Chapter 42 Mechanical Ventilators

Copyright © 2013, 2009, 2003, 1999, 1995, 1990, 1982, 1977, 1973, 1969 by Mosby, an imprint of Elsevier Inc.

Chapter 42

Mechanical Ventilators

Copyright © 2013, 2009, 2003, 1999, 1995, 1990, 1982, 1977, 1973, 1969 by Mosby, an imprint of Elsevier Inc. 2

Learning Objectives

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.


Learning Objectives (cont.)

Explain the indications for the basic modes of ventilatory support.

Describe the application of selected modes of ventilatory support.


Mechanical Ventilator (MV)

4 basic functions1. Input power

• Electrical, pneumatic, manual

2. Power transmission & conversion

3. Control system

4. Output


Control System

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All of the following are functions of mechanical ventilation, except:

A. Turbo power

B. Input power

C. Power transmission and conversion

D. Control system


Power Transmission & Conversion

Drive mechanism 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 Regulates flow of gas to patient Can be just on/off valve or one that modifies output

waveform


Volume & Pressure Ventilation:Characteristic Waveforms

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Control Circuit

System that allows ventilator to manipulate pressure, volume, & flow

May be composed of mechanical, pneumatic, electric, electronic, or fluidic components 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


Control Circuits may be composed of all of the following components, except:

A. Mechanical

B. Pressure

C. Electric

D. Pneumatic


Control Variables

Primary variable ventilator controls to cause inspiration

3 possible explicit variables1. Pressure controlled

2. Volume controlled

3. Flow controlled Only one can be controlled; other two

become dependent variables


All of the following are possible explicit variables except:

A. Pressure controlled

B. Volume controlled

C. Flow controlled

D. Loop controlled


Control Variables (cont.)

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Control Variables (cont)

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Pressure Controller

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


Volume & Flow Controllers

Volume controller 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

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


Phase Variables

Ventilator uses variables to initiate or limit each phase of ventilation Initiation of inspiration (E to I) Inspiration End of inspiration (I to E) Expiration


Phase Variables

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All of the following are variables that initiate or limit each phase of ventilation, except:

A. Initiation of inspiration (E to I)

B. Inspiration

C. End of inspiration (I to E)

D. End of expiration


Initiation of Inspiration

Trigger variable 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


Inspiration: Target Variable

Limits inspiration but does not terminate the phase 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


Target Variable

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End of Inspiration

Cycle variables terminate inspiratory phase Pressure cycled

• Inspiration terminates as preset pressure reached (hit alarm level)

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

Defined by how baseline or end expiratory pressure (EEP) relates to atmospheric pressure PEEP Positive or supra- atmospheric EEP NEEP Negative or sub-atmospheric EEP ZEEP Zero EEP equals sub-atmospheric

pressure


All of the following are all baseline variables, except:

A. ZEEP

B. MEEP

C. PEEP

D. NEEP


Primary Breath Control Variable

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


Two Breath Types

Spontaneous Patient triggers & cycles the breath Patient effort may be supported by manual or

mechanical ventilator Mandatory

Ventilator initiates and/or cycles breath See Box 42-2.


Modes of Ventilation 3 possible sequences of breaths

1. 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 When tied to control variable, nine possible

combinations


Breath Sequence

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Breath Sequence

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Modes of Ventilation (cont.)

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Control Type

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


Importance of Defining Modes

Modern ventilators, modes may look similar on graphics but must be set up differently

Clear understanding & definition of each mode will avoid potentially dangerous patientventilator mismanagement


Ventilator Waveforms

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


Ventilator Waveforms (cont.)

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Chapter 42 Mechanical Ventilators