Assist-control (AC) and synchronized intermittent mandatory ventilation (SIMV) modes

In this video, you will learn about the basis of selecting the AC volume control mode as the initial mode of ventilation.

26th Jan 2017 • 4m read

In this video from our Mechanical Ventilation Essentials course, you will learn about the basis of selecting the AC volume control mode as the initial mode of ventilation.

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Video Transcript

[00:00:00] Now that we have established the need of a mechanical ventilator, let's discuss which initial mode should be selected. Although there are numerous options for selecting the initial mode, according to Esteban, Ferguson, and Meade, assist-control volume ventilation is the most common ventilator mode used throughout the world as the primary initial mode of ventilatory support. Now, the question is why would the AC mode be preferable over other modes like,

[00:00:30] for example, the SIMV (synchronized intermittent mandatory ventilation mode). Don't both of these modes offer good ventilatory support? Well, to answer this question, let's see how the two modes work and then focus in on why the AC mode is preferable over the SIMV mode. Suppose we set our patient on the AC mode and we set the respiratory rate to a rate of 15 breaths per minute. That means that if you do 60 seconds divided by

[00:01:00] 15 breaths in a minute, the patient will receive a breath every four seconds. In this example, the patient is not attempting a breath but every four seconds, a fixed volume will be delivered regardless. Here, we've set a mandatory volume of 500 mL, so every timed breath there will be a volume of 500 mL being delivered. But let's say at the

[00:01:30] two-second mark, the patient attempts to breathe. What happens? Well, in the assist control mode, the patient merely attempts to trigger a breath and the ventilator will take over and deliver the full set tidal volume. Notice how the delivered volume is the same as the other breaths, where there was no patient trigger. This is what makes the AC mode preferable for the initial mode. Remember,

[00:02:00] the goal of initiating mechanical ventilation is to alleviate the work of breathing of the patient and to allow the ventilatory muscles to recover. This mode is ideal for muscle rest and recovery because the patient does not have to work so hard to get a full volume breath. They just merely have to initiate or trigger a breath and the ventilator does the rest. So now let's compare this to the SIMV

[00:02:30] mode. In the SIMV mode, we can copy the same settings with a respiratory rate of 15 and a tidal volume of 500. And if you look closely, you'll see that a breath will be delivered every 4 seconds, just like in the AC mode. So, you should be asking yourself this question, then what's the difference? Well, let's imagine what a patient trigger breath would look like at the two-second mark in both of these modes and remember, initially,

[00:03:00] this patient most likely will not have adequate muscle strength, since they probably have an acute ventilatory muscle dysfunction and fatigue. Notice how in the SIMV mode, the full 500 mL isn't reached. In the SIMV mode, the patient is responsible for the volume received during a spontaneous or a triggered breath. And initially, when the patient is weak and the lungs are impaired, the patient

[00:03:30] may not be strong enough to draw a full breath. So, this type of shallow breathing may actually contribute to hypoventilation and continued respiratory muscle dysfunction. On the flip side, as we've already seen with the AC mode, the patient in the AC mode merely attempts and the ventilator does the rest. So yes, the ventilator will allow full spontaneous breathing during the SIMV mode,

[00:04:00] which in theory seems like a good choice because it gives the patient freedom to determine their own volume. But this may not be such a good idea when initially placing a patient with respiratory impairment on a mechanical ventilator. In other words, even though they're free to take larger or smaller breaths, their respiratory muscles may be so weak, initially, that all they can do is to take smaller breaths, which will not be beneficial for our patients.

[00:04:30] So, when you think about the initial goal of mechanical ventilation, which is to reduce the work of breathing and to allow for ventilatory muscle rest and recovery, then it should come as no surprise as to why the assist-control volume ventilation mode is the most common ventilator mode, as the primary initial mode used throughout the world. Are there inherent challenges with the AC mode? Absolutely, and we'll discuss those later on as we progress.

[00:05:00] But now let's move on to another question that we should be asking. In this lesson, we used a controlled tidal volume as the means to inflate the lungs, from breath to breath to breath, so here's the question. Why did we select volume ventilation? In this case, the 500 mL of volume, instead of filling up the lung, to a set controlled pressure. In other words, why did we use volume ventilation instead of pressure ventilation?

[00:05:30] And this is a great question and I'm really glad you asked it and we'll discuss and answer this very important question in the next video.