I understand teh danger of lung injury in volume ventilation with ARDS (specifically barotrauma and volutrauma). I also understand that plateau pressures are more important than peak airway pressures, adn the plateau pressures are more indicative of alveolar pressure and peak pressures are more indicative of the larger airway pressures and airway resistance. I understand that plateau pressures are static and done on indpiratory hold with no gas flow. I understand that peak pressures are dynamic. I understand that plateau pressures should not be greater than 30 cm H2O. What I do not understand is why my RT department subtracts the peep when determining teh plateau pressures. I may be wrong, but I don't think this is accurrate. If you are on a peep of 12 and do an inspiratory hold and get a plateau pressure of 39, well then isn't your plateau pressure 39? RT was subtracting 12 (peep) from that number and tellimg me that was the plateau pressure. I think that subtracting peep from plateau pressure gives you thoracic compliance, but that you should not be subtracting peep to determine plateau pressure.
This was important because they were telling me the plateau pressures were ok, because 39-12 was only 27 and thus the plateau pressures were < 30, but I don't agree and I think the plateau pressures were actually 39 which is dangerous (patient was maxed out on sedation).
Now, I know the strategies for elevated pressures (ie ... low VT's, pressure control ventilation, pressure regulated volume control ventilation, increased sedation, and even paralytics). I'm not asking that. I just want to know about what the true plateau pressure is (do you or do you not subtract the peep). I honestly don't think you do, because peep increased plateau pressure.
Feb 16, '09
I think you're right. If PEEP is subtracted, you're getting an inaccurate measure of what the alveoli are really "seeing." This CCM tutorial seems to agree, although it is not very explicit about how to measure plateau pressure:
Feb 16, '09
i agree and i found that same tutorial. i had emailed my manager to see if she could find out more and she talked to rt and here was her response:
ok, this is the answer i got from two rrts that just sat for their advanced boards. peak pressure measures airway pressure and will be elevated with biting on tube, plugs etc. plateau pressures show the static compliance of the lung and is useful in detection of ards among other things. in order to check the true static compliance, you measure the plateau pressure without the peep. if you measured with the peep it would not give you a true static compliance, but instead one with the treatment (peep) already added. the rt, rn and mds are all aware the peep is there. if you get a reading of 27 on someone you know has a peep of 12, this indicates non-compliant lungs even though the reading is normal. cathy (rt) stated that this is always the practice and what they are taught in school. she took a refresher course before sitting for her boards and it is presented that way also. i will see if i can hunt up some literature regarding it sometime this week.
i hope this helps a little.
this is my answer and my thinking:
thank you so much! this was a great help, and i learned something new this weekend! something is still bothering me though, and i still have questions. please do forward this onto cathy.
when plateau pressures are routinely recorded, is the peep always subtracted-or only when calculating static compliance? i understand the problem of decreased lung compliance with ards. i think i'm starting to understand that determining true static compliance should not include peep. i guess when evaluating how a patient is doing (improving or worsening), we want to evaluate the patient and not the treatment. so measuring static compliance is an evaluation of the patient.
but don't we measure plateau pressures for other reasons aside from determining compliance? don't we also monitor our pressures to prevent complications from treatment? don't we look at plateau pressures to detect dangerously high pressures on volume ventilation, and with this goal in mind, shouldn't we be looking at the plateau pressure including the peep (since both positive pressure volume ventilation and peep increase the pressure in the alveoli) ? i have always been taught that plateau pressures should not remain above 30 to avoid complications (barotraumas and volutrauma). if my plateau pressures are consistently above 35 with peep, isn't this still dangerous even if they are below 30 when peep is subtracted (because in reality, the peep is there and the elevated pressure is there, especially considering the high levels of peep we sometimes use for ards)? if i am monitoring pressures in order to prevent complications from the ventilator, shouldn't i be looking at what the ventilator does to the pressures??? i know it sounds like i'm splitting hairs, but subtracting peep can make a difference between normal and significantly elevated, action and non-action. i want to know what to do in the future when my patient's plateau pressure reads 39 with peep of 12....(patient assessment aside ).
i think that they are confusing static compliance and plateau pressures? while plateau pressures are needed to determine the static compliance, static compliance is not the same as the plateau pressure--i mean, the pressure is what it is- no? this is my thought.
Jun 29, '09
Peak Inspiratory Pressure(PIP) is a combination of the driving pressure and plateau Pressure. Plateau pressure reading includes PEEP, but not the driving pressure required to deliver the breath.
Static Compliance of the Lung is Tidal Volume/P plateau-PEEP. In other words, how much pressure a certain volume exerts on the lung.
Dan, Respiratory Therapist
Jun 30, '09
Thanks for this thread.
dorimar, what was the consensus after your reply?
Jul 14, '09
There was no consensus.... The RT told me she would meet with me, but I could never catch up with her (She was days & I was nights). I left my job for a teaching post before I ever met up with her, but Dan's post above really helped me. If I read his post correctly, I was right. Plateau pressures include PEEP and help to determine static compliance (which does not include peep). I could have interpreted his post incorrectly though.
What got me questioning teh issue is that peep is end-expiratory (the lowest time of pressure in positive pressure ventilation). My thought is that set Peep never goes away unless we D/C it, it is the minimum pressure. An inspiratory hold should not stop or negate PEEP, nor the pressure it causes. My thought is that inspiratory hold is on top of peep.
Static compliance is wholly different. it involves plateau pressure minus peep. But, for protective lung ventilation, when our goal is for plateau pressures < 30, I truly feel this includes delivered peep (I acutally saw the pressure wave on the inspiratory hold starting at the level of set peep and then saw the RT subtract the peep--I still think subtracting peep calculates static compliance but not plateau pressures). I think this is a very important question since a great deal of evidence-based practice for ARDS relys on plateau pressures, and peep significangly affects this number.
I also realize I may be incorrect, but my logic just will not let it go....
Mar 21, '10
here is the short version. For plateau pressure you are measuring pressure with inspiratory hold...on inspiration. peep is measured on exhalation. positive end EXPIRATORY PRESSURE. exhalation. The circuit is always primed with two pressure constants.. peep and the pressure created from the gas in the tubing... two knowns... the third known... the PIP to create the unknown the Plateau.
This is a basic logic equation.
Last edit by videovixen on Mar 21, '10
: Reason: needed more information
Mar 21, '10
But isn't the end expiratory pressure (peep) the minimum pressure. Isn't inspiratory pressure added on top of that?
Mar 26, '10
Ok, I'm trying to follow this thread, but its a little confusing.
PEEP is the constant, base level, minimum pressure applied to the lungs by the vent, helping to splint the airways open.
Inspiratory Pressure is indeed added on top of that, when a pressure or volume is delivered.
Plateau Pressure is the pressure applied to the lungs at end inspiration, with no flow occuring(why we do the insp. hold). Peak Inspiratory Pressure includes pressure due to flow. Peak Inspiratory Pressure(PIP) is a combination of the driving pressure(flow) and plateau Pressure. Plateau pressure reading includes PEEP.
Static compliance tells us how "healthy" the lungs are. For a certain volume should exert, or create a pressure in the lungs. ARDS = 25mls per cmH2O pressure, normal lungs = 80 mls per cmH2O pressure exerted.
Does this help?
Mar 26, '10
Yes!!! This is exactly my take. So, why was respiratory subtracting PEEP when doing the plateau pressure? It wasn't the static compliance I was asking them for, but the plateau pressure. I told them I thought they were confusing plateau pressure with static compliance. I know it is used to calculate static compliance, but all I wanted to know was just how high the plateau pressure was, because my peak pressures were consistantly high, adn I wanted to know if we should change vent setting or mode....
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