[NO AUDIO]
THOMAS PIRANIO: Hello, my name is Thomas Piraino,
and this is a basic introduction to mechanical ventilation.
The need for mechanical ventilation is based on the equation of motion.
The equation of motion represents the total amount of pressure
that needs to be generated to overcome the combination of pressure
due to resistance and the pressure due to elastance.
An increase in resistance, such as an asthma exacerbation,
would require increased work to overcome this resistance.
A condition such as intra-abdominal hypertension and ARDS
would result in increased work required to overcome the stiffness of the chest
wall or the lung, which is a component of the respiratory system elastance.
In some cases, increased work can be due to gas exchange
issues that are not directly related to resistance and elastance.
There have been reports of patients with COVID-19 having a high respiratory
drive with normal resistance and compliance.
And compliance is just the inverse of elastance.
It should be noted that hypoxemia and hypercarbia can also
increase drive without increased work.
And COVID-19 patients that require mechanical ventilation
are presenting with profound hypoxemia.
Spontaneous breathing patients generate negative pressure to do this work.
And the first widely used type of mechanical ventilation
to assist or take over this work was based
on this concept of negative pressure--
the iron lung.
Modern mechanical ventilation no longer uses negative pressure.
Instead, it generates positive pressure to overcome this work.
Non-invasive ventilation can be provided via facemask.
Non-invasive ventilation is a therapy that
has been shown to significantly reduce the need for intubation and mortality
in patients with chronic lung and heart disease.
But it has demonstrated high failure rates
and lacks recommendations for patients in hypoxemic failure
without chronic lung or heart disease.
This is a term called de novo respiratory failure.
And for this reason, it is not recommended as an approach
to avoid intubation in COVID-19 patients.
Invasive mechanical ventilation is delivered through an endotracheal tube
and addresses the pressure required to overcome work
by delivering a set pressure directly with the mode Pressure Assist
Control or the mode Pressure Support.
It can also directly address the work of breathing
by setting a flow and volume to be delivered with the mode Volume Assist
Control.
In this case, the total pressure that is generated
is directly related to the resistance and elastance
of the respiratory system.
The most common modes of ventilation have criteria for when they start,
what they control, and what determines the cycling or the end
of the breath that is being delivered.
Volume Assist Control has a mandatory frequency of breaths per minute
that is set.
The patient can trigger all the breaths and more,
but if they lack respiratory drive, the ventilator
will provide the minimum set frequency.
Pressure Assist Control has the same criteria
for when a breath is delivered.
Pressure Support, however, requires active effort from the patient,
as there is no set frequency.
It should be noted that a leak around the endotracheal tube and even
cardiac oscillations can trigger a ventilator,
giving the impression that the patient is
breathing when it may not be the case.
For Volume Assist Control, the item that is controlled is the flow.
Even though total volume is set, it is essentially the target.
When the target volume is delivered by the flow that is set and controlled,
the breath has ended.
However, added time can be used to prolong the breath
and provide a measurement of plateau pressure.
This concept is covered further in the basic ventilator settings video.
Pressure Assist Control controls the pressure in the system.
It provides variable flow, essentially whatever flow
is required to maintain the pressure.
The pressure is maintained for a determined amount of time,
which is the iTime, which is a setting.
And that's the point at which the breath cycles off.
Pressure Support also controls pressure and provides whatever
flow is required to maintain it.
But the cycling criteria to end the breath
is based on a percentage of the peak flow.
This is also further discussed in the basic ventilator settings video.
Mechanical ventilation is generally separated into two goals--
provide assist with minute ventilation to remove carbon dioxide
and provide FiO2 and maintain end-expiratory lung volume
with positive end-expiratory pressure, PEEP, to maintain oxygenation.
The elements that are used to adjust this, of course,
are respiratory rate and tidal volume.
Or if using a pressure targeted mode, you
would adjust the pressure to get a tidal volume
to affect minute ventilation and, of course, PEEP and FiO2 for oxygenation.
The breath sequence on the mechanical ventilator
is defined by the flow timescaler.
The inspiratory phase occurs during positive flow, above the line.
And the expiratory phase occurs during negative flow, below the line.
Please note that the word negative just represents
the position on the waveform.
It is not an active negative pressure applied.
It is a completely passive process.
The ventilator screen generally displays two or three scalers that
consist of volume, flow, and pressure.
They're not always in the same order, so you
have to understand what you're looking at, keeping in mind
that flow has settings or measurements, I should say,
that are above and below the line.
So in this example, volume is at the top, flow is in the middle,
and pressure is on the bottom.
Measured values are typically displayed as large numbers
on either the side of the screens or at the top.
And ventilator settings normally appear across the bottom.
Software buttons or hardware buttons on the side of the monitor
provide additional functions and access to additional settings,
either for that specific mode or to change
to a new mode, alarm settings, and maneuvers.
Of course, there is always an Alarm Silence button
somewhere on the monitor.
In this example, it's the bottom right hand screen.
Here's an example of a ventilator screen.
You can see software buttons along the left side,
three wave forms in the center--
pressure at the top, flow in the middle, volume at the bottom.
Settings are all along the bottom, and monitored and measured values
are along the right-hand side.
And there's an Alarm Silence button in the top left.
Here's an example of an additional features
graph that can be displayed, either continuously or momentarily,
for measurements.
Again, monitored values on one side, and software buttons on the other side,
and the ventilator settings are along the bottom.
For this ventilator, the Silence button is not on the screen.
It is actually a hardware button on the physical monitor itself.
This is an example of some of the standard ventilator measurements
and standard ventilator settings.
It is not a complete list because there are many modes that
are not represented here.
However, many of the measurements apply to all mechanical ventilation
modes, such as peak pressure, plateau pressure, mean airway pressure,
tidal volume, minute ventilation, et cetera.
Probably the most common or important monitored values
would be peak pressure, plateau pressure, and minute ventilation.
[NO AUDIO]