An important aspect of understanding mechanical ventilation is knowing how to interpret peak inspiratory pressures (Ppeak) and plateau pressures (Pplat). What’s the difference, and why does it even matter?
Think of the ventilator, inspiratory limb tubing, endotracheal tube, and your patient’s airway as a long, continuous pipe with a diameter much smaller than the overall length. Poiseuille’s law shows that the resistance (‘R’) in this “airway pipe” is directly related to the length of the pipe and indirectly related to the radius raised to the 4th power. In other words: R ~ length / radius4. For example, if the radius of the pipe is cut in half, the resistance increases by a factor of 16.
What can cause this “pipe” to be narrowed? What if the tubing leaving the ventilator is kinked? What if the patient is biting the endotracheal tube? What if the airway lumen is reduced because of bronchospasm or mucous plugs? These are all going to effectively increase airway resistance to varying degrees… but how does this affect the peak and plateau pressures?
I think about the peak inspiratory pressure as the sum of the plateau pressure (pressure used to keep air in the lungs) and pressure used to overcome airway resistance to get the air into the lungs (elastic recoil of the lungs and chest wall, friction, etc.). In other words: Ppeak = Pplat + Presistance. Consequently, Pplat can never be more than Ppeak, because there’s always going to be intrinsic resistance which must be overcome by Presistance.
Pplat is determined by an inspiratory hold maneuver in which the patient is given a fixed volume of air. Ppeak is determined at the end of that inspiration. The drop-off that occurs between Ppeak and Pplat is airway resistance which was overcome during the inspiratory phase by Presistance. The pressure that remains during the hold maneuver is the Pplat and is a product of the lung tissue itself. Decreased pulmonary compliance, pulmonary edema, and interstitial lung disease can all affect this.
Now how does this translate to a real world example? Let’s say you walk into the room and see a Ppeak of 60 cm H2O (normal is 25-30 cm H2O). Let’s say I do an inspiratory hold maneuver, and Pplat is only 20 cm H2O. That means there’s a huge pressure overcoming airway resistance (ie, a very high Presistance)! Now I’m thinking more about things which are decreasing the radius of the airway pipe. For example, the patient was biting his endotracheal tube and there’s a huge kink in the tubing coming off the ventilator.
What if the Ppeak was 50 cm H2O and the Pplat was 45 cm H2O? The high Pplat points towards a lung issue affecting the alveoli or small airways. Did the patient develop a pneumothorax? Is his pneumonia evolving?
Also, it’s important to consider that once the underlying pulmonary issue begins to resolve, pressures will change. Compliance will change. Pressure/volume curves will change. Peak and plateau pressures are a crude way to assess clinical improvement (all other factors being equal).
Drop me a comment below with questions! 🙂