Driving Pressure & PEEP Titration

Taking it a step further

For a given value of PEEP and compliance, PPlat will be higher if you increase Vt.

However, increasing the PEEP will not always lead to increasing TPP. Let’s see why:

As discussed above:    

     PPlat – PEEP = Vt/Crs

Thus,

     PPlat = PEEP + (Vt/Crs)

If you recruit the lung by increasing the PEEP,  the Crs will increase, and thus the PPlat could increase, decrease, or remain the same (depending on how effective the recruitment is). 

At the bedside, one way to determine if you have recruited more lung (improved Crs) by increasing the PEEP is through driving pressure.

Remember, ΔP = Vt/Crs

So for a given Vt,

• If by increasing the PEEP, you successfully recruit (i.e improve Crs), the ΔP will decrease
• If by increasing the PEEP, you cause over-distention of the functional lungs (i.e worsen Crs), the ΔP will increase.
  
• If  ΔP remains the same, it reflects the absence of both recruitment and tidal over-distension

Evidence:

The most significant evidence on effect ΔP on outcomes comes from statistical analyses (including mediation analysis) performed by Amato et al. on ~ 3500 patients enrolled in previous RCTs on ARDS (3). The analysis showed that among the ventilation variables, ΔP had the strongest association with survival. An increase in ΔP was associated with an increasing mortality, with an inflection point around ΔP of 15 cmH20. Furthermore, Vt or PPlat was not associated with mortality when ΔP remained the same. See the classic figure (Fig 1) in the original paper here.

It is very important to note that even though this was an exhaustive analysis of patients enrolled in RCTs, the quality of evidence does not equal that of a prospective RCT. Attention should be paid at the bedside to optimize all the lung protection variables during mechanical ventilation. 

Now let us work through a couple of scenarios to apply ΔP in making clinical decisions.

Scenario 1

35-year-old woman (BMI 40) with COVID-19 ARDS- you just intubated and placed the patient on VC/AC (VC-CMV) with FiO2 60%, Vt of 360 (6 mL/kg IBW), RR 32, and PEEP of 10. The PPlat is measured at 29

ABG is 7.20/55/60

What would you do?

1. Increase Vt or RR to correct acidosis -> let’s get to a pH of 7.4
2. Increase FiO2 to  80% -> more oxygen is good 
3. Increase PEEP to 16 -> but wait, my repeat PPlat is now 31
4. Do nothing! The gas is acceptable for a patient with ARDS

In most cases in critical care, D would be the correct option, however, in this case, you have the possibility to recruit the lungs by increasing the PEEP. 

The driving pressure at PEEP of 10: ΔP = Pplat – PEEP = 29 – 10 = 19

The driving pressure at PEEP of 16: ΔP = PPlat – PEEP = 31-16 = 15

Even though the PPlat is higher than the magic cutoff of 30, the ΔP (PPlat – PEEP) decreased (from 19 to 15)

ΔP  = Vt/Crs

Since Vt is unchanged, the reduction in ΔP shows that you improved Crs by recruitment. Another important point to note is that patients with obesity such as in this case may have mass loading of the chest wall and higher pleural pressures. 

Thus for the same PPlat, the TPP is lower in obese patients (TPP = PPlat – Ppl). So, a cutoff of 30 for PPlat in those with poor chest wall compliance might not be feasible in order to achieve adequate recruitment and prevent atelectrauma. In situations like these, monitoring the ΔP and targeting it to <15 cmH20 might be a reasonable approach. Another alternative is to measure PPl via an esophageal balloon but this practice is limited by experience in accurate positioning and interpretation. 

Scenario 2

A 50-year-old man (BMI 28) with ARDS transferred from OSH for evaluation for ECMO for severe ARDS. Currently on VC/AC (VC-CMV) with FiO2 70%, Vt of 460 (6 mL/kg IBW), RR 30, and PEEP of 24. PPlat measured at 39

ABG 7.22/58/48

What would you do next?

1. Increase FiO2 and PEEP to correct hypoxemia
2. Increase FiO2 and keep PEEP at 24 (since ΔP is acceptable at 15)
3. Increase FiO2 and titrate PEEP to find the optimal range of PEEP
4. Cannulate ASAP

In this patient, the PEEP was titrated from as high as 26 to as low as 8 while maintaining Vt at 460 mL. As shown in the figures, the ΔP for various levels of PEEP were as follows.