The purpose of this video is to provide general information
and education about the care of a critically ill child.
It is in no way a substitute for the
independent decision-making and judgment by a qualified
health care professional.
The information contained in this
video should not be used to make a diagnosis or
to overrule the advice of a qualified health care
provider, nor should it be used to provide advice for
emergency medical treatment.
Introduction to Mechanical Ventilation, by Doctor Gerhard Wolf.
Please note, that in this video we will be
following the guidelines used at Boston Children's Hospital.
Some of this information may need to be modified based
on the equipment, guidelines, and practices in place in
your institution.
My name is Dr. Gerhard Wolf.
I work in the Division of Critical
Care Medicine at Children's Hospital Boston, and I wanted
to give you an introduction about mechanical ventilation.
Introduction
Let's start with indications for mechanical
ventilation.
There are three fundamentally different
reasons why somebody needs to be on a mechanical ventilator.
And it sounds very simple but I actually do think
about those indications every single time I think about
a patient on the ventilator.
A patient might be on the
ventilator because his primary problem is hypoxia,
and hypoxia is often caused by acute lung injury; the
patient has atelectasis, inflammation, may have an
aveolar to arterial 02 gradient, and the main approach to
this is actually that we ventilate the patient with
adequate PEEP to recruit most of his atelectasis and
improve his oxidation.
The next, fundamentally different reason
why a child may be on the ventilator might
be hypercarbia.
The patient who's hypercarbic normally has little
problem with oxygen consumption and oxygen uptake
but the problem of this patient is that he cannot
exhale CO2 because he has obstructive airway
disease.
So the patients with asthma, for example, are in
this category, and those patients are often ventilated
in a different way.
Here, it's not so much PEEP
and oxygenation that the patient needs to be improved
on, it's more that the patient needs an adequate expiratory
time, and needs sufficient time to exhale
CO2.
And the third category of patients actually
would be just loss of airway control.
Those patients presumably have healthy lungs,
but they have a loss of airway control either because they
had a head injury, or they may be intoxicated with
something, or they may have a severe infection, with
altered mental status.
Terminology
Let's review some of the terminology that occurs
when we talk about mechanical ventilation.
There's the peak inspiratory pressure, or the
PIP, which is the highest pressure that the patient
normally sees throughout the respiratory cycle.
Then there is the positive end
expiratory pressure, which is the pressure that is present
during expiration, and prevents the lung from
experiencing end-expiratory collapse.
The mean-airway pressure is the area under the pressure wave
form.
The mean airway pressure is not directly set on
the ventilator, but it's influenced by a lot of other
settings.
For example, if you increase the peak inspiratory
pressure on a ventilator, the mean airway pressure is
increased as well.
If you increase the PEEP on
a ventilator, the mean airway pressure goes up.
If you increase the inspiratory time, the mean airway
pressure also increases, because all of that increases
the area under the pressure waveform.
There's the respiratory rate, and often
there's a rate we set on the ventilator,
but that may also be the rate the patient
is actually breathing, so they may set the ventilator at
twenty, but the patient is breathing thirty, so it's
important to recognize that, and there's the inspiratory
time.
The inspiratory time is set mostly according to the age
of the patient.
And, whereas most neonates may have an
inspiratory time of 0.4 to 0.5
seconds, children sometimes have an inspiratory time of 0.8
to 1 second; older children or adults normally
have a time of 1.2 seconds or 1.4 seconds
inspiratory time.
SIMV means synchronized
intermittent mandatory ventilation.
And what does that mean?
Synchronized means that the patient has the
opportunity to trigger the ventilator within a certain time
frame of a couple of hundred milliseconds.
If the patient triggers the
ventilator, he will actually get a triggered breath, but if
the patient is, for example, too sedated or otherwise not
interested in triggering the breath, the ventilator will deliver
the breath anyway.
SIMV exists in volume
control or in pressure control.
And when you talk about triggering, there's a
flow trigger and a pressure trigger.
Most modern ventilators are equipped now with
a flow trigger, which means that the patient has to
generate a negative flow at the airway opening, and that
will deliver the mechanical breath, or that will trigger
the breath.
Some older ventilators have a pressure
trigger, which means that patients have to
generate negative pressure at the airway opening, and
especially for small children, that can be harder to
trigger, so most of our smaller children, or most of
our children now are set up with a
flow trigger, so they can trigger with a negative flow.
What does compliance mean?
Compliance means the change in volume over the change in
pressure.
Compliance is both age and weight specific,
that is why it is relatively impractical to compare the
compliance between two different patients, but it is
quite useful to look at the compliance in
each individual patient, for example, as the patient gets
sicker, he may need more pressure to generate the
same volume change in the lung, which means the
compliance just got worse.
Modes of Ventilation
Pressure control and volume control: what's the
difference?
In pressure control, the pressure is set, and
the volume that the patient is receiving is actually a
result of his lung compliance.
In volume control, it's the other
way around.
A certain volume is set, for example, an
inspiratory volume of 500 cc's,
and the pressure that is required to get
this volume in the patient's lungs is then a
result of patient's compliance.
And the important difference of both is the shape
of the inspiratory flow.
Here's pressure versus volume control.
On the right side, you see volume control, and
on the left side, you see pressure control.
The top panel is flow, and the
bottom panel is airway pressure.
Let's look at volume control first.
You can see that flow is constant; it's
like the constant flow out of a faucet, the
water flow out of a faucet and the airway pressure rises over the
inspiratory time.
So, with volume control, you have
a constant flow, and the airway pressure is increasing
over the inspiratory cycle.
With pressure control, it's a little
different.
There's a lot of flow in the beginning of
inspiration, and that flow decelerates to almost
zero at the end of inspiration, and we call that
a decelerating flow.
And the result of that is that
the pressure now is more homogeneously distributed
over the inspiratory time.
Let's look at a screen of a typical
ventilator here.
On the bottom you see the numbers that
are actually set on this ventilator, and on the
side are the numbers that are actually read off the
ventilator, and you typically have three curves, which are
pressure on the top, flow in the middle, and then
volume on the bottom.
This patient is in pressure control; you
can actually see on top the relatively square
wave of the pressure wave form, and below the flow
waveform, with a decelerating flow.
And note that the flow decelerates to
zero, both in inspiration and in expiration, and that it's actually
important to set up the ventilator in this fashion.
In this ventilator, a triggered and a non-triggered breath
looks different, and most ventilators have some way
to delineate whether the breath is actually
triggered by the patient, or not triggered.
In this ventilator, it is such
that the non-triggered breath is red, and the triggered
breath is yellow.
Let's talk about pressure support.
Pressure support is a mode of ventilation that is widely
used now, and it means that the patient is breathing
spontaneously, and the inspiratory time is variable.
Now, while on pressure support, there is a
pressure support level set, which means that there is a peak
pressure, and a peak positive end expiratory
pressure.
The inspiratory time is actually determined by
the patient.
The ventilator measures the inspiratory flow, and
once that drops to a certain number, then the
inspiration is regarded to be completed.
So now the patient actually has control over his
respiratory rate, but also over his inspiratory time, so
it's a relatively spontaneous mode of inspiration.
In comparison, in SIMV, the
patient can trigger, but once it triggers, the inspiratory time
is set.
There's a couple of positive effects on pressure
support.
It means that the patient is in control of
the ventilator more than during other modes of ventilation,
the patient is triggering, which means that he is using
his own respiratory muscles to help breathing
on the ventilator.
This can lead to a recruitment
of lung volume, and to an overall increased patient
comfort.
Acute Respiratory Distress Syndrome
Let's talk about oxygenation index and P/F
ratio.
Both are actually important indexes that are useful to be calculated because they delineate
if a patient is getting sicker, or if a patient is getting better.
The first index is the oxygenation index.
It's calculated by the mean airway pressure times the FiO2, divided by the PaO2, times
one hundred.
So, if the oxygenation index is rising, it means the patient is getting worse.
It means that the patient needs more mean airway pressure or higher FiO2 to achieve
the same amount of PaO2.
The next important ratio is the P/F ratio.
It's simply the PaO2 divided by the FiO2.
Now, it's the other way around here.
If the P/F ratio is decreasing, it means that the patient is doing worse, and the P/F ratio
actually has an important role in defining ARDS.
The Pediatric Acute Lung Injury Consensus Conference, or PALICC, provided the first
pediatric-specific definition of acute respiratory distress syndrome, or PARDS for short.
In order to diagnose PARDS, a few components of the patient's condition need to be addressed.
First, patients with peri-natal lung disease should be excluded.
Onset should be acute, occurring within seven days of a known clinical insult.
And in general, the condition is characterized by respiratory failure not fully explained
by cardiac failure or pulmonary edema.
Second, chest imaging.
New infiltrates should be observed that are consistent with acute pulmonary parenchymal
disease.
And last, oxygenation.
As a reminder, the adult definition of ARDS, the Berlin definition, utilizes the PaO2 to
FiO2 ratio in order to quantify the degree of hypoxia.
In the case of pediatric ARDS, the oxygenation index, or OI for short, is computed in order
to quantify this degree of hypoxia.
The OI is computed by multiplying the mean airway pressure by fraction of inspired oxygen,
divided by the PaO2 and multiplied by 100.
The higher this number, the worse degree of hypoxia.
The perceived benefits of using OI, as opposed to P/F ratio, is that it includes both of
the important clinical modulators of oxygenation: mean airway pressure and FiO2.
You can see in the table below that an OI of greater than or equal to 4 satisfies the
criteria for PARDS.
If you do not have access to an arterial blood gas in order to obtain a PaO2 value, you can
also calculate the oxygen saturation index, or OSI.
In this case, instead of PaO2, you insert the SpO2, provided that measured value is
less than or equal to 97%.
In this case, an OSI value of greater than or equal
to 5 satisfies the criteria for PARDS.
You can see in the table the varying degrees of PARDS severity based on both OI and OSI.
Here is an important differential diagnosis in a newborn.
This newborn presented with hypoxia and respiratory distress, and if you look at the
chest x-ray, you actually see you at the patient is intubated and has bilateral infiltrates
on chest x-ray the heart looks slightly generous.
The patient actually does not have neonatal respiratory distress syndrome, the patient
has TAPVR: total anomalous pulmonary venous return; so, it's a cardiac condition that
looks like ARDS, or like respiratory distress syndrome of the neonate in the first place,
but it's actually important to exclude that with an ECHO.
Let's talk about ventilator associated lung injury, because whenever we use a ventilator,
we recognize it's life saving for the patient, but also the ventilator can actually induce
ventilator-associated lung injury and make the patient worse. Ventilator-associated lung
injury can happen in three ways.
We have volutrauma, which means that there is an over-distension of normal lung units,
normal aveoli; there is atelectrauma, which means that there is the repetitive opening
and closing of atelectasis, which is potentially harmful to the lung as well; and then there
is the biotrauma which means that there is a release of inflammatory cytokines, that
now fuel the lung injury we're dealing with.
Here is an image out of John West's chapter.
This is an animal lung under an electron microscope, and you actually can see those fine tears
in the aveoli, and again, through those tears there is a leakage of fluid, protein-containing
fluid that leaks now into the alveoulus and is fueling the inflammatory cascade.
Lung Protective Strategies
Now what are the main strategies
of ventilating a patient with ARDS?
We try to reduce the tidal volumes and
use tidal volumes of no more than 6/kg.
We know from a large multi-
center study, that a tidal volume of 6/kg
is far beneficial compared to a tidal volume
of 12/kg, so we try to use a
low title volume strategy.
We try to use the amount
of PEEP that will give us the adequate lung
recruitment, which is often a PEEP that allows us
to reduce the FiO2 to less than sixty
percent or to sixty percent.
We also involve a strategy called permissive
hypercapnia and permissive hypoxia, which means that
as long as the patient is not acidotic, say his pH
is above 7.25, we can let his CO2 rise
and reduce ventilator settings, that is in and of itself lung protective,
and a patient with ARDS doesn't need to saturate one hundred
percent; we often use a goal PaO2 of fifty-
five to sixty-five, which corresponds to a SAT of eighty-
eight or ninety-two percent, which is called permissive
hypoxia.
Extubation Readiness
When is a patient ready for
extubation?
So once the patient got better, we have
to assure ourselves that the patient is actually awake,
so sedation plays an important role in the
extubation readiness test.
We would stop his feeds, to make
sure he has an empty stomach.
His FiO2 should be weaned to fifty percent
or less, and his PEEP should be weaned to five.
And then here at Children's we use a pressure support
that is actually linked to the size of the endotracheal
tube, because a smaller endotracheal tube will
need a larger pressure to overcome the resistance of that tube.
So, if a small patient has a 3.0 or a
3.5 tube, we actually use a pressure support of
ten, so that patient would be on a ten over
five.
If he has a 4.0 or 4.5
endotracheal tube, we would use a pressure support of
eight, and if he has a
5.0 endotracheal tube or larger, we use
a pressure support of six.
If the patient tolerates that and is
awake, and is not getting tachypneic under those settings
and has a normal blood gas, then the patient is
considered to be ready for extubation.
Thank you very much.
Please help us improve the content by providing us with
some Feedback.
What did or didn't you like about this
video?
Was the content too simple, just right, or
too difficult?
Was the length too short, just right, or
too long?
Any additional comments?
You can either click the 'Start a New Discussion'
button and type in feed back, or send us an
email, at openpediatrics childrens.harvard.edu.
Note, feedback is not required to complete this
activity in the guided learning pathway.