Vent Settings AC vs SIMV | FiO2, Vt (Tidal Volume) | Differences, Advantages, Disadvantages

vent settings for nurses

Confused about ventilator settings in the ICU?  No more!  This podcast covers the basics behind AC (assist control) and SIMV (synchronized intermittent mandatory ventilation) two of the most common vent modes you will see in your ICU/critical care unit.

Click here and you can view and download a FREE PDF document that outlines some of the oxygen support systems that we have for our patients.


The goal of both SIMV and AC ventilator modes is to ensure proper ventilation/gas exchange for our patients.  The major difference has to do with the tidal volume and how/when it is delivered.  This concept is discussed in the episode along with some of the common vent settings like PEEP, Pressure Support, Tidal Volume, and FiO2.

If this podcast has helped you or answered questions for you please leave a kind review in iTunes or Stitcher.

Visit THIS LINK to get the FREE O2 Delivery Guide

Go check out the lesson on O2 Delivery HERE!

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Podcast Transcription

Before we get started into today’s podcast about ventilation, I wanted to tell you about a little resource that we have available. If you go to, that’s,, you can download a PDF file with all the different oxygen support therapies that we have for our patients. It’s a free PDF. You can just go there and download that. No obligation or anything. Just go there. You can download that and have that available. Print it out, whatever. Save it, share it, whatever you want to go. Again, that’s at and it’s a great little resource to help you keep straight the different types of oxygen support that we have available for our patients.


Good afternoon everybody. This is Jon with It’s great to have you back here. Today, we’re going to talk just really briefly about ventilator settings. Two of the most common vent settings and what are some of the indications and what’s the difference between the two.


It can be confusing. We walk into an ICU and see our patient is ventilated to look over at the machine and understand everything that’s going on with all the different numbers and wave forms and alarms and things happening on a ventilator. So it’s important to just kind of understand some of the basics about vent settings and what’s actually happening with the patient and why are they being ventilated and what it all means.


So there are many, many different modes of mechanical ventilation. Every few years, a new mode comes out and people debate between different modes and what the best mode is for their patient. The two modes that you will most likely see with your patients are going to be assist control and SIMV. The SIMV stands for synchronizes intermittent mandatory ventilation. So you have assist control or AC, that’s what it goes by or synchronized intermittent mandatory ventilation or SIMV.


But we use most often in my ICU is SIMV and that’s really the preference of the neurologist who runs the unit. In our surgical and medical ICUs, what is most often used is assist control. And there are many theories behind what is most effective for the patient, why you would use one specific setting over another. But today, we’re really just going to talk about what assist control is and what SIMV is and what it means.


So basically with a ventilation, what we’re doing with mechanical ventilation, what we’re doing is we’re creating an artificial airway to deliver breaths by a machine. Now, that can be done through a tube in the trachea and that’s either done by intubation through the oral airway. It can be done nasally and it can also be done with a tracheostomy. So you can actually insert a tube directly into trachea. You can insert one through the mouth into trachea or through the nose down into the trachea as well.


And those tubes are going to sit right at the carina, which is right near the bifurcation of the right and left lungs. So it’s going to sit just right above that area there to hopefully deliver breaths equally to the right and left lung.


In case of the two modes that we’re going to talk about are AC and SIMV, with assist control ventilation, what happens here is every time the machine delivers a breath, it’s going to deliver a set tidal volume. OK? And tidal volume is the volume of air that is being delivered to the patient and that is going to be based on the patient’s size, patient’s condition, and what their needs are. Usually, AC anywhere from 300 to 800 mLs of air being delivered with each breath.


So you’ll see their tidal volume is set at 500 or so and with each breath that the machine delivers, they’re going to receive that set volume of air. So that the tidal volume is set whether – every time a breath was delivered.


Now, with assist control, the machine is going to deliver breaths at a very set rate. Set rate might be 12 breaths per minute. And so, every few seconds, the machine is going to deliver that breath. Every five seconds, a breath is going to be delivered and the patient can actually trigger breaths as well but what happens with assist control is the patient has to reach a specific limit of respiratory – inspiratory effort. Once they reached that limit of inspiratory effort, the machine is going to go ahead and deliver the set tidal volume even though the patient is attempting to breathe on their own.


So if the patient reaches a set limit of inspiratory effort, the machine is going to deliver the set tidal volume. So no matter what, with assist control, every time a breath is taken, it reaches that limit, a set tidal volume is going to be delivered. OK.


So with this type of ventilation, with assist control ventilation, there’s volume cycle which essentially means that they’re getting a set volume of air every time air is being delivered from the machine, and the patient can only take spontaneous breaths if they reached a specific threshold. Once that threshold is reached, they’re still going to get that same set of volume of air. OK?


Some of the advantages of assist control mechanical ventilation is it can decrease the work of breathing since the patient doesn’t have to really use their respiratory muscles. Those muscles can really kind of relax. And even though the patient might try to initiate breaths, there’s no need for them to do that. And so, with that set inspiratory effort required, they can kind of just relax. They’re still going to get their 12 breaths per minute whether they attempt to take a breath or not.


Some of the disadvantages of assist control ventilation is that it can lead to breath stacking which is essentially if the patient can trap air inside the alveoli as they – so as they take a breath, as they try to take a small breath and maybe don’t reach that inspiratory effort as they’re trying to let that breath out before they get all that air out of the alveoli, another breath maybe delivered from the machine at that set tidal volume and that will leave some air trapped inside the alveoli. That’s called auto-stacking or auto-PEEP, which means that they’re just keeping some air inside the alveoli and that’s not necessarily a good thing because it can lead to excessive wear on the alveoli.


So that’s really kind of what assist control is. The best way to remember assist control is that essentially, it’s a very specific set tidal volume that will be delivered with each breath. So that’s kind of way to think of it is it’s really – the patient will always get a set volume whether they attempt to take a breath on their own or whether it’s the machine initiating everything, that machine is going to deliver that set tidal volume.

So what’s – how is that different from SIMV? So SIMV, you still set a minimum rate on your machine. So you might still set that the machine is going to deliver 12 breaths per minute. OK? But this machine also synchronizes with the patient. So the patient can take breaths on their own and when they attempt to take breaths on their own, they can take in any volume that they want. So that’s really how it differs from assist control.


As the patient takes breath on their own, the machine is going to not deliver that breath and it’s going to allow the patient to take in whatever volume they want. So the patient is still going to get those 12 breaths but on any breath that the patient takes on their own, they’re going to pull in whatever volume they want.


So the machine is going to deliver its 500 mLs of tidal volume every time that it deliver a breath. But the rate is set at 12 when a patient is actually attempting to take one breath over that, so they’re taking an extra additional breath. On that 13th breath, the patient is going to pull in any volume that they want. So it might be a 100, it might 500, it might be a 1,000 but the patient is going to take in their own breaths.


So it’s not going to require that a specific volume is delivered. But with this, as long as the patient is not taking their own breaths, there’s really no difference between this and assist control because you’re still getting your set rate with the set volume. The only way that SIMV differentiates from assist control is when the patient actually is taking breaths on their own over what the ventilator is delivering because as soon as they do that, they take in any volume that they want.


So that’s really the biggest – that’s really the difference between assist control and SIMV. So that’s – otherwise, it’s completely identical …

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