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<rtnovice>
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Hello Forum,

This is my first post, so please forgive me for if this topic has been addressed previously; I could not find anything in the archives.

I wanted to get some insight on the rationale of adding pressure support above the inspiratory pressure during Bilevel? If I am setting my inspiratory pressure at the level of my ventilating pressure why would I want to add pressure above this point? Would this not cause overdistention?
 
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rtnovice,
Think of Bilevel like two sets of CPAP pressures that the vent is altering between. Your Phigh is not like PC, but rather a increase in CPAP that you are going to hold for normally 4-6 seconds. The PS on top of this pressure is so that the patient can breath spontaneously (we need and want this by the way) without having to overcome the Raw of the ETT. If set low 5-10cmH2O the patients lungs will not be seeing this increase in pressure becasue the ETT is absorbing it. But what it does do is remove the Raw associated with the ETT. remember the numbers shown on the vent screen are being measured at the wye not at the bottom of the ETT.

You can not think of Bilevel, Bivent, APRV as conventional mecahnical ventilation. I look at it as physiologic ventilation. We use the Phigh, Thigh and Tlow as parameters to recruit the lungs open giving us a better V/Q matchup and hince making any air I do push in and out of the patients lungs more effective. The spontaneous breaths that patients do are not meant to "remove CO2" in the traditional sense but rather move the CO2 from the alveoli and dump it into the large AW's so that when I do release the pressure we will have a bulk movement of CO2. This is why spontaneous volumes of 100-200 ml by the patient is okay.

When using a vent that does not have ATC (not a Drager), I think it is okay to use low levels of PS to avercome Raw, but not okay if you are using them to generate larger volume movements of gas (don't try to get 6-8ml/kg), just let the patient do what they want with their own breaths.


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

Doesn't the PB 840 have tube copmensation? Wouldn't this be the way to overcome the resistance of the artificial airway instead of utilizing PS? And are you saying that a PS of 5-10cmH20 will not be seen by the lungs at all and will get absorbed by the artifical airway? Is that true of any size airway?
 
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The 840 does have tube comp, but it is only available as a mode by its self. You can not add it to other modes. As far as 5-10 of PS being be seen or not is depends on the size of the ETT. If a 6.0 is in place I would use 10 of PS, vs a 8.5 ETT I would use 5ps. These are ingerneral not seen at the bottom of the ETT. We are doing a poster presentation on this at the AARC conferenc if you want to look it up.


Light
 
Posts: 104 | Location: Springfield, MO | Registered: March 08, 2004Reply With QuoteReport This Post
<respkid>
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Interesting topic. Light you mention APRV being described as 2 levels of CPAP, but adding Pressure Support on top of the upper pressure no longer makes it 2 levels of CPAP; does it? A trigger is involved and the gas delivery is completely different. There are countless articles out there discussing the advantages of spontaneous breathing as it relates to lung recruitment, increased CO etc. Since Pressure Support assures a set pressure and accompanying flow regardless of patient requirements I see this method as a poor way to assure reduction in airway resistance. We all know the resistance of the airway changes with secretions and changes in ett rigidity as it warms, so I am not sure how you can guarantee a Pressure Support of 10 with a 6.0 tube will assure adequate flow? I would be curious to see if your results change as you change the resistance of the artificial airway. Thanks.
 
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<rtnovice>
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Please forgive my ignorance, but I need to clarify some things. So on the PB 840 if my Bilevel settings are PEEPH 20, PEEPL 0, TimeL 0.7 with a frequency of 9 and I have a patient with a 6.0 ETT I should set the PS at 10 to overcome the resistance of the airway; right? So that means I need to set the PS at 30cmH20 in order to get the 10 of PS needed to overcome this resistance during spontaneous breathing at the upper pressure; am I close? And this PS does not see the lungs because is gets lost or absorbed in the ETT; how am I doing? How did you verify this pressure support did not see the lungs because again it displays it on the graphics.
 
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Respkid,
Yes you are correct the PS of 5-10 may on one breath be to much and on another be too little depending on the patient flowrate. And yes the literature, mainly Habashi (who helped teach me APRV)states that PS above Phigh is bad. What I said is that if you do not have a drager and ATC, the use of LOW levels of PS is okay, in my opinion. If you did not add this low level of PS the patient would be pulling these volumes and flows through the Raw of the ETT, which inreturn would increase their WOB. Most of the patients that APRV is used on can no tolorate this increase WOB, this increase WOB would also increase their oxygen consumption. So yes they might see this pressure some of the time, but if my choices were to increase their WOB or cause slight changes in their pulmonary pressures, I will choice slight changes. Even with these slight variances in pressure the patient would still be pulling the breath in if the lungs were only seeing small pressure differences, remember these patients lungs a normally very stiff so 1-2 cmH20 would not be pushing in much volume. Also you stated that the addition of PS to CPAP no longer makes it CPAP, so I ask you this wouldn't that hold true to the addition of ATC(second pressure and increase of flow) to CPAP? and yet Drager still calls it CPAP!

rtnovice,
Please remember that the pressures seen on the screen are what are being expressed at the tip of the ETT, not at the patients lungs. Think Poiusselle law, as you increase the pressure at the top of the ETT the pressure will drop as it filters down the ETT. For those of you that have dragers and ATC you see this aswell with ATC. Look at the PIP's when you are in APRV with ATC, do they equal your Phigh setting? NO, becasue the vent increases the pressure at the top of the ETT to overcome the Raw of the ETT so that the patient does not experience the Raw of the ETT, does not mean that they are seeing this pressure being measure on the vent. You can also see this on the vent with Exp ATC on the drager with the temporary removal of PEEP untill the tracheal pressures meet the set PEEP.

Think also of VC your PIP reading lets say is 40 cmH2O, is your patient lungs seeing a pressure of 40? NO, check your Ppl. I bet it is lower.

Once again please do not take my comments as PS is just as good as ATC when in APRV. ATC is commpletly better addition to APRV than PS, but only the drager has this advantage. So wat I am saying is that if you have the 840, orservoi the use of LOW levels of support can be used.


Light
 
Posts: 104 | Location: Springfield, MO | Registered: March 08, 2004Reply With QuoteReport This Post
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Respikid,
How can you Guarantee that ATC is enough to overcome tueb Raw? The only study I have was done with the Algorythm of the 840 and it showed to not be enough to over come Raw of ETT. All I am saying is that guarantess are hard to come by in the medical field!


Light
 
Posts: 104 | Location: Springfield, MO | Registered: March 08, 2004Reply With QuoteReport This Post
<respkid>
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I think that is an excellent question; does adding ATC to CPAP still mamle CPAP, CPAP? In other words is ATC a mode? I don't have the answer to that; I'll leave that to those who right the definitions. However, if you examine the delivery of flow between PS (decelerating) and ATC (sinusoidal) and the ability to maintain the function of the diaphragm during ATC, I think CPAP with ATC simulates spontaneous breathing more than PS with CPAP.

I believe there is a paper out there stating that same fact; ATC on the 840 was not enough to overcome Raw of the ETT. Would this hold true of other ventilators?

I also did some testing here at our hospital over the weekend and found the opposite of what you had stated; with pressure support added to Bilevel, the pressure support of 10 with a 6.0 ETT was transmitted to our Michigan Test lung. TC can be added to Bilevel during spontaneous breathing and I would recommend this instead of PS.
 
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Once again remember that the vent graphics are seen at the patient wye, not at the lungs. As the PS breath is given, yes the vent shows a decelerating waveform, but what does the patient see? At the first part of the breath the bottom of the ETT would have a slower flow than what the vent shows(due to Raw of tube), as the breath continues the flow delivered by the vent slows down so the Raw lowers and the difference between the top of the tube and the bottom of the tube gets smaller. Since the difference is smaller the flow does not slow as much as it did in the begining, so the PATIENT sees a sinusidal flow, but the vent shows a decelerating.

My 840 does not allow for addition of TC to Bilevel, I will have to check on updates.

As far as the experiment, how did you set it up? We placed a ETT in the michigan to where the Proximal AW pressure monitor was at the bottom of the ETT, so we could measure carinal pressures. We then set the Breath simulator at enough of a amplitude to trigger spontaneous volumes of around 300ml. We also tried different PS levels for different size ETT's. We did not find that the low levels pf PS on average where delivered to the carina (proximal AW pressure) We will be presenting this at the AARC conference if anyone would like to come by and see all of it.

Also Once again ATC is better than PS in APRV, but if ATC or TC not available, then LOW levels of PS I think is okay (for what that is worth)


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