A/C vs. SIMV

diveRN · May 12, 2008

I'd like to hear your thoughts on ventilator modes. Which is best A/C vs. SIMV and in what cases. Trauma/Surg employs SIMV more often; MICU uses A/C almost always! I asked the fellow, and he said "you just make them work harder on SIMV." Let's see...

"Best" is a matter of preference and on the goal of the mechanical ventilation.

AC delivers a preset Vt and the pt can breath above the preset rate. The upside is that it rests respiratory muscles. The downside is that it's not natural to take the exact same sized breath each time. I've seen patients panic because they can't control their breathing. Additionally, AC can lead to a lazy diaphragm. If the goal is to rest the muscles short term, AC works. I don't like it used more than 12 to 18 hours for a unsedated/unparalyzed patient.

SIMV delivers a preset rate and Vt, but if the patient is breathing on their own above the preset rate, then the vent won't deliver the prescribed volume for those breaths. It'll deliver what the patient wants with each breath above the rate. Now, if you add norcuron and versed, you can control the breathing and Vt completely. Preset rate, preset volume... in this case, it works like AC because the vent is doing all the work. SIMV is more natural for a responsive patient breathing on their own. It exercises respiratory muscles, but the downside to that is that it can lead to complete respiratory failure in patients who have little reserve left ... that's when the preset rate and Vt kick in. The patient just has to be monitored to make sure they're not overworking.

Personally, I prefer SIMV in the ICU. For patients who aren't going to get trach'ed, the goal is to get the ETT out as soon as the pt can tolerate it. If you take away a person's drive to breathe by doing it all for them, then they get lazy which makes weaning that much more difficult.

Just my :twocents:

poppy07 · May 12, 2008

what confuses me with the A/C mode is that you will read the tidal volume by the pt on a set volume of let's say 500 reading 460, 520, 730, 340...etc with a rate of 18-24 on set rate of 12. So, if it truly is A/C then why are each of these breaths determined by the pt and not the set volumes?

diveRN · May 12, 2008

what confuses me with the A/C mode is that you will read the tidal volume by the pt on a set volume of let's say 500 reading 460, 520, 730, 340...etc with a rate of 18-24 on set rate of 12. So, if it truly is A/C then why are each of these breaths determined by the pt and not the set volumes?

"RT to the ICU Forum please, RT to the ICU forum please."

Dunno on that one. Theoretically, each breath SHOULD be a set volume. I'm no vent expert by any stretch and it's been a good while since I've seen a pt on AC.

I'll ask. I work with some great RTs.

allnurses Guide · May 13, 2008

I pretty much agree completely with diveRN. I much prefer SIMV - there's some evidence that providing some work of breathing leads to quicker vent weaning.

Vikingnurse · May 13, 2008

Could someone please explain what A/C stands for? Perhaps we just call it something else in our little country? It sounds a bit like the MMV (Mandatory Minute Volume Ventilation) on the DrägerXL...

If the preset ventilations in A/C mode is given with regular intervals without synchronisation with the patients inspiration, it would seem as if the patient is working against the ventilator. It could explain the different readings... The ventilator starts an inspiration, the patient isn't ready for it and works against the ventilator leading to a small tidalvolume. Next the patient takes a deep breath (I understand that spontaneous ventilation is possible in A/C mode) to compensate for the interupted inspiration.

KevinS · May 13, 2008

A/C= Assist Control

joeyzstj, LPN · May 17, 2008

what confuses me with the A/C mode is that you will read the tidal volume by the pt on a set volume of let's say 500 reading 460, 520, 730, 340...etc with a rate of 18-24 on set rate of 12. So, if it truly is A/C then why are each of these breaths determined by the pt and not the set volumes?

Poppy...........I am both an RRT and a Critical care RN, so from both standpoints I will try to explain this the best that I can. With A/C Rate of 12, Vt of 500 as you mentioned, the ventilator is set at a MINIMUM of 500 cc's to be delivered 12 times a minute.........MINIMUM. Now, if the patient "decides" to do some work on his/her own, they can take in a larger tidal volume and a faster rate if desired. So, the patient can breath as fast as they want, they just cant breathe any slower then 12 times a minute. The same with tidial volume (Vt). The patient can breathe as much volume as they want theoreticlally, however they cant breathe less than 500 cc's or the set tidal volume.........whatever that may be. You listed several ovserved volumes above in your post (460, 520, 730, 340). I think there is either the possibility that the patient wasnt actually in A/C, they had a leak or they were high pressuring the ventilator and the set volume wasnt being delivered. In A/C of 500, and no problems were present, the tidal volume would NEVER go below the set volume, so you would never see a tidal volume of 460 or 340 as you mentioned. This can get a bit confusing, but sometimes, depending on what ventilator you are using and how old it is, it will display settings of A/C or SIMV on the screen, however the vent itself might be set on something different. A popular ventilator, the PB 7200 often shows on the display screen all of the settings for A/C but can acutally be in pressure support. Back to the tidal volumes you listed above such as the 520 and 730. In the case of the ventilator being set on 500 cc's, the patient will be given 500 cc's by the vent, however if the patient decides " hey, I want to take a deeper breath in and do a little of the work on my own", the patient can breathe in deeper.........sucking in 100,200,300,400, ect..........cc's of more tidal volume. The volume that you are seeing on the screen that is changing IS NOT measuring what is delivered to the paient. It is measuring what the patient exhales with each breath.

SIMV and A/C both have their benefits and downfalls, and I personally would argue the fact that its not good for more then a day or two. It is really a physicians preference. SIMV is better for Post op hearts due to the risk of the cyclic build-up of intrathoracic pressure and subsequently the risk of compression of the vessles and additional strain on the heart. SIMV allows for a periodic release of pressure during spontaneous breathing. A/C is usually preferred in patients who are sedated and/or patients who cannont tolerate spontaneous breathing such as in in acute respiratory distress, ARDS, ALI, ect. I hope this helps.

diveRN · May 17, 2008

Could someone please explain what A/C stands for? Perhaps we just call it something else in our little country? It sounds a bit like the MMV (Mandatory Minute Volume Ventilation) on the DrägerXL...
If the preset ventilations in A/C mode is given with regular intervals without synchronisation with the patients inspiration, it would seem as if the patient is working against the ventilator. It could explain the different readings... The ventilator starts an inspiration, the patient isn't ready for it and works against the ventilator leading to a small tidalvolume. Next the patient takes a deep breath (I understand that spontaneous ventilation is possible in A/C mode) to compensate for the interupted inspiration.

I talked with an RT about this the other day and she pretty much said the same thing. In AC mode, the vent is not synchronized with the pt. If the patient attempts a breath or is exhaling as the vent is delivering a breath, then peak pressures rise will rise above the pre-established limits and the vent will stop short of delivering the full volume. If the timing of the pt's own breath happens to be synchronized with the vent's delivery of one, then you may see higher volumes than what the vent was set for.

The point in the pt's respiratory cycle that this happens will determine the actual volume, which is a possible reason why you were seeing varying volumes in AC mode.

Hope this helps.

ICURRT · Oct 30, 2008

what confuses me with the A/C mode is that you will read the tidal volume by the pt on a set volume of let's say 500 reading 460, 520, 730, 340...etc with a rate of 18-24 on set rate of 12. So, if it truly is A/C then why are each of these breaths determined by the pt and not the set volumes?

In A/C the volume is pre set, as is the rate. The patient can breathe above the set rate, but the volume will remain the same.

RRT2RN2CRNA · Nov 11, 2008

This is always such a can of worms.

If I feel like messing with a new MICU resident I'll just say "So can we try pt in room 128 on SIMV?" Some of them totally freak out. (It's very fun.)

Honestly, I really don't see what all the fuss is about. I don't see the need to make a unit an "SIMV only" unit or an "A/C only" unit. Every pt is different and I think that we should be able to tweak settings and use all the tools in the vent box when trying to comfortably ventilate a pt.

My only rules are that I always use AC when a pt is not going to be taking any kind of spontaneous breaths anyway (anoxic-neuro, totally and completely sedated). Not that SIMV would do any harm, it's just that the basic AC is all that's needed.

I always use SIMV with my post-open heart and thoracic patients because I don't want them to possibly stack breaths and created auto-peep as Joeystzj said.

Other than that, I think the Physician and RRT should use all the modes and settings and see what works for that particular pt.

bethem · Nov 11, 2008

What do you guys (US) call what I know as Pressure Control Ventilation plus Assist? It's a Draeger mode on the Evita models. It delivers breaths at a set rate, with a pressure limit. The breath is delivered until the pressure limit is reached, therefore the tidal volumes can change from breath to breath. The 'assist' part of it allows the patient to spontaenously trigger breaths, which are then delivered according to the pressure limit set. You can control the Tinsp on this mode as well which I understand you can't on SIMV on the Draeger ventilators.

So what is it called in the US?