What is PEEP? How to do a PEEP study?

PEEP is an abbreviation for Positive End Expiratory Pressure. It's a small amount of pressure above what is in room air that remains at the end of expiration.

The benefits of PEEP are.

1. Increased Residual Capacity. This essentially means that it increases the amount of air that stays in the lungs. This works to...
2. Recruit collapsed (atelectic) alveoli. This makes it so they participate in gas exchange. It also works to...
3. Keep alveoli from collapsing. It keeps alveoli open so the effects of fluid or atelectasis do not cause shunting. This also helps to reduce V/Q mismatching. This also makes it so you have an...
4. Increased PaO2 for a given FiO2. It's a good way of improving oxygenation. 
5. Decreases Cardiac preload and afterload. It reduces the amount of blood returning to the heart, and thereby reduces the amount of blood leaving the heart. In this way, it can help patients who are in heart failure (pulmonary edema) by reducing the amount of work their heart has to do to pump blood through your body. This also means that too much PEEP can be observed by drops in cardiac output, which can be measured by bloodpressure and oxygen saturation (SpO2). 
6. Reduction in tissue injury and inflammation. It prevents the alveoli from constantly opening and closing and thereby inuring them and causing inflammation, which may be associated with the development of ARDS. Studies have shown that it is protective against "ventilator induced lung injury." This is often called volutrauma. Volutrauma was more prevalent back in the days when it was thought that people on ventilators should be on higher tidal volumes, hence the old formula of setting tidal volumes based on 10-15cc/kg ideal body weight. This has now been lowered to 6-8cc/kg ideal body weight in order to prevent volutrauma. 

There are disadvantages of this.

1. Over-distention of alveoli. It causes too much air to stay in the lungs resulting in decreased cardiac output, as would be shown by blood pressure and SpPO2. There are certain instances where you would benefit from higher PEEP, although too much PEEP can lead to over-distention and volutrauma, which may mimic respiratory disease states. So, in such instances, you would want the highest PEEP that doesn't cause over-distention. (Described below is how to accomplish this with a PEEP study). Over-distention results in increased dead space, increased work of breathing, and medical disorders such as ARDS. 
2. Diminished Cardiac Function. As noted, PEEP that is set too high can decrease venous return and cardiac output. This can be measured by complex formulas, although the simplest way is by taking a blood pressure and monitoring pulse oximetry. 
3. Diminished Renal Function. May decrease renal blood flow resulting in diminished urinary output. So, this is another reason to keep PEEP as low as clinically possible, especially when you have a patient in heart or kidney failure. 
4. Increased Intracraneal Pressure. When venous return decreases, intracraneal pressure may increase. This is usually not clinically significant. However, if you have a patient who already has an elevated intracraneal pressure (ICP), such as due to a head trauma, this is something you'll need to watch out for. This is another reason to raise the head of the bed, as this may offset any increase in ICP (the other reason for raising the head is to prevent GERD, which can increase the risk for ventilator associated pneumonia). 

Now that you know about PEEP, along with its benefits and disadvantages, we can now get into how to perform a PEEP study. The purpose here is to determine the perfect PEEP for an individual patient at any given moment in time. Keep in mind here that the ideal PEEP may increase or decrease over time, especially as a patient's medical condition worsens or improves.

Here is the basics of any PEEP study.

1. Increase PEEP by 2-3 cwp every 20 minutes and continue to monitor the patient. You should write down the patients blood pressure and SpO2. If desired, you can also jot down the patients P/F Ratio and static compliance.
2. If static compliance, P/F Ratio &/or SpO2 increase, you know it's working. 
3. Stop when the patient's blood pressure and SpO2 start to drop. Also stop when the P/F Ration is equal or greater than 200. Also stop when the static compliance decreases. 
4. The required PEEP should be set at the PEEP setting used just prior to where the hazards of PEEP were observed. 
5. Do not increase PEEP if systolic BP is less than 90
6. Also, keep mean airway pressure (MAP) less than 15. This is one of the newer markers of too much PEEP. When it starts to drop, this is an early indicator that cardiac output is about to decrease. 
7. Ideally, static complliance should be between 60-100.

I also have a shortcut. Maybe I shouldn't teach you this, but here goes: essentially, based on the wisdom we learned above, all you really need to do is monitor pulse oximetry and blood pressure. If either starts to drop, then you know it's time to lower your PEEP by 2 cwp, which would be your ideal PEEP. This makes it simple. 

The optimal goal of any PEEP study is to find the optimal PEEP to maintain a desired SpO2 and PO2.

If any of my fellow respiratory therapists has anything further to add (any tips), please feel free to share.

(Post originally published on 8/9/08. It has been edited and updated by RT Cave Staff). 

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References:

1. Vincent, Jean Louis, editor, "Intensive Care Medicine: Annual Update 2002," 2002, Springer, pages 302-303
2. Criner, Gerard J., Rodger E. Barnette, Gilbert E. D’Alonzo, editors, “Critical Care Study Guide: Text and Review,” 2nd edition, 2010, Springer
3. Kacmarek, Robert M., James K. Stoller, Albert J. Heuer, “Egan’s Fundamentals of Respiratory Care,” 10th edition, 2013, Elsevier Mosby
4. Saura, Pilar, Lluis Blanch, "Conference Proceedings: How to set Positive End Expiratory Pressure," Respiratory Care, http://www.rcjournal.com/contents/03.02/03.02.0279.cfm, accessed 4/11/17
5. Respiratory Update: "Benefits, Contraindications, Adverse Effects for PEEP/CPAP," http://www.respiratoryupdate.com/members/Contraindications_for_PEEPCPAP.cfm, accessed 4/17/17
6. Valenza, et al., "Positive end-expiratory pressure delays the progression of lung injury during ventilator strategies involving high airway pressure and lung overdistention," Critical Care Medicine, 2003, July, 31 (7), pages 1993-08, https://www.ncbi.nlm.nih.gov/pubmed/12847394, accessed 4/11/17
7. Respiratory Therapy Cave: Respiratory Failure Lexicon
8. Respiratory Therapy Cave: ABG Lexicon