mechanical ventilation part 2

1. Dr.Radhwan Hazem Alkhashab
Consultant anaesthesia & ICU
2022

2. Howtosettheventilatorright?
Mechanical ventilation is an important tool in ICU for treatment of
various ventilatory failure.
To set the ventilator right you should know the ventilator language.
In ventilator language we have variables and these form breaths and
breaths form modes of ventilation.

3. Variablesare5types
1. Control or target.
2. Trigger.
3. Limit.
4. Cycle.
5. Baseline.

4. 1st variableiscontrolortarget
 The definite thing or parameter you are aiming in ventilatory support
to your patient.
 It is either volume control {target} or pressure control {target}.

5. ModesofVentilation
Fixed TV with each breath
Peak airway pressure can
vary with each breath
depending on:
– Resistance to airflow
during inspiration
– Patient’s lung-chest wall
compliance
Fixed peak airway pressure
with each breath
TV can vary with each breath
depending on:
– Resistance to airflow
during inspiration
– Patient’s lung-chest wall
compliance
Volume Targeted
Modes
Pressure Targeted
Modes

6. 2nd variableistrigger
This is the parameter that tells the ventilator to start inspiration.
1. Pressure trigger: Breath is delivered when ventilator senses
patients spontaneous inspiratory effort. • sensitivity refers to the
amount of negative pressure the patient must generate to receive a
breath/gas flow. • If the sensitivity is set at 1 cm then the patient
must generate 1 cm H2O of negative pressure for the machine to
sense the patient's effort and deliver a breath. • Acceptable range -
1 to -5 cm H2O below patient s baseline pressure.
2. Flow trigger: Ventilator senses a preset value of inspiratory flow
due to patient effort.
3. Volume trigger: Ventilator can not sense volume but it calculate
volume from dividing flow over time.
4. Time trigger: A preset interval of time passes then the ventilator
starts inspiration this is usually in mandatory breath.

7. 3rd variableislimit
This defines the extent of inspiratory flow (volume) or pressure that
cannot be exceeded it is often the same as the target (control) but they
are different variables.

8. 4th variableiscycle
This is reverse of trigger that means it will tell the ventilator to end
inspiration or in other words to switch from inspiration to expiration .
Its either the ventilator senses a preset pressure , volume , flow or it will
cycle by time usually in mandatory modes .
5th variableisbaseline
Which can be zero or positive like in PEEP or ZEEP

9. Definitions
Peak Inspiratory Pressure (PIP):The peak pressure is the maximum
pressure obtainable during active gas delivery. This pressure a function
of the compliance of the lung and thorax and the airway resistance
including the contribution made by the tracheal tube and the ventilator
circuit. – Maintained at <45cm H2O to minimize barotrauma • Plateau
Pressure.
The plateau pressure is defined as the end inspiratory pressure during
a period of no gas flow. The plateau pressure reflects lung and chest
wall compliance.
Mean Airway Pressure- The mean airway pressure is an average of
the system pressure over the entire ventilatory period.
End Expiratory Pressure- End expiratory pressure is the airway
pressure at the termination of the expiratory phase and is normally
equal to atmospheric or the applied PEEP level.

10. Typesofbreath
1. Mandatory breath: this is triggered, limited and cycled by the
ventilator and the patient has no role to do. We usually find this
type in controlled modes where the breaths triggered and cycled
by time.
2. Assisted mandatory or assisted controlled breath: this is
mandatory but with modification that it can be triggered by the
patient or by ventilator {time trigger} otherwise it is like mandatory
breathe limited and cycled by ventilator.
3. Spontaneous breath: this is normal natural breath triggered,
limited and cycled by the patient.
4. Supported spontaneous breath: this is simply a spontaneous
breath supported by pressure to make it more effective .It is still
triggered, limited and cycled by patient.

11. Mandatorybreath
This is triggered, limited and cycled by the ventilator and the patient has no
role to do. We usually find this type in controlled modes where the
breaths triggered and cycled by time.

12. Assistedmandatoryorassistedcontrolledbreath
This is mandatory but with modification that it can be triggered by the
patient or by ventilator {time trigger} otherwise it is like mandatory
breathe limited and cycled by ventilator.

13. Spontaneousbreath
This is normal natural breath triggered, limited and cycled by the
patient.

14. Supportedspontaneousbreaths
This is simply a spontaneous breath supported by pressure to make it
more effective .It is still triggered, limited and cycled by patient.

15. Modesclassifiedaccordingtocontrol
1. Volume targeted modes (CMV,A/CMV and SIMV)
2. Pressure targeted modes (PCMV,PA/CMV,SIMV,PS-PEEP and
CPAP)

16. Controlledmechanicalventilation{CMV}
This is an old mode consists of mandatory breaths only leaving no role for
the patient so it requires deep sedation with or without muscle relaxation.
Hazards of this mode?

17. Assisted/controlledmechanicalventilation
This mode differs from CMV by combining both mandatory and
assisted mandatory breaths.
Assisted breaths are triggered by the patient.
If the patient does not trigger for any reason then the ventilator will
trigger by time according to a preset backup respiratory rate.

18. AC(Assistcontrolventilation)mode
Indications:
1. Myasthenia gravis.
2. GBS.
3. Post cardiac / resp arrest.
4. ARDS.
5. Pulmonary oedema.
Advantages:
Minimal work of breathing and patient controls RR which helps
normalize PaCO2.

19. Synchronizedintermittentmandatoryventilation
This mode was invented aiming to give more comfortability to
patient by giving him chance to breath spontaneously yet
giving him a preset number of mandatory breaths to
guarantee minute ventilation.
This mode can be used as a starting ventilatory mode because
it guarantee a fixed minute ventilation and as a weaning
mode by gradual decrease in number of mandatory breaths.

20. Synchronizedintermittentmandatoryventilation
This mode combines both spontaneous and mandatory breaths .
In the past the mandatory breaths of this mode were not synchronized
with spontaneous breaths and the mode was called intermittent
mandatory ventilation.
This asynchronization created a problem called stacking which means
that mandatory tidal volume will buildup over spontaneous tidal volume
.
Stacking lead to patient discomfort and may lead in severe cases to
volutrauma and/or barotrauma.

21. Synchronizedintermittentmandatoryventilation
Stacking

22. Synchronizedintermittentmandatoryventilation
This problem was solved by synchronizing the inspiration of the
mandatory breath with inspiration of the patient or with an interval of
absence of respiratory effort.
Note: synchronization is not triggering and the patient does not
trigger the mandatory breaths in this mode.

23. Synchronizedintermittentmandatoryventilation
This mode has two disadvantages:
1. Hyperventilation because the patient spontaneous minute
ventilation will be added to mandatory minute ventilation.
2. Hypoventilation during weaning.

24. Pressuretargetedmodes
Here the constant parameter or variable is the pressure.
These include (PCMV, PA/CMV, P/SIMV, PS and CPAP).

25. Pressuretargetedmodesadvantages
In addition to lung protection pressure targeted modes have other
advantages which are compensation for limited leak and variable
flow rate during inspiration which offers better distribution of
ventilation and more patient comfortability.

26. PCMV
Pressure controlled (mandatory) mechanical ventilation (PCMV):
This mode has the advantages of pressure targeted modes but still
needs deep sedation with or without muscle relaxation.

27. P/SIMV
Pressure - synchronized intermittent mandatory ventilation (P/SIMV):
Pressure are used to support patient`s effort during SIMV , this help to
augment the tidal volume.

28. Pressuresupport(PS)
Supports spontaneous breathing of the patients.
• Each inspiratory effort is augmented by ventilator at a preset level of
inspiratory pressure.
• Patient triggered, flow cycled and pressure controlled mode.
• Applies pressure plateau to patient airway during spontaneous
breathing.
• Commonly applied to SIMV mode during spontaneous ventilation to
facilitate weaning PSV (Pressure Support Ventilation) mode

29. Pressuresupport(PS)
Cycling primarily done by flow cycling by reaching either a
percentage of peak inspiratory flow rate or a preset
absolute value of flow.
This type of cycling is most comfortable for the patient
because it usually matches normal cycling.
If this cycling mechanism fail (due to leak for example)
then 2ndary mechanism works by time will cycle.

30. Pressuresupport(PS)
This is the best mode for patients who have mild to moderate lung
injury at the same time have good respiratory efforts because it
guarantees maximum patient-ventilator synchrony and at the same
time if the PS is set right then it will provide good ventilation.

31. DisadvantageofPS
This mode does not guarantee fixed minute ventilation because :
• It does not guarantee fixed VT because it depends on patient
efforts.
• It does not guarantee fixed RR because it depends on patient
rate.
The problem of rate solved in some types of ventilator by setting
a backup rate and in other ventilators by combining P/SIMV with PS.

32. (CPAP)
This is the same as PEEP but CPAP is more technically accurate term
because the pressure is keep positive during the whole respiratory
cycle not only at the end of expiration.
It was known for long time that artificial ventilation affects oxygenation
badly this is because it increases both ventilation/perfusion mismatch
and basal collapse. So doctors used high FIO2 to manage this problem
but this is injurious and the maximum FIO2 can be given with least
harm is 50%. So doctors looked for another solution and that is CPAP
or PEEP.

33. AdvantagesforCPAPorPEEP
1. Improve oxygenation by:
 Opening collapsed alveoli during inspiration: so increasing
number of ventilated alveoli and decreasing percentage of
shunt. This is occurring especially in basal areas and diseased
areas. Actually before the use of CPAP doctors used high VTs
(10-15 ml/kg Bwt) to open basal collapses. But this turns to be
harmful to lung and the use of high VTs was abundant.
 Prevent collapse during expiration: by increase FRC above
closing capacity.
 Ventilate alveoli filled with edema fluid: by shifting fluid out of
them.

34. AdvantagesforCPAPorPEEP
2. Improve compliance of the lung by:
 Opening collapsed alveoli (recruitment)
 Shifting edema fluid out of alveoli and interstitial space.
3. Prevent lung injury.