"What is the difference between assist control and SIMV?" This is a question I would get all the time, or at least some variation of it. A while back, a colleague asked me this question to answer on the RSI podcast. But The RSI Podcast's purpose is short and to-the-point answers, and I didn't think I could adequately answer the question in a short time frame... Especially without pictures 😁
Now, most recently, the question, “What is the difference between CMV and SIMV,” was asked by someone who utilizes the Hamilton T1 ventilator. For anyone unfamiliar with the Hamilton T1 ventilator, it is a very specific and unique vent with its own challenges when attempting to bridge into learning mechanical ventilation. The T1 is a completely pressure-driven ventilator that allows you to choose between volume-targeted or pressure breath types and the typical A/C and SIMV modes... There is NO true volume mode, only pressure-targeted or pressure-regulated volume-targeted, as some may call it (Adaptive or PRVC for others 🤪 ).
For anyone who uses the Hamilton and is reading this may be asking, "Did you say A/C and SIMV modes?" Yes. Although the T1 doesn't have a mode labeled A/C, the modes (S)CMV+ and PVC+ operate as the same "assist-control" many of us learned in our initial vent education.
** Picture obtained from The Hamilton Medical website **
According to the Hamilton Medical Group, referencing the picture above, CMV modes have two sub-groups: assist-control modes and genuine CMV modes. The "CMV" modes we encounter on the T1 DO allow the patient to trigger a breath, lumping it into the A/C category of the CMV modes umbrella.
Side Track: CMV can stand for Controlled OR Continuous Mandatory Ventilation, with "Controlled" being an old nomenclature. Most CMV modes seen in newer generation vents are referred to as CMV, with the C standing for "continuous," and allow the patient to take, or trigger, a breath between the normal mandatory set respiratory rate. You WILL see both of these CMV terms used interchangeably, unfortunately. I try to avoid this confusion myself by using the term assist-control instead of CMV, and if I am truly referring to the old school barbaric genuine CMV that doesn't allow a patient trigger, I refer to it as genuine CMV, again as seen in the picture above from the Hamilton Medical website.
Continuous Mandator Ventilation (CMV) = Assist-Control; patient can trigger
Controlled Mandatory Ventilation (CMV) = Genuine CMV; patient cannot trigger
It's nomenclature/verbiage at the end of the day. Here is a neat little blurb about the history of CMV from an online textbook.
Back from my side tack... The two primary modes that we have to choose from on the Hamilton T1 are CMV (assist-control) and SIMV, with volume-targeted and pressure breath types available. For the sake of simplicity, referencing the question that led to the creation of this blog, we will avoid overcomplicating differentiating the modes by using traditional volume modes to describe genuine CMV, assist-control, and SIMV modes. Later, getting into the pressure and volume-targeted domain.
** Remember, this is just a basic intro to these modes. If you're looking for a more in-depth dive into the modes or other components of mechanical ventilation, I suggest enrolling in the FOAMfrat EMS refresher course and taking their very well-put-together 12-hour course on mechanical ventilation. **
Controlled Mandatory Ventilation (Genuine CMV)
CMV stands for Controlled or Continuous Mandatory Ventilation, as I stated above. In this section, I am referencing "Controlled" Mandatory Ventilation or, as I prefer to call it, genuine CMV. As with most other ventilation modes, in genuine CMV, your patient will have a set tidal volume or pressure control and a set respiratory rate. But, it has no bearing on breath attempts that the patient may make — In essence, the ventilator WILL NOT respond to patient-initiated breaths.
So, if we were to set a patient up in a genuine CMV mode with a volume breath type with numbers such as a tidal volume of 500 and a respiratory rate of 12, the patient would receive 12 breaths in one minute at 500 mL. The ventilator gives no 💩's whether you are attempting to take your own breaths or not — it will deliver 500 mLs every 5 seconds.
(60 seconds in a minute divided by 12 breaths per minute equals 5 — one mandatory breath every 5 seconds. The respiratory cycle is 5 seconds)
*** Continuous Mandatory Ventilation (in a volume breath type) delivering the preset tidal volume of our input 500 mL at a rate of 12; delivering a breath, every 5 seconds ***
This is bad for patients who are not paralyzed and sedated and who want to take a breath independently. If the patient does attempt to take a breath, they'll be taking that breath on their own without any assistance from the ventilator (and in some cases - on some vents - genuine CMV won't allow them to take a breath at all, as if you just clamped the ETT).
I'm unsure of any commonly used vents that are traditionally used in the critical care transport environment that don't let patients take that "in-between" breath, but if you've used these, you know:
CMV modes essentially have two sub-groups, as I stated earlier, according to the Hamilton Medical Group: assist-control modes and genuine CMV modes. The two groups are very similar, except for that patient triggering part. I spoke above about genuine CMV modes not having a patient trigger, right? Well, assist-control is a subgroup of the CMV umbrella where triggering the ventilator to deliver a breath IS POSSIBLE by the patient. But again, NOT POSSIBLE in a genuine CMV mode.
Assist-control is a mode that requires the least amount of effort from the patient. Just like in the genuine CMV mode, if you set the patient up in assist-control using a volume breath type with a respiratory rate of 12 and tidal volume of 500, the patient will receive - at a minimum - 12 breaths in one minute at 500 mL; It will deliver a mandatory breath of 500 mLs tidal volume every 5 seconds. BUUUUUTTT, this ventilator mode DOES give a 💩 whether your patient is attempting to take a breath or not. When the patient triggers the ventilator, and EVERY TIME the patient triggers the ventilator, the vent will deliver a full breath at the tidal volume you input (500 mL in this case). With assist-control, EVERY BREATH (time-triggered mandatory breaths or patient-triggered) is a machine breath delivered at the preset tidal volume you, as the clinician, chose.
** Assist Control mode (in a volume breath type) delivering the preset tidal volume of our input 500 mL at a rate of 12; delivering a breath every 5 seconds **
Synchronized Intermittent Mandatory Ventilation (SIMV):
SIMV stands for Synchronized Intermittent Mandatory Ventilation. Once again, you will choose a respiratory rate and tidal volume (if you are using a volume, not a pressure breath type). So, let's stick with a respiratory rate of 12 and a tidal volume of 500. Again, this mode will ensure that the patient receives a minimum respiratory rate of 12 and a tidal volume of 500 every minute (one mandatory breath every 5 seconds) … sounds familiar, right?!
*** Synchronized Intermittent Mandatory Ventilation mode delivering the preset tidal volume of our input 500 mL at a rate of 12; delivering a breath every 5 seconds ***
Sooo… what is the difference, then? This comes back to the patient-triggered breaths. Once again, this ventilator DOES care whether your patient is attempting to take a breath. BUT, instead of receiving a full preset tidal volume as we would in assist-control, a patient-triggered breath does not result in a machine-delivered mandatory breath. The patient "gets what they can get" when they take their own breath. They receive a tidal volume that is 100% dependent on their own power, which will vary depending on the patient's current respiratory status, respiratory strength, sedation, and pain level. The patient gets whatever volume he or she can pull through the ventilator circuit. This is of worry and could put the patient at risk for increased respiratory muscle fatigue.
This is where pressure support (PS) comes in, and pressure support is another thing that we have to talk about regarding SIMV. Pressure support is a way for us to “augment” a patient’s spontaneous breath to reduce dead space and reduce the patient's workload. This will reduce the risk of increased respiratory muscle fatigue, as mentioned above. Pressure support is just like BiPAP but with an intubated patient. This feature is exclusive to the SIMV mode, is not available in genuine CMV or assist-control modes, AND is only utilized during spontaneous patient-triggered breaths. In my humble opinion, no patient in an SIMV mode should be set up without pressure support. They are taking an extra breath, likely because they want it, and it seems inhumane to me not to give that extra pressure or “umph” of air when they are taking their own breath. I mean, they are breathing through a tube and a long ventilator circuit.
* DO NOT get (PS) confused with pressure control (PC); these are two different things. *
A good analogy I once read regarding pressure support in Eric Bauer's Ventilator Management textbook was, “If your pressure support is inadequate, think of this as you sucking a very thick milkshake out of a straw. As you add pressure support, this thins the milkshake, and the thinning milkshake becomes easier to suck and drink out of the cup.” As you add pressure support, you make that breath easier for them to take with their potentially weak respiratory effort.
You may also be asking yourself what the "synchronized" in Synchronized Intermittent Mandatory Ventilation means. The easiest and simplest way to explain it is that the ventilator is very intelligent and detects if the patient's effort is very close to the next timed or mandatory breath. If we keep the same respiratory rate and tidal volume that we've previously been using, the ventilator will deliver a breath every 5 seconds. In SIMV, if the patient attempts to breathe or triggers the breath at or near the next mandatory breath (nearly 5 seconds from the last breath), the ventilator will sense this and decide to give the preset tidal volume established by the clinician (500 mL's in this case) instead of them pulling in only the amount they can muster. So, a patient-triggered breath, in this instance, will result in a full tidal volume breath instead of the smaller breath dependent on their own power.
^^^ So, I don’t know if ya’ll noticed, but the images I used to convey what the vent in each mode is doing… WAS THE EXACT SAME IMAGE! Only with the mode label changed in each…
That’s because if the patient doesn’t trigger the breath… THERE IS NO DIFFERENCE IN THE MODES… The difference is the patient-triggered breaths! And yeah, yeah, yeah, before any of you vent nerds follow up with, “Well, that’s not the only difference, Jared.” I know… This is just my way of simplifying the terminologies for those out there learning the basics with this exact question.
😳 Hamilton T1 😱
Near the beginning of this blog, I mentioned that the T1 is a completely pressure-driven ventilator that allows you to choose between volume-targeted or pressure breath types and the typical A/C and SIMV modes... There is NO true volume mode, only pressure-targeted or pressure-regulated volume-targeted breath types (or volume-targeted with adaptive pressure control as some will say). That's where the Hamilton T1 complications come in... It's just different... Kinda.
Regarding the Hamilton T1's pressure breath types, they stay the same as with most ventilators that have pressure as an option for breath delivery. Whether they are in an assist-control or SIMV mode, it acts the same as I mentioned above, but instead of the vent delivering an input volume, it delivers an input pressure that you, the clinician, select. The big difference here is that we are now attempting to guarantee a pressure, where volume becomes the variable we must trend (in a volume mode, we trend the pressures -- PIP, Pplat).
Now for the pressure-regulated volume-targeted breath types. Depending on how your Hamilton T1 is set up, these may have different names (lame I know). But, they mean the same thing.
(S)CMV+ = APVcmv
SIMV+ = APVsimv
APV = Adaptive Pressure Ventilation
The "+" signifies the mode is adaptive
If you're not aware, many ventilator names are "proprietary." Ventilator manufacturers are free to name the modes of their ventilators differently, and this can be confusing. But, if you are familiar with PRVC (pressure-regulated volume control), you already have an understanding of how these adaptive Hamilton modes work.
The Hamilton T1 adaptive modes will be set up the same as all modes. You either choose assist-control or SIMV (APVcmv and APVsimv, respectively). Then, you'll still need to identify your patient's IBW, targeted tidal volume, respiratory rate, PEEP, I-time, and FiO2 as you normally would and input them the same way you would with any other ventilator. You will also set a Plimit, which is a maximum pressure that can be applied in these modes, typically 30 cmH2O (Your Plimit will always be 10 cmH2O below your high-pressure alarm, so if you change a high-pressure alarm, you are also changing the Plimit). Then, you let the ventilator perform witchcraft.
The ventilator will typically deliver a mandatory breath at an inspiratory pressure of around 15 cmH20. After that breath is delivered, the ventilator assesses the exhaled tidal volume and compares it to the targeted tidal volume that you input. The ventilator automatically regulates the inspiratory pressure/Ppeak in an attempt to match your input tidal volume at the lowest level possible. In the older software version, prior to the 3.0 updates, the ventilator would only adjust the pressure by 1 - 2 cmH2O with each breath. The SW update 3.0 and newer will make larger adjustments to obtain the input tidal volumes faster.
The ventilator recalculates the minimal inspiratory pressure needed to achieve the target volume as lung characteristics change -- one could say that it... adapts! 😏... The ventilator will continuously reassess the exhaled tidal volumes and adjust inspiratory pressures as needed to maintain your targeted tidal volume, never exceeding your Plimit.
You want pictures? I would too, so here you go:
We're gonna use a healthy adult 5 foot 8 inches (174 cm) male, which would be me.
Mode: (S)CMV+, which is also APVcmv, remember?
Targeted Tidal Volume: 500 mL, for the sake of consistency from the above examples (410 mL is the 6 mL/kg target per IBW)
Respiratory Rate: 12 breaths per minute
FiO2: 0.5 (50%)
^^^ If you look at the portion of the image highlighted in yellow, you'll see a Ppeak (PIP) of 20 cmH2O. That's the initial pressure given. Remember I mentioned that the T1 typically starts with an inspiratory pressure of 15? Well, if you add PEEP (set at 5 cmH2O), you get your Ppeak (PIP), which in this case is 20 cmH20. Unfortunately, the vent doesn't tell you what inspiratory pressure it's currently using, but if you subtract your PEEP from your Ppeak (Ppeak - PEEP), you get a glimpse of the pressure it's using. Highlighted in green, you'll see my input targeted tidal volume. Highlighted in red, you'll see the exhaled tidal volume of 762 mL. This is too high. Fortunately, this very smart adaptive mode will recognize this and make changes.
^^^ The vent made adjustments, reducing the pressure delivered to decrease the exhaled tidal volume to match your targeted tidal volume. But it's still not quite where we'd like to be. Again, this very smart adaptive mode will recognize this and make changes.
^^^ The vent again made some adjustments, reducing the pressure delivered to decrease the exhaled tidal volume to match your targeted tidal volume. But this time, the T1 overshot (or undershot 😏) your targeted tidal volume, now less than the 500 mL you input. While I would be very happy with this exhaled tidal volume, this very smart adaptive mode isn't done.
^^^ Well, would you just look at that! The T1 knew that it had just provided an exhaled tidal volume that was just a tad too low and "adapted" by increasing its pressure. We now have an exhaled tidal volume that is damn close to the volume we input and would like the vent to "target."
Oof, that was a long one, but I hope that it helped and clarified a few things regarding ventilator terminology and some of the differences with the Hamilton T1. And please don't eat me alive for using the 500-12 vent settings 😬😬😬 (believe me, it hurt a little, but it's also a very good set of example numbers for teaching).
Here is an image received from UpToDate.com that some may find useful:
💥 BOOM 💥
Killin' It Since 1989
Jared Patterson, CCP-C, One Rad Medic
If I can suggest some inexpensive and great reads regarding mechanical ventilation, look into purchasing these textbooks:
Check Out The Rapid Sequence Interrogation (RSI) Podcast:
Don't need em! They're all in here 🧠
Bauer, E., n.d. Ventilator management. 2nd ed. FlightBridgeED, LLC.
Owens, W., 2018. The ventilator book. 2nd ed. Columbia, SC: First Draught Press.
Brown, C., Sakles, J. and Mick, N., 2018. The Walls Manual of Emergency Airway Management. 5th ed. Wolters Kluwer.
Cairo, J., 2016. Pilbeam's Mechanical Ventilation. 6th ed. Elsevier.
College.hamilton-medical.com. 2021. Ventilation Modes ABC. [online] Available at: <https://college.hamilton-medical.com/auth/RepositoryEntry/57966604/CourseNode/99319570301624> [Accessed 4 March 2021].
Hyzy, MD, R. and Jia, MD, S., 2021. Modes of Mechanical Ventilation. [online] Uptodate.com. Available at: <https://www.uptodate.com/contents/modes-of-mechanical-ventilation?search=Continuous%20Mandatory%20Ventilation&source=search_result&selectedTitle=8~150&usage_type=default&display_rank=8#H4> [Accessed 4 March 2021].
Swearingen, C., 2016. Vent Hero. 1st ed.