When it comes to acute respiratory distress syndrome (ARDS), most providers will immediately say 6mL/kg tidal volumes. This seems to be the big takeaway from all the work done on ARDS. It has become dogma and a mantra that is repeated for intensivists, fellows, and residents. In fact, just suggesting to not do 6mL/kg tidal volumes is met with anger and accusations about the level of care.
What if I said the idea of 6mL/kg tidal volumes should be abandoned and has led to a huge misunderstanding of what is best for patients?
Many people are probably reading this with skepticism. Others may get defensive and stop reading. Either way, I hope it will inspire them to read more to try to prove me wrong.
In 2000, ARDSNet published the ARMA trial (1), which found that 6mL/kg tidal volume was better than 12mL/kg.
With all the new data coming out now, I think that the ARMA trial accidentally found the right answer but focused on the wrong results.
I do not want to take away from its contribution to ARDS. I think it showed that less ventilator support was needed and made a huge difference in mortality. This is similar to Rivers and early goal-directed therapy (EGDT). He showed that early recognition of sepsis was important and changed the management of sepsis. ARDS management was improved significantly with ARDSNet, but it may be time to move forward.
In 2015, Amato described driving pressure and, in my opinion, laid the foundation for the new management of ARDS and ventilator-induced lung injury (VILI) (2). It was a retrospective look at multiple ARDS papers including ARMA. In this paper, it was found that driving pressure affected mortality. The lower the driving pressure the better the outcomes.
Let’s look at driving pressure. How did they get pressure data from volume data?
Controlled Volume → Pressure Data
The majority of the patients in these trials were in volume control. This means the flow was constant and the pressure varied. This would make it more difficult to get reliable pressure data since it varies. They were able to achieve this with the first formula for driving pressure which is the tidal volume divided by static compliance. Manipulating this equation, the second driving pressure equation can be derived which is plateau pressure minus PEEP.
Compliance Formula:
Static Compliance:
Calculating Driving Pressure
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It was found that having a driving pressure ≤15cmH20 led to improved outcomes in ARDS (2).
Figure 1: Driving Pressure and Mortality (2)
Taking a deeper look at the paper, they controlled the other ARDSNet variables and looked at high vs low PEEPs and Pplat >30cmH20 vs. Pplat ≤30cmH20 while keeping driving pressure low. It was found that plateaus >30cmH20, low PEEP, or high PEEP did not change outcomes when driving pressure was ≤15cmH20. Additionally, when the Pplat ≤30cmH20 but the driving pressure was >15cmH20 there were worse outcomes.
Figure 2: Driving Pressure, PEEP and Plateau Pressure (2)
The overall picture is that shoving a lot pressure into the lung is worse than shoving a little bit of pressure in the lung. It also calls into question the recommendations of ARDSNet. If the PEEP and the plateau ≤30cmH20 did not matter when the driving pressure was appropriate, and the volume is the first variable that should be changed according to ARDSNet, is it time to reevaluate the ventilator algorithm for ARDS.
How I approach the ventilator in ARDS:
When I am trying to optimize someone on a ventilator I focus on both equations for driving pressure:
I spend time a lot of time optimizing the compliance of the lung. By optimizing compliance, in volume control, it will decrease driving pressure since static compliance is in the denominator. In a pressure mode, it will increase the tidal volume for a given driving pressure and allow you to lower ventilator settings.
How do you optimize compliance?
The key is to look at the pressure-volume loop. Due to surfactant and transpulmonary pressure (TPP), the alveoli operate best when they remain open at rest. When they collapse it leads to atelectasis and they are much harder to open, leading to damage.
Here is a great representation from derangedphysiology.com. This is an amazing website for physiology if you have not already seen it.
Figure 3: Pressure-Volume Curve
https://derangedphysiology.com/main/cicm-primary-exam/required-reading/respiratory-system/Chapter%20554/interpreting-shape-pressure-volume-loop
Optimizing the ventilator for ARDS:
Step 1: Optimize compliance/Find the transpulmonary pressure (TPP)
Try to identify the lower inflection point (LIP). This is not easy but can be done in a number of ways. There will be another post looking at this in more depth. By determining the TPP, which is the pressure to keep the alveoli open, you can recruit as many alveoli as possible and keep the patient above the lower inflection point. You can see from Figure 1 that above the LIP is the optimum compliance.
Setting the PEEP to be slightly above the LIP will ensure the best compliance
Step 2: Ensure driving pressure is ≤15cmH20/Understanding baby lung
When a patient has an alveolar disease like ARDS, the number of diseased alveoli is unknown, which means the number of usable alveoli is unknown. This amount of usable lung, described by Gattinoni, is known as baby lung (3). Baby lung is the idea that the amount of usable lung is the size of a small child's lungs. If you try to put 400mL of volume in alveoli that can only expand to accept 300mL, you will get high pressures and injury.
By using driving pressure, whatever the volume ends up being, it is safe and will minimize damage. This could be 250mL, 300mL, or 500mL, it all depends on the disease process
Spontaneous effort: Please take into account spontaneous effort. The 15cmH20 driving pressure was done in non-spontaneously breathing patients. The diaphragm adds to the pressure. If the Pplat – PEEP is 15cmH20 and the patient has high work of breathing the diaphragm can add more than 10mH20 pressure and the true driving pressure is >25cmH20 and cause a lot of damage.
Step 3: Escalate if needed
If the patient cannot be ventilated with safe driving pressure then the patient needs escalation to paralytics, proning, or VV ECMO.
By reducing the tidal volume, the amount of driving pressure needed will drop. So, it physiologically makes sense that 6ml/kg was better than 12ml/kg, it was not the volume that was the answer, it was the decrease in the driving pressure. I think that if we looked at ARDS from the two iteration of the driving pressure equation there would be better ventilator synchrony, safer ventilator settings and better outcomes.
References:
1. Acute Respiratory Distress Syndrome Network, Brower RG, Matthay MA, et al. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med. 2000;342(18):1301-1308. doi:10.1056/NEJM200005043421801
2. 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):747-755. doi:10.1056/NEJMsa1410639
3. Gattinoni L, Pesenti A. The concept of "baby lung". Intensive Care Med. 2005;31(6):776-784. doi:10.1007/s00134-005-2627-z