So we've talked about the importance of safe mechanical ventilation and one way of quantifying safe mechanical ventilation during pressure support. We've also discussed driving pressure as an important parameter during mechanical ventilation, as it is directly related to mortality. Now get this, it's possible to measure driving pressure during pressure support! Sure, there are some caveats and considerations, but why am I so excited about this?
During COVID-19 we've obviously had a tremendous influx of patients with ARDS. A lot of these patients require mechanical ventilation for weeks. In early ICU admission phases, these patients are usually ventilated with a controlled mode, which makes it easy to guide safe mechanical ventilation and also measure driving pressure and compliance. Static compliance can be used as a decent way of quantifying “how sick their lungs are”. And up until now, was only available during controlled modes.
Note: grain of salt required! Compliance of the respiratory system does not necessarily reflect compliance of the lungs. Stay tuned for a separate article on this.
Now, especially these COVID patients start triggering the ventilator and quickly require pressure support. It's on us as clinicians to evaluate if this is safe (see Safe Pressure Support ventilation) with regard to their pulmonary status. If we were able to measure driving pressure during pressure support, that would definitely help to guide safe PS ventilation, as it is also related to mortality just like during controlled ventilation. But the main reason I'm personally so excited about this is the following:
If we're able to measure driving pressure during pressure support, we have a way of calculating compliance, so this can be used as a follow up parameter to see if the patients lungs are improving!
So how do we do this?
Exactly the same as you would during controlled ventilation: by performing an inspiratory hold and noting plateau pressure. So that's easy. However, more importantly, it's vital for you to know when you can interpret these values and when you cannot!
First, consider this:
- During PS, Pplat can be higher than Ppeak! How is this possible? Well, during PS, the patient generates a negative pressure. Now at the moment that we apply an inspiratory hold (flow is 0) and the patients respiratory muscles relax, this negative pressure is recorded as a positive pressure on the airway tracing.
- During holds, flow has to be zero! So this leads to the following requirements for reliable measurements:
Requirements for reliable measurements
The Pplat obtained during PS is considered reliable when
- The inspiratory hold is held for 2–3 seconds
- The plateau portion is flat:
▸ There are no dips, i.e. no patient effort (flow is 0)
▸ There is no slow decline, i.e. no air leak in the system (flow is 0)
- Tidal volumes should be nearly equal per breath
- Patients should not have large amounts of intrinsic PEEP
So obviously, if your patient has a high respiratory drive (high respiratory rate) this method probably isn't reliable.
Calculate driving pressure and static compliance:
How to interpret these findings:
- Use driving pressure to guide safe mechanical ventilation
- Use compliance as a follow up parameter for the patients pulmonary status