An important parameter to guide safe mechanical ventilation is driving pressure. In this article, we learn how to calculate driving pressure, its origin, and most importantly, how to use it in clinical practice.
Driving pressure (DP or ΔP) is calculated by taking the difference between the pressure at end inspiration (when flow is 0) and the pressure at end expiration (when flow is 0). This means that during volume controlled ventilation, driving pressure is the difference between plateau pressure (Pplat) and total PEEP. Measurement thus requires an inspiratory and expiratory hold.
Amato et al. NEJM 2015 showed that mortality is directly related to driving pressure:
Amato defined that the target DP should be <15 cmH2O. But you can imagine that the lower the driving pressure, the better (if ventilation permits).
Now what do we do in clinical practice? We aim for a driving pressure <15, which of course is valid, but does require some nuance. In my view, merely aiming for a driving pressure below 15 is too simplistic. Driving pressure should be viewed within the context of the patients pulmonary status. Let me explain what I mean.
Compliance of the respiratory system is defined as
As compliance can be seen as a descriptive state of the respiratory system, at constant tidal volume, driving pressure is an indirect measure of compliance. In other words, and this is key for your understanding and the usefulness in daily practice:
For a standardized tidal volume (for instance 6ml/kg),
driving pressure = compliance
That means that driving pressure should always be interpreted in light of tidal volume. If you have a driving pressure of 8, you're probably pretty satisfied. However, if the patient only receives tidal volumes of 200 ml, this means that their compliance is 25, which is horrible. However, if they receive tidal volumes of 500 with those driving pressures, this patient likely has healthy lungs.
How to lower driving pressure
The obvious answer is by lowering tidal volume. Remember that lowering tidal volume decreases minute ventilation, so you might need to increase respiratory rate as a result. Please note that this could also increase mechanical power.
Another attempt of lowering driving pressure is by increasing PEEP levels. Please note that this only works for some patients, namely patients that exhibit PEEP absorbing behavior: when increasing PEEP does not increase Pplat. This just means that increasing PEEP resulted in recruitment of certain collapsed parts of the lung. But remember, if increasing PEEP also increases Pplat, return to the lower PEEP value, as driving pressure stayed the same but Pplat increased.
So, these are my take home messages for driving pressure:
- Always interpret driving pressure in light of tidal volume, as driving pressure is equal to compliance for standardized tidal volumes
- If ventilation permits, aim for a driving pressure as low as possible, not merely <15