THOMAS PIRAINO: Hello, my name is Thomas Piraino,
and I'm going to discuss with you waveform analysis.
The waveforms I'm going to focus on are called scalars
because they plot something versus time.
We will discuss volume versus time, flow versus time, and pressure versus time.
Let's first look at volume-assist control.
This is a patient with normal lung conditions.
Let's take a closer look at the waveform.
With volume-assist control, it is very common to have a constant flow pattern.
This is why it's straight.
When something doesn't change over time, it's flat.
So here we have constant flow being delivered.
Once the tidal volume is reached, the breath
is technically supposed to be terminated.
However, we've added a 0.2-second pause, as you can see here.
So when it reaches the volume, it's actually held in the lungs,
and there is no flow.
The breath is held, so flow is zero.
And because of that, we have this volume being held in the lung.
And because there is no flow, we have this plateau pressure
which represents the pressure in the alveoli when the breath is held.
This pressure here is due to the resistance in the airways
when flow is constant.
Now let's look at pressure-assist control.
With pressure-assist control, the pressure is held constant over time.
So the flow increases rapidly to reach the pressure,
and then it slows down to maintain the pressure.
If it continued at the same speed, this would be like volume-assist control,
and the pressure would continue to rise.
Instead, it slows down over time to maintain this pressure.
The speed at which it reaches the pressure is called the rise time.
So it's how fast this flow increases.
And right now, the rise time is in seconds, and it's 0.01.
So it's a very rapid increase to the pressure.
If we increase this time, let's say to 0.1 seconds.
And let's start the waveforms again.
You'll see it's a much more gradual increase to the peak,
as you can see here.
Sometimes this will eliminate some overshoot
that occurs with pressure control and pressure support.
For now, let's keep it like this.
Sometimes pressure-assist control is thought
to be a plateau pressure that is read.
However, keep in mind that unless flow reaches zero,
it's not the true plateau pressure.
So here you can see flow is not reaching zero.
So although it's close to zero, it's not quite representing the plateau.
In order to do a plateau, you would have to add an inspiratory pause,
and this pressure would be slightly higher than the plateau pressure
on a patient that is passive.
Let's see.
As you can see, the plateau pressure is 12.9, and the peak pressure here is 15.
So when flow reaches zero, the pressure in the system drops.
So some of this is due to resistance.
When patients begin interacting with the ventilator,
it makes sense that it would cause changes to the ventilator waveforms.
However, their effort is most likely going
to alter the parameter that is not controlled.
So in volume-assist control, the flow is controlled.
Therefore, effort will not cause changes to that waveform.
It will more likely cause changes to the pressure
because the pressure is not controlled.
In pressure-assist control and pressure support,
patient effort is less likely to change the pressure parameter because pressure
is controlled and targeted.
Therefore, it will change the flow waveform.
This is very important for understanding how to assess the patient's interaction
with the ventilator at the bedside.
When assessing waveforms at the bedside, it's extremely important
to understand the term asynchrony.
It can refer to a number of specific types of ventilator patterns
with the waveforms.
However, it's a general understanding that it's
a difference between the neurological timing of the patient's
respiratory drive and the ventilator timing.
It's basically out of sync.
So it's either the ventilator cycling off too late or too early
compared to what the patient wants.
Another form of asynchrony or dyssynchrony
is the ventilator does not meet the demand of the patient,
and the patient can get what's called flow starvation.
This typically happens in something like volume-assist control
when the patient has high drive, which I'll describe in just a moment.
Another form is when the patient is unable to meet the trigger
criteria for various reasons.
This typically can happen when the cycling of the ventilator is delayed,
so it is prolonged.
And when the patient goes to take another breath,
they still have a lot of volume and pressure in the system,
meaning the respiratory system, and they are
unable to trigger the ventilator, either due to extreme weakness
of respiratory muscles or simply because they are just
dynamically hyperinflated.
Let's first look at volume-assist control.
I've introduced patient effort into the system.
It's represented by this pink line.
Every time there's an effort, it decreases.
This would represent the pleural pressure or the PMOS
that's being generated by the patient.
And as you can see, the flow has no indication of effort
because flow is held constant where you see
the patient effort is in the pressure waveform because it is not controlled.
So you see a scooping that is consistent with the patient effort.
The patient's effort can be measured on the ventilator using
the occlusion pressure p 0.1.
In this example, it is 2.35, which is considered normal.
This effort is not excessive.
Now let's look at excessive effort.
Now the p 0.1 is 4.70.
This is considered high.
And you can see there's much more scooping of the pressure
wave form, again no indication in the flow.
Now some ventilators may allow the patient to pull some flow.
But in general, if it's not allowing it, it should not change.
And what you should see is scooping and the pressure, which you can see
is much more drastic than it was in the previous example.
This is considered excessive effort.
We are hearing reports that patients with COVID-19
have excessive efforts when they begin to interact with the ventilator.
So one parameter that can be easy to look at
is not only the occlusion pressure, which is available on every ventilator,
but also pay close attention to what parameter would
be changing on the ventilator screen.
Now let's look at pressure-assist control.
Here we have the normal drive again, the p 0.1 is 2.35.
What you can notice is that the patient makes the effort.
And when they're done, you actually notice a change in the flow waveform.
Again, you should not see any obvious change in the pressure
because this is what's controlled in this mode.
So when you see effort, you should see a change in the flow
because it's not controlled.
Now the flow is a little bit more rounded up here than normal.
Typically, it has a peak, and then it decelerates immediately.
So it's a bit rounded.
So that's one indication that the patient is making an effort.
But then the second indication is here when the patient basically ends
inspiration into passive exhalation.
You can see there's a change in shape here.
Now the criteria for cycling off this breath, if you recall,
in pressure-assist control is time 0.8 seconds is what is set.
So the breath goes on to 0.8 seconds, but the patient
is actually stopped breathing here.
So this is a perfect example of the mismatch
between the patient's neurological time and the ventilator time.
So, of course, one way we can address this is we can change the time.
Now you can see the breath cycles off much more closely
to the patient's effort.
This is one way to optimize a ventilator mode.
When you see changes in the flow, for example, in pressure-assist control,
you can try to address this by shortening the time
so that it does not have this shape here.
Now let's look at high effort.
Now the p 0.1 is 4.7.
There's not much changing here, except the flow
has become more rounded at its peak.
There's much less of an obvious peak to the flow,
and the deceleration is normal.
However, everything seems more rounded.
This means the patient is doing more work.
Either you don't see an obvious spike in the flow--
It's much more rounded--
however, the timing is enough to know that it
cycles off and then into exhalation, but there's no delay in cycling off here.
Now let's look at pressure support.
With pressure support, there is no inspiratory time
in seconds that is set.
If you recall from the basic ventilator settings video,
it's a cycle off percent.
So it's 25% of this peak flow is when the breath cycles off.
And in this patient example, it's actually
relatively in sync with the patient's neurological drive.
Now this is not always the case.
And often, this requires adjusting specifically
in patients with high airway resistance, which
I will talk about in just a moment.
But as you can see, the drive is normal, 2.35 of a p 0.1 measurement.
Now let's look at the patient with a high drive.
Now the p 0.1 has gone back up.
And similar to pressure-assist control, you see more rounding
in the flow waveform here.
However, the cycling off seems to be still reasonably OK,
but there's a little bit of dampening here in the expiratory flow.
This can be addressed by simply lowering the cycle
off criteria to a lower level.
Let's see how that works.
So you can see there's less dampening here.
So that's one way to manage it.
However, little adjustments like that are less
likely to reduce the patient's drive.
So it's more important to try and increase the pressure support
to a level that may reduce their drive.
If they respond to the increase in pressure support and the drive
decreases, as it does here, then, as you can see, the cycle criteria is good,
and let's increase it back to 25 as well.
You can see it still looks good at the normal default setting.
So what you really want to do is try to reduce the patient's drive.
However, if increasing support, which increases tidal volume
delivery to the patient, does not decrease the drive,
you really have to be concerned whether or not
the patient is causing potential injury to themselves.
Now this patient is on a moderate level of PEEP at 10 centimeters of water,
and the FiO2 requirements are 50%.
So this patient having a very high drive is actually concerning.
So you want to keep the drive in this range.
So this is an OK and acceptable drive, but when it was at 4.7,
this is not the kind of patient you want to continue to have that drive.
You want to be able to try and address it and try
to reduce the drive anyway you can because they still need a bit of oxygen
and PEEP and it's important to try and reduce that drive.
Now PEEP can sometimes affect drive in a negative or positive way,
so you can try affecting the drive by adjusting PEEP.
However, if that does not improve or reduce the drive,
you need to find other means of doing so.
Now let's look at cycling of the ventilator breath.
Here we have a patient in pressure-assist control
with an inspiratory time of 0.8 seconds.
As you can see, the 0.8 seconds is holding the pressure, but at the end,
you're seeing a pressure that's rising.
Now this can be due to active exhalation or sometimes
just passive relaxation of the respiratory muscles
can cause a little bit of a spike here.
One way to determine whether or not it is active
and the patient is really trying to end the breath at that moment
is if you have an abrupt spike in expiratory flow.
Let's look at an actual ventilator waveform
screen to see what that might look like.
Here you can see there's a drastic increase in pressure,
which is caused by delayed cycling.
The patient wants to exhale, but the machine has not cycled off yet.
And as you can see in the expiratory flow,
there's a rapid increase in flow at the onset of exhalation,
indicating that there is some force behind it.
Now by shortening the inspiratory time to 0.7 seconds,
you can see there's still a tiny blip in the pressure waveform.
However, the rapid deceleration in the experts airway form
is no longer present.
Therefore, this is probably the optimal timing
you're going to experience without overshooting
and making the breath be too short, which is called premature cycling.
With premature cycling, the patient's effort
lasts longer than the ventilator's cycle-off criteria.
So in this example, the cycle-off criteria
again is 0.8 seconds and pressure-assist control.
It cycles off here.
But as you can see, the patient is just not quite at the point
where they are beginning to exhale.
And, therefore, there is a dampening in the expiratory flow waveform
because the effort continues on past the cycle-off criteria.
This can also occur in pressure support.
If the cycling is set too high, the machine
will cycle off before the patient effort is complete.
Let's take a look at it on a real ventilator waveform.
In this example, the cycle-off percent is
60% of inspiratory flow, which is too short.
And there's clear evidence that the patient
is continuing to inspire after the ventilator cycles off.
Sometimes if the effort is strong enough, what can happen
is the ventilator can sense that the patient wants a breath
because the effort is strong enough to meet the trigger sensitivity criteria,
and in this example, you can see a second breath is delivered
to the patient with the same effort.
Now let's discuss a very common form of asynchrony called ineffective efforts.
Ineffective efforts generally occur when patients are overassisted, either
with too much pressure support or if they have very high airway resistance,
for example.
So here in this case, we have a patient with a very weak effort that they
trigger the ventilator, but due to their airway resistance, which is quite high,
it takes a very long time to meet the criteria, which
is 25% of peak flow to cycle of the breath.
So all of this breath is excessive compared to their drive.
So this patient is essentially being passively ventilated by pressure
support ventilation because it requires this criteria to be
met prior to cycling off.
Now one thing you could consider is shortening the time, which,
of course, will cycle of the breath.
But if the peak flow is being met by the patient,
it may not stimulate them to actually make a stronger effort.
So usually, the first step, when you have this,
is to assess your tidal volume, which here you
can see it's over 8 mL per kilo.
Now it's only picking up 17 breaths per minute,
but the patient that I've set up here is actually
breathing 28 breaths per minute.
So they're a little bit too tachypneic.
So the tidal volumes they probably would be bringing in
would be a little bit lower than this for sure.
So let's see what happens when we reduce the pressure support.
The p 0.1 was 1.4, and now it's increasing to just over two.
So the patient's drive has increased.
The tidal volumes are lower, but still acceptable
considering the respiratory rate of the patient.
And as you can see, they're now cycling every breath
and triggering every breath.
So you have to consider that the patient effort has a lot
to do with whether or not the machine is overassisting and taking
too long to cycle off.
So that condition I described was delayed
cycling, which is actually the number one reason for ineffective efforts
to occur.
And in fact, one of the mechanisms people
think is the problem is the trigger sensitivity.
Now unless someone has set it abnormally high,
the default trigger sensitivity on most ventilators
should be sufficient for a patient to trigger.
So if the patient cannot trigger a breath,
look for signs of delayed cycling, as I've just described.
Finally, I will discuss the asynchrony type called reverse triggering.
It's complicated to understand, but very common, so it's worth noting.
Reverse triggering is different from normal patient ventilator interaction.
The normal way a patient and ventilator interact
is that the patient triggers a breath from the ventilator
by setting off the trigger sensitivity.
In reverse triggering, the ventilator delivers a breath first
without patient demand, but then that breath that's being delivered
triggers or rather stimulates an effort from the patient.
It's very common in sedated patients or those transitioning off sedation,
but whether it's reflex or just bad timing
is up to the clinician to decide by making ventilator adjustments.
When it's reflex, it can be difficult to manage.
There are things to try, which I'll review,
but sometimes those things that you can try
may not be safe to do in a patient, for example,
with acute respiratory distress syndrome or, in other words, patients
with COVID-19.
It may not be ideal to do some of these maneuvers
in order to try and relax the reflex.
Now just to clarify, reverse triggering does not
require a ventilator breath to be triggered
in response to that patient reflex.
However, it can happen, and this is called breath stacking,
which I'll describe in the next slide.
So reverse triggering, as you can see on the left,
the machine delivers a breath.
And as you see, there's an esophageal pressure away from the bottom.
That indicates the patient's making an effort
after the ventilator delivers a breath.
So in this example, the patient is on a volume
assist control mode that actually allows the patients to pull flow
from the ventilator, even though it's supposed to be controlled,
but only when they demand and have a huge drop in pressure,
which you can see in the second breath, which is
why flow is delivered to the patient.
On the one on the right, they're also in volume control.
They're triggering the first breath.
The second one, the machine comes first, and then the patient
makes an effort that although it may be difficult to see
in the inspiratory pressure waveform, although there is a loss in plateau which
you can see, you can clearly see that the effort continues on
into the expiratory phase as there is a large increase in flow
as the patient is making an effort.
All of the phenomenon in these slides or in these graphics,
except for the first breath in the image b, are all reverse triggered breaths.
The final one is the only one that actually
resulted in a second delivered breath, which is called breath stacking.
But reverse triggering the term itself does not require a breath stack breath
to occur in order for it to be classified as something
called reverse triggering.
Now let's demonstrate.
The first thing to do is to first see is this reflex or just bad timing?
If you look at this paused waveform graphic here,
this is volume assist control.
You can see the effort is represented by this pink line here,
and the machine delivers a breath.
And the patient responds with an effort afterwards.
Now in volume-assist control, you're looking for changes in pressure.
So you can see this scooping that occurs here,
but not at the beginning of the breath.
So this could be classified as a reverse trigger.
Machine comes first and stimulates the patient to make an effort--
same with this breath here.
The machine comes first.
The patient makes an effort, and the effort
continues on past the point at which the breath cycles off.
And you can see this little dampening in the expiratory flow.
So if you saw this sign here in volume-assist control
or a drop in pressure here, you may think that the patient
has reversed triggering.
Now is this bad timing, or is this truly a reflex action?
The first thing to do is to simply reduce the respiratory rate.
So if I reduce the respiratory rate-- let's
would start up the simulator here.
If I reduce the respiratory rate to 10, I
can see if my patient has an intact respiratory drive,
and perhaps I'm just beating them to the punch.
So now that the ventilator rate has been reduced to 10,
you can see my patient is triggering every breath.
So this patient no longer has the phenomenon known as reverse triggering.
All it was was bad timing.
So that's one way to determine whether or not
the patient has reverse triggering.
However, in many patients when you reduce the rate,
the phenomenon of reverse triggering continues, and of course,
reducing the rate would reduce the minute ventilation.
And this may not be appropriate.
So let's discuss actions to take if you reduce the rate and the patient
continues to reverse trigger at the lower rate,
and therefore, you lose valuable minute ventilation.
So if you've already reduced the respiratory rate and the patient
does not take over with sufficient minute ventilation,
turn off sedation, when applicable, or if it can be done.
Otherwise, if you cannot increase tidal volume to a maximum of eight
milliliters per kilogram of ideal body weight,
if it isn't already at that level, keeping in mind that we should keep
plateau pressures less than or equal to 0.27 centimeters of water when
possible, now in COVID-19 patients, the recommendations are to follow those
of typical ARDS guidelines, which is to minimize or limit plateau pressure less
than 30 centimeters of water.
Now if there is a known injurious pattern and you cannot correct reverse
triggering-- and the injurious pattern I'm speaking of is breath stacking,
for example--
and if medical management is still a concern--
in other words, you cannot turn off sedation--
then consider the use of neuromuscular blocking agents to protect the lung
and minimize the possibility of barotrauma.