Pulmonary Artery Occlussion Pressure (Wedge)

Nurses General Nursing

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Specializes in ICU, CVICU, Surgical, LTAC.

Hello,

Okay so I am not totally lazy. I attempted to research this question before posting but could not find any literature that breaks this down in lamens terms. Can anyone explain the concept of "end-expiration" when obtaining a wedge pressure in a non-ventilated vs ventilated patient and also how this relates to ventilated patients receiving "peep"? And also when you obtain the wedge reading, what exactly are you measuring on the monitor? I understand the concept of wedging, how it measures preload in the LV, and how the balloon inflates in the pulmonary artery (occluding it so to speak) and a pressure reading is obtained, but I am still a little confused about what I am looking at when obtaining the wedge waveform, and how the measurement is actually obtained. I am a fairly new critical care nurse and am just trying to put all the peices together.

Thanks to all who respond!

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End exhalation is just that, at the end of exhalation. Changes in pulmonary pressure during inhalation can decrease venous return (preload) and cause you to misinterpret information if pressures are obtained during inhalation.

Wedge pressure is simply an indirect method of looking at the left ventricle (LV). It is a representation of left ventricular end diastolic pressure. (Pressure in the left ventricle during diastole) Looking at this can indirectly tell us how the left ventricle is doing. For example, if the left ventricle is in good shape, it can clear (pump out) the blood it receives from the left atrium (LA), and the pressures will be normal. However, if it is not doing well, it cannot clear this blood, and the blood backs up. This back up causes pressure to increase in the LV, then the pressure will increase in the LA, and finally, the pressure will increase in the pulmonary artery (PA). By looking at increased PA pressure, we can infer that the pressure is increased in the LV and assume LV dysfunction.

Clearly, wedge pressure is indirect and pulmonary pathology such as pulmonary HTN can cause false positive results so to speak. Therefore, it is critical not to look at these numbers in a vacuum and always correlate our findings clinically.

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Specializes in ICU, CVICU, Surgical, LTAC.
End exhalation is just that, at the end of exhalation. Changes in pulmonary pressure during inhalation can decrease venous return (preload) and cause you to misinterpret information if pressures are obtained during inhalation.

Wedge pressure is simply an indirect method of looking at the left ventricle (LV). It is a representation of left ventricular end diastolic pressure. (Pressure in the left ventricle during diastole) Looking at this can indirectly tell us how the left ventricle is doing. For example, if the left ventricle is in good shape, it can clear (pump out) the blood it receives from the left atrium (LA), and the pressures will be normal. However, if it is not doing well, it cannot clear this blood, and the blood backs up. This back up causes pressure to increase in the LV, then the pressure will increase in the LA, and finally, the pressure will increase in the pulmonary artery (PA). By looking at increased PA pressure, we can infer that the pressure is increased in the LV and assume LV dysfunction.

Clearly, wedge pressure is indirect and pulmonary pathology such as pulmonary HTN can cause false positive results so to speak. Therefore, it is critical not to look at these numbers in a vacuum and always correlate our findings clinically.

thank you very much for your response. I really was looking more for an answer regarding reading the waveform on the monitor when you obtain the wedge and how you measure it on the monitor. I understand what the wedge value represents. I understand what end expiration means but i appreciate your explaination of how it affectsthe value.

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Reading the waveform is similar to any other hemodynamic waveform. You slowly wedge and look the PA waveform to transition to a wedge waveform. A PA waveform will typically have a dicrotic notch like any other arterial waveform, and you will see a change to what looks very similar to a CVP waveform. Your monitor should be printing the entire time you wedge. After you are done look at the strip and identify the exhalation phase, this will typically be higher (higher pressure) than the series of waveforms during inhalation. You should be able to identify the up and down of the baseline that is caused by exhalation and inhalation.

The waveform will be similar to the CVP waveform. Try to identify small prominent bumps and larger prominent bumbs. (Small = a and large = b.) Utilising a strait edge, you can draw a line from the top of the a waveforms and appreciate the pressure. Likewise, your monitor may actually calculate a mean wedge pressure for you.

I am not sure I can do much more on a public forum in the absence of a face to face discussion. Do you have access to a hemodynamic monitoring course, or a provider who would be willing to go over this material with you?

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Maybe this?

"empty" I edited my link out. Gee, don't know if I can post that one... it might have usage rules, so I took it off... easy subject to search tho!

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http://domain675291.sites.fasthosts.com/anae/paop.gif

Try this. Notice the large and small bumps of the PAOP waveform? Also, note that the top of the small waveform is at the 10 mmHg mark, for convenience I am sure. This is how the pressure is measured. (Simply; systole-diastole-systole-and so on) There are no valves behind you, therefore no dicrotic notch separating systole from diastole. Remember, inflating the balloon blocks out input from the pulmonic valve. Hence, the reason the wave form changes so drastically.

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Specializes in Pediatric Critical Care, Cardiac, EMS.
Hello,

Okay so I am not totally lazy. I attempted to research this question before posting but could not find any literature that breaks this down in lamens terms. Can anyone explain the concept of "end-expiration" when obtaining a wedge pressure in a non-ventilated vs ventilated patient and also how this relates to ventilated patients receiving "peep"? And also when you obtain the wedge reading, what exactly are you measuring on the monitor? I understand the concept of wedging, how it measures preload in the LV, and how the balloon inflates in the pulmonary artery (occluding it so to speak) and a pressure reading is obtained, but I am still a little confused about what I am looking at when obtaining the wedge waveform, and how the measurement is actually obtained. I am a fairly new critical care nurse and am just trying to put all the peices together.

Thanks to all who respond!

End-expiration: You want to get your measurements of PAWP (and CVP for that matter) at end-expiration because the movement of air into the thoracic cavity (i.e., inspiration) creates a substantial negative pressure inside the chest. These pressures equalize at their highest point at end-inspiration, and their lowest point at end-expiration. With me so far? So when we measure pressures that are sensitive to outside influences (like intra-thoracic pressures) we want to get the "cleanest" number possible - one that reflects the actual intervascular pressure.

If your patient is awake, have them take a deep breath, then let it out and hold for 2-3 seconds - that's the numbers you should record, because the pressures have equalized. If vented, watch the wave patterns on the vent and get your numbers after the vent waveform (the square one) drops off. (It's easier to time this on a vented patient, naturally.)

With PEEP, according to the AACN (and translated a bit) - if the PEEP setting is less than 10 cm H20, there should be a good correlation between LAP and PAWP (barring other complicating factors). If PEEP is more than 10, than some interference with the accuracy of the numbers is expected. Remember that you are measuring not only left heart pressures, but the pressure of the surrounding pulmonary vasculature until you "wedge" - and even after wedging, the catheter measurement can be affected by pulmonary vascular bed pressures.

You can correct for high PEEP by the following formula.

  1. Convert the applied PEEP from centimeters of water to millimeters of mercury (1.36 cm H2O =1 mm Hg).
  2. Subtract half the applied PEEP in millimeters of mercury from the measured PAWP.

Generally, you should question the accuracy your Swan measurements if you have a) significant respiratory variation in the PAWP waveform (it wiggles a lot up and down from baseline), b) PAWP is greater than (unwedged) PAD, or c) the difference between your PAD and PAWP is greater than 4. If any of these are happening, look at your patient's position in bed, volemic status and RH function.

So - a Swan catheter with the balloon uninflated "looks" at the pressure in the surrounding pulmonary vascular bed - but when wedged, the catheter can only look forward - directly into the left heart. Therefore, PAWP and LAP should correlate unless there are other factors. This is why a Swan is useful to us - we can look at both sides of the heart and get a good picture of hemodynamic function. What you measure from are the tops of the larger "bumps" on the PAWP waveform (and most monitors do this for you these days.)

A lot of information that is useful to the beginning ICU nurse (and the experienced one, for that matter) can be found at icufaqs.org - I also recommend pacep.org, the Pulmonary Catheter Education Project for anyone who is going to be working with Swans on a regular basis. You have to register for the latter, but it's free, and it's a great course in how PA catheters work. If you want to take CCRN at some point, it's an invaluable resource.

Just my :twocents: and hope this helps!

Ted

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pacep.org

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Specializes in CTICU.

Don't forget in a ventilated patient, end expiration is the lowest point of the waveform (inhalation is positive pressure) while in a spontaneously breathing patient, end expiration is the high point since they have negative pressure inspiration.

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Specializes in ICU, CVICU, Surgical, LTAC.

I appreciate all of your feedback thus far, but really a lot of this is still going over my head. I feel really stupid that I don't get it :( I keep trying to read but its so confusing...

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Specializes in Pediatric Critical Care, Cardiac, EMS.
I appreciate all of your feedback thus far, but really a lot of this is still going over my head. I feel really stupid that I don't get it :( I keep trying to read but its so confusing...

Don't feel stupid. This is a lot of information all at once. Try checking out the sites that have been recommended - especially pacep.org, they helped me a lot when I first started working with Swans. ICUfaq.org is another good one that really simplifies things.

The most important thing to remember right now is just to keep a thought process (or even make a "cheat sheet") of expected values, and keep an eye on the numbers you find. Ask experienced colleagues questions about things that fall outside those norms - and as you go on you'll find the numbers "fall into place" with the clinical conditions you see.

Just knowing what the numbers "look at" is the first part of understanding them. Once you get that, you'll be able to understand the rest as you go on. It generally takes at least six months of working with PA caths to start to really understand what they're reading and what you're treating.

So don't be so hard on yourself - the fact that you're looking for the information and the answers says a lot for you as a Critical Care nurse!

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Don't know if this is exactly what you are looking for, but this website is a great learning tool for pulmonary artery catheter monitoring and such. http://www.pacep.org

It does require registration, but it's all free. Pretty extensive power-points. Maybe it will help?

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