Vent strategy

[cfdavid]

So, we've all seen the typical 108 kg woman with the BMI of 42 come in for a laparoscopic hysterectomy. So, obese, insufflation pressures, steep trendelenberg........

Any vent management strategies would be very welcomed. I've had some luck adding PEEP, obviously using PCV, and playing with I:E ratios (more like 1:1.5), but today I still had a pretty tough time maintaining adequate tidal volumes.

I WILL be reading about this topic very soon, but any practical advice from some of the senior dudes would be greatly appreciated.

Thanks in advance.

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[Mman]

but today I still had a pretty tough time maintaining adequate tidal volumes

"adequate tidal volumes" in what way? Were you having trouble oxygenating? Couldn't keep the sat above 92? Couldn't blow off the CO2 with ET reading in the 60s?

Adequate tidal volumes on a ml/kg basis kinda go out the window to some extent in that situation. Sometimes you just need to crank the respiratory rate up to compensate for the small volumes. Trial and error.

 

[RT2MD]

So, we've all seen the typical 108 kg woman with the BMI of 42 come in for a laparoscopic hysterectomy. So, obese, insufflation pressures, steep trendelenberg........

Any vent management strategies would be very welcomed. I've had some luck adding PEEP, obviously using PCV, and playing with I:E ratios (more like 1:1.5), but today I still had a pretty tough time maintaining adequate tidal volumes.

I WILL be reading about this topic very soon, but any practical advice from some of the senior dudes would be greatly appreciated.

Thanks in advance.

Well, I'm not one of the above highlighted individuals, but what about even going closer to a 1:1? As long as your airways are ok, the chest wall should help in your favor for avoiding air trapping... what does your waveform look like? Do you have room to play there?

Looking forward to other replies.

 

[Ketafol20]

What peak pressures where you dealing with? Vent pressures are what I'm talking about.

 

[Idiopathic]

yeah i think its a scenario where you need to titrate to clinical picture more than to vent numbers.

i give my obese lap bands/lap colectomies/historectomies a 40-50 cmH2O recruitment breath immediately following induction for 8-10 seconds, and then add 10 of PEEP immediately, it helps decrease intraop atalectasis (there is a study, Ill look for it) and it can help offset that time during which you have suboptimal oxygenation.

edit: http://www.ncbi.nlm.nih.gov/pubmed/19809292

 

[Mman]

an example of not terribly uncommon vent settings I've used for this type of patient.

PCV, Rate 20, IP 30-35, PEEP 5, tweak the FiO2 and I:E to maintain adequate oxygenation and ventilation. I try to keep the ETC02 at least around 45 (if not normal) and keep the sat at 95 or above. Periodic PEEP maneuvers are quite helpful with oxygenation. Close the Popoff valve and manually hold them to an airway pressure of 30 for about 10 seconds. Definitely opens up some alveoli and small airways that weren't being used.

 

[cfdavid]

Thanks for all the replies you guys. Great info. I've come to realize that you really can't "titrate" to vent numbers, but rather take in the entire clinical scenario.

I think I've been reluctant to ramp up the RR that high for some reason. So, I've mainly been playing with other parameters, more focused on Vt's.......

So, hypothetically, if RR was set at 40, and Vt's were something like 150, then MV would be 6 LPM. Not bad! But, basically, you're ventilating deadspace (in this extreme example). So, while ETCO2's might actually drop, so then would Sp02, which would be the key factor indicating you're way off base. It's not as if you're going to be jugging along, with ETC02's in the 30's, with an ABG that WOULD look pretty horrible in that scenario, AND at the same time, still be oxygenating well. Correct?

 

[MTGas2B]

Thanks for all the replies you guys. Great info. I've come to realize that you really can't "titrate" to vent numbers, but rather take in the entire clinical scenario.

I think I've been reluctant to ramp up the RR that high for some reason. So, I've mainly been playing with other parameters, more focused on Vt's.......

So, hypothetically, if RR was set at 40, and Vt's were something like 150, then MV would be 6 LPM. Not bad! But, basically, you're ventilating deadspace (in this extreme example). So, while ETCO2's might actually drop, so then would Sp02, which would be the key factor indicating you're way off base. It's not as if you're going to be jugging along, with ETC02's in the 30's, with an ABG that WOULD look pretty horrible in that scenario, AND at the same time, still be oxygenating well. Correct?

Your biggest problem with oxygenation won't be from hypoventilation (low MV/high dead space ventiation). You have to be really far off base to hypoventilate someone to hypoxemia. Most of your problems with oxygenation in this scenerio will be from shunt.

If you had a fancy ICU vent this would a great scenerio for esophageal pressure measurements. First, you could measure static pressure (which by itself may tell you that a high peak pressure is nothing to worry about), second you could use the esophgeal pressure to calcuate your transalveolar pressures, and now you can comfortably crank that peak pressure to 50 and or set your peep at 15 and know those little alveoli are nice and safe.

 

[dhb]

Think about the forces at play on your lungs: if obese + trend + pneumo: everything is pushing the lungs up so use PEEP + periodical recruitment to counter atelectasis formation and reduce your I/E ration to 1/1 or even less 1.5/1 if possible. Look at your waveform and check for auto peep.

 

[RemyMcswain]

Your biggest problem with oxygenation won't be from hypoventilation (low MV/high dead space ventiation). You have to be really far off base to hypoventilate someone to hypoxemia. Most of your problems with oxygenation in this scenerio will be from shunt.

If you had a fancy ICU vent this would a great scenerio for esophageal pressure measurements. First, you could measure static pressure (which by itself may tell you that a high peak pressure is nothing to worry about), second you could use the esophgeal pressure to calcuate your transalveolar pressures, and now you can comfortably crank that peak pressure to 50 and or set your peep at 15 and know those little alveoli are nice and safe.

Hey, could you talk more about using esophageal pressures to titrate PEEP? Im always confused on this topic.

 

[RxBoy]

My approach
1)First off TV and Weight have nothing in common... Its a misconception that has been around forever. Adequate TV is a measure of IBW which is related to only height. Ever see a 500 pounders CT of the thorax? Little bitty lungs and a massive amount of subq tissue. I usually shoot for 8 mL/kg of IBW. Quick and dirty numbers I memorized are:
50 kg for the 5' (about 400)
60 kg for 5'6'' (about 500)
75 kg for 6'0'' (about 600)

2) PC is the only way to go in this situation. Without getting into too much detail its the difference of trying to squeeze a lot of air into a small hole. PC forces as much of that air it can in the first milliseconds whereas VC tries to do this with a steady flow meaning it takes a lot longer to deliver that same tidal volume (many times unable to deliver). So I set my PC to reach the above TV's.

3) Start with a FiO2=50% with no peep. FiO2 100% fixes problems quickly but after 5 or so minutes absorption atelectasis takes place and your much worse off.

4) Maintain good CO. From the literature I read, tberg does not increase CO (although it does decreases in rterg). This is why I shy away from PEEP/huge TV's and use colloids upfront. Like someone said earlier, shunting is usually the problem. Pulse pressure usually increases in this situation (high afterload so large SBP but very minimum change in DBP), I always follow the MAP so I don't fall into the trap normal SBP=Good.

5) Try I:E ratio of 1:1.5 or 1:1. RR really doesn't effect alveolar MV that much in these situations under PC (granted you are delivering adequate MV). As you increase the rate you increase pressures needed for the same TV and hence TV decrease and like you stated earlier this decreases alveolar ventilation (better TV with low RR is much better than the opposite).

6) I make sure to give good muscle relaxation. Ask surgeons to reduce pneumo pressures if possible. Sometimes a pressure of 15 isn't needed, 12 may work. I have even seen them set it to 20 mm Hg for optimal viewing just because it helps without them having any idea what they are causing on the other side of the drapes.

7) All else falls, intermittent recruitment breaths (CPAP of 40 for 8 sec), FiO2 of 100% followed by PEEP. Each time you increase PEEP, do a recruit breath first. You may need a PEEP of 20 in some cases but know that the patient will just have to tolerate hypercapnia.

8) Little trick I read in Dorsch... Check the FiO2 versus your EtO2. If there is a difference >5%, you're likely hypoventilating.

 

[MTGas2B]

Hey, could you talk more about using esophageal pressures to titrate PEEP? Im always confused on this topic.

Good read about the use of it was in NEJM
http://www.nejm.org/doi/full/10.1056/NEJMoa0708638

Lead author is even a critical care anesthesiologist

So, my simplified explanation. When we normally talk about peep and plateau pressure we make an assumption. That assumption is that pleural pressure is about zero. Truly, what we are worried about is trans alveolar pressures. That is the pressure in the alveoli minus the pressure in the pleura. If pleural pressure is zero, and you have peep set at 10 cmH20 then at the end of expiration the transalveolar pressure is 10. But what if the pleural pressure is higher, what if its 15? Now your trans alveolar pressure is -5 cmH20. When pleural pressure is greater than PEEP, the pleural pressure can be transmitted to the alveoli and lead to collapse, which increases shunt, and basically defeats the whole purpose of PEEP. In ARDS when your oxygenation sucks already this is a big problem.

So, how can we measure pleural pressure. Its kind of invasive and probably high risk to place a pressure transducing catheter into the pleura. Its been established that esopageal pressures are a very good approximation of plueral pressures. In the NEJM study they basically did a pilot study using esophageal pressures to guide PEEP.

So, they took PEEP minus the esophageal pressure. The goal was to keep this difference between 0 and 10 cmH20. So, if esophageal pressure was 15, PEEP should be at at least 15 and as high as 25 to maintain the difference. They had a PEEP ladder for consistent FIO2.

On the other side, (that is end inspiration) its really the transalveolar pressure determines the pressure the alveoli see at inspiration. We like to use plateau or static pressure for this calculation. Again, the plateau pressure minus pleural pressure is really what the alveoli are seeing. In the NEJM study their goal was to keep this difference less than 25 cmH20.

Things to remember, this was a pilot. They were able to show oxygenation was improved with balloons vs. standard. But, remember oxygenation isn't a surrogate for improvements in mortality.

I believe they are working on bigger followup study. I will be curios to see their results.


So, back to the OP. Where I think this could help in the OR. When you can't keep this big girl's sats up what you might find is that her esophageal pressure could be 20 cmH20, and your 10 of PEEP isn't cutting it. Knowing that you could comfortably crank her PEEP up. You might still pay the hemodynamic price for PEEP though. On the other side, inspiration, if you're having trouble ventilating her because of high pressure alarms knowing that her pleural pressure is 20 with a plateau pressure 25 would let be more comfortable with living with the high peak pressure. This limited since our OR ventilators don't measure plateaus.

Hope that help.

 

[jetproppilot]

So, we've all seen the typical 108 kg woman with the BMI of 42 come in for a laparoscopic hysterectomy. So, obese, insufflation pressures, steep trendelenberg........

Any vent management strategies would be very welcomed. I've had some luck adding PEEP, obviously using PCV, and playing with I:E ratios (more like 1:1.5), but today I still had a pretty tough time maintaining adequate tidal volumes.

I WILL be reading about this topic very soon, but any practical advice from some of the senior dudes would be greatly appreciated.

Thanks in advance.

At my new gig I made sure our new anesthesia machines had

PCV-VG capability

precisely

for the scenario you describe.

A vent on an anesthesia machine that has

Pressure Controlled Ventilation Volume Guaranteed Mode

is quasi

revolutionary

especially for deep Trendelenberg laparoscopic cases.

If your machines don't have it,

YOU NEED TO GET IT.

I was amazed at PCV-VG's efficacy first time I used it.

Still amazed.

 

[RT2MD]

At my new gig I made sure our new anesthesia machines had

PCV-VG capability

precisely

for the scenario you describe.

A vent on an anesthesia machine that has

Pressure Controlled Ventilation Volume Guaranteed Mode

is quasi

revolutionary

especially for deep Trendelenberg laparoscopic cases.

If your machines don't have it,

YOU NEED TO GET IT.

I was amazed at PCV-VG's efficacy first time I used it.

Still amazed.

I do like whatever flavor of volume targeted pressure control your manufacturer carries... ESPECIALLY if you set your alarms correctly (ie high pressure). I'm used to the draeger XL, so it's called "auto flow"... why? I have no idea... seems like a dumb name to me.

Most anesthesia machines don't measure plateau pressure? Can you at least do an inspiratory pause and guestimate it? I might have a harder time with the OR vents than I thought... I'll miss my cushy ICU vents!

The point made about esophageal pressure monitoring is pretty sweet... are there ICU vents that can measure this? Or are you just talking about dropping an additional pressure transducer?

Alright... I've finally gotten my awake son to join his sleeping brother. Time for me to go back to bed!

 

[MTGas2B]

I do like whatever flavor of volume targeted pressure control your manufacturer carries... ESPECIALLY if you set your alarms correctly (ie high pressure). I'm used to the draeger XL, so it's called "auto flow"... why? I have no idea... seems like a dumb name to me.

In the institutions I've worked the machines haven't had anything else besides the basics, volume control, pressure control, maybe pressure support. The newer Tiro-M's I have out here have more features than the older Draeger Fabius's we had where I trained. We do have two new Draeger Apollo's back at my usual place, they're primarily used on peds and hearts. I really haven't used them much. I left shortly after they arrived as well.

Most anesthesia machines don't measure plateau pressure? Can you at least do an inspiratory pause and guestimate it? I might have a harder time with the OR vents than I thought... I'll miss my cushy ICU vents!

You'd have to have a machine that can do the pause, and measure it. Probably easier in the OR, since we frequently use neuromuscular blockade. But again, I haven't seen any anesthesia machines that can do this.

The point made about esophageal pressure monitoring is pretty sweet... are there ICU vents that can measure this? Or are you just talking about dropping an additional pressure transducer?:

This feature is already built into some newer ICU vents. The newer vents at one of the hospitals I trained at were Viasys AVEA's, and they could do it. You just had to drop in a disposable esophageal pressure catheter, and confirm position by checking the waveform on the vent. Most (but not all) of our RTs could place them.

 

[jwk]

So, we've all seen the typical 108 kg woman with the BMI of 42 come in for a laparoscopic hysterectomy. So, obese, insufflation pressures, steep trendelenberg........

Any vent management strategies would be very welcomed. I've had some luck adding PEEP, obviously using PCV, and playing with I:E ratios (more like 1:1.5), but today I still had a pretty tough time maintaining adequate tidal volumes.

I WILL be reading about this topic very soon, but any practical advice from some of the senior dudes would be greatly appreciated.

Thanks in advance.

Vt is not the concern, and has been over-emphasized for years. Use PC as others have indicated. I never set Vt anymore - everything is pressure controlled on any patient on a vent. I hope we get some PCV-VG vents like Jet has to try soon.

5 of PEEP is standard for us nowadays, sometimes a little more - when I was in school 30 years ago, PEEP was akin to devil worship. I've discovered that it is not.

I'm much more tolerant on mild hypercarbia than I used to be as well. High 40's, low 50's? Probably fine with most of our <2hr procedures and our patient population.

One other thing - you can always tell the surgeon "I'm not ventilating/oxygenating well enough - you'll have to change position / decrease pressure / do something else so your patient will make it through surgery". Open surgery is always a backup plan for laparoscopy.

 

[RxBoy]

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