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hello this is Eric strong again and this
is the seventh lecture in this series on
mechanical ventilation the topic today
is ventilated modes if you've watched
the first six lectures you may have
started wondering when I was ever going
to get around to discussing how actually
the program the ventilator well this is
the point where I will begin to make the
transition from the physiology
background to the practical application
the learning objectives of this lecture
are as follows first to understand that
three variables which define a
ventilator mode next to be familiar with
the most commonly used ventilator modes
that is assist control simv pressure
control and a pressure support
ventilation finally to know how to
choose modes for a specific situation
and be aware of the advantages and
disadvantages of each before I go any
further I want to point out two things
first the topics of ventilator modes and
ventilator options are each difficult to
understand without first learning about
the other therefore you may not fully
appreciate this lecture until after
seeing the next one and may even benefit
from watching it again afterwards second
as problematic and inconsistent as
terminology is within the general
subject of mechanical ventilation I find
personally that there's no specific
topic in which this is more of an issue
than with ventilator modes people will
often use different terms to refer to
the exact same form of ventilation and
sometimes even use the same term to
refer to different forms of ventilation
this is partially a consequence of
different brands of ventilators coining
fancy sounding names for a basic vet
mode which could be known by a different
name when delivered by a different brand
of vents the most common place this
becomes a difficulty is when physicians
and respiratory therapists disagree with
the terminology about what mode a
particular patient is on usually a
physician bases their language on what
they've read in the textbook about
ventilators in the general sense while
the RT basis the language on the
specific ventilator being used whenever
there seems to be a disagreement I
advised just
discussing the specifics of the form of
ventilation being delivered and usually
you'll discover that you are both
describing the same thing patterns of
ventilation also known as ventilator
modes can be partially defined based on
the following three variables the
trigger variable the control variable
and the cycling variable I will discuss
each one at a time the trigger variable
defines how the ventilator determines
when to initiate a machine driven breath
for patients with no spontaneous
respirations the passage of a specific
duration of time is a trigger for breath
for patients with spontaneous
respirations this variable may be set to
either pressure triggers or flow
triggered such that the ventilator
delivers a breath when either a
threshold negative pressure or a
threshold degree of flow is detected
both pressure triggered and flow
triggered ventilation are equally
effective and there is rarely an
advantage to switching from one to the
other with the exception of patients
with COPD who may do better with a flow
trigger triggering options used less
frequently include volume chest wall
electrical impedance and motion the
control variable defines what aspect of
inspiration is the primary variable
controlled by the ventilator during
inspiration the most common options
include pressure controlled and flow
control in pressure controlled
ventilation the pressure delivered to
the airway is constant regardless of
airway resistance or compliance
therefore if pressure is preset and
constant changing resistance or
compliance results in changing tidal
volumes in flow controlled ventilation
the flow is controlled as a function of
the preset tidal volume and the
additional designation of flow pattern a
flow pattern also known as flow contour
will be discussed later in this lecture
as well as in lecture 8 if flow is
predetermined changes in resistance or
compliance results in changes in area
pressures it should be noted that most
clinicians and even many textbooks refer
to flow controlled volume cycle
ventilation
such as most forms of assist control and
simv as quotes volume controlled
although this seems like a logical
description as the clinician is directly
specifying and thus controlling the
total volume to be delivered the
ventilator is actually controlling the
flow rates
some people would therefore state that
usage of the term biocontrol is
technically erroneous while others would
brush off as criticism as
inconsequential semantics the cycling
variable now defines what signals have
been delayed are to terminate
inspiration for example with volume
cycle ventilation the ventilator will
cease inspiration after a preset volume
has been delivered with flow cycle
ventilation the ventilator will cease
inspiration after airflow drops below a
preset threshold typically approximately
25% peak flow ventilators can also be
time cycled or pressure cycled the
recycling variable has some overlap with
the control variable and as kind of
mentioned already the two are frequently
used interchangeably although some would
say this is incorrect usage of the terms
so here is a summary of common options
for the three mode variables a mode can
either be time triggered flow triggered
or pressure triggered it can be flow
control or pressure controlled and it
can be flow cycled volume cycled or time
cycled typical settings for these
variables are listed to the rights to
add to the confusion of ventilator modes
you should also be familiar with the
terms volume targeted and pressure
targeted volume targeted ventilation
describes any mode in which the
clinician is able to ensure the patient
receives a specific title volume
pressure target ventilation describes
any mode which the clinician is able to
ensure the patient's inspiratory
pressure does not exceed a maximum
pre-specified value remember from
lecture two that lung compliance is
equal to the change in volume over the
change of pressure therefore by setting
either volume
or pressure loan compliance will dictate
the value of the remaining parameter for
example in volume targeted ventilation
high compliance of the lung will results
in low area pressures and low lung
compliance will result in high airway
pressure while in pressure targeted
ventilation high lung compliance will
results in high lung volume while low
lung compliance will result in low lung
volume ventilator modes also distinguish
between three different types of breaths
mandatory breaths are triggered by the
ventilator and receive full level of
support the number of mandatory breaths
per minute is dictated by the
respiratory rate set by the clinician
assisted breaths are triggered by the
patient and may receive either full
support or only partial supports finally
spontaneous breaths are those which the
patient takes which do not trigger any
response from the ventilator and
therefore receive no support most modes
allow fully supported assisted breaths
to count as mandatory breaths in order
to improve patient comfort prevent
patient ventilator to synchrony and
prevent hyperventilation there are four
basic common ventilator modes used in
the ICU which I will talk about in this
lecture
they are assist control synchronized
intermittent mandatory ventilation more
commonly known as simv a pressure
controlled ventilation and the pressure
support ventilation a fifth common mode
is controlled mandatory ventilation or
CMV however as this mode usually
requires deep sedation and paralysis it
is rarely done outside of the o R and I
won't be talking about it more at this
time as I mentioned at the beginning of
this lecture there is a severe lack of
consistency with mode terminology
therefore what I am referring to as
pressure controlled ventilation may be
called something different by other
clinicians also in the common usage the
terms assist control and simv imply
volume targeted ventilation however our
pressure targeted assist control and
simv also exist though they are much
more rarely used
to best understand the differences
between these four ventilator modes
I will first provide a brief description
of the mode as well as its possible
options for the trigger control and
cycling variables I will then display
graphs of airflow airway pressure and
volume as functions of time here each
hash mark along the horizontal axis will
represent one second peep for all
examples will be set at five centimeters
of water lastly I will list the
advantages disadvantages and indications
for each mode the first mode I will
discuss is assist control this mode has
a mix of mandatory and fully supported
assisted breaths all breaths once
triggered are treated the same and have
a consistent tidal volume the trigger
for assist control may be either time
pressure or flow the control variable is
technically flow though as mentioned
before many people will refer to this as
volume control and the cycling variable
is typically time here is the typical
appearance of flow pressure and volume
graphs for the assist control mode in
assist control the ventilator senses an
inspiratory effort by the patient and
responds to each by delivering a preset
tidal volume the patient is able to vary
respiratory rate but a backup rate is
set to prevent hypoventilation such that
if a certain period of time passes
without the patient initiating a breath
the ventilator will give one in this
particular example in the pressure
tracing you can see the small downward
deflections just before the first third
and fourth breaths these are the results
of the patient trying to take a breath
on his own
this small but relatively negative
pressure triggers the ventilator to
deliver a fully supportive breath the
second breath has no such proceeding
downward deflection but the ventilator
has given a breath anyway because the
maximum allowable time without a breath
has elapsed in this case you can see
that just over four seconds elapses
between the moment when the first breath
is pressure triggered by the patient
and the moment when the second breath is
time triggered by the ventilator thus
you can infer that the backup rate for
this patient is probably about fourteen
breaths per minute also you can see that
the clinician has preset the total
volume to be about 600 milliliters the
ventilator then calculates the necessary
flow rate to provide that tidal volume
in the amount of time designated for
each breath advantages of assist control
are that it guarantees a minimum minute
ventilation and requires low work of
breathing on the part of the patient
disadvantages are that it can lead to
respiratory alkalosis and auto-peep and
hypotension in hyperventilating patients
now this is because every patient
triggered breath receives a full level
of support
therefore if the e-rate is set to 14 and
the patient suddenly begins to breathe
at a rate of 28 the patient's minute
ventilation will literally double
leading to a drop in arterial pco2 and
significantly increased airway pressures
indications for assist control are
critically ill patients requiring full
of vent supports and in whom
fluctuations in tidal volume are
undesirable as this control is overall
the most commonly used vent mode in the
world let's now talk about simv
although simv can be used without
pressure support it is very rarely done
so the following will assume that
pressure support is being used SI mb is
a mix of mandatory breaths
some of which are synchronized with
spontaneous breaths and assisted breaths
the mandatory non synchronized breaths
are time triggered flow controlled and
time cycled while the assisted breaths
which are synchronized with the
mandatory breaths are pressure or flow
can trigger flow controlled and time
cycled lastly the non synchronous
assisted breaths are pressure or flow
triggered pressure controlled and flow
cycled these non synchronized assisted
breaths are only partially support
but in this example of simv the first
and fourth inflation's are synchronized
pressure triggered fully supportive
breaths tidal volume has been set for
approximately 600 milliliters the second
third and fifth inflation's are
spontaneous non synchronized breaths
assisted by pressure support in this
particular case pressure support has
been set for 10 centimeters of water
which is the difference between the
maximum inspiratory pressure on the
partially supported breaths and the peep
one advantage of simv is that like
assists control it guarantees a minimum
minute ventilation
however unlike assist control it
generally leads to lower mean airway
pressure and can provide a wide range of
respiratory support depending upon the
set respiratory rates a major
disadvantage of Si and B is that it
requires more worker breathing on part
of the patients I've also heard multiple
clinicians and textbooks claim that simv
can reduce cardiac output in patients
with LV dysfunction however I am unaware
of both the original source of this
claim and the mechanism by which this
would occur so therefore I can't vouch
for its accuracy the indication for Si
MB is a critically ill patient who
requires a relatively high level of vent
support but who are hyperventilating or
otherwise prone to auto peep or high
airway resistance the major difference
between assist control and si and B is
that in assist control spontaneous
breaths in excess of the set respiratory
rate receive full support
while in simv access to spontaneous
breaths receive partial supports
therefore in a patient with no
spontaneous breaths for example a
patient who is deeply sedated and
paralyzed assists control and si MB are
identical the next mode to discuss is
pressure control ventilation this mode
has only mandatory breaths in its more
common formulation and the patient is
unable to trigger the ventilator it
time-triggered pressure controlled and
time cycled you are the flow pressure
and the volume tracings the inspiratory
pressure is set at 25 centimeters of
water the flow starts high with each
inspiration and rapidly declines this is
known as a decelerating flow contour and
with pressure control ventilation it is
simply a consequence of lung mechanics
there are many advantages to pressure
control ventilation it helps prevent
excessive airway pressures avoids
regional alveolar or distension which
can lead to lung injury and worsened VQ
mismatch and may lead to earlier
liberation for mechanical ventilation
unfortunately it is very uncomfortable
and requires deep sedation plus -
paralysis it is also unable to guarantee
a minimum minute ventilation the major
indication for pressure control
ventilation is a particularly high risk
of Barrow trauma the final mode I will
discuss in any detail is pressure
support ventilation in pressure support
ventilation there are no mandatory
breaths thus every breath must be
triggered by the patient by pressure or
flow it is pressure controlled and flow
cycled in this example the pressure
support is set at approximately 15
centimeters of water remember that the
pressure support will be equal to the
maximum inspiratory pressure minus the
peep which in all these examples has
been set at five also notice in the flow
tracing how inspiration is terminated by
removal of the precious port once the
flow drops to 25 percent its maximum the
major advantage of pressure support
ventilation is that it is probably the
most comfortable mode for the awake
conscious patient there are many
disadvantages including the patient must
trigger each breath a minimum minute
ventilation cannot be guaranteed it is
associated with poor quality sleep than
other modes and is generally incapable
of providing full ventilator support its
indications are conscious patients
and as a stepping stone immediately
prior to excavation here is just one
more relationship between vent modes
which may help you to understand them a
little bit better so pressure support
ventilation with peep BiPAP or BiPAP as
it may discussed in the last lecture and
si MB with peep and with respiratory
rate set to zero are all equivalent
they will provide the exact same form of
ventilatory support to a patient recent
advances in technology have given rise
to an entirely new class of ventilator
modes known as dual control modes dual
control modes use instantaneous feedback
to control aspects of lung volume and
airway pressure simultaneously remember
that traditional modes target only
volume or pressure but not both and
allowed lung compliance to fully dictate
the other parameter examples of dual
control modes include pressure regulated
volume control this mode is a form of
pressure control ventilation in which
the pressure limit is continuously
adjusted in small increments to maintain
a tidal volume as close as possible to
the desired volume some ventilators have
a setting known as auto flow which is
very similar to prvc there's also volume
support this is a form of pressure
support ventilation when the amount of
pressure support is adjusted up or down
with each breath in order to maintain a
minimum minute ventilation and minimum
tidal volume there's volume assured
pressure support this is a form of
pressure support ventilation where the
ventilator will provide additional
airflow near the end of an individual
inspiration if it predicts that the
tidal volume from that inspiration will
otherwise fall below a preset minimum
allowed volume dual control modes are
somewhat more complicated to use and
understand than the other modes we've
discussed in more detail and are best
left to clinicians and respiratory
therapists with significant ventilator
experience
I hope you found this lecture on vent
modes to be both interesting and useful
please continue to lecture aids on
ventilator options which will further
round out your understanding of how
ventilators work and how they can be
programmed to treat patients with a
variety of clinical disorders
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you