Have you ever been in a situation where increasing PEEP does not seem to be improving oxygen saturation, despite being on 100% fiO2? How high should we crank the PEEP before we meet diminishing returns? This week on the podcast we are going to discuss everything PEEP. But first, let's set the stage with some pre-show thoughts.
PEEP & MAP
Positive end-expiratory pressure (PEEP) is one way to increase mean airway pressure (MAP) and improve oxygenation. This technique works by increasing the amount of air that is left in your lungs at the end of exhalation beyond physiological norms. However, the correlation between increasing PEEP and improving oxygenation are not linear. At some point, different in every patient, positive pressure can distend the alveoli too much and tamponade the alveolar blood supply. The trick is finding the sweet spot.
PEEP Goals
Our goal with PEEP is to improve oxygenation, promote recruitment, and decrease work of breathing. This is performed by keeping the alveoli partially open between breaths. As my daughter begins to blow up the balloon, she sees very small changes in volume for a given pressure. This is because she must first overcome the air-surface tension. The point in which the balloon actually starts to take form, is called the lower inflection point.
Eventually, the balloon becomes maximally inflated and changes in volume for a given pressure are smaller, this stage is called the upper inflection point. With this illustration in mind, we can say that our first goal with PEEP, is to be above the lower inflection point but below the upper inflection point.
What Governs Alveolar Expansion?
In the balloon example above, the atmospheric pressure is the opposing pressure against inflation. If we inflate a balloon to 10 cmH2O at sea level, compared to 10 cmH2O on top of Mt. Everest, which balloon will be larger?
If you guessed Mt. Everest, you are correct. Atmospheric pressure decreases with elevation. This means you get more inflation for a given pressure. In our body, the opposing force to alveolar expansion is pleural pressure. Because the lower third of the esophagus is exposed to pleural pressure, this value can be measured with an esophageal probe (Grieco, 2017).
Things that influence pleural pressure are illustrated below.
Why is this important? We commonly use plateau pressure as a surrogate for alveolar expansion, but in reality, this is only one part of the equation. For example, consider a situation in which I inflate the lungs until 10 cmH2O of pressure is reached. The variable that will govern alveolar expansion is pleural pressure. This means that your plateau pressure of 30 cmH2O, may mean different amounts of alveolar inflation.
An obese patient will require a higher alveolar pressure in order to overcome the body habitus weighing down on the lungs. There is a reasonable theory that alveoli are more damaged by over-distention and not pressure, which is why many say the term volutrauma is a more accurate than barotrauma when it comes to describing ventilator associated lung injury (VILI).
Pleural Pressure & PEEP
Some of the PEEP you provide for a patient will be counteracting pleural pressure and avoiding atelectasis. Preventing atelectasis means holding PEEP above the lower inflection point and is key to avoiding VILI (Gattinoni, 2017).
But remember how we said alveoli are damaged by over-distention? There are a few ways we over-distend alveoli.
1. Providing Too Much Volume
Providing too much volume is the obvious one. We give too much volume and the alveoli expand beyond physiological norms. The best way to prevent this is by utilizing appropriate tidal volumes based on ideal body weight.
2. Atelectasis Regional Distention
Another form is alveolar distention is the alveoli caught between atelectatic lung and healthy lung. Even at low tidal volumes, the alveoli between atelectatic lung and healthy lung will be forced to distend as they are stuck in the middle between a static and dynamic neighbor.
I demonstrated this phenomenon with a GIF. The middle hexagons are barely moving and represent atelectatic lung. The alveoli adjacent to those are caught between dynamic lung and static lung. This causes stretching and distention of those alveolar units.
By recruiting collapsed alveoli, we reduce alveolar distention in the blue area illustrated. However, too much PEEP may over-distend healthy alveoli in your attempt to recruit collapsed alveoli. How do we know the tipping point between recruitment of alveoli and over-distention? This is the million dollar question and in general, you need more pressure to recruit lung than you do to maintain. This means that when the patient gets better after the hospital increased the PEEP to 15, they may not need you told hold them at 15 for the remainder of transport. In fact, you may actually see a decrease in SPO2 if you do. It is this reason that I have built in specific PEEP stop-points.
PEEP Stop-Points
As a general rule, I use 10 cmH2O of PEEP as a stop-point. This is not to say that I won't go higher, but I use this as a checkpoint to make sure I am working smart in terms of improving oxygenation. As mentioned earlier, one of the goals of PEEP is to increase oxygenation. The reason PEEP increases oxygenation is because it increases your mean airway pressure. PEEP is only one of four ways to increase mean airway pressure.
GIF by Bryan Winchell
I can think of many times early in my career when my strategy for improving oxygenation was cranking fiO2 and escalating PEEP. Sometimes this works, and other times it doesn't. In fact, some patients may actually require a decrease in PEEP in order to improve oxygenation.
When May You Need Higher Levels of PEEP?
There are occasions in which a patient may require higher levels of PEEP to avoid atelectasis and maintain adequate oxygenation, but I think these are actually pretty rare. A meta analysis published in BMC, evaluated a total of nine randomized control trials evaluating high and low PEEP strategies (Guo, Xie, & Huang, 2018). High PEEP strategies utilized a PEEP > 10 cmH2O and showed no mortality benefit outside of a small sub-group analysis that looked at "positive oxygenation response to PEEP." The way this was defined was ANY improvement in P/F ratio from baseline. The study stated:
"In ARDS patients with positive oxygenation response to PEEP (5 RCTs), 28-day mortality was lower in the high PEEP group than the low PEEP group (RR = 0.83; 95% CI, 0.67 to 1.01; P = 0.07)."
Even though this was a sub-group analysis of a meta analysis, I believe the overall message that some patients may require PEEP >10 holds true. One situation may be the patient with a large body habitus and higher pleural pressures.
One reason P/F ratio may improve with PEEP is due to a reduction in cardiac output. If less blood is flowing through the lungs, we reduce the rate of right to left shunt. It is important to look at the entire clinical picture and make sure we aren't chasing a P/F ratio while sacrificing cardiac output (Gattinoni, 2017).
I believe the best way to conclude this article is with one of my favorite excerpts from a blog by Brandon Oto.
Peer Review
Kristin Ireland EMT, RRT
Thanks for asking me to peer review your blog, Tyler! I could picture exactly what you mean when you talk about how increasing PEEP may mean different things in each patient, and how the whole clinical picture is mandatory when ventilating someone. I think there is value in highlighting your paragraph about how in our attempts to recruit collapsed alveoli, we damage the good ones. In pulmonary medicine we tend to focus on the diseased portion of the lung, because that's that portion of the lung(s) that is causing us problems in the acute phase. However, are we causing collateral damage with the methods we use to temporize these areas? That's a question worth asking! There are certainly strategies that cause the healthy portions to take on a very hefty workload.
I have also seen this concept you describe on BiPAP through the vent during a spontaneous breathing trial. When the patient is losing their minds, sometimes it just takes a decrease in the IPAP to help them breathe easier. Just because we see increased WOB, doesn't necessarily mean they need more pressure. I also like to use a recruitment maneuver before I dial up that PEEP. I simply use the manual breath or inspiratory option on the ventilator until I see micro changes on the SpO2. I do this slowly and with care, of course.
Leon Eydelman, MD
Hey really like your blog post! There is some interesting stuff about driving pressure and PEEP titration to optimize that, same thing functionally as being at the best inflection point for compliance but that might be something to discuss. I'm not sure the verdict is in regarding pressure versus volume as main cause of lung damage. Also, some thought that it could relate more to sheer stress with drive pressure multiplied by respiratory rate. I will say we routinely use PEEPs greater than 10, especially with cold a lot of these people are huge. I think the highest I've used is 24 and not as a recruitment maneuver. Totally agree with the arbitrariness of trying to maximize your P:F ratio - it doesn't take into account PEEP, nitric, or oxygen consumption.
The inflection points are a good way to check for best compliance peak, I usually will not mess with that and just change my PEEP by few then check a plateau a little bit later to see if it increased or decreased my drive pressure, as long as it doesn't increase the driving pressure by more than the amount I increase the PEEP, I don't think i'm causing over distention.
I've also definitely been in situations where oxygen saturations respond to PEEP despite already being over distended. Just like you mentioned, those people seem to be the ones with most benefit of higher PEEP. Maybe it's best to just titrate to peep to oxygenation anyway and not worry too much about compliance until your plateau is greater than 30 or something.
It's seemingly more about the drive pressure or the dynamic strain as opposed to the total plateau, that's why I will stop going up on peep at the point where increasing peep leads to increasing drive pressure but until then probably not an issue. That being said I'm always uncomfortable when my plateaus are greater than 30, regardless of how low the drive pressure is, because we do have to remember ARDSnet's ARMA trial was the only thing prospectively studied in humans.
References:
Gattinoni L, Collino F, Maiolo G, Rapetti F, Romitti F, Tonetti T, Vasques F, Quintel M. Positive end-expiratory pressure: how to set it at the individual level. Ann Transl Med 2017;5(14):288. doi: 10.21037/atm.2017.06.64
Grieco, D. L., Chen, L., & Brochard, L. (2017). Transpulmonary pressure: importance and limits. Annals of translational medicine, 5(14), 285. https://doi.org/10.21037/atm.2017.07.22
Guo, L., Xie, J., Huang, Y. et al. Higher PEEP improves outcomes in ARDS patients with clinically objective positive oxygenation response to PEEP: a systematic review and meta-analysis. BMC Anesthesiol18, 172 (2018). https://doi.org/10.1186/s12871-018-0631-4