Pulmonary Mechanics in Non-Invasive Positive Pressure
Ventilation by Dr. Brian McAlvin.
Hello.
My name is Brian McAlvin, and I'm
an intensivist in the division of Medicine Critical Care
at Boston Children's Hospital.
Today, I'll be speaking about Non-Invasive Positive Pressure
Ventilation.
Review of NIPPV.
First, let's start with some terminology.
NPPV or NIPPV refers to non-invasive positive pressure
ventilation.
There are two varieties of NPPV that we'll be talking about--
CPAP and BiPAP.
CPAP stands for continuous positive airway pressure.
BiPAP stands for bilevel positive airway pressure.
You'll see on the slide, that there are three graphs.
The top two represent patients on CPAP.
The top one shows a continuous positive airway pressure of 5,
and the respiratory cycle where pressures
go below and above that value throughout
the respiratory cycle.
During inspiration, the patient generates a negative pressure,
and during expiration, generates a positive pressure.
In the second graph, that also represents CPAP,
but some ventilators have a setting
whereby the pressure can be reduced during exhalation
for patient comfort, represented by the red arrows on the graph.
Ventilators at Boston Children's Hospital have that setting.
Finally, the third graph shows a patient on BiPAP,
where just like CPAP, there is a continuous positive airway
pressure on the bottom known as EPAP or expiratory positive
airway pressure, as well as an inspiratory positive airway
pressure that's given to the patient when
they draw a breath.
It's an assisted breathing pattern.
And you see that the pressures cycle between the end
positive airway pressure, and the peak, inspiratory positive
airway pressure.
Three more curves to consider-- very similar to the ones
that we looked at, but this, instead,
is a measurement of alveolar pressure.
If we were to measure the pressure in the alveolus,
this is what you would see.
The top left shows a normal patient
without any positive pressure ventilation,
where the pressures cycle below and above a value of zero
throughout the respiratory cycle.
The top right figure shows a patient on CPAP,
and the bottom figure shows a patient on BiPAP.
So let's talk about the advantages
of non-invasive positive pressure ventilation.
First off, it offers the benefits
of mechanical ventilation without many
of the risks of intubation.
It reduces the need for sedation,
and the patient can retain the ability to communicate.
Often, it will allow for intermittent eating
and drinking, but only if the clinician deems
it appropriate after having ruled
out concerns for aspiration.
It also avoids the risks of ventilator-associated
pneumonia.
So what are some of the indications
for using non-invasive positive pressure ventilation?
First off, respiratory failure as well as dyspnea, tachypnea,
and use of accessory muscles.
If there are gas exchange abnormalities,
you may consider using NPPV.
Evidence exists for acute hypercapnic respiratory failure
in COPD, cardiogenic pulmonary edema,
acute hypoxemic respiratory failure
in immunocompromised patients, facilitation of extubation,
and for patients with obstructive sleep apnea.
Insufficient evidence exists for pneumonia, asthma, and ARDS.
I've compiled a list of diagnoses
that we often consider and use non-invasive positive pressure
ventilation.
These include pneumonia, asthma, some cases of ARDS depending
on the circumstances, including severity and patient's wishes
for intubation, neuromuscular diseases,
end-stage lung disease-- for example,
cystic fibrosis or transplant recipients with graft
rejection--
and patients with a do not intubate status.
There are some absolute contraindications to consider.
For example, a comatose patient, patients who
are having cardiac or respiratory arrest,
if there's facial trauma or facial deformities that
don't allow proper fitting of a mask,
and severe encephalopathy.
The caveat here is that when the encephalopathy is
secondary to hypercapnia, you may consider a trial of BiPAP
to see if you can reverse the hypercapnia
and regain mental status.
There are some relative contraindications as well,
and I've listed them here for your review.
Physiology of NIPPV.
So let's discuss the physiology of non-invasive positive
pressure ventilation.
Going back to first principles, this graph is useful.
It represents the respiratory system divided
into individual components--
the lungs, the chest wall, and the combined system.
On the x-axis is the transmural pressure.
On the y-axis is the lung volume reported as a percentage
of vital capacity.
The curve I'd like to focus your attention on is the solid line.
You'll note that there are two dashed lines.
One represents the chest wall compliance,
one represents the lung compliance,
and the solid line is the composite of the two.
The important take-home message here
is that the chest wall favors expansion,
and the lungs have an intrinsic recoil property
that favors collapse.
Where the forces of expansion by the chest wall
and the forces of collapse by the lungs are equally opposed,
that represents functional residual capacity.
It's important to consider these dynamics
as you think about the diseases you might be taking care of.
This graphic represents two extremes you might encounter--
emphysema, pulmonary fibrosis, and a normal lung.
You'll note that in the case of emphysema,
expanding forces predominate and the FRC is high.
And in fibrosis, the FRC is low.
These are adult diseases that you may not
encounter so much in pediatrics, but I've
compiled a list of diagnostic considerations
that may help you think about this moving forward.
So factors that could affect chest wall compliance
include obesity, edema and ascites, neuromuscular
weakness, and many others.
Factors that could affect lung compliance
include a variety of things that we commonly
encounter including pulmonary edema, ARDS, and many others.
So let's look at a specific example.
We could choose among many, but this one's useful.
So patients who have neuromuscular disease,
the expanding forces of the chest wall
are quite weak due to neuromuscular weakness.
And therefore, the collapsing forces of the lungs
predominate.
By instituting non-invasive positive pressure ventilation,
you can restore lung expansion, alveolar recruitment,
and improve gas exchange.
Let's look at some pressure volume
curves to understand what we need to be thinking about.
You'll note, on the x-axis is positive pressure applied
to the lung, and on the y-axis is lung volume.
Represented is the inspiratory curve
with the three different zones, the lower and upper inflection
points.
And you can see, to the right of this graph
are representative alveoli and their changes in volume
with positive pressure.
In this region of the curve, you'll
note that the alveoli are not recruited
and their change in size is minimal.
But in this region, you have an optimal expansion and recoil
of alveoli.
And finally, the alveoli are overdistended.
BiPAP.
So let's look at BiPAP specifically.
As you recall, BiPAP can be divided into IPAP and EPAP.
First, let's look at IPAP, inspiratory positive airway
pressure, and its influence on work of breathing.
On the x-axis, you'll look at different levels
of pressure support with a PEEP of 0, ranging from 5 to 20.
So for example, to the far right of the graph,
patients receive support in the form of 20 over 0.
And on the far left, patients are spontaneously breathing.
And on the y-axis, you see the pressure time product,
which is a measure of the work of breathing.
Regardless of whether or not patients were
normocapnic or hypercapnic, both experienced drastic reductions
in work of breathing and a dose response
according to the amount of IPAP that was used.
This was a linear relationship, and the effect
was more dramatic in normocapnic patients.
If we study the effect of EPAP, we, again, see--
you're familiar with the solid black line, which
represents the IPAP alone, but now we've
added two additional curves.
For the squares, you see patients
that are supported with BiPAP and a PEEP of 5,
and for the triangles, patients supported with BiPAP
using a PEEP of 10.
And you observe that with increases in EPAP,
work of breathing, again, is reduced
until you reach an EPAP of approximately 10,
at which point little is gained from increasing it further.
So what this graph shows, again, adult patients,
various disease states, looking at use
of spontaneous breathing, CPAP, pressure support
with a PEEP of 10 or pressure support with a PEEP of 5.
On the y-axis, you see the pressure time product,
which is a measure of work of breathing.
And what you find is that as you move further
to the right, using BiPAP, whether 20 over 10 or 20
over 5, work of breathing is dramatically reduced.
So what does this mean?
What we observe is that IPAP reduces
work of breathing by providing a greater
proportion of the transpulmonary pressure during inspiration,
the so-called push-pull effect, whereas EPAP
reduces work of breathing due to the fact
that it counterbalances intrinsic PEEP.
In other words, it reduces the threshold load to inspiration.
It also increases respiratory system compliance
by reducing the elastic load to inspiration.
An important observation here, though,
is that the optimal settings with respect
to work of breathing may not match what patients report
to be the most comfortable settings,
and these two graphs show different measures
where patients report discomfort with the BiPAP settings
that they were using.
Both scales show a very similar distribution.
What this shows is patients reported comfort
using two different scales, and you find that the settings
patients report to be more comfortable
don't actually represent the settings that maximally
reduce work of breathing.
So if you consider the previous slides
that we reviewed with settings of 20 over 10 or 20 over 5,
those are actually on the left portion of both of these curves
and are not in the most comfortable range for patients.
So the take-home message here is that optimal settings
may have to be a balance of reducing work of breathing
as well as patient comfort.
So let's dig deeper into the physiology.
This slide is important and it represents
the lung of a patient spontaneously breathing, awake,
and sitting upright.
It's divided into three different zones.
These are the West zones that you may
have learned in medical school.
In zone 1, the alveoli are overdistended
and the alveolar pressure exceeds the arterial pressure
in the capillary bed, which means
although ventilation may be good to that region of the lung,
perfusion is poor.
Moving to zone 2, the alveolus is recruited in a way
that alveolar pressure is less than arterial pressure,
and both are greater than the venous pressure.
This establishes conditions where perfusion is perfectly
matched to ventilation.
Moving down to the dependent regions of the lung, or zone 3,
you find that the alveoli tend towards atelectasis,
and that the pulmonary arterial pressure and venous
pressure exceeds alveolar pressure so that perfusion
is good but ventilation to that region of the lung is poor.
So zone 2 represents a region where ventilation and perfusion
are properly matched.
And zones 1 and 3 represent regions where V/Q mismatching
occurs.
So what does this mean for your patient?
I want you to focus on this slide, where on the x-axis,
it represents lung volume, and on the y-axis,
it represents pulmonary vascular resistance.
You'll note the further to the right you move on the x-axis,
the closer you come to total lung capacity.
And the further left you move, the closer
you get to residual volume.
Either way, if lung volumes are low, you favor atelactasis.
If lung volumes are high, you favor overdistension.
The result under either circumstance
is that pulmonary vascular resistance is increased.
By finding the proper lung volume that approaches FRC,
pulmonary vascular resistance is optimized
and V/Q matching can be enhanced.
These are important considerations
as you titrate your ventilator settings.
So as we work to optimize V/Q matching,
some things we may need to consider
are the West zones that we talked about previously
and the compliance curves that we looked at in prior slides.
If you look at the curve on the right, where the alveoli are
poorly recruited and don't change much in response
to changes in pressure, that's analogous to West zone 3.
The linear portion of the curve is
analogous to West zone 2 where alveoli are optimally
recruited.
And West zone 1, at the top of the curve,
represents where alveoli are overdistended.
Summary.
So in summary, as we put this all together,
I'd like you to think about non-invasive positive pressure
ventilation for the following frameworks.
You might use it to enhance oxygenation.
You potentially could use it for cardiac unloading,
it improves lung and respiratory muscle functions.
Thank you very much for watching.
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