna be transmitted to the respiratory bronchioles and finally to the alveolus as well, because that pressure is left in the lungs and that pressure is just going to get added to this intrathoracic pleural space.
人们通常将PEEP设置为5，但您可以将PEEP设置为10 12 13。 PEEP会传播到细支气管，但也会传播到呼吸性细支气管，最后也传播到肺泡，因为这种压力留在肺中，而这种压力只会增加到胸腔内胸膜腔。
That intrathoracic pleural space is going to go from negative to positive to even more positive. So just as going from negative pressure ventilation to positive pressure ventilation caused an increase in the intrathoracic pressure, going from regular positive pressure ventilation to high PEEP, or increased PEEP ventilation, is going to do the same even more, so the higher you increase that PEEP, the likelihood or chance you have of causing a decrease in venous return, which correlates to a decrease in cardiac output, which finally corresponds to a decrease in blood pressure.
So when I’m at the ventilator, and I’m ventilating a patient, and I’m having difficulty with oxygenation, I’m going to go ahead and increase the PEEP, knowing full well that I’m going to recycle that blood pressure after I do it to make sure that I haven’t crossed the line and cause the blood pressure to drop because of drop in the venous return.
It’s really good with I’ve got an arterial line so I can get real-world data back right away. It’s also helpful to know that when I’m going to increase the PEEP on a patient because of oxygenation, I may have to increase the volume to overcome that intrathoracic pressure and that barrier to venous return. Okay, so that’s positive end-expiratory pressure. And so this is all a continuum: negative to positive, to even more positive with positive end-expiratory pressure.