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Vent Setting Explained?


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#1 JClayborne

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Posted 08 February 2010 - 10:16 PM

Mode: SIMV
Rate: 10
Pressure Control: 18
PEEP: 5
Pressure Support: 8

SIMV delivers "factory breaths" at the set rate, in this case 10 per minute, but in synchrony with the patient’s respiratory effort. The first patient triggered breath is assisted to reach "factory standards", in this case that is a pressure level of 18 cmH2O. So we are saying air flows in until 18 cmH2O pressure is equal to airway pressure then air flow stops (which is why Vt is variable with PC). Subsequent breaths in the same breath period are not "factory breaths". If no breath was triggered at the start of a breath period (apnea limit or in this case 6 seconds) a factory breath is given. Now where the pressure support comes in to play is if the patient tries to take their own breath. The vent will see this and provide “pressure support” of 8 cmH2O to help them take that breath but the patient only takes as much or as little air as they want.

How did I do explaining that? I thought I had this information down but it has been so long since I actually had to explain this stuff I'm loosing it with the rest of my mind and confusing myself LOL. Thanks
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#2 MSDeltaFlt

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Posted 08 February 2010 - 10:34 PM

Mode: SIMV
Rate: 10
Pressure Control: 18
PEEP: 5
Pressure Support: 8

SIMV delivers "factory breaths" at the set rate, in this case 10 per minute, but in synchrony with the patient’s respiratory effort. The first patient triggered breath is assisted to reach "factory standards", in this case that is a pressure level of 18 cmH2O. So we are saying air flows in until 18 cmH2O pressure is equal to airway pressure then air flow stops (which is why Vt is variable with PC). Subsequent breaths in the same breath period are not "factory breaths". If no breath was triggered at the start of a breath period (apnea limit or in this case 6 seconds) a factory breath is given. Now where the pressure support comes in to play is if the patient tries to take their own breath. The vent will see this and provide “pressure support” of 8 cmH2O to help them take that breath but the patient only takes as much or as little air as they want.

How did I do explaining that? I thought I had this information down but it has been so long since I actually had to explain this stuff I'm loosing it with the rest of my mind and confusing myself LOL. Thanks


Close. To assist the patient in to getting "factory standards" would be the use of the pressure support of 8. Add that to the PEEP of 5, you'd only get assisted to a pressure totalling 13cmH2O.

As far as the pressure support goes, it will help the patient take a breath, but only as much as both the pt will allow and as much as compliant as the pt's lungs are. It depends on agitated the pt is and how diseased the pt's lungs are.
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#3 JClayborne

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Posted 08 February 2010 - 10:57 PM

Thanks Mike! That is where I was confusing myself when we were talking about it. I was getting 13 cmH2O. Would the clinician have set the pressure support to 8 and the PEEP to 5 (and on the LTV 1000 we don't have PEEP compensation) so the "real" pressure support being 3 cmH2O? This simply to compensate for the added resistance of the ventilator tubing and such. Not to help the patient take a "factory breath" pre se? Or am I way off base here?
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#4 MSDeltaFlt

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Posted 09 February 2010 - 12:41 AM

Thanks Mike! That is where I was confusing myself when we were talking about it. I was getting 13 cmH2O. Would the clinician have set the pressure support to 8 and the PEEP to 5 (and on the LTV 1000 we don't have PEEP compensation) so the "real" pressure support being 3 cmH2O? This simply to compensate for the added resistance of the ventilator tubing and such. Not to help the patient take a "factory breath" pre se? Or am I way off base here?


Your real pressure support is still what you have dialed in. If you have true PEEP then the total pressure reading you will see on any assisted breaths will be that dialed in pressure support in addition to the PEEP dialed in. If you do NOT have true PEEP (as in you only have a PEEP valve) then the pressure you get from the pressure support is just the pressure support dialed in.

Make sense?
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#5 TexRNmedic

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Posted 09 February 2010 - 01:34 AM

Thanks Mike! That is where I was confusing myself when we were talking about it. I was getting 13 cmH2O. Would the clinician have set the pressure support to 8 and the PEEP to 5 (and on the LTV 1000 we don't have PEEP compensation) so the "real" pressure support being 3 cmH2O? This simply to compensate for the added resistance of the ventilator tubing and such. Not to help the patient take a "factory breath" pre se? Or am I way off base here?


Think of PEEP (positive end expiratory pressure) as the pressure the vent keeps in the airway at the end of the exhalation to recruit and maintain alveoli and hopefully improving FRC.

Think of pressure support as the amount of assistance the vent gives on inhalation. This is support above you baseline pressure. So this will be above and beyond any PEEP. Higher the driving pressure the more work the vent is doing, lower the pressure the more work for the patient. Also increased for lower lung compliance. If your vent doesn't compensate for PEEP then you will need do a little math and adjust your PS setting.

Think of pressure control as a way for the vent to regulate tidal volumes based on the airway pressure (usually in an effort to prevent barotrauma in a poorly compliant lung). Tidal volumes may vary a bit. When using pressure control you will also need to set the I:E ratio. Also, when using PC in SIMV the PC and PS should be the same.

SIMV + PS can give three types of breaths-controlled, synchronized and pressure supported. So to use your example of rate of 10: 60sec/10rr= 6 seconds per respiratory cycle. If the patient doesn’t initiate a breath in the expected amount of time to keep the dialed in rate the vent will deliver a controlled breath (once every 6 seconds). If the patient initiates a breath the vent will detect the negative pressure and synchronize the breath with the patient. If the patient breaths faster than the set rate (more than once every 6 seconds) than the vent will deliver a pressure supported ventilation. In my opinion, usually not the best mode for someone with lungs sick enough for pressure controlled ventilation.

As Mike already said, effectiveness of ventilation depends a lot on airway compliance and the patient’s ability to tolerate mechanical ventilation. High pressures and long I times are usually pretty uncomfortable and tough for poorly sedated patient to tolerate.

I’m sure the RRTs around here can give a much more technical answer for you but I hope this helps answer your question.
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#6 TexRNmedic

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Posted 09 February 2010 - 04:24 AM

Think of pressure control as a way for the vent to regulate tidal volumes based on the airway pressure (usually in an effort to prevent barotrauma in a poorly compliant lung). Tidal volumes may vary a bit. When using pressure control you will also need to set the I:E ratio. Also, when using PC in SIMV the PC and PS should be the same.


I reread what I wrote about pressure control and I think its a little misleading. Pressure control itself doesn't directly cause the vent to alter TV. The vent doesn't stop inspiration at a pressure limit or anything like that. The vent will cycle based on the I:E ratio. Example RR=10. I:E =1:2. So inspiration is 2 seconds and expiration is 4 seconds. During that time the airway pressure is limited to what you have dialed in. The vent adjusts TV and flow to maintain the set pressure, I:E ratio and RR. This allows airway pressures to reach and maintain a higher (but regulated) airway pressure during more of I time, allowing better gas distributing throughout the lung. Air tends to go to the areas of least resistance first. So this allows areas of "bad" lung to catch up with "good" lung. Many factors impact how much air can get in during I time such as airway compliance, turbulence, RR, I:E ratio, chest wall compliance, volume exhaled, level of sedation, secretions, size of the ETT, even water accumulation in the vent circuit. With all that said, you can see how SIMV might not be the best mode to use with PC. I didn’t want to steer anyone the wrong way and I’m sure I’d hear from one of our great RRTs if left unclarified. If I said anything goofy I hope one of them sets me straight.

Regards,
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#7 LearRRT-CCEMTP

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Posted 09 February 2010 - 05:27 AM

I reread what I wrote about pressure control and I think its a little misleading. Pressure control itself doesn't directly cause the vent to alter TV. The vent doesn't stop inspiration at a pressure limit or anything like that. The vent will cycle based on the I:E ratio. Example RR=10. I:E =1:2. So inspiration is 2 seconds and expiration is 4 seconds. During that time the airway pressure is limited to what you have dialed in. The vent adjusts TV and flow to maintain the set pressure, I:E ratio and RR. This allows airway pressures to reach and maintain a higher (but regulated) airway pressure during more of I time, allowing better gas distributing throughout the lung. Air tends to go to the areas of least resistance first. So this allows areas of "bad" lung to catch up with "good" lung. Many factors impact how much air can get in during I time such as airway compliance, turbulence, RR, I:E ratio, chest wall compliance, volume exhaled, level of sedation, secretions, size of the ETT, even water accumulation in the vent circuit. With all that said, you can see how SIMV might not be the best mode to use with PC. I didn’t want to steer anyone the wrong way and I’m sure I’d hear from one of our great RRTs if left unclarified. If I said anything goofy I hope one of them sets me straight.

Regards,



A little clarification.....In PCV the ventilator DOES NOT adjust Vt and Flow to maintain the set pressure. There is no VT in PCV! The ventilator adjusts purely flow to maintain the desired pressure for the length of the set Ti. With PCV it's best if you completely forget about volume ventilation, VT, etc. You need to think purely in regards to set inspiratory pressure and mean airway pressure. The rate and the IP control you PaCO2 and the Mean Airway Pressure and FiO2 control your PaO2. Your Mean Airway Pressure is basically everything under the pressure curve. You can adjust it by increasing the baseline (PEEP), by lengthening the curve (iT), and slightly by increasing Insp Pressure but then you are also altering ventilation so I prefer NOT to use IP for mean control unless all else is maxed. SIMV-PC is not a mode that is widely used in adults. It is used mostly with Neonates and peds. Most pulmonary guru's will tell you that SIMV in general is pretty useless. However is is the most popular mode of ventilation among non critical care trained physicians and surgeons due to lack of experience. Basically the only difference between ACV and SIMV is when the patient breaths spontaneously. If the patient is sick enough to require a rate then support them fully with ACV. If they are capable of breathing spontaneously then allow them to do so with pure PSV! Where folks get into trouble is with the thought process that PSV is a weaning mode or an adjunct with SIMV. PSV is the most physiologic mode of ppv that we have. But the problem is that it is misused. Folks will place patients on 8 or 10 of PS and then the patient's minute volume is insufficient and they requite a rate and VT as well. I actually RSI asthmatics and COPD'ers and allow them to come out of the sux then place them on high levels of PSV. Sometimes as much as 20 or 30 or even higher. I basically titrate the PS to reach not only an exhaled VT within the set range (8 - 12 ml/kg)but also to give them a comfortable feeling by watching their body language, RR, and VS. The nice thing about pure PSV in obstructive and restrictive lung patients is that they will control their own I:E and will not take another breath until they fully exhale. You will find they normalize their pH much quicker and don't run near the risk of becoming vent dependent. What happens is we as clinicians are way too quick to take over ventilation completely. We intubate a COPD'er, place them on the vent, over ventilate them, they in turn dump all the bicarb that they have spent months to years building up in order to have a normal pH. Then we can't get them off the vent and wonder why? Let them normalize their own pH's with pure PSV from the start. Of course there will be those that are just way too sick or tired out to breath spontaneously in those cases fully support them on ACV until they are rested then change them over to pure PSV. Now this does not matter whether you are ACV - Pressure (AKA Pressure Control) or ACV-Volume (AKA Assist-Control), it works both ways! I hope this helped!
Stay Safe,
Dave
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David W. Garrard, BHS, RRT, RCP, CCEMTP, PNCCT
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#8 Flightgypsy

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Posted 09 February 2010 - 05:27 AM

Also, when using PC in SIMV the PC and PS should be the same.


Actually that would make it Assist Control versus SIMV PC/PS.
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#9 LearRRT-CCEMTP

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Posted 09 February 2010 - 06:09 AM

"The vent adjusts TV and flow to maintain the set pressure, I:E ratio and RR. "

I mentioned in my last post that there is no VT and that purely Flow maintains the desired pressure. I forgot to also mention that neither VT nor Flow controls the I:E Ratio or RR. These are controlled entirely by the microprocessor which regulated the Ti resulting in the desired I:E ratio which results in the desired RR.


The easiest way to understand PCV is to look at the circuit. I wish I could draw on here but lets use the settings you mentioned. You did not tell us the set I:E Ratio so I am going to use 1:2.

Mode: SIMV
Rate: 10
Pressure Control: 18
PEEP: 5
Pressure Support: 8
I:E Ratio: 1:2
RCT = 60/RR thus 60/10=6 second RCT
Ti = RCT/Sum of the I:E thus 6 sec /(1+2)= 3 second Ti

So at the beginning of the breath the ventilator inflates the expiratory valve thus closing the expiratory limb off to atmosphere and fires the maximum amount of flow capable by make/model until the desired pressure is reached. It then holds that pressure while decelerating the flow for the length of time set by the Ti. Once that Ti is reached it then deflates the expiratory valve opening the circuit to atmosphere allowing the patient to exhale to baseline for the set Te then once the Te is met it begins the process all over again.

So for your patient it would Close the valve off, fire 160 lpm of flow until a plateau pressure of 18 cmH20 is reached. It then decelerates the flow while maintain that 18 cmH20 for 2 seconds the opens the expiratory valve dumping the pressure and flow for 4 seconds when it starts the process all over again.

The advantage of PCV comes from two things. First you are controlling the pressure thus reducing baratrauma and second you are utilizing a decelerating flow pattern. The decelerating flow pattern allows for alveolar recruitment by having that initial burst of maximum flow. It pops open alveoli that was otherwise collapsed thus increasing compliance allowing you to ventilate a larger area of lung and clear a great amount of CO2 with a lower amount of pressure. Also the decelerating waveform creates a higher mean airway pressure resulting in a higher FRC thus greater oxygenation. The biggest issue with PCV is the fact that we as clinicians wait until it's too late to use it. There is a great misconception that PCV is uncomfortable. It is only uncomfortable when you utilize prolonged Ti or inverse. Otherwise it is actually somewhat physiologic and patients tolerate it well! The other thing to keep in mind is that most clinicians stress over PIP. They see a PIP of 40 and freak out. PIP mean nothing! It is merely a reflection of the pressure seen by the airways and as we know, baratrauma occurs in the alveoli and nOT in the airways. We need to be concerned with Plateau Pressure! PPlt is a reflection of the pressure in the Alveoli and it is very ;possible to have high PIP's and normal PPlt's. IF a patient has a small ETT, bronchospasm, need for suction can all create increased airway resistance this increased PIP's while maintaining a normal PPlt. Only when the PPlt is elevated should we be concerned with barotrauma! An elevated PIP is merely a "hey, look at me" alert for us to more closely examine the patient! We as transport clincians should be monitoring PPlt and Mean Air Pressure during transports and documenting these measurements because they are far more important to vent management that a PIP!

I hope this helps!
Take care and good night!
Dave
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David W. Garrard, BHS, RRT, RCP, CCEMTP, PNCCT
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#10 TexRNmedic

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Posted 12 February 2010 - 10:17 PM

Thanks Dave for the RRT clarification. I am a big fan of PC and PRVC modes. Just takes a little getting used to in order to manipulate CO2 and O2 levels.
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#11 James

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Posted 17 March 2010 - 05:50 AM

Hello All,

Thank you first of all for this interesting discussion. As a result I have a question about the monitoring of the Mean Airway Pressure (MAP) and the Plateau Pressure (Pplat) (in VC mode). If I understand it correctly the MAP is a calculation of the pressure applied during the entire ventilatory cycle. And the Pplat reflects Alveloair Pressure directly. The question: Would you monitor Pplat Q 1 hour or Q 4 hours and with changes in PEEP and/or Tv (ARDS.net)- and in between monitor the MAP (as the LTV measures this continously)?

Hope this makes sense,

Thank you,

James.
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#12 LearRRT-CCEMTP

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Posted 17 March 2010 - 10:53 AM

Hello All,

Thank you first of all for this interesting discussion. As a result I have a question about the monitoring of the Mean Airway Pressure (MAP) and the Plateau Pressure (Pplat) (in VC mode). If I understand it correctly the MAP is a calculation of the pressure applied during the entire ventilatory cycle. And the Pplat reflects Alveloair Pressure directly. The question: Would you monitor Pplat Q 1 hour or Q 4 hours and with changes in PEEP and/or Tv (ARDS.net)- and in between monitor the MAP (as the LTV measures this continously)?

Hope this makes sense,

Thank you,

James.




James,
There is no set schedule, they should be officially monitored and recorded at the same time and as often as you record all the other ventilatory parameter and alarm settings. That said, the actual eye balling of these parameters should be patient determined. Example, if my protocols say that I should monitor and record the ventilatory parameters every 30 minutes in flight and I have a patient that acutely has a rise in PIP, then as part of my trouble shooting a differential diagnosis of that rise, I will check the Pplt. If the Pplt is increased as well then I am going to know that it's a change in compliance and troubleshoot accordingly. If I make changes in PEEP or Ti then I am also going to more closely monitor the MAP to keep track of the effect that the change had on it, but keep in mind that it may take a few moments before you fully see the effect. Again, if I am going to changes from Assist Control to Pressure Control then I am also going to check both the Pplt to obtain a approximate starting point for my Delta P (the titrate for desired exh VT) and the MAP to ensure that I don't reduce it with the settings I apply. Usually with the LTV-1000 or 1200 I just set the graphics monitor so that I can see both the vent graphics and measured parameters, that way I can just glance at all of it periodically. Of course you have to manually measure the Pplt. I hope this helps!
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David W. Garrard, BHS, RRT, RCP, CCEMTP, PNCCT
Program Director and Vice President
AAC - Air Ambulance Caribbean, Inc. D/B/A Flight 4 Life
Charlotte Amalie, St. Thomas, USVI