Ventilator Pressures: Static -vs- Plateau

With every ventilator check it is important to measure how much pressure is needed to deliver a tidal volume. There are two different pressures that we typically check: Peak Inspiratory Pressure (PIP), and Static Pressure, also known as plateau pressure (p-plat).

Now to define these two pressures:

• PIP:  This is the pressure at peak inspiration with flow.  
• p-plat:  This is the pressure at peak inspiration after holding your breath.  This is the measure of pressure without flow. 

The best way I can explain these two pressures is by having you take in a deep breath.  Take in a breath as deep as you can.  PIP is the pressure right at the end of inspiration.  Now, hold your breath and relax your chest while still holding your breath.  This is the pressure minus flow.  It is called static or plateau. 

These two pressures are important.  They should both be recorded with each ventilator check.  Now, here is how they can be used.

1.  To monitor resistance and compliance.  

• Both PIP and p-plat go up, or are trending up together, this is probably due to the fact that the patient's lungs are becoming stiff, or less compliant.  In this case, the static compliance may be decreasing.   Efforts should be made to keep the static compliance under 30, such as decreasing tital volume.  This is one reason why low tidal volume strategies are used on patients with ARDS. 
• PIP goes up and p-plat stays the same.  This indicates increased resistance.  It can be measured by taking PIP and subtracting p-plat = resistance.  It may indicate increased resistance, meaning the patient work of breathing (WOB) will be increased, or the patient has to work hard to obtain a desired tital volume.  There are three causes of this:
• Water in the circuit.  Solution is to empty this water (empty water traps)
• Secretions in airway.  Solution is to suction
• Bronchospasm.  Treatment is bronchodilator

Squiggly lines may indicate water in tubing or secretions in airway.

There is another simple method of observing if secretions or water in the circuit are the cause of increased resistance.  When this happens the you will probably observe squiggly lines on the graphics.  When the water is removed from the circuit, or after suctioning, the lines will be normal again.  

A rule of thumb is, if you see squiggly lines, check the circuit for water.  If that doesn't solve the problem, suction should be the next thing to try.  

A recheck of PIP and p-plat after resolving these problems SHOULD result in PIP decreasing, and thus lowering the resistance.  The patient should now be able to breathe easier, or work of breathing should be reduced.

2.  To determine readiness to wean.  Such as, if the patient is requireing more than resistance to obtain adequate ventilation, then the patient is not ready to wean.  Determine resistance by the formula PIP minus p-plat. This is one of the nice things about the servo ventilators, because they have volume support.  In this mode the patient determines his own PS and flow, and therefore you can see how much PS is needed to obtain a tidal volume.  When we want to see if a patient is ready to wean, we turn the patient into volume support.  If the support drawn in by the patient is greater than resistance, the patient is not ready to wean.

Example.  The patient is in assist control or pressure regulated volume support.  Check the PIP and p-plat. Use the formula: PIP minus p-plat = resistance.  PIP =15, P-plat = 10, resistance = 5.  Switch the patient to volume support.  If the pressure is using a PIP of 10, then you know this patient is requiring too much assistance to maintain an adequate tidal volume, and is not ready to wean.  If the patient is requiring only a PS or 5, then he is probably ready to wean.

However, determining readiness to wean involves more than just looking at numbers.

3.   To determine adequate pressure support (PS).  Frequently it occurs that a physician, or a therapist, just makes up a number for PS.  Yet the purpose of PS is to make up for resistance caused by the circuit and endotracheal tube, to make it so it doesn't feel to the patient that he is breathing through a straw.

• PIP minus p-plat = resistance of tubing, endotracheal tube, and airway.  
• Example.  PIP 20 and p-plat 15 = resistance of 5
• 5 should be more than what is needed to make up for the resistance of tubing and ETT, and should make ventilator breaths feel more like normal breathing.  
• Usually this number is somewhere around 5.  However, in patients with lung disease, it may be higher.
• If PS is set at lower than resistance, in this case 5, this results in increased WOB, and this can cause anxiety.  It may result in unnecessary sedation, and failure to wean. 

So, hopefully this information will help you better manage your ventilator patients.  If you find this useful, please let me know.  If you have more tips to add, please feel free to leave a comment below. 

This post was originally published on August 22, 2008 on respiratory therapy cave.  It was updated and edited for accuracy and simplicity by Rick Frea. 

Further reading:

• Medscape, Benjamin D. Singer, Basic Invasive Mechanical Ventilation
• Considerations for readiness to wean
• Acute Respiratory Distress Syndrome
• Respiratory Therapy Formulas and Normal Values