Vent Modes

[Carlos Danger]

As far as I know, no ventilation specific ventilation strategy is superior to any other in terms of outcomes.

E tank is right about tidal volumes. Put a patient on VCV at any given volume. Then switch them to PCV with the IP set at whatever the peak pressure was being generated by the first mode. You will almost always see a significantly increased tidal volume.

 

[VentMonkey]

As far as I know, no ventilation specific ventilation strategy is superior to any other in terms of outcomes.

This is turning into a good thread. To kind of bring it back to a novice ventilator management perspective though, this isn't to say that you cannot cause harm to your patients with poor vent management, specifically with long transport times in the out of hospital environment.

 

[E tank]

In PCV, PIP=Pplat.

Like I said, my purpose wasn't to engage in a "less filling, tastes great" debate on minutiae. It was to de-clutter some of the confusion that the less experienced bring to the table when mechanically ventilating a patient.

It's evident that different patients benefit from different modes of ventilation, both hemodyamically and with regard to gas exchange, PCV being one. The literature is there for the mining, for what it's worth.

As far as "bucking" goes, if the issue were just isolated to the intrapulmonary/thoracic pressures, there would less of an issue. But it's not. It raises ICP, MAP, and the HR as well. Healthy patient? Who cares? But I don't think that's what we're talking about.

 

[Brandon O]

As far as I know, no ventilation specific ventilation strategy is superior to any other in terms of outcomes.

E tank is right about tidal volumes. Put a patient on VCV at any given volume. Then switch them to PCV with the IP set at whatever the peak pressure was being generated by the first mode. You will almost always see a significantly increased tidal volume.

1. This is not magic. The PCV is probably just ending up with a longer I time.

2. This is not good. Set tidal volume based on what's lung protective and what's needed for ventilation. Bigger is worse, not better.

3. Ignore peak pressures. Plateau is what's relevant to most of our interests.

 

[E tank]

1. So what? The volume is delivered at a lower pressure and that is done without adjusting anything else and it's "protective"

2. What's not good? Delivering more tidal volume at a lower pressure?

3. For all practical purposes, PIP = Pplat when giving PCV

@Remi, FWIW there are patients that PCV is better than other modes. Kind of obscure for this forum, for the obese laparoscopic surgery patient, PCV, in at least one paper that I saw, is demonstrably better than VCV (PC-volume guarantee being the best) and the reasons are all pretty intuitive. I'll kick it your way if you're interested.

 

[Carlos Danger]

And all this time, I thought it was magic.....

Would love to see that paper, @E tank.

 

[E tank]

And all this time, I thought it was magic.....

Would love to see that paper, @E tank.

PM'd you.

 

[TXmed]

@E tank

So if in PCV, PIP=Pplat

Does that mean in an bronchospastic patient with a PIP of 35 but TV are down to 2-3ml/kg, your Pplat is still 35? Or what about an obese patient, pneumo patient, etc ?

 

[E tank]

@E tank

So if in PCV, PIP=Pplat

Does that mean in an bronchospastic patient with a PIP of 35 but TV are down to 2-3ml/kg, your Pplat is still 35? Or what about an obese patient, pneumo patient, etc ?

With the ventilators that I use, yes, and if the one's that you use are configured so that there is no flow at the end of the cycled breath, then yes there too.

Did I get your question or did I miss something?

 

[TXmed]

@E tank you answered it sir

 

[Brandon O]

As far as "bucking" goes, if the issue were just isolated to the intrapulmonary/thoracic pressures, there would less of an issue. But it's not. It raises ICP, MAP, and the HR as well. Healthy patient? Who cares? But I don't think that's what we're talking about

If you're pointing out that patient-ventilator dyssynchrony can elevate the variables you mentioned, obviously this is true. But this is primarily a result of discomfort and sympathetic activation. I can also elevate the heart rate by vigorously poking the patient with a stick. The solution is to resolve the stimulus (i.e. adjust the vent to allow better synchrony, or increase sedation). While I certainly agree there are times when PCV is better tolerated than VCV, it sounds like you're talking about something different.

I think it is also probably untrue that raising the ICP by a purely valsalva-type mechanism is harmful per se. Just like the transpulmonary gradient, the gradient across the cerebral vessels is not elevated in this case; since the MAP and ICP elevate simultaneously there is usually no change in the CPP. IMO; obviously there may be exceptions.

2. What's not good? Delivering more tidal volume at a lower pressure?.

I think this is the crux of the issue here. Why would this be good?

 

[Brandon O]

@E tank

So if in PCV, PIP=Pplat

Does that mean in an bronchospastic patient with a PIP of 35 but TV are down to 2-3ml/kg, your Pplat is still 35? Or what about an obese patient, pneumo patient, etc ?

The inspiratory pressure you set in PCV is the driving pressure created by the vent (on top of PEEP); no pressure within the airway will exceed this. Early in the breath, much of this pressure will be from airway resistance, and the "plateau" is low here (although not measurable). As flow decreases later in the breath, less of that peak figure is made up from airway resistance and more by alveolar elastance (i.e. the plateau pressure). If flow drops to zero at the end of the breath, which it should in a properly-configured PCV mode (inspiratory time must ideally be long enough to allow this), then resistance is gone and all of the peak pressure is from the plateau.

This can be a little confusing, but the point is to remember than in PCV, *we* set the total airway pressure, and the vent maintains it. What that pressure consists of, however, depends on the flow. So in a patient with high airway resistance (bronchoconstriction), you would still have the same PIP; you simply might not get much volume squeezed into each breath, and the plateau would be low (probably less than the PIP). Likewise with poor chest/lung compliance (ARDS, obesity), you will not exceed the set PIP and hence not exceed that plateau pressure, but the tidal volumes might be quite small.