Difference Between CMV and SIMV..
This is a question I get ALL OF THE TIME, or at least something similar to it. “Jared, what is SIMV? What is the difference between assist control and SIMV?” There have been many different variations of it over the years. A colleague recently asked me this question to answer on the podcast. But since The RSI Podcast’s purpose is short and to the point answers, I didn’t think I could answer the question in a short time frame, adequately… and 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. The Hamilton T1 is a very specific and unique ventilator with its own challenges when attempting to bridge into learning to use a mechanical ventilator.
The two primary modes that we have to choose from on the Hamilton T1 are (S)CMV+ and SIMV+, and you can choose between the volume or pressure breath types with each, with the pressure breath types names of PCV+ and PSIMV+, respectively. Even though we have two breath types to choose from, we will be referencing volume breath types only, for the sake of simplicity, for the remainder of this post.
This mode’s title is a tad misleading because the (S)CMV+ mode works more like an assist control than a CMV. So before I dive into delineating between (S)CMV+ and SIMV+, I would like first to discuss each ventilator mode's basic definitions as they are commonly seen in the transport environment.
** Remember, this is just a basic intro into 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/Continuous Mandatory Ventilation (CMV)
CMV stands for Controlled, or Continuous Mandatory Ventilation (Controlled being an old nomenclature). As with most other ventilation modes, in 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 CMV with a volume breath type with numbers such as a tidal volume of 500 and a respiratory rate of 12, the patient will receive 12 breaths in one minute at 500 mL (CC's for those of you in the back). The ventilator gives no shits whether you are attempting to take your own breaths or not — it will deliver 500 mL's 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 ***
*** An example of a breath stacking in a CMV mode. The patient is struggling to pull in a tidal volume, and before he/she can finish exhaling, the next timed mandatory breath is delivered. You can see where this may lead to patient discomfort or even excessively large tidal volumes. ***
This is bad for patients who are not paralyzed and sedated 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 by the ventilator (and in some cases - on some vents - CMV won't allow them to take a breath at all). Think of the patient attempting to take a breath in this mode the same way you would imagine someone taking a breath through a pinched straw. So what happens if the patient tries to take a breath and hasn't completely exhaled just before the ventilator wants to give the next breath? The patient will begin to breath stack… Not good!
Assist Control (A/C):
Why am I bringing the ventilator mode Assist Control into a question regarding the difference between CMV and SIMV? And that's because the (S)CMV+ mode on the Hamilton T1 doesn't act as a traditional or genuine CMV mode even though the letters C-M-V are in the name; it works more like Assist Control. A basic understanding of Assist Control will help you further understand generalized mechanical ventilation and how it works on the Hamilton T1 ventilator.
CMV modes essentially have two sub-groups, 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 CMV modes not having a patient trigger, right? Well, Assist Control is a subgroup of CMV where triggering the ventilator to deliver a breath is POSSIBLE by the patient. But again, NOT POSSIBLE in genuine CMV modes.
We'll talk more about the "(S)" and the "+" in the (S)CMV+ later, as they are specific to the Hamilton T1 ventilator. Let’s stick to the basics for now.
Assist Control is a mode that requires the least amount of effort from the patient. Just like in CMV mode, if you set the patient up in an Assist Control in 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 mL's tidal volume every 5 seconds. BUUUUUTTT, this ventilator mode DOES give a shit 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 **
** Assist Control delivering a patient-triggered breath between the time-cycled mandatory breaths **
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." 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 could put the patient at risk for increased respiratory muscle fatigue.
Pressure support (PS) 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. Pressure support is just like BiPAP, but with an intubated patient. This feature is exclusive to the SIMV mode and is not offered in a traditional CMV or Assist Control mode, 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.
* 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 Bauers 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 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 with this exact question.
😳 Hamilton T1 😱
Remember at the beginning of this post, and I mentioned the Hamilton T1 having two primary volume modes? They consist of (S)CMV+ and SIMV+. Both of these “volume” modes are volume-targeted, pressure-regulated. I put the word “volume” in quotations because it is not a true volume mode. Yes, it delivers a volume, but it delivers that targeted volume using pressure; hence why I said volume-targeted, pressure-regulated (very similar to PRVC). So, (S)CMV+ and SIMV+ are volume-targeted, pressure-regulated, meaning that the ventilator automatically regulates the inspiratory pressure (pressure used to give the breath) and flow to maintain a targeted tidal volume.
On the Hamilton Ventilator, the “+” in (S)CMV+ and SIMV+ means that the mode is adaptive. So what the hell does adaptive mean?! Well, it means exactly what I put in the last paragraph: “The ventilator automatically regulates the inspiratory pressure and flow to maintain a targeted tidal volume.” You, as the operator, set the targeted tidal volume, respiratory rate, PEEP, and the high-pressure alarm limit. When the ventilator gives a breath, it then compares the exhaled tidal volume to your input targeted tidal volume and adjusts the inspiratory pressure that is applied to the lowest possible level to achieve the targeted tidal volume (volume you input). The ventilator at no time will adjust the inspiratory pressure more than 2 cmH2O.
The ventilator recalculates the minimal inspiratory pressure needed to achieve the target volume as lung characteristics change -- one could say that it... adapts! 😏... So the ventilator will continuously reassess the exhaled tidal volumes and adjust inspiratory pressures as needed to maintain your targeted tidal volume.
What about that damn “(S)” in the mode (S)CMV+. The (S) stands for synchronized. And in a similar fashion to the “synchronized” in SIMV, this mode will synchronize the patient’s breath if the patient-triggered breath is at or near the patient’s next timed mandatory breath.
Why do I bring up the adaptive modes into this? That's because although they are their own volume targeting beasts, they still work in the same fashion as the traditional modes I explained above. They will still give a set respiratory rate and tidal volume, based on the parameters you set.
SIMV+ = Adaptive Synchronized Intermittent Mandatory Ventilation
Oof, that was a long one, but I hope that it helped clarify a few things regarding ventilator terminology and some of the differences with the Hamilton T1.
Here is an image received from UpToDate.com that some may find useful:
Furthermore, many of the ventilation mode names are "proprietary." Ventilator manufacturers are free to name the modes in their ventilators differently. This can become confusing to providers. This is where your inner nerd should come out and you read the darn manual for the ventilator your service utilizes.