Principles of Mechanical Ventilation

1
Principles of Mechanical Ventilation

• RET 2284
• Module 2.0
• Classification of Ventilators
• How a Breath is Delivered

2
How a Breath is Delivered

• Factors Controlled and Measured by the Ventilator
  During Inspiration
• To deliver inspiratory volume, the operator most
  commonly sets either a volume or a pressure
• The primary variable the ventilator adjusts to
  achieve inspiration is called the control
  variable

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How a Breath is Delivered

• CONTROL VARIABLES
• The primary variable the ventilator adjusts to
  achieve inspiration
• Mechanical ventilators can control four
  variables, but only one at a time
• Pressure
• Volume
• Flow
• Time

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How a Breath is Delivered

• CONTROL VARIABLES
• Because only one of these variables can be
  directly controlled at a time, a ventilator must
  function as either one of the following
• Pressure controller
• Volume controller
• Flow controller
• Time controller

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How a Breath is Delivered

• Pressure Controller
• When the ventilator maintains the pressure
  waveform in a specific pattern, the breathing is
  described as pressure controlled (also pressure
  limited or pressure targeted)
• The pressure waveform is unaffected by changes in
  lung characteristics

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How a Breath is Delivered

• Pressure Controller
• The pressure waveform will remain constant but
  volume and flow will vary with changes in
  respiratory system mechanics (airway resistance,
  compliance)

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How a Breath is Delivered

• Volume Controller
• The ventilator maintains the volume waveform in a
  specific pattern, the delivered breath is volume
  controlled (volume limited, volume targeted)
• The volume and flow waveforms remain unchanged,
  but the pressure waveform varies with changes in
  lung characteristics (resistance and compliance)

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How a Breath is Delivered

• Volume controller
• Volume and flow waveforms will remain constant,
  but pressure will vary with changes in
  respiratory mechanics (airway resistance and
  compliance)

Pressure
Volume
Flow
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How a Breath is Delivered

• Flow Controller
• Directly measures flow and uses the flow signal
  as a feedback signal to control its output
• Most newer ventilators measure flow and are flow
  controllers volume becomes a function of flow as
  follows
• Volume (L) Flow (L/sec) x Inspiratory Time
  (sec)

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How a Breath is Delivered

• Flow Controller
• Flow and volume waveforms will remain constant,
  but pressure will vary with changes in
  respiratory mechanics (airway resistance and
  compliance)

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How a Breath is Delivered

• Time Controller
• Measures and controls inspiratory and expiratory
  time
• Pressure and volume waveforms vary with changes
  in resistance and compliance (neither pressure or
  volume are used as a control)

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How a Breath is Delivered

• Inspiration is commonly described as pressure
  controlled or volume controlled (although flow
  and time ventilation have been defined, they are
  not typically used).

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How a Breath is Delivered

• PHASE VARIABLES
• Ventilator-supported breath may be divided into
  four distinct phases
• The initiation of inspiration
• Inspiration itself
• The end of inspiration
• Expiration
• To understand a breath cycle, you must know how
  the ventilator starts, sustains, and stops
  inspiration and you must know what occurs between
  breaths

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How a Breath is Delivered

• PHASE VARIABLES
• The phase variable is a variable that is measured
  and used by the ventilator to initiate some phase
  of the breath cycle
• Trigger variable causes a breath to begin
• Limit variable limits the magnitude of any
  parameter (pressure, flow, volume) during
  inspiration
• Cycle variable causes the end of inspiration
• Baseline variable determines what happens
  during expiration

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How a Breath is Delivered

• Trigger Variable
• Determines the start of inspiration
• Any one of the following may be sensed/measured
  by the ventilator and used to initiate
  inspiration
• Pressure
• Volume
• Flow
• Time
• Either the ventilator or the patient initiates a
  breath. If the machine initiates the breath, the
  trigger variable is time

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How a Breath is Delivered

• Time Triggered
• Ventilatory initiates a breath according to a
  predetermined time interval
• Rate control on ventilator is a time-triggering
  device
• Example Rate 10 (breaths/min) One breath every
  6 seconds
• An alternative approach is to provide separate
  timers for inspiration and expiration. Changing
  either or both of these timers will alter the
  breathing rate

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How a Breath is Delivered

• Time Triggered
• Ventilatory initiates a breath according to a
  predetermined time interval

18
How a Breath is Delivered

• Pressure Triggered
• Occurs when the patients inspiratory effort
  causes a drop in pressure within the breathing
  circuit

19
How a Breath is Delivered

• Pressure Triggered
• Typically, trigger levels (sensitivity) range
  between 0.5 to 1.5 cm H2O below the patients
  baseline, or end-expiratory pressure
• Setting the sensitivity to a higher number, e.g.,
  3 cm H2O makes the ventilator less sensitive and
  requires the patient to work harder to initiate
  inspiration. Conversely, setting the trigger
  level lower make the ventilator more sensitive

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How a Breath is Delivered

• Pressure Triggered
• The operator must set the sensitivity to meet the
  patients needs. If it is not set correctly, the
  ventilator may not be sensitive enough to the
  patients effort, and the patient will have to
  work too hard to trigger the breath.
• If the ventilator is too sensitive, it can
  autotrigger (i.e., the machine triggers a breath
  without the patient making an effort)

21
How a Breath is Delivered

• Quiz
• A patient has a baseline pressure of 10 cm H2O
  during mechanical ventilation. The trigger
  sensitivity is set at 1 cm H2O. At what
  pressure will the ventilator sense a patient
  effort and start inspiration?

Answer 9 cm H2O
22
How a Breath is Delivered

• Flow Triggered
• Ventilator-supported breath is initiated when the
  ventilator detects a drop in flow through the
  patient circuit during exhalation

Continuous flow through circuit Base Flow
23
How a Breath is Delivered

• Flow Triggered
• Between breaths, the flow coming out of the main
  flow control valve and the flow going through the
  exhalation valve are equal
• During the patients inspiratory effort, the flow
  through the exhalation valve falls below the flow
  from the output valve
• The difference between these two flows is the
  flow trigger variable

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How a Breath is Delivered

• Flow Triggered
• Ventilator graphic representing flow triggering

25
How a Breath is Delivered

• Quiz
• The operator decides to use flow triggering for a
  patient and sets the base flow at 6 L/min and the
  trigger flow at 2 L/min. The base flow
  measurement must drop to what value before the
  ventilator will begin the inspiratory phase?

Answer 4 L/min.
26
How a Breath is Delivered

• Volume Triggered
• Occurs when the ventilator detects a small drop
  in volume in the patient circuit during
  exhalation. The machine interprets this drop as
  a patient effort and begins inspiration
• Dräger Babylog
• Cardiopulmonary Venturi

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How a Breath is Delivered

• Manual Triggered
• A button or touch pad on the control panel
  labeled Manual breath or Start breath
• When this control is activated, the ventilator
  delivers a breath according to the set variables

PB 840
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How a Breath is Delivered

• Inductive Plethysmography Triggered
• Infants ventilator-supported breath is initiated
  when a when chest expands

Sechrist IV-200
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How a Breath is Delivered

• NAVA (Neurally Adjusted Ventilator Assist)
• A esophageal probe senses the electrical activity
  of the diaphragm, which triggers the ventilator

Maquets Servo-I NAVA
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How a Breath is Delivered

• Limit Variable
• A limit variable is the maximum value a variable
  (pressure, flow, volume) can attain. This limits
  the variable during inspiration but does not end
  the inspiratory phase.
• Do not confuse this with cycle variable, which
  always ends inspiration

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How a Breath is Delivered

• Pressure Limiting
• Allows pressure to rise to a certain value but
  not exceed it

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How a Breath is Delivered

• Pressure Limiting
• Remember in pressure ventilation, the pressure
  the ventilator delivers to the patient is
  limited however, reaching the pressure limit
  does not end the breath
• Example of pressure-limiting modes
• Pressure support
• Pressure control

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How a Breath is Delivered

• Volume Limiting
• The volume is set by the operator however,
  reaching that volume does not necessarily end
  inspiration

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How a Breath is Delivered

• Flow Limiting
• Ventilator flow to the patient reaches but does
  not exceed a maximum value before the end of
  inspiration

35
How a Breath is Delivered

• Maximum Safety Pressure
• Pressure Limiting vs. Pressure Cycling
• All ventilators have a maximum pressure limit
  control, which is used to prevent excessive
  pressure from reaching a patients lungs
  reaching the maximum high pressure limit ends the
  inspiratory phase
• AKA
• High pressure limit
• Upper pressure limit
• Pressure limit

36
How a Breath is Delivered

• Cycle Variable
• The variable a ventilator measures to determine
  the end of inspiration is called the cycling
  mechanism once cycling occurs, expiratory gas
  flow begins
• Cycle variables
• Pressure
• Volume
• Flow
• time

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How a Breath is Delivered

• Pressure Cycled
• Ventilator will deliver flow until a present
  pressure is reached, at which point inspiration
  ends and expiratory flow begins
• The most common application of pressure-cycling
  is for alarm setting (e.g., high pressure alarm)
  and IPPB

38
How a Breath is Delivered

• Volume Cycled
• Ventilator will deliver flow until a present
  volume is reached, at which point inspiration
  ends and expiratory flow begins

39
How a Breath is Delivered

• Flow Cycled
• Ventilator will deliver flow until a present
  level is met, at which point flow stops and
  expiration begins
• The most frequent application of flow cycling is
  pressure control mode ventilation (to be
  discussed in a future module)

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How a Breath is Delivered

• Time Cycled
• Expiratory flow starts because a present time
  interval has elapsed

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How a Breath is Delivered

• Limit Variable

A
B
C
This figure illustrates the importance of
distinguishing between the terms limit and cycle.
A, Inspiration is pressure limited and time
cycled. B, Inspiration is flow limited and volume
cycled. C, Inspiration is both flow limited and
volume limited (because flow and volume reach
preset values before inspiratory time ends) and
time cycled (after the preset inspiratory hold
time).
42
How a Breath is Delivered

• Inflation Hold (Inspiratory Pause)
• Designed to maintain air in the lungs at the end
  of inspiration
• Used to obtain PPlateau for calculating CS
• Occasionally used to help increase peripheral
  distribution of gas and improve oxygenation

43
How a Breath is Delivered

• Inflation Hold (Inspiratory Pause)
• Calculation of CS requires accurate measurement
  of the plateau pressure the PPlateau value is
  inaccurate if the patient is actively breathing
  when the measurement is taken

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How a Breath is Delivered

• TYPES of Breaths
• Spontaneous
• Started by the patient (patient triggered) and VT
  delivery is determined by the patient (i.e.,
  patient cycled)
• Volume and pressure delivered are based on
  patient demand rather than a value set by the
  ventilator operator

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How a Breath is Delivered

• TYPES of Breaths
• Mandatory
• Ventilator determines the start time (time
  triggered) or VT (or both)
• The machine triggers and/or cycles the breath

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How a Breath is Delivered

• BASELINE VARIABLE
• The variable that is controlled during the
  expiratory phase
• Note Most commonly, pressure is controlled
  during the
• expiratory phase
• CPAP
• PEEP

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How a Breath is Delivered

• BASELINE VARIABLE
• Continuous Positive Airway Pressure (CPAP)
• The application of pressures above ambient to
  improve oxygenation is a spontaneously breathing
  patient
• Can be applied through a freestanding CPAP system
  or a ventilator

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How a Breath is Delivered

• BASELINE VARIABLE
• Continuous Positive Airway Pressure (CPAP)
• Simplified pressure-time waveform showing
  continuous positive airway pressure (CPAP).
  Breathing is spontaneous. Inspiratory positive
  airway pressure (IPAP) and expiratory positive
  airway pressure (EPAP) are present. Pressures
  remain positive and do not return to a zero
  baseline.

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How a Breath is Delivered

• BASELINE VARIABLE
• Positive End Expiratory Pressure (PEEP)
• The application of pressures above ambient at the
  end of exhalation to improve oxygenation during
  mechanical ventilation

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How a Breath is Delivered

• BASELINE VARIABLE
• Positive End Expiratory Pressure (PEEP)
• Positive end-expiratory pressure (PEEP) with
  intermittent mandatory breaths (also called
  intermittent mandatory ventilation IMV with
  PEEP). Spontaneous breaths are taken between
  mandatory breaths, and the baseline is maintained
  above zero.

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How a Breath is Delivered

• BASELINE VARIABLE
• Positive End Expiratory Pressure (PEEP)
• Helps prevent early airway closure and alveolar
  collapse at the end of expiration
• Recruits collapsed alveoli
• Augments FRC
• Facilitates better oxygenation
• DOES NOT PROVIDE VENTILATION!