Positive end-expiratory pressure

Probably already familiar with these, but let's go over them anyway.

Tidal volume, respiratory rate, FiO2, and PEEP.

Tidal volume is the amount of gas delivered with each breath.

We base a lung predictive tidal volume on predicted body weight, which comes from height and gender rather than the patient's actual weight.

Room air is 21% and the ventilator can deliver concentrations up to 100%.

We titrate toward the lowest concentration that meets the patient's oxygen target because prolonged exposure to a high FiO2 raises the risk of oxygen toxicity and absorption atelectasis.

Then of course we have PEEP or positive end expiratory pressure.

PEEP is the pressure left in the lungs at the end of exhalation.

Your pulmonary pressures in general are quite low.

And so the effects of positive pressure ventilation can be quite significant, especially when you start getting above 10, 15 of PEEP.

you're also going to extrinsically compress the left heart.

Now, most of the time, this doesn't really affect the left heart so much because your left heart, it's strong, it's muscular, it's thick-walled.

On the other hand, your lungs are kind of bad.

If you are in this situation, you're kind of in trouble.

Because on the one hand, adding more volume to the system will help you insofar as if you do that, your hemodynamics are better going to tolerate increased PEEP.

On the other hand, that may not be the cleverest idea.

And this is a representation of what driving pressure is, but is not as clinically useful.

The more clinically useful equation, which is the one that most people use, is that driving pressure is the plateau pressure minus the PEEP.

And this equation is best applied to a patient who is passive, so they don't have a lot of ventilator desynchrony or interaction with the ventilator, and they're on volume control.

So they can't be spontaneously breathing, they can't be desynchronous.

You pump a breath in and then you just hold and you check the pressure.

And this is more representative of things like lung compliance or alveolar compliance.

And the driving pressure then is the plateau pressure minus the PEEP, the PEEP being the positive end-expiratory pressure.

And if we really think about this, what we're saying is I'm going to push a breath in and hold it, which is going to help me just see kind of alveolar pressures, lung compliance.

You can search Whiteboard Medicine driving pressure, Whiteboard Medicine stress index, and they'll pop right up.

But if you do have some foundational understanding or you're just looking to dive right into it, by the end of this, we hope you will have kind of a good grasp on how these two measurements compare and how we can kind of use them to better understand the optimal kind of PEEP and distension of those lung units.

As always, none of this is intended to be acted upon as medical advice.

Please read this disclaimer in its entirety before moving on.

So driving pressure, right, is looking at a singular number, the pressure generated when you deliver a tidal volume.

The stress index is looking at the shape of the pressure time curve as you are delivering that tidal volume to help understand how the compliance is changing when you're delivering that breath.

So going into the driving pressure a little bit more, driving pressure is the pressure applied above the PEEP to deliver a breath.

And it's a mathematical equation.

And that is what the stress index, which is a measurement on the ventilator, can help us understand.

It can help us titrate our PEEP, titrate our tidal volume, but mostly our PEEP and ensure that we're trying to optimally distend those alveolar units to limit the amount of ventilator-induced lung injury.

So why does the stress index work? And then we're going to get into exactly what it is.

Well, to measure the stress index, to understand the stress index is to understand how we measure it.

And that is under distension.

That's a bad thing.

That patient might need more PEEP, more positive pressure to try to keep those alveoli recruited so that they don't collapse all the way down when the breath is done.

whereas a stress index a pressure time scaler that is um instead of ballooned out it's kind of sucked down these are words we're trying to use for those listening but again the drawing is going to be way better but it's kind of pushed down right the curve bends inward and that is that compliance worsens during inspiration So as you're giving that breath, the compliance that at first was pretty smooth is now worsening because you are over distending those lung units.

We're gonna watch driving pressure, often aiming around fifteen or less.

We're gonna increase respiratory rate as needed up to about thirty-five in an ARDS-style protocol while checking for auto-PEEP.

I think one thing we don't wanna reflexively do is just think if we increase the resp rate at a set tidal volume, that's gonna result in improved CO2s.

If we're not giving patients enough time to exhale, they will trap an auto-PEEP, and that certainly can make the situation worse in terms of a respiratory acidosis.

And then when it comes to the PaCO2, we're not chasing a normal number at the expense of lung protection.

ARDS Net recommends targeting a pH of seven point three to seven point four five.

Two position and patency, secretions, ruling out complications of barotrauma like an unrecognized pneumothorax.

Check the circuit.

It reflects two things, airway resistance and lung compliance.

PEEP or positive end-expiratory pressure is the pressure left in the lungs at the end of exhalation.

Its job is simple, to keep alveoli open and to prevent collapse.

Plateau pressure is the pressure inside the alveoli when all flow is paused.

The lungs are stiff.

Think ARDS physiology, pulmonary edema, or bilateral pneumonia.

Driving pressure, which is the plateau pressure minus the PEEP, tells you how much pressure is stretching the lung.

Higher driving pressures means that you are at a higher risk of lung injury.

It really is.

You've got like ARDs as PEEP, VAP, SVT, ICP.

It feels like you need a secret decoder ring just to walk onto the ward, like SVT, for instance.

Superventricular tachycardia.

If we never lift the fog of sedation, we'll never know if they are strong enough to come off the life support machine.

Okay, let's get into the heavy physics of the ventilator settings themselves.

I want to explore PEEP, P-E-P, positive and expiratory pressure.

I have to admit, it sounds like a cute Easter candy.

After admission, they were randomized to CPAP or NIPPV.

Nasal CPAP was given with a PEEP between 6 to 8 and an FIU2 between 21 and 40%.

NIPPV, standard stuff really, PEEP of 6 to 8, PIP of 15 to 20, high time of 0.3 to 0.4, respiratory rate 20 to 40 breaths per minute, FIU2 21% to 40%.

Now, if maximal parameters were insufficient to maintain pre-ductal SAT of 90 to 95%, FIU2 was gradually increased up to 40%.

At that point, not at that point, actually, surfactant administration.

What I see floating around a lot on the socials is, you know, increasing the peep.

The one point about increasing the peep is like, we don't know if you increase the peep.

depending on where you are, sort of like in the compliance curve of the lung, you know, increasing the PEEP might just overall increase the pressure in the system.

So the way you make the driving pressure less than 15 with PEEP is if you're recruiting lung, i.e. you recruit lung, therefore the same amount of volume takes less of a change in pressure because you've recruited lung that that total volume will now sort of flow into.

So, you know, by definition, your compliance is better.

But I think that we've made it really simple and just said like increase the peep and the driving pressure will get better.

So that pressure is spread out over a larger area and we've recruited lung, which is what we want to do because when we put in a certain amount of total volume with six CCS, you know, we can argue about that, but you know, a little bit of total volume sort of on not quote unquote normal, we want to make sure that that volume is not descending the alveolus and causing barotrauma.

We want to make sure that the volume in of itself is like a safe quote unquote volume, because otherwise you get something like volume trauma.