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Ventilator Basics
Goals
 Understand volume-preset mode of ventilation
 Understand the difference between SIMV and AC
 Understand the meaning of plateau pressure and peak
pressure in volume mode ventilation
 Learn how to use peak pressure and plateau pressure as
additional “vital signs” in a ventilated patient
 Learn how use interpret expiratory waveform to diagnose
obstruction in a patient on a ventilator
 Learn how to measure auto-PEEP and to decrease autoPEEP
Volume-Preset Mode
Ventilation
 Volume-preset mode ventilation are modes where the
tidal volume is determined by the clinician
 In comparison, pressure-preset mode ventilation are
modes where the pressure is specified by the clinician.
 In pressure-preset model ventilation, the tidal volume that
the patient receives is determined by mechanics of the
patient’s lung and airways in response to the pressure
specified by the clinician
 E.g., In non-compliant lungs, a given pressure setting would
result in less tidal volume delivered to the patient
Volume-Preset Mode
Ventilation
 We will focus exclusively on volume-preset mode
ventilation
 The primary trials in ARDS were done in volumepresent mode ventilation
 It is easier to measure mechanical properties of the
respiratory system
 Measurement of peak pressure and plateau pressure
 It is easier to understand how to manipulate the
ventilator
Volume-Preset Mode
Ventilation
 AC = Assist Control
 Tidal volume is set by the clinician as well as the
respiratory rate
 Control means that the ventilator delivers the tidal volume
at the set respiratory rate
 Any additional breaths over the respiratory rate set by
the clinician is guaranteed to be the same tidal volume
set by the clinician
Volume-Preset Mode
Ventilation
 AC = Assist Control
 Example: AC with tidal volume 450 mL, respiratory rate
24 breaths/minute
 In a paralyzed patient, the patient receives 24
breaths/minute with each breath at a tidal volume of 450
mL
 In a non-paralyzed patient with the same ventilator
setting, the patient is forced to breathe 24 breaths/minute
at a tidal volume of 450 mL.
 If a patient breathes above the rate of 24 breaths/minute,
each of those additional breaths are also guaranteed to be
at 450 mL
Volume-Preset Mode
Ventilation
 SIMV = Synchronized intermittent mandatory
ventilation
 Tidal volume is set by the clinician as well as the
respiratory rate
 Any additional breaths over the respiratory rate set by
the clinician is not guaranteed to be the same tidal
volume set by the clinician
 The tidal volume of the additional breaths are dependent on
patient effort
 Because the extra breaths are dependent on patient
effort, this mode of ventilation is not recommended in
patients with sepsis/ARDS because they put additional
strain on the patient
Volume-Preset Mode
Ventilation
 SIMV = Synchronized intermittent mandatory ventilation
 Example: SIMV with tidal volume 450 mL, respiratory rate 24
breaths/minute
 In a paralyzed patient, the patient receives 24 breaths per
minute with each breath at a tidal volume of 450 mL
 In a non-paralyzed patient with the same ventilator setting, the
patient is guaranteed 24 breaths/minute with a tidal volume of
450 mL during those mandatory breaths
 However, if a patient breathes above the rate of 24 breaths/minute,
each of those additional breaths are not guaranteed to be at 450
mL and the tidal volume generated depends on the patient’s effort
Oxygenation
 To improve the oxygenation of the patient on a
ventilator, you can either increase the FiO2 or the PEEP
 Increase FiO2
 Increased FiO2 helps to increase oxygenation by increasing
the oxygen gradient between the air in the alveoli and the
blood
 Increase PEEP
 PEEP helps to recruit alveoli, thus helping to improve
oxygenation
 This is important in ARDS
 A caveat is that too high of PEEP can potentially lower
venous return and as a result, stroke volume and cardiac
output thus causing hypotension
Ventilation
 To adjust the ventilation of the patient on a ventilator,
you can either increase the tidal volume or the
respiratory rate
 Increase tidal volume
 A caveat is that this may increase the plateau pressure
 In general, in ARDS, it is essential to decrease the tidal
volume if the plateau pressure is > 30 cm H2O
 Refer to ARDS lecture slides and subsequent discussion here
 Increase respiratory rate
 A caveat is that if the patient is already breathing above the
respiratory rate that you are setting on the ventilator, then
ventilation will not be improved
Volume Preset Modes

Examples:



AC (assist control)
SIMV (synchronized
intermittent mandatory
ventilation)
Important Points:



Tidal Volume set
Flow rate set
Pressure in the system
develops in response
to the volume pushed
in by the ventilator
Volume Preset Modes

Refer to the pressure-time graph
(top graph) for this discussion

Pressure response depends
on the respiratory system



In patients with a very stiff lung,
the peak pressure for a given
tidal volume will be higher
Similarly, in patients with very
tight airways (high resistance),
the peak pressure for a given
tidal volume will be higher
Note that pressure increases
during the inspiratory phase
when the ventilator pushes in the
tidal volume set by the clinician

This is the active phase of
ventilator where the ventilator
pushes in the tidal volume
specified by the clinician
Volume Preset Modes

Refer to the flow-time
graph (bottom graph)

Note that the expiratory
phase is passive

The flow drops to zero
quickly during the
expiratory phase of
ventilation

Remember this as we will
see what obstruction
looks like in the expiratory
phase of ventilation
Pressure Response
 Given a specific tidal volume, the pressure it takes to
overcome the respiratory system is equal to the pressure
needed to overcome the resistance of the airways (Presist)
and the pressure it takes to expand the alveoli against the
elastic recoil of the lung and the chest wall (Pelast)
 Total pressure (Ppeak) = Presist + Pelast + PEEP
 This is the total pressure measured at the end of inspiration
 Assuming the patient is not actively breathing against the ventilator
 The patient should be passive during this measurement otherwise it
will not be accurate
 For the following discussion, we will leave the PEEP out for simplicity
Pressure Response
 Total pressure (Ppeak) = Presist + Pelast + PEEP
 We are leaving PEEP out during this discussion for
convenience
 Presist gives you insight into the resistance of the airways
in the respiratory system
 Presist depends on flow, according to Ohm’s law
 Presist = flow x resistance
 Pelast gives you insight into how difficult it is to inflate the
alveoli
Pressure Response
 Total pressure (Ppeak) = Presist + Pelast
 This is assuming that there is no PEEP
Presist
Pelast
 On a ventilator, we can measure Ppeak and Pplat
 Pplat = Pelast + PEEP
 Therefore, we can calculate Presist = Ppeak - Pplat
Utility of Pressure Response

Total pressure (Ppeak) = Presist + Pelast + PEEP
 Knowing what happens to Presist and Pelast in your
patient allows you to assess another “vital” sign in the
ICU
 Knowing Presist can tell you that there is something wrong
with the airways in your patient
 Knowing Pelast can tell you that there is something wrong
with the compliance of the lungs in your patients
Utility of Pressure Response

Total pressure (Ppeak) = Presist + Pelast + PEEP
 This gives you a window into what is wrong with your
patient’s airways and/or lungs
 And, if you track this data over time, it gives you an
idea of whether your interventions are working or not
Utility of Pressure Response

Total pressure (Ppeak) = Presist + Pelast + PEEP
 Following Presist over time
 If Presist increases over time in a patient with COPD, what
does that tell you about the disease?
 COPD airways disease is worsening
 If Presist decreases over time in a patient with asthma as
you are giving albuterol, what does that tell you about
your intervention?
 Bronchoconstriction in asthma is improving with albuterol
Utility of Pressure Response

Total pressure (Ppeak) = Presist + Pelast + PEEP
 Following Pelast over time
 If Pelast increases over time as you watch an infiltrate grow
on a patient’s chest x-ray, what does that tell you about what
is going on?
 Pneumonia worsening in the patient causing decreased lung
compliance
 If Pelast decreases over time as you diurese a patient with
CHF, what does that tell you about your intervention?
 Lung compliance improving as you diurese the pulmonary
edema out of the patient’s lungs
Utility of Pressure Response

Therefore, there is utility to measuring Presist and Pelast in
monitoring a ventilated patient

If only there was a way to measure these pressure
responses
Utility of Pressure Response

Total pressure (Ppeak) = Presist + Pelast + PEEP
 Ppeak is measured at end inspiration
 This is the highest pressure that is reached after the tidal volume
is pushed in
 The patient must NOT be actively breathing while the pressure is
measured
 An elevated Ppeak tells you that something is wrong with the
patient’s airways and/or lungs but does NOT tell you which one is
the problem
 Is the problem in the resistance of the airways?
 Is the problem in the elastance of the lungs?
 If Ppeak > 30 cm H2O, you should starting trying to figure out what is
causing the pressures to be so high
 Similarly, if there is an increase in Ppeak, you should try to find out
why there is an increase in Ppeak
Utility of Pressure Response

Total pressure (Ppeak) = Presist + Pelast + PEEP
 Once Ppeak is elevated, you should try to figure out
which part is the problem (i.e., it takes more pressure to
ventilate your patient which is NOT a good thing)
 Given the exact same ventilator settings, it is NOT a
good thing that more pressure is required to ventilate your
patient
 Is the problem with the airways (Presist) and/or with the
lungs/chest wall (Pelast)?
Utility of Pressure Response







Total pressure (Ppeak) = Presist + Pelast + PEEP
Ppeak = (Flow x resistance) + Pelast + PEEP
 Presist = Flow x resistance by Ohm’s law
If you put a pause at end-inspiration, the flow drops to zero
(i.e., Presist = 0), allowing you to measure Pelast + PEEP.
This end-inspiratory pause pressure is called the plateau
pressure (called static pressure on Care Connect)
Designated as Pplat
Pplat = Pelast + PEEP
If we ignore PEEP or if PEEP = 0, then Pplat = Pelast
Utility of Pressure Response
Total pressure (Ppeak) = Presist + Pelast + PEEP
 Pplat, however, can still be considered a measure of how
hard it is to overcome the elastance of the lung
 Notice that the Pplat is always lower than the Ppeak
 For our discussion here, we can think of Pplat as Pelast

 The next slide will demonstrate what it looks like on the
ventilator and how it’s done.
Pressure at airway opening, Pao
 Pao = Pressure at airway
opening needed to expand
the lungs and overcome
airways
 At the highest pressure,
 Ppeak = Pelast + Presist + PEEP
 Pelast = pressure needed to
expand alveoli against the
elastic recoil of the lung and
chest wall
 Presist = pressure needed to
drive gas across inspiratory
resistance
 PEEP = pressure in alveoli
present before inspiratory
flow
 Note how Ppeak is measured at end-inspiration (point b, highest pressure)
 Inserting an end-inspiratory pause (point x), the measured pressure drops. This
measured pressure is the Pplat = Pelast + PEEP
Pressure at airway opening, Pao
 Pelast = pressure needed to
expand alveoli against the
elastic recoil of the lung and
chest wall
 PEEP = pressure in alveoli
present before inspiratory
flow
 Pplat = Pelast + PEEP
 Note how Ppeak is measured at end-inspiration (point b)
 Inserting an end-inspiratory pause (point x), the measured pressure drops. This
measured pressure is the Pplat = Pelast + PEEP
How to find Ppeak and Pplat on
Care Connect
 The Ppeak and Pplat are measured by the respiratory therapist as
part of their assessment of the patient
 When a patient develops new respiratory distress, you can
have the respiratory therapist measure the Ppeak and Pplat for
you to give you an idea of what is going on with your patient
 You can find this information under the RT Data Flowsheet in
Care Connect
 It is under the Ventilator section of the RT Data Flowsheet
 Care Connect calls Ppeak the PIP (Peak inspiratory pressure)
 Care Connect calls the Pplat the static pressure
Presist
 Presist = pressure needed to drive gas across inspiratory
resistance
 Recall Ohm’s law
 Pressure = flow x resistance
 Dependent on flow rate which is set by the physician
 Dependent on the resistance of the airways
 Once we measure the Ppeak and the Pplat, we can
determine the Presist
 This is simply done by substracting Pplat from Ppeak
Presist
 Presist = pressure needed to drive gas across inspiratory
resistance

Ppeak = Presist + Pelast + PEEP

Ppeak = Presist + Pplat


Presist = Ppeak – Pplat


Recall that Pplat = Pelast + PEEP
Recall, that we can measure Ppeak and Pplat
In essense, the pressure needed to overcome the resistance
of the airways is the difference between the Ppeak and the
Pplat
Presist
 Presist = pressure
needed to drive gas
across inspiratory
resistance
 Presist = Ppeak - Pplat
Presist
 Presist = Ppeak – Pplat
 If there is a large difference between the Peak pressure
and the plateau pressure (typically greater than 10),
then there is likely increased resistance in the airway
(an obstruction)
 If Presist > 10 cm H2O, then there is likely a problem with
the resistance of the airways
Pelast
 Pelast = pressure needed to expand alveoli against the
elastic recoil of the lung and chest wall
 Pelast = DV x Ers, where Ers = elastance of the
respiratory system
 Dependent upon tidal volume, set by the physician
 The higher the tidal volume set, the higher the Pelast
 In essence, it takes more pressure to inflate a balloon to a
higher volume
 Dependent upon Ers, in a sense how “stiff” the
respiratory system is
 For a given tidal volume, the stiffer the lung, the more
pressure is required
 In essence, a stiffer balloon will take more pressure to inflate
to the same volume as a more compliant balloon
Plateau Pressure, Pplat
 Pplat = Pelast + PEEP
 Measured by inserting a
inspiratory pause at endinspiration and allowing the
pressure to fall from the Peak
pressure
 Note that Pplat should be less than
Ppeak.
 If Pplat > Ppeak then the patient
may be actively breathing out
while the plateau pressure is
being measured leading to
incorrect measurement
Example: Airway Obstruction
 The large Presist here represents a
Peak pressure
Plateau pressure
case of increased resistance in the
airways, in this case status
asthmaticus.
 Presist = Ppeak – Pplat
 Note the large difference
between Ppeak and Pplat
 Note also the prolonged
expiratory flow that does not
reach zero before the next breath
is delivered in the diagram on the
bottom.
 This prolonged expiratory flow is
End-inspiratory pause
characteristic of obstruction.
 Recall that normal expiration
should be passive and drop to
0 flow very quickly
Example: Airway Obstruction
 The large Presist here is almost 60
Peak pressure
Plateau pressure
cm H2O.
 Presist = Ppeak – Pplat
 This is significantly greater than
10 cm H2O
 Suggestive of obstruction (in
this case, status asthmaticus)
 Note that the Pplat is less
than 30 cm H2O
 This is suggestive that the
lung compliance is normal
End-inspiratory pause
Example: Airway Obstruction
 Note also the prolonged
Peak pressure
Plateau pressure
expiratory flow that does not
reach zero before the next breath
is delivered in the diagram on the
bottom.
 This prolonged expiratory flow is
characteristic of obstruction.
 Recall that normal expiration
should be passive and drop to
0 flow very quickly
 The fact that the flow does not
reach 0 L/s before the next
breath is initiated predisposes the
patient to develop auto-PEEP
End-inspiratory pause
Example: Decreased Lung Compliance
 The small Presist here
Peak pressure
Plateau pressure
represents a case of
increased Pelast
 Presist = Ppeak – Pplat
 Note the minimal
difference between Ppeak
and Pplat
 This suggests that it does
not take much pressure to
overcome the airways
 In this case, the plateau
pressure was measured to
be 50 cm H2O which is
elevated suggestive of
decreased lung
compliance
Example: Decreased Lung Compliance
 Note that the expiratory
flow drops quickly to 0 L/s
before the next breath
 This is typical of normal
airways and suggests no
significant airway
obstruction
Expiratory Flow Example
 Emphysema
 Initial high expiratory flow is
caused by the collapse of the
airways
 The final prolonged slow
expiratory flow is due to the
reduced elastic recoil of the
emphysematous lung
 Again, note that the expiratory
flow does not reach zero before
the next breath is given
 This is secondary to obstruction
and can lead to auto-PEEP
Auto-PEEP
 What is auto-PEEP?
 Auto-PEEP is PEEP that develops when there is incomplete
expiration before initiation of the next breath
 The auto-PEEP effect occurs when there is insufficient time for the
respiratory system to return to functional residual capacity by endexpiration.
 Short expiratory times, high minute volumes, and increased
expiratory resistance contribute to auto-PEEP, but all of these need
not be present.
 Auto-PEEP is present in the majority of ventilated patients with
asthma and COPD (and in many during spontaneous
breathing), but it is also seen in ARDS and other settings with
high minute ventilation.
Why Is Auto-PEEP Harmful?
 Auto-PEEP increases the work of breathing and impairs the
patient's ability to trigger the ventilator.
 The patient must overcome the auto-PEEP before the ventilator can be
triggered for a breath.
 Severe auto-PEEP can decrease venous return causing
hypotension and pulselessness
 In many regards auto-PEEP acts like PEEP to impede venous
return, heighten the risk of barotrauma, and improve oxygenation.
 For these reasons, it is imperative to monitor routinely the presence
and amount of auto-PEEP in mechanically ventilated patients.
Determining Auto-PEEP
 Auto-PEEP is present when the expiratory flow tracing
reveals persistent end-expiratory flow
 Expiratory flow that does not decrease to 0 L/s before the
next breath is initiated.
 This leads to hyperinflation and auto-PEEP
Time at which next breath is initiated
• Note that expiratory flow does not
reach 0 L/s before the breath is
initiated
Persistent end-expiratory flow
Before the next breath is initiated
Determining Auto-PEEP
 End-expiratory port occlusion (end-expiratory pause)
allows determination of auto-PEEP.
 This is like the end-inspiratory pause used to determine the
plateau pressure, except this is performed at endexpiration.
 Note: The patient should be passive during the process,
otherwise the measurement of auto-PEEP will be
inaccurate
 The patient can not be actively inspiring or expiring during the endexpiratory pause
Determining Auto-PEEP
Presence of flow
at end-expiration
End-expiratory
port occlusion
resulting in no flow

Auto-PEEP determined
by the end-expiratory
port occlusion technique
(end-expiratory pause)

Note that the PEEP is
actually 10 cm H2O
 In this case, the PEEP is
caused by auto-PEEP
as the ventilator was set
to 0 PEEP

In this case, auto-PEEP is
suspected because the flow
was not 0 L/s before the
next breath is initiated
Ways to Decrease Auto-PEEP
 Because auto-PEEP is due incomplete expiration before the next breath is initiated,
maneuvers to decrease the auto-PEEP are directed at helping to increase complete
expiration
 Low tidal volume ventilation, especially important in patients with
asthma/COPD and also in ARDS
 By decreasing the tidal volume, there is less “breath” to empty with each
expiration thus lowering the amount of incomplete expiration
 Can consider decreasing the respiratory rate
 Less helpful, as the innate respiratory drive for a patient with respiratory
failure is likely very high
 As a last resort, may have to increase sedation or initiate paralysis to
decrease the respiratory rate of the patient
 Increase inspiratory flow in volume-preset modes.
 This decreases the time needed to push in the tidal volume thus increasing
the time available for more complete expiration
 Medications to decrease airway obstruction, if obstruction is present
 Bronchodilators and systemic steroids in asthma/COPD
Difficulty Triggering Vent with Auto-PEEP
 Patients that develop auto-PEEP have more difficulty
with triggering an inspiratory breath
 This is because the patient has to over the auto-PEEP in order
to trigger the breath
 Example: Assume presence of auto-PEEP which is measured to
be 8 cm H2O
 If triggered sensitivity is -2 cm H2O meaning that the patient has to
inspire to bring down the pressure to -2 cm H2O before the ventilator
gives a breath, the patient has to inspire hard enough to bring down
the 8 cm H2O of auto-PEEP to -2 cm H2O before the ventilator will
initiate the breath
 If the auto-PEEP was actually 0 cm H2O, the same patient would have
to work less hard to initiate the next breath by the ventilator
Difficulty Triggering Vent with Auto-PEEP
 In patients with auto-PEEP, increasing the PEEP on the
ventilator can help with triggering breaths and lowering the
work of breathing
 Example: Assume presence of auto-PEEP which is measured
to be 8 cm H2O
 Once the auto-PEEP is measured, setting the applied extrinsic
PEEP (PEEP set on the ventilator) to 50-85% of the measured
auto-PEEP can help to trigger breaths more easily
 This will not, in general, increase auto-PEEP
 Do not set the PEEP (on the ventilator) higher than 85% of
auto-PEEP as this can lead to increased auto-PEEP
What To Do If Auto-PEEP Results in
Pulselessness
 If auto-PEEP becomes so high that venous return is
interrupted and the patient becomes pulseless, unplug the
ventilator from the endotracheal tube
 This will allow a prolonged expiratory phase to empty the
hyperinflated lung, reducing auto-PEEP
 During this time, the ventilator needs to be adjusted to
ensure that auto-PEEP does not occur again
 Decrease tidal volume
 Increase inspiratory flow in volume-preset modes of ventilation
 Consider decreasing respiratory rate
 Again this may not be effective if the patient has a high respiratory
drive
 May have to sedate/paralyze the patient
Etiologies of Increased Presist = Flow x Resistance
 High flow rate
 Bronchospasm
 COPD/Asthma
 Secretions
 Kinked/obstructed tubing, including endotracheal tube
 Airway edema
 Airway tumor/mass
 Airway foreign body
Etiologies of Increased Pelast = DV x Ers
 High tidal volume
 Chest wall





Kyphoscoliosis
Rib deformity
Pleural disease
Obesity
Abdominal distention (e.g., abdominal compartment syndrome)
 Lung






Interstitial lung disease
Lung resection
Atelectasis
Pulmonary edema, including cardiogenic and ARDS
Pneumonia
Alveolar hemorrhage
Pressures
 In general, maintain plateau pressures below 30 cm
H 2O
 Keep plateau pressure < 30 cm H2O to decrease mortality
in ARDS
 If there is an increase in peak pressures, measure the
(Ppeak – Pplat) difference to characterize the new
process causing the increased pressure
 Note: Peak and plateau pressures have to be measured in
volume-preset modes of ventilation
 If (Ppeak – Pplat) > 10 cm H2O, suggestive of an increase
in Presist or obstructive process
Example 1
 Previous day
 Peak pressure is 15 cm H2O
 Plateau pressure is 14 cm H2O
 Today with no changes in tidal volume or ventilator
settings
 Peak pressure is 40 cm H2O
 Plateau pressure is 38 cm H2O
 What is the problem?
 Is it a lung compliance problem or a resistance problem?
 If there is a problem, what could be the causes?
Example 1
 This is primarily a compliance problem in that the
compliance of the lungs worsened overnight
 The peak pressure increased from 15 cm H2O to 40 cm
H2O overnight
 This increase in peak pressure should alert you that
something is amiss
 Next calculate the Presist = Ppeak – Pplat
 Presist = 40 – 38 = 2 cm H2O
 Since there is minimal difference between the peak and
plateau pressures, there is no problem with the airways
 In this case, the explanation for the increased peak
pressure is that the compliance of the lungs worsened
overnight
Example 1
 You can also tell that there is a compliance problem
because the plateau pressure increased from 14 cm
H2O to 38 cm H2O overnight
 Since the tidal volume did not change, an increase in
plateau pressure suggest that it takes more pressure to
distend the lungs to the same tidal volume
 In this case, the patient aspirated leading to pneumonia
and development of ARDS overnight
 Similarly, this may also be seen in a patient if the patient
developed a massive myocardial infarction leading to
congestive heart failure overnight
Example 1
 Another possible explanation for the worsening plateau
pressure (in the right clinical setting) may be abdominal
compartment syndrome
 Abdominal compartment syndrome develops typically
in trauma cases where the patient has had lots of fluids
and transfusions, leading to distended abdomen
 Abdominal compartment syndrome can worsen the
compliance of the lungs because the diaphragm
separates the abdomen from the thorax
 A sudden increase in the amount of fluid in the abdomen
can make it difficult to distend the lungs causing the lung
compliance to worsen
Example 1
 Suppose that the plateau pressure increase from 14 cm H2O to 38 cm
H2O was due to development of ARDS in the setting of septic shock.
 Again, this suggests that the lung compliance has worsened.
 However, is there something that we should do with that information?
 Recall that this is a case of ARDS (acute respiratory distress
syndrome)
 In ARDS, the first thing to do in an intubated patient is to lower
the tidal volume to 6 mL/ideal body weight
 However, if the plateau pressure is > 30 cm H2O, then the next
thing to do is to lower the tidal volume by increments of 1
mL/ideal body weight until the plateau pressure is < 30 cm H2O
 Please refer to the ARDS slides for more detail
Example 2
 Previous day
 Peak pressure is 15 cm H2O
 Plateau pressure is 14 cm H2O
 Today with no changes in tidal volume or ventilator
settings
 Peak pressure is 40 cm H2O
 Plateau pressure is 14 cm H2O
 What is the problem?
 Is it a lung compliance problem or a resistance problem?
 If there is a problem, what could be the causes?
Example 2
 This is primarily an airway resistance problem in that
the airway resistance worsened overnight
 The peak pressure increased from 15 cm H2O to 40 cm
H2O overnight
 Again, this increase in peak pressure should alert you that
something is amiss
 Next calculate the Presist = Ppeak – Pplat
 Presist = 40 – 14 = 26 cm H2O
 Since there is large difference (greater than 10 cm H2O)
between the peak and plateau pressures, there is a problem
with the resistance of the airways
 In this case, the explanation for the increased peak
pressure is that the airway resistance worsened overnight
Example 2
 You can tell that there is no problem with compliance in
this case because the plateau pressure did not change.
It remained unchanged at 14 cm H2O overnight
 Because the Presist increased dramatically overnight, you
need to determine what is the cause of this airway
problem
 The patient’s asthma could have worsened
 There could be a kink in the endotracheal tube
 There may be a mucus plug in the endotracheal tube
Example 3
 Previous day
 Peak pressure is 15 cm H2O
 Plateau pressure is 14 cm H2O
 Today with no changes in tidal volume or ventilator
settings
 Peak pressure is 60 cm H2O
 Plateau pressure is 40 cm H2O
 What is the problem?
 Is it a lung compliance problem or a resistance problem?
 Could there are two problems?
Example 3
 In this case, this is a problem with both worsening
compliance and increased airway resistance
 The peak pressure increased from 15 cm H2O to 60 cm
H2O overnight
 Again, this increase in peak pressure should alert you that
something is amiss
 Next calculate the Presist = Ppeak – Pplat
 Presist = 60 – 40 = 20 cm H2O
 Since there is large difference (greater than 10 cm H2O)
between the peak and plateau pressures, there is a problem
with the resistance of the airways
 In this case, the airway resistance did increase overnight
contributing to the increased peak pressure
 But, how about the compliance? Did that change?
Example 3
 There is also a problem with worsening compliance as
the plateau pressure increased from 14 cm H2O to 40
cm H2O overnight
 Given the same tidal volume, an increase in plateau
pressure suggests that it is more difficult to inflate
the lungs to the same volume
 In other words, the patient’s lung compliance also
worsened overnight
Example 4
 What would you expect to see in a tension
pneumothorax?
 Would you see a high Presist?
 Would you see a high Pplat?
 Can you explain your findings?
Example 4
 To answer this question, you have to realize that the
ventilator is not smart and will happily put in the desired
tidal volume even if something has changed in the
patient
 Give it some thought before going to the next slide
Example 4
Ventilator
450 mL
Before pneumothorax
Ventilator
450 mL
After pneumothorax
Example 4
 For instance, assume that the tidal volume that you set was 450
mL
 Overnight, the patient developed a tension pneumothorax and
completely collapses the left lung
 The ventilator happily goes on to push in 450 mL of air with every
breath
 Because the ventilator continues to push in the same tidal volume
(450 mL) into approximately half as much lung, the plateau
pressure increases
 In essence, because the ventilator continues to inflate only the
right lung to 450 mL while before it was trying to inflate both
lungs to 450 mL, the plateau pressure increases