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Mechanical Ventilation

Alexandra Wilson MD

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Ventilator modes and strategies

Indications for ventilation
Modes of ventilation -conventional
Selection of CV mode and settings
Trouble shooting
Non-conventional modes of ventilation*
Complications
Weaning off support

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Indications for support

Impending/acute respiratory failure

Apnea, hypoxia, hypercarbia

Cardiac Insufficiency

eliminate work of breathing
reduce oxygen consumption

Neurologic dysfunction

central hypoventilation/ frequent apnea
inability to protect airway
control of ICH

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Nomenclature

Airway Pressures

Peak Inspiratory Pressure (PIP) peak inspiratory pressure - the highest pressure created on inspiration.
Positive End Expiratory Pressure (PEEP)

Pressure maintained in airways at end of exhalation

CPAP = equal to PEEP used in refer to pt on NIV (CPAP/BIPAP)
Mean airway pressure (MAP)

an average of the airway pressure throughout the respiratory cycle

These are some of the basic terms used with ventilators. CPAP is equivalent to PEEP except the term is usually used when referring to patients who are not intubated (i.e., on nasal CPAP).

PEEP therapy can be effective when used in patients with a diffuse lung disease that results in an acute decrease in functional residual capacity (FRC), which is the volume of gas that remains in the lung at the end of a normal expiration. FRC is primarily determined by the elastic characteristics of the lung and chest wall. In many pulmonary diseases, FRC is reduced because of the collapse of the unstable alveoli. This reduction in lung volume decreases the surface area available for gas exchange and results in intrapulmonary shunting (ie, unoxygenated blood returning to the left side of the heart). If the FRC is not restored, a high concentration of inspired oxygen may be required to maintain the arterial oxygen content of the blood at an acceptable range.

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Nomenclature

Inspiratory Time or I:E ratio

(inspiratory time + inspiratory pause time):expiration
usually set to 1:2 to mimic usual pattern of breathing
in general longer I time improve oxygenation by:

increasing the mean airway pressure
allowing re-distribution of gas from more compliant alveoli to less compliant alveoli

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Nomenclature

But…. increased I time can increase risk of gas trapping, intrinsic PEEP and barotrauma by reducing expiratory time

Tidal Volume: the size of the volume of the breath

Usually set at 7-8 cc/kg

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Nomenclature

FIO2 - fraction of inspired oxygen - the percentage of oxygen from 21% ( room air ) - 100% ( the maximum ).
PS - pressure support – amount of pressure provided to overcome resistance of the endotracheal tube. Helps simulate normal respiration.

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Nomenclature

Trigger: How the vent knows when to give a breath

patient effort
elapsed time

The patient’s effort can be “sensed” as a change in pressure or a change in flow (in the circuit)

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Mechanical Ventilation

What we can manipulate……

Minute Ventilation (increase respiratory rate, tidal volume)
FiO₂
Surface Area “recruitment”

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SIMV

Breaths are synchronized with pt’s effort

(i.e. the vent senses patient’s spontaneous breaths) to deliver rate you set

Patient takes “own” breaths in between
Can use SIMV VC or SIMV PC
improved weaning, prevention of respiratory muscle atrophy ?�

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Assist Control

every breath is fully supported by the ventilator
you set a minimum rate and all triggered breaths above that rate also fully supported.

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Case 1

You have been taking care of a 2 week old infant in the PICU who was admitted with pneumonia 5 days ago. The attending says it is time to start weaning the patient from the ventilator and then walks away. The RT asks you “so what do you want to do doc?”

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Case 1

The patient is currently in a mode where every breath the patient initiates is fully supported with a full pre-set TV. What is this mode called?

A. SIMV volume control
B. Assist control
C. SIMV pressure control.

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Case 1

Can you “wean” the patient in this mode?

A. Yes

B. No

What mode might you want this patient to be in?

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Mechanical Ventilation

If volume is set, pressure varies…..if pressure is set, volume varies…..

….according to the compliance…...

COMPLIANCE =

Δ Volume / Δ Pressure

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Pressure vs. Volume�control

Pressure Limited

You set the pressure (PIP)
Decelerating flow pattern (lower PIP for same TV, improved gas distribution)

Volume Limited

You set Tidal volume, PIP changes with compliance
Guaranteed MV

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Pressure Volume

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Pressure vs. Volume�control

Pressure

tidal volume can change suddenly as patient’s compliance changes
this can lead to hypoventilation or over expansion of the lung

Volume

Changes in compliance can lead to high PIP and barotrauma
Square wave(constant) flow pattern results in higher PIP

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Pressure vs. Volume�control

Just remember you need to choose!

If you set the volume the pressure will change (as compliance varies)

And if you set the pressure the volume will vary (as compliance varies)

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Case 2

15 year old male presents to your ER in west Texas s/p MVA. He was intubated in the field for a GCS of 6. CXR is clear , patient is easy to bag. You are asked to choose mode of ventilation for transport to the trauma center. Do you prefer…

Pressure control
Volume control

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Pressure regulated volume control (PRVC)

The ventilator varies inspiratory pressure in response to the lungs current compliance to deliver preset target TV.
If measured tidal volumes changes, the pressure levels will make small adjustments (3 cmH2O) to maintain the preset tidal volume.
Combines the benefit of a volume mode (“guaranteed minute ventilation”) with the benefits of a pressure mode (decelerating flow pattern)
Can be used in SIMV mode or AC mode

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Pressure support mode

Used to help patient overcome additional work imposed by breathing a patient has to breathe through a narrow long ETT
May be used with SIMV or with CPAP
Patient’s spontaneous breaths are supported by the ventilator with the pressure level you set.

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Set up ( what to do if you are in Timbuktu)

Mode : PC or VC
TV to give 8 cc/kg or PIP to achieve 8cc/kg
Rate infant 30/ child 20 / teen 15
PEEP 4
Fio2 100%

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What to manipulate

Oxygenation

FiO₂
PIP
I-time
PEEP

Ventilation

Rate
Tidal Volume

MV

MAP

The decision to choose volume or pressure as a mode is based on what the clinician is more interested in directly affecting. Volume modes offer a guaranteed minute ventilation while pressure modes allow one to directly manipulate the MAP. Adjusting blood gases with ventilator settings �

Respiratory acidosis: CO2 greater than 45

RR - increasing the rate will help decrease the CO2. �PIP - increasing the PIP will give a larger breath and help decrease the CO2. �Vt - increasing the Vt will give a larger breath to help decrease the CO2.

*** as the CO2 is decreased the pH will increase to assist in resolving the acidosis. �

Respiratory alkalosis: CO2 less than 35

RR - decreasing the rate will increase the CO2. �PIP - decreasing the PIP will give a smaller breath and help increase the CO2. �Vt - decreasing the Vt will give a smaller breath and help increase the CO2.

*** as the CO2 is increased, the pH will decrease to assist in resolving the alkalosis. �

Low PaO2: below 60

PEEP - increasing the PEEP will decrease FRC and keep the alveoli open to improve � oxygenation. Cardiac output should be monitored because an increase in PEEP � can decrease cardiac output. �PIP - increasing the PIP can indirectly improve oxygenation with extra pressures. �Vt - increasing Vt can indirectly improve oxygenation, also.

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Case 3

2 week old infant with RSV

ABG 7.16/70/55/24

How would you describe this blood gas?

A. metabolic acidosis with resp compensation

B. resp acidosis without compensation

C. resp acidosis with metabolic compensation

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Case 3 continued

7.16/70/55/24

The patient is in PC mode with a PIP of 28 and rate of 22 and PEEP of 4 and Fio2 of 40%. The patient’s TV = 10cc/kg

How might you adjust the ventilator?

A. Increase the PIP and Fio2

B. Increase the rate and PEEP

C Decrease the PEEP and Increase the PIP

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Troubleshooting

If it is not working

Look at the patient !!
Listen to the patient !!
Pulse Ox, ABG, EtCO₂
Chest X ray
Look at the vent-to check for changes in compliance/leak (PIP/expired TV)

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Troubleshooting

When in doubt, DISCONNECT THE PATIENT FROM THE VENT, and begin bag ventilation.
Ensure you are bagging with 100% O2(use, but this caution in which group of patients??).
This eliminates the vent circuit as the source of the problem.
Bagging by hand can also help you gauge patient’s compliance

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Troubleshooting

Still not working…..

Right Mode ?
Leak around ETT too large
Does the patient need to be more sedated ?
Does the patient need to be extubated ?

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Troubleshooting

Patient - Ventilator Interaction

Vent must recognize patient’s respiratory efforts (trigger)
Vent must be able to meet patient’s demands (response)
Vent must not interfere with patient’s efforts (synchrony)

In order for a patient to breathe comfortably on the ventilator, their demands must be recognized and quickly met. The delay between recognition of a patient’s effort and the opening of the inspiratory valve has decreased markedly with newer generation ventilators. The less work the patient needs to do to trigger the vent, the more work they can spend on generating a tidal volume. Also, the ventilator and patient should not conflict. That is to say that the ventilator should not try to give a breath when a patient is exhaling. You do not want a situation in which both the ventilator and the patient are working hard - one should be asked to work less. If the patient is ready to do more, than decrease the vent settings. If the patient is not ready, then increase the settings in order to decrease the patient’s distress.

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Troubleshooting

Improving Ventilation and/or Oxygenation

can increase respiratory rate (or decrease rate if air trapping is an issue)🡺 ventilation
can increase tidal volume/PIP to increase tidal volume 🡺 ventilation
can increase PEEP/itime to help recruit collapsed areas 🡺oxygenation
can increase pressure support and/or decrease sedation to improve patient’s spontaneous effort

Okay, you have now dismissed the vent as the source of the problem; there doesn’t appear to be any patient-ventilator dysnchrony and any acute problems have been addressed. You are left with a patient that has unacceptable ventilation and/or oxygenation. What can you do?

Recall the basics - you can increase the FiO₂ or PEEP to improve oxygenation; you can increase the rate or tidal volume to increase ventilation. Before deciding what to do, you must first decide what the problem is. Is hypercarbia due to inadequate tidal volumes? Is there increased dead space? Has the metabolic production of carbon dioxide changed? Is the worsening oxygenation due to increased V/Q mismatch? Is this due to decreased ventilation? Is there increased dead space? Increasing the PEEP may improve alveolar ventilation to a greater extent than it increases dead space. Increasing the rate may improve alveolar ventilation such that both oxygenation and ventilation improve. Increasing the FiO₂ may be all that is needed to bring the SaO₂ back to an acceptable level.

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Knowledge check

The following are true about benefit of PC compared to VC

PC Provides more homogeneous gas exchange with reduced peak pressure and guaranteed TV
PC Provides Variable TV but more homogeneous gas exchange with reduced peak pressure
PC Provide guaranteed TV with square waveform