Mechanical ventilation is routinely utilized in critical care, so understanding the various modes, weaning parameters, and troubleshooting common problems is essential to any medical ICU rotation. An important concept is interpreting peak (inspiratory) pressures versus plateau pressures.
Think of the ventilator, inspiratory limb tubing, endotracheal tube, and airways as a long, continuous pipe with a diameter much smaller than the overall length. Now let’s apply Poiseuille’s law, an infamous fluid dynamics equation, which shows that the resistance in this “continuous 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.
Now let’s think of a patient – 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 large airway edema? These are all going to effectively increase airway resistance to varying degrees.
Now let’s go back to peak and plateau pressures: Peak inspiratory pressure (PIP) = plateau pressure (Pplat) + pressure required to overcome airway resistance. Consequently, plateau pressure can never be higher than the peak, because there’s always going to be intrinsic resistance which must be overcome. Plateau pressures are determined by an inspiratory hold maneuver in which the patient is given a fixed volume of air. The PIP is determined at the end of that inspiration. The drop-off that occurs between the PIP and plateau pressure is airway resistance which was overcome during the inspiratory phase. The pressure that’s left (plateau pressure) is a product of the lung tissue itself – decreased pulmonary compliance, pulmonary edema, interstitial lung disease, etc 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 PIP of 60 (normal is 25-30 cm H20). Let’s say I do an inspiratory hold maneuver, and the plateau pressure is only 20. That means there’s a huge pressure overcoming airway resistance! Now I’m thinking more about things which are decreasing the radius of my “airway pipe.” Behold, the patient was biting his endotracheal tube and there’s a huge kink in the tubing coming off the ventilator.
What if the PIP was 50 and the plateau was 45? The high plateau pressure 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).