Ventilator Basics.

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 auto- PEEP

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

We will focus exclusively on volume-preset mode ventilation The primary trials in ARDS were done in volume- present 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

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

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

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

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: Important Points: 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 Presist
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

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

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

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

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

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

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)?

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

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 Recall that Pplat = Pelast + PEEP Presist = Ppeak – Pplat 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 end- inspiration 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 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 characteristic of obstruction. Recall that normal expiration should be passive and drop to 0 flow very quickly Peak pressure Plateau pressure End-inspiratory pause

The large Presist here is almost 60 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 Peak pressure Plateau pressure End-inspiratory pause

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 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 Peak pressure Plateau pressure End-inspiratory pause

Example: Decreased Lung Compliance
The small Presist here 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 Peak pressure Plateau pressure

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 end- expiration. 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

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 end- expiration. 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 end- expiratory pause

Presence of flow at end-expiration 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 End-expiratory port occlusion resulting in no flow

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 H2O
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 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 Ventilator After pneumothorax Before pneumothorax
450 mL 450 mL Before pneumothorax 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

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