Driving pressure, the bullet or the damage?


 

 

One of the best human made movies I have ever seen is Bourne Identity, that’s because The Godfather and Schindler’s List were note made by humans, like the pyramids. Have you saw it? Well, you should, it’s fantastic. The movie tells a story of a man who woke up in a boat in the middle of nowhere, having no idea who the hell he is. It starts with a big cliché scene: Bourne, the main character, is unconscious on a table, while some fisherman-surgeon removes bullets from his back. After a while Bourne wakes up and immobilizes the guy, just like that! The show of people being cured from shots like a magic trick, like in Rambo, are a huge hit only TV shows can provide us! But we all know between having a bullet removed and be ready to kill 300 terrorists you have a long walk.

Well, today we’re gonna talk about this! Rehabilitation after gunshot wounds! OF COURSE NOT! This was my intro, to get you all in the mood! Our subject is driving pressure, which for me, is a bullet. It really can kill a patient, but, in some patients, we cannot decrease it. Therefore, we cannot decrease mortality by removing the bullet from their bodies.

I know what you are thinking. “Nonsense! This guy is talking shit, speculations, he has no proof whatsoever!” Well my friends, [Morpheus voice enabled] what if I told you that the best evidence we have says we cannot save lives trying to decrease the driving pressure? [Morpheus voice disabled]. And I will show you that this “theory” is bullet-proof. Let’s start with the basics: Whattafuck is driving pressure? In a simple way: Driving pressure is the difference between PEEP and plateau pressure, it represents the difference between the “lowest” and the “highest” static pressures in a mechanical controlled ventilation cycle. To cut the cheese: it’s the stretching pressure. Inflate->Deflate->Lesion->Death.

Does driving pressure kills?

Yes, it kills. In fact, in some countries, kills more than bullets! Brazil not included. In 2015, Amato and colleagues[1] published a fantastic paper showing how obvious is the driving pressure killing potential. The bigger the driving pressure, the bigger the mortality (something similar to McDonald’s meals). They showed that driving pressure has a positive linear relationship with mortality. “Doesn’t matter” what you do with the plateau pressure or PEEP.

It was about time to find something new and interesting regarding our beloved ARDS, specially if it is something we can interfere with! Scientific community went crazy, and this paper was already cited in 217 articles. Fireworks around the globe! HeliOx parties! Everyone was excited with all this lives about to be saved. Doesn’t matter if you have a God or not! But keep the driving pressure below 15!

If we try to decrease driving pressure, will mortality decrease?

If you read the paper, Amato himself says in the end: “Our work is a post hoc observational analysis. Clinical trials need to be designed in which ventilator changes are linked to achieve changes in driving pressure, in order to determine whether our observations can be translated into changes that may be implemented at the bedside”. Well, there is a problem. Imagine the money, time and effort to design an international multicenter RCT, with 767 adults with ARDS, everybody ventilated with low tidal volume, with a safe limit of plateau pressure (30cmH2O), a control arm with standard ventilation (PEEP 5-9cmH2O) and an intervention arm with the lowest driving pressure possible (meaning, the highest PEEP with a plateau pressure <30cmH2O). Oh dear friend. [Morpheus again] what if I told you this amazing trial already exists? It’s the EXPRESS trial[2].

What a minute! Are you nuts?! The EXPRESS trial was conducted to analyze the effect of High vs Low PEEP on mortality! One of the EXPRESS authors’, Brochard, is also one of the Amato’s driving pressure authors, and in the supplement you have a beautiful table showing that driving pressure was also directly associated with mortality in the EXPRESS trial. Yes, you have bigger mortality because driving pressure kills, however, “treating”the driving pressure does not decrease mortality for two reasons: 1- You usually cannot decrease driving pressure in a meaningful way. 2- The reduction in driving pressure we can achieve does not decreased mortality. And I’ll show you why!

High skill performed graphic derived from the EXPRESS trial below:

EXPRESS trial: Standard (minimal distension) X Increased Recruitment (High PEEP with Plateau < 30cmH2O). In other words -> lowest possible driving pressure.

Take a look on day 1: In the increased recruitment group (lowest driving pressure) we have doubled the PEEP (7,1 -> 14,6cmHO), but the difference in driving pressure was only 1,1 (14 -> 12,9). That’s because the majority of PEEP was translated into plateau pressure, keeping the driving pressure almost untouched (the majority of patients are not recrutable when you respect the safe limit of plateau pressure <30cmH2O). On day 7 the driving pressure was even higher in the high PEEP group, once, even those patients with recruitable lungs on day 1 were not recruitable anymore (ok, that’s something you can argue). Now, to the mortality results the EXPRESS trial:

Although the survival curves appear different, this difference was not statistically significant.

To wrap it up: Driving pressure kills. We cannot decrease the driving pressure in the majority of our patients, at least in a meaningful way, and even if you can do so, the decrease is because you have a better (more recruitable) lung. These thoughts are for a population level, it doesn’t mean you patient fit into it. Driving pressure kills because higher driving pressure means worse lungs. Unfortunately we cannot change lungs. The EXPRESS trial showed we cannot change mortality this way, and finally, I don’t believe Rambo could kill 300 terrorists after removing a bullet from his shoulder either!

 

 

1.     Amato MB, Meade MO, Slutsky AS, et al. Driving pressure and survival in the acute respiratory distress syndrome. N Engl J Med. 2015;372(8):747–55.

2.     Mercat A, Richard JM, Vielle B, et al. Positive End-Expiratory Pressure Setting in Adults With Acute Lung Injury and Acute Respiratory Distress SyndromeA Randomized Controlled Trial. JAMA. 2008;299(6):646-655.

 

 

Photo Credit

Bourne Identity


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