pressure control volume guarantee

[narcusprince]

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A bit confused as to the naming of this ventilatory mode. In my mind its a volume controlled ventilation with decellerating pressure to keep it even over the inspiratory phase of ventilation with the computer calculating how much pressure over what time would generate the tidal volume. Whats your take?

 

[Planktonmd]

A bit confused as to the naming of this ventilatory mode. In my mind its a volume controlled ventilation with decellerating pressure to keep it even over the inspiratory phase of ventilation with the computer calculating how much pressure over what time would generate the tidal volume. Whats your take?

In this mode the tidal volume and rate are predetermined and the airway pressure is predetermined too the only variable is the I/E ratio.
Basically the length of inspiration is increased to deliver the required volume while maintaining the desired pressure.
It is a good alternative to pure pressure control where the tidal volume will decrease when the the pulmonary compliance decrease.
It still can contribute to atelectasis if the pressure is set too low.

 

[narcusprince]

In this mode the tidal volume and rate are predetermined and the airway pressure is predetermined too the only variable is the I/E ratio.
Basically the length of inspiration is increased to deliver the required volume while maintaining the desired pressure.
It is a good alternative to pure pressure control where the tidal volume will decrease when the the pulmonary compliance decrease.
It still can contribute to atelectasis if the pressure is set too low.

Plank
You cannot set the pressure in PCV-VG you can only set the tidal volume and the rate. The machine sets the pressure based on the desired tidal volume. I really like this mode of ventilation it lowers the peak pressures you have to generate in straight volume controlled mode. I believe you can alter the I/E ratio. Really I think it should be renamed volumed controlled minimal pressure ventilation.

 

[ssmallz]

IMO it uses a computer to calculate the inspiratory pressure needed to deliver a given tidal volume and delivers that tidal volume via pressure control ventilation at a constant pressure with the parameters of TV, RR, I:E, and PEEP set. This is unlike VCV where the pressure gradually increases untill TV is achieved

 

[Planktonmd]

Plank
You cannot set the pressure in PCV-VG you can only set the tidal volume and the rate. The machine sets the pressure based on the desired tidal volume. I really like this mode of ventilation it lowers the peak pressures you have to generate in straight volume controlled mode. I believe you can alter the I/E ratio. Really I think it should be renamed volumed controlled minimal pressure ventilation.

You are probably right about the pressure being set by the machine.
But my question here is it really beneficial to keep the PIP at the lowest number possible for a certain volume?
Why are people so afraid of increased PIP?
Can it be that some patients with decreased compliance and increased airway resistance might actually need a higher positive pressure to achieve adequate alveolar recruitment?

 

[proman]

I think it's very similar to PRVC with each preceeding breath determining the pressure used for the next one (PRVC uses an average of last 3 breaths). You control:
TV, RR, I:E, PEEP, Pmax, rise rate. By setting a Pmax, you indirectly control the inspired pressure used (it'll always be at least 5 less than Pmax).

So in PCVG, the patient gets a PC breath with a targeted tidal volume. The PI changes based on the compliance determined on the previous breath.

 

[proman]

You are probably right about the pressure being set by the machine.
But my question here is it really beneficial to keep the PIP at the lowest number possible for a certain volume?
Why are people so afraid of increased PIP?
Can it be that some patients with decreased compliance and increased airway resistance might actually need a higher positive pressure to achieve adequate alveolar recruitment?

I don't think so. Volutrauma is the real damaging factor. There's some thought that atelactasis can also produce lung injury. Pressure probably doesn't directly impact lung health as much as we think it does (but does increase risk of ventilator associated injury like pneumothorax). The thing I like about PCVG mode is that it limits the potential volume that PCV can expose the patient to.

 

[Planktonmd]

I don't think so. Volutrauma is the real damaging factor. There's some thought that atelactasis can also produce lung injury. Pressure probably doesn't directly impact lung health as much as we think it does (but does increase risk of ventilator associated injury like pneumothorax). The thing I like about PCVG mode is that it limits the potential volume that PCV can expose the patient to.

The problem is we don't really know if it's better or worse to lower the PIP in a patient with pathologically increased airway resistance.
I know that the current approach is to use smaller tidal volumes but does that apply to all patients?
Even those with obstructive disease and decreased compliance?

 

[proman]

The problem is we don't really know if it's better or worse to lower the PIP in a patient with pathologically increased airway resistance.
I know that the current approach is to use smaller tidal volumes but does that apply to all patients?
Even those with obstructive disease and decreased compliance?

Low tidal volumes aka "protective" ventilation strategy is clearly better in injured lungs. Everything that I've seen in non-injured lungs shows improvement in inflammatory markers like IL-6 but hasn't been shown to improve morbidity or mortality. It might make a difference in patients who have suffered one "hit" like radiation to the chest for esophageal/stomach cancers. Low FiO2 is also another component. It reduces free radical formation but more importantly reduces atelactasis during positive pressure ventilation. I suspect that atelactasis, free radicals etc may represent that second "hit" that leads to lung injury.

Low pressures are harmful if they produce large tidal volumes (likely from the volume). Based on what I've read, I'd say that delivered TV is more important than keeping the pressure low. Speaking of lungs in particular, peak pressures don't mean much to me. I think mean airway pressure is more important because that's what the alveoli are seeing. I think you do need to provide enough TV and PEEP to keep the lungs from getting atelactatic, even if that means tolerating an increased pressure.

 

[RT2MD]

I think it's very similar to PRVC with each preceeding breath determining the pressure used for the next one (PRVC uses an average of last 3 breaths). You control:
TV, RR, I:E, PEEP, Pmax, rise rate. By setting a Pmax, you indirectly control the inspired pressure used (it'll always be at least 5 less than Pmax).

So in PCVG, the patient gets a PC breath with a targeted tidal volume. The PI changes based on the compliance determined on the previous breath.

👍👍👍

 

[amyl]

hey np (whoops almost typed your real name n outted, ya 🙂
ask the engineers but i think the pressure delivered in a breath is determined by the pressure needed in the last few breaths (i think its three but i don't want to say for sure, ask them). but the breaths are time cycled like PCV with a constant pressure delivered over the course of the breath like PCV. the end of the breath is not triggered by acheivement of the tidal volume (like VCV), it is triggered by time (like PCV). thats why the machine alarms "tidal volume not given" -- because for the pressure its calculated based on previous breaths this breath did not deliver the whole TV. xo, a

 

[Narcotized]

volume controlled ventilation with decellerating pressure to keep it even over the inspiratory phase of ventilation

volume controlled ventilation with decellerating flow to keep pressure even over the inspiratory phase of ventilation

It sounds like you know what it is, but just adding the typo correction for those that don't.

 

[epidural man]

A bit confused as to the naming of this ventilatory mode. In my mind its a volume controlled ventilation with decellerating pressure to keep it even over the inspiratory phase of ventilation with the computer calculating how much pressure over what time would generate the tidal volume. Whats your take?

I haven't seen this.

What machine/vent has this mode?

 

[jetproppilot]

A bit confused as to the naming of this ventilatory mode. In my mind its a volume controlled ventilation with decellerating pressure to keep it even over the inspiratory phase of ventilation with the computer calculating how much pressure over what time would generate the tidal volume. Whats your take?

Fabulous ventilatory mode. Impressive to watch in a deep Trendelenberg laparoscopic case.

PCV-VG was a must for me when I was ordering anesthesia machines for our new place.

 

[Pietrantonio]

volume controlled ventilation with decellerating flow to keep pressure even over the inspiratory phase of ventilation

It sounds like you know what it is, but just adding the typo correction for those that don't.

VC has a constant "square" flow waveform. What you described is PC. PC is characterized by a decelerating flow waveform with a constant "square" pressure waveform. Same goes for PRVC.

My experience comes from working the Servo i ventilator.

 

[Pietrantonio]

Fabulous ventilatory mode. Impressive to watch in a deep Trendelenberg laparoscopic case.

PCV-VG was a must for me when I was ordering anesthesia machines for our new place.

Pressure Control Ventilation - VG? What does the VG stand for?

 

[proman]

VG=Volume Guarentee. PCVG uses the previous breath, PRVC uses an average of 3 to calculate inspired pressure.

 

[Pietrantonio]

VG=Volume Guarentee. PCVG uses the previous breath, PRVC uses an average of 3 to calculate inspired pressure.

I have no experience with anesthesia machines, but that sounds awesome. I'll have to read up on it and learn more about it.

Thank you for the clarification!!

 

[Narcotized]

VC has a constant "square" flow waveform. What you described is PC.

That's what this is, pressure control, but with a volume guarantee.

 

[Pietrantonio]

You are probably right about the pressure being set by the machine.
But my question here is it really beneficial to keep the PIP at the lowest number possible for a certain volume?
Why are people so afraid of increased PIP?
Can it be that some patients with decreased compliance and increased airway resistance might actually need a higher positive pressure to achieve adequate alveolar recruitment?

In reference to keeping PIP's low with a certain volume is dependent on the patients lungs. A constant Vt will yield variable pressures to maintain that set Vt. So, over time, if you have low PIP's and Pplat's with your set Vt, then the patient has good lung compliance and vice versa.

High PIP's are something to take into consideration and can tell you what degree of airway resistence and dynamic compliance is present. There are other options to consider such as suctioning the patient, giving a bronchodilator treatment in-line, increasing the flow, et al.

Remember that your PEEP setting is what will keep the alveoli recruited because you're keeping a certain amount of positive end expiratory pressure in the lungs. Patients with decreased Cst (ALI, ARDS, etc etc) will most likely need higher PEEP settings with lower Vt's in the 4-6 cc/kg IBW range. As it was stated in a later post, it's all lung protective strategies. You can also check out ARDSnet for some good information.

However, more importantly is keeping an eye on Pplat's. Our standards (as RRT's) is to keep our Pplat < 30 cmH2O. Pplat's are more important than PIP's, but that's not to say they are not important either.

Hopefully this helps (from an RRT's standpoint). I admit I have no experience working with anesthesia ventilators, but I would imagine that the same principles of mechanical ventilation apply.

 

[Pietrantonio]

That's what this is, pressure control, but with a volume guarantee.

That's awesome. Any literature on this mode of ventilation. You have my interest....🙂

 

[RT2MD]

Yes, as most people have said - it's a pressure control mode, but with a target volume. Most times the vent gives 3 test breaths to determine lung compliance, then attempts to deliver the target volume... I know that draeger uses an average of 3 breaths to determine pressure needed for the target volume. It will then adjust the pressure in increments of 3cmH2O up to 5 of the Pmax. If it cannot deliver the target volume at the Pmax, it will give you a Volume Not Constant alarm. FYI: on the drager XL icu vent, if auto flow is on, you are in a volume targeted pressure control.