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

4th Jan 2021

Now that we have established the need for 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 (AC) volume ventilation is the most common ventilator mode used throughout the world as the primary initial mode of ventilatory support1.

Now the question is, why would the AC mode be preferable over others such as the synchronized intermittent mandatory ventilation (SIMV) 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 on why the AC mode is preferable over the SIMV mode.


Why is AC mode the first choice?

Suppose we set our patient on the AC mode, and we set the respiratory rate (RR) to a rate of 15 breaths / minute. That means that if you calculate 60 seconds divided by 15 breaths in a minute, the patient will receive 1 breath every 4 seconds. In this example, the patient is not attempting a breath, but regardless, every 4 seconds a fixed volume will be delivered. Here we set a mandatory tidal volume (VT) of 500 mL, which means for every timed breath there will be a volume of 500 mL delivered. 

Happy patient and graph of tidal volume changes over time in assist-control mode mechanical ventilation. Illustration.

Figure 1. Assist-control (AC) mode of mechanical ventilation with a respiratory rate (RR) of 15 breaths / minute and a fixed tidal volume (VT) of 500 mL. This mode is ideal for the initial setting because the patient merely triggers a breath and the ventilator does the work, which allows for respiratory muscle recovery.

But let’s say at the 2-second mark, the patient attempts a breath. What happens? Well, in the assist-control mode, 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? It’s this characteristic that makes the AC mode preferable initially. Remember, the goal of initiating mechanical ventilation is to alleviate the work of breathing to allow the patient’s ventilatory muscles to recover. So, because in AC mode the patient merely has to initiate or trigger a breath, and the ventilator does the rest, this mode is ideal for muscle rest and recovery.

So now, let's compare this to the synchronized intermittent mandatory ventilation, or SIMV, mode.

In the SIMV mode, we can copy the same settings with a respiratory rate (RR) of 15 breaths / minute and a tidal volume (VT) of 500 mL.

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Unhappy patient and graph of tidal volume changes over time in synchronized intermittent mandatory mode mechanical ventilation. Illustration.

Figure 2. Synchronized intermittent mandatory ventilation (SIMV) mode of mechanical ventilation with a respiratory rate (RR) of 15 breaths / minute and a fixed tidal volume (VT) of 500 mL. This mode is not ideal for the initial setting because the patient may not be strong enough to do the work, which could result in hypoventilation.

And if you look closely, you'll see that 1 breath will be delivered every 4 seconds, just like in the AC mode. So then you may be asking, what’s the difference?

Well, let’s imagine what a patient-triggered breath would look like at the 2-second mark in the SIMV mode. And remember, initially, this patient will most likely not have adequate muscle strength since they probably have acute ventilatory muscle dysfunction and fatigue.

Notice how in the SIMV mode, the full 500 mL isn’t reached? That’s because in this mode the patient is responsible for the volume received during either a spontaneous or triggered breath. And, while giving the patient the freedom to determine the volume may seem like a good choice, the patient’s lungs may not initially be strong enough to draw a full breath.  So, all they can do is take smaller breaths, and it’s this type of shallow breathing that may contribute to hypoventilation and continued respiratory muscle dysfunction.

So when you think about the initial goal of mechanical ventilation—to reduce the work of breathing and to allow for respiratory muscle rest and recovery while still delivering an optimal volume breath—then it should come as no surprise that AC volume ventilation mode is the world-wide choice for the initial ventilator setting. 

Are there inherent challenges with the AC mode? Absolutely, and we’ll review those later. 

But now, let’s move on to another question that we should be asking. Why did we select volume ventilation, in this case, the controlled 500 milliliters of volume instead of filling up the lung to a pre-set controlled pressure? In other words, why did we use volume ventilation instead of pressure ventilation? This is a great question, and I’m thrilled you asked! We'll discuss and answer this later.


But, that’s it for now. If you want to improve your understanding of key concepts in medicine and improve your clinical skills, make sure to register for a free trial account, which will give you access to free videos and downloads. We’ll help you make the right decisions for yourself and your patients.

Recommended reading

  • Esteban, A, Ferguson, ND, Meade, MO, et al. 2008. Evolution of mechanical ventilation in response to clinical research. Am J Respir Crit Care Med. 177: 170–177. PMID: 17962636
  • Hess, D. 2001. Ventilator modes used in weaning. Chest. 120: 474S-476S. PMID: 11742968
  • Tobin, MJ, and Lodato, RF. 1989. PEEP, auto-PEEP, and waterfalls. Chest. 96: 449–451. PMID: 2670461
  • Mechanical ventilation protocol summary. NIH-NHLBI ARDS Clinical Network.