What are the initial settings for assist-control (AC) volume control ventilation?

In the previous article, we reviewed the five initial parameters that you need to consider when placing a patient on a ventilator in the assist-control (AC) volume control mode. Now, let’s see what the initial settings (or values) are for each one.

First up is the tidal volume (V_T). Setting the right tidal volume is dependent on a few factors including gender, height, and ideal body weight. That said, we really only need to know the first two—gender and height—because with those two values we can calculate the patient’s ideal predicted body weight.

It’s important to note that the ideal body weight is often different than the patient’s actual body weight. But we use the calculated ideal body weight because the lungs grow in proportion to the patient’s height and not necessarily in proportion to the patient’s weight. And what we’re trying to do is estimate the size of the patient’s lungs.

So, how much tidal volume should we initially set? Some textbooks suggest an initial setting of 6–8 mL of tidal volume for every kilogram of the patient’s calculated ideal body weight, while others suggest 5–7 mL / kg. In reality, both are acceptable initial ranges, and the main point is not necessarily to land on a specific number, but rather to choose a setting that is initially safe and then make modifications.

In the past, the initial range for tidal volume was actually nearly double of what is commonly accepted today. In fact, 10–15 mL / kg was an acceptable initial setting. However, because an initial larger tidal volume to an injured lung may prove to be dangerous, opting for the lower tidal volume range is the current accepted practice

So, next up is the respiratory rate (RR). Normally, the respiratory rate is set with a tidal volume in mind. And that’s because, as we’ve already discussed, both the respiratory rate and the tidal volume are responsible for the patient’s minute ventilation. And, setting an initial respiratory rate between 10 and 20 breaths / min is acceptable as long as the tidal volume and respiratory rate are both within an acceptable range, and meet the patient’s minute ventilation needs.

But how do you know if you’re meeting the patient’s needs? Well, that’s a great question and we’ll answer it in another article. For now, let’s remember that the initial respiratory rate can be set between 10 and 20 breaths / min.

Next up is the positive end-expiratory pressure, or PEEP. Depending on the type of lung injury, the amount of PEEP can vary significantly and may need frequent readjusting depending on the progress of the lung injury.

However, the initial acceptable setting is 0–5 cmH₂O.

Next is the fraction of inspired oxygen, and its setting also depends on the type of lung injury. The acceptable practice is to begin on 100% and then titrate as soon as possible, since high levels of oxygen for an extended amount of time may exacerbate the lung injury.

Now, of course, you should be asking, how do I know when it’s possible to titrate the F_IO₂? Another great question which we’ll cover in another article. For now, just remember that an initial F_IO₂ of 100% is generally accepted.

Last is the flow. To prevent the patient from becoming anxious we must set an appropriate flow rate to get the tidal volume to the patient in a reasonable amount of time. An initial flow rate of 40–60 L / min is generally accepted, with many clinicians choosing to begin at a flow rate of 60 L / min.

So now here’s an example of what all of this actually looks like on a patient.

Suppose I have a male patient whose calculated ideal body weight is 65 kg. Let’s choose some initial settings that fall within our acceptable ranges.

First up, the tidal volume. In this case, I’ll choose the 6 mL / kg option as my initial setting. This calculates to an initial tidal volume of 390 mL. Because I have some room to move up, and because we all like nice round numbers, I’ll actually select the tidal volume of 400 mL as my initial setting.

Next up is the respiratory rate, and together with my tidal volume, the minute ventilation will be set. So here I’ll set a respiratory rate of 15 breaths / min, which gives me an initial minute ventilation of 6 L / min (because 15 breaths times 400 mL comes equals 6 L / min). But will this be enough? We don’t know yet. This is something we'll check after the initial set-up is complete.

Then we’ll set the PEEP. And because I do not want the lung to deflate to a pressure of zero on exhalation, I will choose an initial PEEP of 5 cmH₂O.

And the F_IO₂? Because I have no history on my patient just yet, I’ll set an initial F_IO₂ of 100%.

And finally, the flow, which I’ll initially set to 60 L / min to deliver the tidal volume at an acceptable speed that meets the demand of most adult patients.

Now, how will we know if these are the right settings for this patient? We'll need to assess if the patient's needs are being met with an arterial blood gas analysis (BGA) and modify these initial settings based on that.