timely wake ups?

[BobBarker]

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Good point on the benzos. I did a carotid the other day, no midaz given, woke up much sharper than normal. Used Iso for my volatile.

 

[soonerfrog]

All great points and I think Sevo has nailed it (providing of course he meant: keep flows on avg, > .3L/min not >3L/min; assumed this was typo).

 

[Doctor4Life1769]

This is a good low flow anesthesia topic.

BB is correct. If you have a 50/50 mixture of O2 and N2O you are likely not meeting metabolic needs.

VO2 (oxygen consumption) is around 4-7 cc/kg/min in a normal healthy resting adult. Probably more like 4 cc/kg/min under GA as it decreases 02 consumption.

Take a 70 kg patient. 70kg x 4cc/kg/min = 280cc of O2 consumption per minute.

@ .35 l/m with 50/50 N20/O2 mixed in you are only delivering about 175 cc/min. 175 cc’s is 105 cc/min. off the calculated VO2.

Keep in mind there is some variability. Super healthy, fit people with a low VO2 may only need 3cc/kg/min. Those with more issues such as obesity, PVD, cardiac shunts, will need a higher delivery of fresh O2.

For adults, I don’t usually go under 3L/M of fresh gas flow.

Great topic to discuss as it is very clinical.

Been real busy and didn't return to thread until now.

I should have specified. It wasn't equal Nitrous/O2. It was 70% O2 and 30% N2O. I get I was still under the calculated VO2 but not by a whole lot and it was only for 20 mins.

0.35 at 70% is ~ 245cc/min I believe. Is this significant for a short case? Difference is 35cc/min.

 

[RxBoy]

Been real busy and didn't return to thread until now.

I should have specified. It wasn't equal Nitrous/O2. It was 70% O2 and 30% N2O. I get I was still under the calculated VO2 but not by a whole lot and it was only for 20 mins.

0.35 at 70% is ~ 245cc/min I believe. Is this significant for a short case? Difference is 35cc/min.

250 cc/min is an awake pt at a MET of 1. General anesthesia substantially decreases the metabolic rate so the number is probably smaller. If you really want to know if your under oxygenating someone, simply compare the FiO2 to EtO2 (a number few look at). If the gradient is greater is >5 then you are either underoxygenating or underventilating.

 

[pgg]

I dont think you were meeting her oxygen requirement with .35L total flow of nitrous/o2 mixture.

Even if you're not meeting the patient's O2 requirement, it's not like you're going to fail to notice. The machine's O2 sensor and your gas analyzer are going to alarm as the FiO2 drops below 18%, or if circuit reservoir (bag) volume gets too low the vent will alarm as it's unable to achieve set tidal volumes / apnea flow, or a spontaneously ventilating patient will entrain room air through the relief valve. You're going to notice, unless you're asleep. 🙂



While we're being esoteric and academic 🙂 I'll throw this out there - when your fresh gas flows are very low, the actual composition of inspired gases generally won't correlate well with where your knobs are set. The lower the flows and the longer the case has been going on, the greater the difference between set and read values will be. Believe what the gas analyzer tells you.

Very low flows can produce some counterintuitive mixtures. For example, if you have the des vaporizer set to 6% and you're 4 hours into the case with .3 or .4 LPM O2, you're going to see inspired des up around 7% because there's a concentration effect - minimal uptake of des, consumption of O2, and CO2 absorbed.

I don't know how many times I've seen people declare that their gas analyzer was broken or the vaporizer wasn't calibrated right because it's "impossible" to have a steady state inspired volatile reading higher than the vaporizer setting.

 

[pgg]

250 cc/min is an awake pt at a MET of 1. General anesthesia substantially decreases the metabolic rate so the number is probably smaller. If you really want to know if your under oxygenating someone, simply compare the FiO2 to EtO2 (a number few look at). If the gradient is greater is >5 then you are either underoxygenating or underventilating.

(FiO2 - EtO2) x MV = actual O2 consumption

In most adults I find it's usually in the 300s around 3-4 cc/kg/min even under GA, peds 5-6 cc/kg/min just like the book says.

 

[RxBoy]

My wake up strategy for longer cases (>2 hrs)

Between 10 - 20 min before extubation (depending on length of case and volatile used):
-Titrate gas down early but not change flows (flows are usually low)
-give 25-50 of fentanyl ( I avoid opiates last 1-2 hours case until near extubation). Opiates have powerful antitussive effects to minimize bucking plus calm as gas washed out.
-Keep paralyzed at 1 twitch

When I predict there is 5 min left:
-Do not reverse
-3-4 cc's of 4% down the tube then baggin 5 or so times..
-Suction mouth, table head up
-Increase flows to >MV to insure no rebreathing and keep vent on
- Will never build CO2 beyond EtCO40. EtCO2 is a gauge of anesthetic depth. If the volatile is still blunting your CO2 responsive curve to the point of needing a high EtCO2 they are still too deep.
-Wait for one rebreath pattern on capno then reverse. Pt reverse much easier when there isn't any synergistic effects from volatile + paralytics
-Will not touch patient or ever touch tube. Will not allow any personale to touch pt. At a MAC of 0.1 even taking off a blood pressure will wake them up. Never ever ever shake or yell at pt to open eyes.
-Pulling good tidals, rr, and MAC<=0.2, will gently ask them to open eyes. Then squeeze fingers. Immediately pull tube and suction. No bucks, no wild flailing of limbs.

Less<2 hour case
-shut off gas near skin, iincrease flows
-Suction
-reverse early and have them breath
-Have them blow off residual gas
-pull tube

 

[codeb1ue]

My wake up strategy for longer cases (>2 hrs)

Between 10 - 20 min before extubation (depending on length of case and volatile used):
-Titrate gas down early but not change flows (flows are usually low)
-give 25-50 of fentanyl ( I avoid opiates last 1-2 hours case until near extubation). Opiates have powerful antitussive effects to minimize bucking plus calm as gas washed out.
-Keep paralyzed at 1 twitch

What do you mean by the last part. Would you give additional paralytics with 20 minutes left if its more than 1 twitch? I'm really curious because I, for one reason or another, am deathly afraid of giving paralytics towards the end of an operation, out of concern I'll delay wakeup.

What's the absolute latest one can give an additional dose prior to the end of an operation?

 

[MRW]

In regards to the debate over breathing off gas SV vs CV, I'll throw pressure support into the mix. You can get the patient breathing, monitor their RR and utilize PS or preferably PSVPro to supplement their tidal volumes to help the gas blow off while titrating in narcotics. I have had the most "patient moves themselves to the stretcher" moments with this approach.

 

[soonerfrog]

Side bar for first years: what do you think the difference in PSV and PSV-Pro?

 

[RxBoy]

What do you mean by the last part. Would you give additional paralytics with 20 minutes left if its more than 1 twitch? I'm really curious because I, for one reason or another, am deathly afraid of giving paralytics towards the end of an operation, out of concern I'll delay wakeup.

What's the absolute latest one can give an additional dose prior to the end of an operation?

What I meant was I like at least 1 strong twitch so I know I'll be able to reverse. But lets say I have 4 twitches, I sometimes give a 1/2 cc of vec or roc. If you dose really small toward the end, it won't burn bridges. I do this especially for the very obese where I went the vent to do all the work until the very end. However when I actually reverse I want there to be as many twitches as possible because reversal works much faster and more complete.

I agree with PSVPro. I absolutely love it but only our satellite hospitals have the Rolls Royce machines. Best for LMAs. But even with GETA, they make a nice transition to SV without compromising low MV.

 

[LovingBaltimore]

What's the absolute latest one can give an additional dose prior to the end of an operation?

We do a ton of neuro procedures with the head pinned. I give vec routinely until the head is out of pins... knowing I'll be waking the patient up minutes later. I.E. I give it right up until the end.

 

[heathermed]

thank you so much everyone for all the suggestions. It's really helping me get a better idea about the art of anesthesia.

some questions that I was wondering about

1) for those using nitrous at the end, when do you do the switch and turn the volatile off and go to 70% nitrous. when they're starting to close? when they're on the fascia? Do patients tend to move with just nitrous and a tapering volatile?

2) how do you dose your roc at the end? I've had patients move with just 2 switches the past week and I'm kinda timid. I've been running volatiles on high after this experience. But this is leading to longer wake ups. Any suggestions?

thanks!

 

[BobBarker]

You weren't accurately measuring your twitches. Either that or the patient didn't really move. If you have .7 mac nitrous, some volatile, opiates, and residual midaz and paralytic you should not have pt movement.

 

[Jay K]

1) for those using nitrous at the end, when do you do the switch and turn the volatile off and go to 70% nitrous. when they're starting to close? when they're on the fascia? Do patients tend to move with just nitrous and a tapering volatile?

In residency, when they were on fascia. In PP it depends on the surgeon, but for the most part they close a lot faster, so you'll have to do it sooner; You'll get to know your surgeons. My patients do not move during closure, given sufficient narcotic dosing throughout the case and at end. For your purposes, until you get the hang of things, you can keep your propofol syringe handy for 1-3mL during closing should you need it.

2) how do you dose your roc at the end?

I don't. Make sure you've titrated enough narcotic to make your patient comfortable, and/or dosed your epidural, done an adeq nerve block, etc. Perhaps for your purposes save a little fentanyl for the stimulation from closure.

Above in bold. You'll get better.

 

[Oggg]

In residency, I did a lot of yelling at the patient to wake up. Now, in PP, we had a new candidate come try out and he started doing that and it looked so... bush league. I used to tap pts on the forehead (hard) in residency -- that's lame too. They're both signs that you woke up the patient waaaaaaaaaay too slow or you gave too much narcotic.

If you find yourself in that position, I've been smartly advised to distract everyone while continuing to wake the pt up. Chat them up, ask staff to go get blankets, move the pt to bed for extubation, pack your EKG cable away extra neatly, ask for a new IV bag, clean up your desk, etc. if you stare at your pt in silence, people become more aware that you are delaying the exit time. Every time you yell at the patient you remind everyone how you're out of control. Every time you whack the pts forehead, the nurses consider writing you up.

 

[cchoukal]

This is a good low flow anesthesia topic.

BB is correct. If you have a 50/50 mixture of O2 and N2O you are likely not meeting metabolic needs.

VO2 (oxygen consumption) is around 4-7 cc/kg/min in a normal healthy resting adult. Probably more like 4 cc/kg/min under GA as it decreases 02 consumption.

Take a 70 kg patient. 70kg x 4cc/kg/min = 280cc of O2 consumption per minute.

@ .35 l/m with 50/50 N20/O2 mixed in you are only delivering about 175 cc/min. 175 cc's is 105 cc/min. off the calculated VO2.

Keep in mind there is some variability. Super healthy, fit people with a low VO2 may only need 3cc/kg/min. Those with more issues such as obesity, PVD, cardiac shunts, will need a higher delivery of fresh O2.

For adults, I don't usually go under 3L/M of fresh gas flow.

Great topic to discuss as it is very clinical.

Your calculations are fine, but what seems to be missing is the rest of the oxygen in the circuit. Sure, you're adding fewer molecules per minute than are being used, but the circuit has plenty of them. How do I know? The bag is full and the O2 analyzer reads some # above 21%. As time goes by, the patient will consume oxygen and produce CO2. The CO2 gets absorbed, so the circuit volume (bag) will get smaller and smaller. Sometimes I have to add a little O2 back to the circuit, but this will dilute the remaining agent and may hasten emergence. Obviously over the hours of a low-flow/closed-circuit case, this is unsustainable, but for 15 minutes at the end of the case, there's more than enough oxygen in the circuit, and I often spend considerable time at NO flow at the end.

Oh, and I just read Oggg's post above and really, really like it.

 

[Doctor4Life1769]

In residency, I did a lot of yelling at the patient to wake up. Now, in PP, we had a new candidate come try out and he started doing that and it looked so... bush league. I used to tap pts on the forehead (hard) in residency -- that's lame too. They're both signs that you woke up the patient waaaaaaaaaay too slow or you gave too much narcotic.

If you find yourself in that position, I've been smartly advised to distract everyone while continuing to wake the pt up. Chat them up, ask staff to go get blankets, move the pt to bed for extubation, pack your EKG cable away extra neatly, ask for a new IV bag, clean up your desk, etc. if you stare at your pt in silence, people become more aware that you are delaying the exit time. Every time you yell at the patient you remind everyone how you're out of control. Every time you whack the pts forehead, the nurses consider writing you up.

I never tap on a patient's forehead, just looks tacky.

I will attempt to stimulate a patient as I'm calling their name out by lightly pressing on their chest or suctioning their mouth, or will press in along the TMJ region. of course, i'll look at the eyes to see if they're still in stage II in addition to looking at Et of agent, and if there's still agent circulating then i'll know it's pointless to keep calling out the pt's name and rather try to get the gas off by breathing for the pt or inc RR at the expense of EtCO2.

Your ideas above are great ideas and something i'll look to incorporate in the case of a delayed wake-up.

 

[BobBarker]

I use tetany.

 

[codeb1ue]

leave the vent as is. as you've already observed, turning the gas and minute ventilation down at the same time won't get you anywhere. if you want to wake the pt up on spont vent, they have to be breathing very early ie usually not possible in a belly case.

the reason for going to low flow with gas off super early can be illustrated by the following example which i used to see often with junior residents.

get called to room for wake-up - fat (american average) patient for belly case, gas kept somewhere around 3/4 mac until fascia is closed, then resident turns flows way up, gas off, keeps vent on (MV 5L/min).

at around 0.5-0.6 ET volatile pt sputters and coughs. resident turns vent off, pt slowly starts to breathe (MV 2-3L/min).

ET volatile on spont vent comes up at 0.8-0.9?!? resident makes comments about novel patient quality of spontaneous generation of volatile anesthesia - where's that gas coming from?!

patient takes another 10-15min to breathe the rest of the volatile off. 15min x 4patients can be an extra hour for that room.

for the residents - how is possible that the same pt will have an et iso of 0.5 on the vent with MV of 5L/min, but when they start breathing on their own with MV of 2L/min the et iso comes back at 0.8?

Any answer to this question?

Also just to clarify, once turning gas off and going low-flow, what are the options if your MAC eventually drops to 0.6 and surgeons are still working on fascia. i know propofol is an option but if that is not readily available, do some of you turn the gas back on at that point to keep MAC at 0.6? Or go nitrous?

I ran into this issue today and just ended up turning on nitrous at that point. Of course I then got into a self-debate about whether to keep flows low (<2) or to just turn it way up and blow off the sevo and go pure nitrous. I went with the latter (70/30) and then ended up running into issues with hypoxia. lol. I'm such a noob at this still.

 

[nap$ter]

Any answer to this question?

Also just to clarify, once turning gas off and going low-flow, what are the options if your MAC eventually drops to 0.6 and surgeons are still working on fascia. i know propofol is an option but if that is not readily available, do some of you turn the gas back on at that point to keep MAC at 0.6? Or go nitrous?

I ran into this issue today and just ended up turning on nitrous at that point. Of course I then got into a self-debate about whether to keep flows low (<2) or to just turn it way up and blow off the sevo and go pure nitrous. I went with the latter (70/30) and then ended up running into issues with hypoxia. lol. I'm such a noob at this still.

you essentially gave the answer - redistribution from fat (and other less important compartments). once your inspired gas concentration reaches zero, if your MV-flow product is greater than delivery of gas to the alveoli (a function of the uptake equation), then the mass of volatile in the alveoli becomes diluted. when the MV drops as the pt begins to spontaneously ventilate, (CO) has time to "catch up" and the expired concentration of gas increases.

In answer to your second set of questions - i generally prefer bumps of propofol to avoid the hypoxia you ran into, but those bumps can have unpredictable durations of action sometimes if the anesthetic/patient is complicated,so less is more. i consider propofol to be one of the emergency drugs, so it should always be available

 

[Oggg]

I sometimes use stun doses of sevo at the end of case when there's early movement, if I've got an LMA in, or if I've used desfl for the case. If the pt is older than 18 or so, I pull the LMA at all an any stages of wakeup since I'm not afraid of laryngospasm though I'm alert for it. If I have used desfl for the case, a 10-30sec burst of 8%sevo at 4-8LPM does a good job of stunning the patient for a few minutes and still washes out quick. Between sevo stun and saving 20mg PPF from the first induction syringe, I almost never draw up a second PPF.

 

[codeb1ue]

I sometimes use stun doses of sevo at the end of case when there's early movement, if I've got an LMA in, or if I've used desfl for the case. If the pt is older than 18 or so, I pull the LMA at all an any stages of wakeup since I'm not afraid of laryngospasm though I'm alert for it. If I have used desfl for the case, a 10-30sec burst of 8%sevo at 4-8LPM does a good job of stunning the patient for a few minutes and still washes out quick. Between sevo stun and saving 20mg PPF from the first induction syringe, I almost never draw up a second PPF.

Would you mind explaining why sevo burst as opposed to des? And why only if pt is on des do you do that. What about in cases of sevo as maintenance.

 

[Doctor4Life1769]

My guess is that Des is less soluble, hence would be blown off faster, and you may have to keep the pt on Des longer to "stun" the pt. Might as well just do maintenance with Des. I've read Des is more irritating to the airways than Sevo, too.

Of course, the above is just my thoughts. Not sure what Oggg or others have to say about it.

 

[Oggg]

Desfl is expensive at high flows, and can cause airway irritation and hypertension when the % is raised too quickly.

If I'm trying to blow sevo off at the end, I don't like to turn the sevo back up to stun. If the sevo is <0.3 and I need to stun, I might consider doing the sevo stun but I would expect an unpredictable duration.