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’slaw 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?
During an inspiratory hold maneuver, a fixed volume of air is delivered into the lungs. Ppeak is determined at the end of that inspiration. This breath is then held in (ie, the patient is not allowed to exhale). During this time, the airway pressure drops down to a new level – this is the plateau pressure (Pplat). The degree of drop-off is the amount of pressure that was required to overcome resistive forces (Presistance) such as chest wall/lung elastic recoil, friction, etc. during inspiration. In other words: Ppeak = Pplat + Presistance
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). Pplat is only 20 cm H2O during inspiratory hold. 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, narrowing of the endotracheal tube, mucous plugs, and circuit kinks could be causes.
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 lung itself such as decreased pulmonary compliance, edema, pneumothorax, or evolving parenchymal disease like pneumonia or ILD.
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