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WOW,
amazing thread. Thanks for this great discussion everyone! Respkid and rtnovice hardly sound like a kid or a novice at all. Your intelligent inquisitiveness is great. Thanks again Light for your highly educated insights. I apologize for the delays in clearing out the moderation queue. I have had a couple of days in there with 14 pages of blocked items, all but a couple absolute trash. The filters are working, but occasionally catch real posts. I only hope I didn't delete anyone's contribution. Once again I request that if you plan to post regularly please join the forum. It is much easier to separate out and approve member's posts. Thanks Light for the personal Email to make me aware that a portion of this discussion was hung up, also this has really whet my appetite for your upcoming poster presentation at Vegas.
 
Posts: 147 | Location: Buckeye Az | Registered: January 27, 2006Reply With QuoteReport This Post
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Yeah, I figure this poster might get people fired up, but the Peak pressure and average pressure seen at the bottom of the ETT does not change with low levels of support and I have the Graphs from the Michigan that show it. My flame retardant suit will be on, as will the former students that are actually doing the presentation (it was a research topic of theirs from last spring, they have since graduated)


Light
 
Posts: 104 | Location: Springfield, MO | Registered: March 08, 2004Reply With QuoteReport This Post
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Respkid,
As far as your question of what I did before ATC with APRV, I can not answer. The Drager E4 was the first vent I used that had APRV and it also had ATC, so the two have always gone together.


Light
 
Posts: 104 | Location: Springfield, MO | Registered: March 08, 2004Reply With QuoteReport This Post
<respkid>
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Light,

Were you able to measure the amount of flow being delivered by the set pressure support for a given ETT ID?

Also are sure TC is not available with Bilevel; for some reason I believe it is. If so why do you think pressure support is still an option with Bilevel? My assumption is that most clinicians set PS above PEEPH to target a specific spotaneous tidal volume and not to overcome ETT resistance.
 
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Respkid,
TC is available on Bilevel on the 840, I had misspoke earlier. I do not know why PS is available with TC on the 840. I have discussed the use of PS in APRV with people that have used it to target volumes of 5 ml/kg. Kinda of like APRV and ARDSnet ran into each other and created a ugly mess. I completly agree that this use of PS is completly inappropiate! I only state that the use of LOW levels of support to overcome the RAW of the ETT is okay if ATC, AAC, TC are not available on your vent, so basically the Servoi.

I have heard some will titrate their Phigh for release volumes of 5ml/kg, once again trying to marry APRV and ARDSnet together. In my opinion this is completly inappropiate as well. APRV ventilation and ARDSnet ventilation are completly different and to try and get one to fit the other is wrong. The only principle I can see between the two is to try and keep Ppl under 30-35 cmH2O. That is it.

As far as flow being delivered by PS, NO we did not look at it. Maybe that will be next, we were just looking at the pressure differential. To look at the flow will be difficult, becasue the flow at the top of the ETT is different than the bottom, so at the begining of the breath the patient will have a higher RAW assocaited with the ETT so the flow will decrease dramatically when compared to the bottom of the ETT. Then as the breath continiues the flow decelerates and the two flows get closer together (graphically it should appear sinusidal at the bottom of the tube). But this does not take into account patient effort, which is all why ATC is better than PS if given a choice.


Light
 
Posts: 104 | Location: Springfield, MO | Registered: March 08, 2004Reply With QuoteReport This Post
<respkid>
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Light,

I believe we are saying the same thing and in agreement that ATC is the better choice over PS when being applied to spontaneous breaths during PEEPH if it is an aption.

I guess where we might disagree is the ability of pressure support to overcome resistance and meet patient demand. Patient ventilatory demand is not constant, so flow in a spontaneous breathing patient is variable and the pressure required to overcome the resistive forces of the artificial airway vary according to patient effort. PS utilizes a fixed pressure level which does not vary with changes in patient effort (flow). Based on the ever-changing resistance and demands of the patient I am not sure how one can say 10 of PS will compensate for the resistance of a #8 ETT. Just an opinion.
 
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Respkid,
At least we agree on one thing, that is a start. ATC is better than PS on Phigh.

As far as PS overcoming Raw, In comparisson to a situation to where the choices are PS or No PS to overcome Raw, I choose PS. The Square pressure pattern is only seen at the top of the ETT, so the patient is not seing this anyways. The flow of PS is variable so it can meet the patients inspiratory demands, just try and take a hiccup breath in PS. It will let you becasue the flow is variable. The flow is not however the same at the bottom of the tube like it is shown on the vent at the top of the tube. Your comment on PS of 10 to overcome a 8 ETT being hard to say. I would first say I would not use 10 for a 8 ETT, but thats not the point. Is 10 of PS enough to overcome ALL RAW of ETT on every breath, no. Is going to over shoot sometimes, yes. Is going to be right on sometimes, Yes. Raw in the ETT is mainly a componet of flow so as flow changes Raw will as well, I will not argue this. In my opinion PS vs no PS (not vs ATC) PS is a better option to relieve WOB associated with the RAW of the ETT.

It has been great discussing this with you, hopefully you will come by our poster and talk with me. I will be the larger of the three people standing at the poster, the other two are former students that did the research this spring in their last semester of school.


Light
 
Posts: 104 | Location: Springfield, MO | Registered: March 08, 2004Reply With QuoteReport This Post
<respkid>
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If the pressure drop across the ETT is flow dependent how can a constant pressure correctly compensate for ETT resistance?
 
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I said it was not perfect.

1)If a person has only the Servoi and the patient is in Bivent how are they supposed to relieve the WOB associated with the ETT?

2) ever noticed that a patient with ATC on will get a higher spontaneous volume than without it (does this cause overinflation in APRV)? If all we are looking for in a "sponaneous" breath is the pull of the diaphragm, then why even add ATC? Why not just have the patient have to pull the breath through the ETT and create an even creater negative pressure in the thorax to assist in distribution of air?

3) As I have posted before the end of the ETT did not see the pressure of PS and the pressure scalars put out by the michigan did not show them either, so where did they go?


Light
 
Posts: 104 | Location: Springfield, MO | Registered: March 08, 2004Reply With QuoteReport This Post
<brandx>
posted
Light,

I would suggest this article if you can get it. Excellent explaination of why Pressure Support ventilation can not overcome airway resistance adequately; a must read.

Minerva Anestesiol. 2002 May;68(5):369-77.
Automatic tube compensation (ATC).
Guttmann J, Haberthur C, Mols G, Lichtwarck-Aschoff M.
 
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