Auto-PEEP

Auto-PEEP.  We've all heard of autopeep.  Most times when people think about it, they're thinking that it's "intrinsic PEEP," and they know it's bad--and they'd be right.  But what causes it and what makes it happen?

Look at the image below

This is a flow vs time diagram.  Above the line is inspiration, and below the line is exhalation.  Note how the flow below the line does not come back to zero.  This means that the patient starts their inspiration before they have a chance to completely exhale.  Try it--it's not comfortable.

What does this mean for a patient on a ventilator?  Well it means that the ventilator will try to give a breath before the patient is done exhaling, and airway pressures will go up and tidal volumes may drop. 

What causes it?   Auto-PEEP is caused by one of three things.

1. High minute ventilation.  Minute ventilation is caused by high respiratory rate and/or high tidal volumes
2. Increased I:E ratio.  If the inspiratory time is too high, relative to the expiratory time, the patient will not have enough time to exhale (see the graph above)
3. Airway trapping, like in asthma or COPD

So how to treat it?
To treat it, you have to think of the three reasons above.

1. High minute ventilation.  This is the easiest to deal with.  Decrease the minute ventilation.  First start with decreasing the respiratory rate, and then the tidal volume.
2. Increased I:E ratio.  Again, decrease the respiratory rate, this will increase the inspiratory time.  You can also try increasing the flow in the ventilator cycle so more of the air goes to the patient at the beginning of the breath.  
3. Airway trapping.  Decreasing the respiratory rate will give time for the air to leave the lungs (think asthma).  Also, increasing the PEEP can help in COPD-related airway trapping.  The way this works, is if, say the auto-PEEP is 10 cm from airway trapping, and the patient has to initiate a breath, they need to pull at least -10 cm before the ventilator will trigger.  Adding PEEP to this will counteract this.  So, say, in this example there is a PEEP of 7, the patient will only need -3 cm to trigger the vent.