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How to set the adequate PEEP ?

First, let’s recall what the algorithm of resuscitation states: in the presence of spontaneous breathing and a heart rate of 100bpm or higher or more, after checking breathing efforts and cyanosis, then apply CPAP or supplemental Oxygen therapy. According to the NCPR, either way is accepted in such cases. However, is the meaning of both the same?
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In this “PEEP review”, I believe that “pressure and oxygen”, both of which contribute to oxygenation, are actually inseparable, therefore knowing the adequate PEEP setting is also knowing how to apply oxygen.

Now, how do you know the adequate PEEP?

This is the center of this essay, I think that PEEP setting will naturally help to minimize oxygen administration.
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First of all, keeping mean lung capacity is also an important factor in oxygenation, in addition to oxygen partial pressure difference in alveolar and pulmonary artery, diffusion area is also required for oxygenation. The mean lung capacity means, for example, the lung’s mean volume in an axis over time including deep inhalations and exhalations, as well as normal breathing, in terms of vital capacity (VC).
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In the NICUs, when a baby becomes apneic, due to a quick drop in SpO2, alarms are activated worrying family members. However, for adults to get an alarm activated due to a SpO2 below 90% they would need to hold their breaths until it becomes very painful in order to attain that condition.
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This difference can be explained by lung’s functional residual capacity. In a healthy adult functional capacity will be maintained unchanged even if he stops breathing, but in case of premature newborns the lung capacity is still unstable, so the functional residual capacity is literally “functional” meaning that its value (impossible to measure) is considered to fluctuate greatly.

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Although a little bit early, I would like to talk about lung-friendly ventilation management.
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For lung-friendly ventilation management using small tidal volumes it’s necessary to prevent pulmonary injury due to pulmonary hyperinflation (volutrauma) in addition, it is necessary to suppress the shear stress caused by excessive expansion and collapse of the alveoli (atelectrauma).
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If PEEP is inadequate during artificial ventilation, the lung will be injured every time it collapses and expands. Such lung injury can be minimized by adequate PEEP.
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To show this point, imagine the shear stress involved in getting your arm through a gown sleeve when the gown’s sleeve is bent.
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Click here to play the video.
Shear Stress movie

Or when the gown’s sleeve has been expanded making easier to get your arm through its sleeve. You see?  It’s definitely easier to wear the expanded gown, isn’t it?
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Click here to play the video.
PEEP movie

Together with the above mentioned functional residual capacity (FRC), FRC is unstable in premature newborns, their volumes are rising and falling in a considerable width. Therefore, when the FRC falls SpO2 falls meaning that their lungs have collapsed. It can be said that PEEP seems to be relatively insufficient even if it is applied with the same value as before.
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The macroscopic role of PEEP is to keep oxygenation by maintaining functional residual capacity (FRC), microscopically, PEEP’s lung protective role is to prevent alveolar collapse.
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So how to set the proper PEEP? Let me return to our first question. The key is in the administration of oxygen as mentioned at the beginning.
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During artificial ventilation, optimal PEEP setting will be visible by trying to minimize oxygen administration. Oxygen toxicity is often said to cause pulmonary injury. However, I think that Atelectrauma plays a major role than toxicity of oxygen itself, especially at low concentrations of 30% or less. Even if PEEP is  inadequate, increasing oxygen saturation will improved oxygenation, hence microscopically hidden atelectrauma won’t be recognized.
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In the clinical environment, this is easy to understand when a patient is going to be transferred to nasalCPAP after been extubated.
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For example, which settings are better for nasalCPAP after extubation?

If PEEP setting of 6cmH2O allows to stop oxygen administration. Then we can say that 6cmH2O is the PEEP required for the lungs.
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However, high PEEP also affects circulatory dynamics. It becomes the same as during HFO, circulatory dynamics such as renal blood flow and cerebral blood flow should be evaluated when applying higher pressures.
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Again, in respiratory management oxygen administration and PEEP setting are inseparable from each other in respiratory management. Knowing PEEP helps to know how to apply oxygen.

I hope this essay may help you to review the way to set PEEP and administer oxygen to your patients.
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