Chapter 42
Mechanical Ventilators
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Learning Objectives
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Discuss the basic design features of
ventilators.
Classify ventilators and describe how they
work.
Define what constitutes a mode of ventilation.
Classify and discuss modes of ventilation.
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Learning Objectives (cont.)
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
Explain the indications for the basic modes of
ventilatory support.
Describe the application of selected modes of
ventilatory support.
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Mechanical Ventilator (MV)

4 basic functions
1.
2.
3.
4.
Input power
• Electrical, pneumatic, manual
Power transmission & conversion
Control system
Output
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Control System
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All of the following are functions of mechanical
ventilation, except:
A.
B.
C.
D.
Turbo power
Input power
Power transmission and conversion
Control system
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Power Transmission & Conversion
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Drive mechanism
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Generates force needed to deliver gas to patient
under pressure
Mechanisms can either be
• Gas from pressure-reducing valve
• Driven by electric motor or compressor
Output control valve
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
Regulates flow of gas to patient
Can be just on/off valve or one that modifies output
waveform
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Volume & Pressure Ventilation:
Characteristic Waveforms
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Control Circuit
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System that allows ventilator to manipulate
pressure, volume, & flow
May be composed of mechanical, pneumatic,
electric, electronic, or fluidic components
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Most modern vents combine two or more
May be advantages to components used
• MRI: Fluidic controls have no metal & are immune to
failure due to electromagnetic interference
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Control Circuits may be composed of all of the
following components, except:
A.
B.
C.
D.
Mechanical
Pressure
Electric
Pneumatic
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Control Variables
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
Primary variable ventilator controls to cause
inspiration
3 possible explicit variables
1.
2.
3.

Pressure controlled
Volume controlled
Flow controlled
Only one can be controlled; other two
become dependent variables
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All of the following are possible explicit variables
except:
A.
B.
C.
D.
Pressure controlled
Volume controlled
Flow controlled
Loop controlled
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Control Variables (cont.)
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Control Variables (cont)
Copyright © 2013, 2009, 2003, 1999, 1995, 1990, 1982, 1977, 1973, 1969 by Mosby, an imprint of Elsevier Inc.
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Pressure Controller
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Ventilator controls pressure (P), but volume &
flow vary with changes in compliance (C) &
resistance (Raw)
Pressure waveform will be square (constant)
during inspiration
Positive or negative pressure controlled

i.e., iron lung controls with negative P
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Volume & Flow Controllers
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Volume controller
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Ventilator controls volume so will be constant
• Flow is volume/time, so flow is also constant
Pressure will vary with changes in C & Raw
Flow controller
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As above, flow & thus volume constant
Pressure varies with changes in C and Raw
Old neonatal ventilators used flow interruption to
deliver volume during inspiration
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Phase Variables
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Ventilator uses variables to initiate or limit
each phase of ventilation
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Initiation of inspiration (E to I)
Inspiration
End of inspiration (I to E)
Expiration
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Phase Variables
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All of the following are variables that initiate or
limit each phase of ventilation, except:
A.
B.
C.
D.
Initiation of inspiration (E to I)
Inspiration
End of inspiration (I to E)
End of expiration
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Initiation of Inspiration
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Trigger variable
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Machine triggered
• Time: determined by rate control
Patient triggered
• Pressure
• Flow (least work for patient to trigger)
• Volume (rare)
Most ventilators provide a manual breath button
that operator activates
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Inspiration: Target Variable
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Limits inspiration but does not terminate the
phase
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Pressure limited
• Limits peak inspiratory pressure (PIP) during inspiration
Volume limited
• Limits amount of tidal volume (VT) delivered during
inspiration to set amount

Flow limited
• Limits the amount of flow during inspiration
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Target Variable
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End of Inspiration
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Cycle variables terminate inspiratory phase
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Pressure cycled
• Inspiration terminates as preset pressure reached (hit
alarm level)
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Volume cycled
• Inspiration terminates at preset VT
Flow cycled
• Inspiration terminates when flow drops to preset value
(PSV)

Time cycled
• Inspiration terminates when set inspiratory time is
reached

Includes any inspiratory holds
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Expiration: Baseline Variable
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Defined by how baseline or end expiratory
pressure (EEP) relates to atmospheric
pressure
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PEEP Positive or supra- atmospheric EEP
NEEP Negative or sub-atmospheric EEP
ZEEP Zero EEP equals sub-atmospheric
pressure
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All of the following are all baseline variables,
except:
A.
B.
C.
D.
ZEEP
MEEP
PEEP
NEEP
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Primary Breath Control Variable
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Volume control: VT/flow set, while P depends on
those settings & pulmonary mechanics
Pressure control: P set, VT /flow depend on P
setting & pulmonary mechanics
Dual control: Mixture of volume & pressure

Either starts breath in volume control & ends with
pressure control or the reverse
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Two Breath Types
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Spontaneous
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Mandatory
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Patient triggers & cycles the breath
Patient effort may be supported by manual or
mechanical ventilator
Ventilator initiates and/or cycles breath
See Box 42-2.
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Modes of Ventilation
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3 possible sequences of breaths
CMV: all breaths mandatory, “full support”
• Patient & machine-initiated breaths are same
2. CSV: all breaths spontaneous
• Patient triggers & cycles all breaths
3. IMV: Breaths can be mandatory or spontaneous
1.

When tied to control variable, nine possible
combinations
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Breath Sequence
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Breath Sequence
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Modes of Ventilation (cont.)
Copyright © 2013, 2009, 2003, 1999, 1995, 1990, 1982, 1977, 1973, 1969 by Mosby, an imprint of Elsevier Inc.
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Control Type
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Open loop control: Most basic early
ventilators were flow, volume, or pressure,
are determined by pulmonary mechanics &
ventilator system
Closed loop control: Flow/volume or pressure
are set & measured, with feedback to drive
mechanism altering output to maintain
desired (set) levels
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Importance of Defining Modes
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Modern ventilators, modes may look similar
on graphics but must be set up differently
Clear understanding & definition of each
mode will avoid potentially dangerous
patientventilator mismanagement
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Ventilator Waveforms
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
Ventilator graphics are to ventilator
management , what ECGs are to managing
the heart, or pressure waveforms from a PA
catheter are to hemodynamic management
Graphics provide wealth of information at a
glance
Copyright © 2013, 2009, 2003, 1999, 1995, 1990, 1982, 1977, 1973, 1969 by Mosby, an imprint of Elsevier Inc.
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Ventilator Waveforms (cont.)
Copyright © 2013, 2009, 2003, 1999, 1995, 1990, 1982, 1977, 1973, 1969 by Mosby, an imprint of Elsevier Inc.
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