Mechanical ventilation V: Hybrid modes

Here we discuss “hybrid” modes of ventilation.

Read the other entries from this series here: (1), (2), (3), (4), (6)

We’ve seen how both volume- and pressure-type ventilation can have advantages. Many providers, therefore, end up with this line of reasoning:

“I like the simplicity of volume control,” they think. “But I like how pressure control breaths are shaped. I like their high initial flow that matches patient demand, and the longer tail to optimize recruitment. I like knowing that the peak airway pressures will be limited to whatever I set.”

They sigh, gazing out the ICU window. “The trouble is that when I use PCV, I have to basically stand here and fiddle with the vent for ages—I set an inspiratory pressure, and give a few breaths, and look at the tidal volumes I’m getting. Then if they’re too high—either for the patient’s metabolic demand or for lung protection—I have to dial down the pressure and repeat the process. If they’re too low, I have to dial it up and check that too. Whenever something changes with the patient, I have to start over. I can just have a med student stand in the room playing with the pressure all day, but they complain about bathroom breaks and work hours. What’s the answer?”

Woe betide us all. The answer, according to clever vent manufacturers, was a “hybrid” or dual-control method of ventilation, which led us to…

Pressure Regulated Volume Control (PRVC)

(Being a fairly recent development, the name of this mode tends to vary by manufacturer)

PRVC is essentially pressure control—with a twist.

The breaths are shaped like PCV breaths. However, rather than setting the inspiratory pressure, we instead set a goal or target tidal volume.

So we set, for instance, a goal volume of 500ml. This is not the tidal volume delivered, because this is not a volume control mode; it is still pressure control. So for the first breath or several breaths, the vent starts with an arbitrary pressure, such as 20 cm H2O. It then monitors how much volume was delivered under those settings.

If the vent delivers a breath at 20 cm H2O and discovers that it has delivered 400 ml, then we clearly haven’t reached our volume goal. So for the next breath, the vent will increase the pressure, say to 22 cm H2O. It then gives another test breath or sequence of test breaths.

Maybe now we’re getting volumes of 450 ml. So the vent increases the pressure more. The vent keeps adjusting the pressure from breath to breath until we hit our goal tidal volume, or get very close. Then it keeps it there.

If anything changes, such as lung compliance or patient effort, the pressure will be adjusted to continue to maintain the goal volume.

In principle, therefore, we have the best of both worlds: all the benefits of pressure control, with the consistent tidal volumes of volume control—without the labor of frequent, manual adjustments to the inspiratory pressure. It’s like a tiny robot that stands in the room, titrating the pressure so you don’t have to.

Theoretically, this makes it essentially self-weaning, since as the patient’s lungs improve and we wean their sedation, they will do more work of breathing on their own, and the vent will need to give less and less support to reach the set volume. Unfortunately, many vents will not actually show you the inspiratory pressure used for each breath, but it is easy to determine by looking at the peak pressure: if the peak is 20 and the PEEP is 5, you know the last breath used 15 cm H2O of support to hit your volume (20 minus 5). If you come back tomorrow and see the peak is 8 with a PEEP of 5, you know that now, only 3 cm H2O of support is needed, and the patient is doing most of their own work of breathing.

PRVC, like PCV or VCV, is not a mode of triggering but a breath type. It can therefore be combined with, say, SIMV (PRVC-SIMV), or I suppose with CMV, but the standard combination is with AC (PRVC-AC).

What’s that? You say you want this same functionality, but without a set rate? That would be…

Volume Support Ventilation (VSV)

Volume support uses the same dual-control functionality as PRVC, but without a set rate. It is therefore to pressure support as PRVC is to pressure control.

In this mode, all breaths are patient triggered. However, once triggered, they receive a level of support that is determined by the vent, not by us. We set a goal tidal volume, and the vent titrates support to reach it.

Just like PRVC, this makes it rather low-maintenance. Rather than saying, “Let’s try him on PSV,” and then standing in the room adjusting the amount of support until the patient’s tidal volumes look about right, we just set the tidal volume we want, and the vent does the adjustment. If it takes 20 of support to hit our volumes, the patient’s not helping out very much; if it only takes 5, it’s exactly like a minimal pressure support of 5.

This is rather useful for patients who aren’t yet moving quite enough air, but who you expect to improve rapidly—prototypically the post-op patient still chewing through their sedation. It may take them several hours to be ready to extubate, and nobody has time to keep coming back to down-titrate their pressure support. So slap them on VSV, and they’ll have a guaranteed minute ventilation, but will still wean off their support as they grow stronger.

Tune in next time when we talk about some more… unusual modes of ventilation.

Read Part VI here

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