mechanical ventilation: a basic introduction
TRANSCRIPT
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Mechanical Ventilation: A Basic
Introduction
John Denny MD
Director Section of Critical Care
Professor
Department of Anesthesia
Rutgers Robert Wood Johnson Medical
School
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Historical Features
Ventilator Origins
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Origins
“…an opening must be attempted in the
trunk of the trachea, into which a tube of
reed or cane should be put; you will then
blow into this, so that the lung may rise
again…and the heart becomes strong…”
Andreas Vesalius (1555)
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Origins
Polio epidemic of 1955
Sweden
Emerson Company, Boston
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Early ventilators
Pressure Cycled
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Early ventilators
Pressure Cycled
Volume Cycled
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Indications for Mechanical
Ventilation
Useful aphorisms from Marino ICU Book:
“Thinking of it”
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Indications for Mechanical
Ventilation
Thinking of it
Intubation is not an act of weakness
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Indications for Mechanical
Ventilation
Thinking of it
Intubation is not an act of weakness
ET tubes are not a disease, and ventilators
are not an addiction
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Strategies for Mechanical
Ventilation
Tidal Volume:
Old settings 10 ml/kg….
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Strategies for Mechanical
Ventilation
Tidal Volume:
Old settings 10 ml/kg
After ARDSnet study:
Lung Protective 5-8 ml/kg
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Ventilator Induced Lung Injury
ARDS, pneumonia-pathology not uniformly
distributed. However, ventilator volumes
are distributed preferentially to NORMAL
lung areas, overdistending normal areas and
producing stress fractures in alveolar walls
and adjacent pulmonary capillaries.
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Ventilator Induced Lung Injury
Can lead to pneumomediastinum, PTX
-damage to pulmonary capillaries can result
in leaky capillary type of pulmonary edema
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Ventilator Induced Lung Injury
May be due to
barotrauma (pressure injury) or
volutrauma (volume distention injury)
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Strategies for Mechanical
Ventilation
End Inspiratory Pressure:
Old setting: Peak < 50 cm H20
Lung Protective: Plateau < 30 cm H20
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Strategies for Mechanical
Ventilation
Positive End Expiratory Pressure (PEEP):
Old setting: Use PEEP to keep FIO2 < 60%
Lung Protective: 5-15 cm H20
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Strategies for Mechanical
Ventilation
ABGs:
Traditional setting: Usual pCO2, pH 7.36-
7.44
Lung Protective: Hypercapnia Ok IF
needed, pH 7.20-7.44
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Adjusting the Ventilator
How to increase the pO2?:
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Adjusting the Ventilator
Increasing the pO2:
Increase FIO2
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Adjusting the Ventilator
Increasing the pO2:
Increase FIO2
Increase PEEP
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Adjusting the Ventilator
Increasing the pO2:
Increase FIO2
Increase PEEP
Try different mode of ventilation
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Adjusting the Ventilator
To decrease pCO2?:
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Adjusting the Ventilator
To decrease pCO2:
Manipulate Minute Ventilation
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Adjusting the Ventilator
To decrease pCO2:
Manipulate Minute Ventilation
(T.V. X Rate)
Either increase TV or increase rate
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When making changes
Change one variable at a time
(unless patient is in extremis)
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Auto PEEP
Results from incomplete alveolar emptying
at end expiration
Aka intrinsic PEEP
Measure by occluding expiratory tubing at
end of expiration
Newer vents, simply hit button to measure
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Auto PEEP, consequences:
Decreased Venous Return
Decreased CO
Alveolar Rupture
Artificial > in PIP
Increase in Work of Breathing
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Auto PEEP
Predisposing Factors:
Patient:
• COPD
• Reactive airway disease
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Auto PEEP
Predisposing Factors:
Patient:
• COPD
• Reactive airway disease
Ventilator:
• High TV
• Rapid Rates
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Auto PEEP
Management:
Avoid excessive TV
Allow for adequate exhalation, avoid rapid
RR
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Weaning: Factors to consider
Inspiratory Loading due to Auto PEEP:
Normal vent trigger threshold ex: –1cm
H2O
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Auto PEEP-Can Increase WOB
Normally, pts spontaneous breaths create
neg. pleural pressure which triggers
pressure support assistance from vent
Need NIF of -1 min. to trigger
Auto Peep increases needed NIF to trigger
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Weaning: Factors to consider
Inspiratory Loading due to Auto PEEP:
Normal vent trigger threshold ex: –1cm
H2O
If Auto PEEP present, pt must overcome
both it and trigger….
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Auto Peep in Weaning
If trigger requires 1 cm H2O NIF
and if Auto Peep is 14,
pt must generate NIF of 15 EVERY breath to
trigger vent
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Counteracting Auto Peep:
By adding external PEEP to the inspiratory
circuit , you reset the sensitivity level. SO
in this ex, with auto PEEP of 14, by adding
14 of external PEEP the vent circuit
sensitivity will be reset to –1 cm H2O for
this patient’s inspiratory muscles
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Auto PEEP in Weaning
RX:
• Aggressive tx of underlying airway obstruction
• Add external PEEP equal to auto PEEP
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Indications for Mechanical
Ventilation
Hypoventilation
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Indications for Mechanical
Ventilation
Hypoventilation
Arterial pH < 7.30
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Indications for Mechanical
Ventilation
Hypoxia
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Indications for Mechanical
Ventilation
Hypoxia
Face Mask CPAP
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Indications for Mechanical
Ventilation
Hypoxia unresponsive to more conservative
measures warrants intubation
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Indications for Mechanical
Ventilation
Respiratory Fatigue
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Indications for Mechanical
Ventilation
Respiratory Fatigue:
Excessive (WOB) Work of Breathing
(tachypnea, dyspnea, use of accessory
muscles, nasal flaring,diaphoresis,
tachycardia)
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Indications for Mechanical
Ventilation
Airway Protection
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Indications for Mechanical
Ventilation
Airway Protection:
Decreased mental status
Increased aspiration risk
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Goals of Mechanical Ventilation
Provide adequate alveolar ventilation (pH,
pCO2)
Provide adequate oxygenation
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Goals of Mechanical Ventilation
Use lowest possible FIO2
Histological signs of O2 Toxicity:
100%-12 Hours
80%-24 hours
60%-36 hours
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Goals of Mechanical Ventilation
Promote pt.-ventilator synchrony
Avoid alveolar overdistention
PEEP to maintain alveolar recruitment
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Modes of Mechanical Ventilation
AC or Assist Control
Pre-set volume is delivered
Patient can initiate (“assisted”)
Else machine initiates (“controlled”)
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Modes of Mechanical Ventilation
Problems with AC:
Rapid breathing produces respiratory
alkalosis
Decreased exhalation time produces auto-
PEEP
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Modes of Mechanical Ventilation
IMV or Intermittent Mandatory Ventilation
IMV delivers preset volume cycled breaths
at a preset rate
But also allows for spontaneous patient
breathing
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Modes of Mechanical Ventilation
IMV or Intermittent Mandatory Ventilation
Synchronized IMV= machine breaths are
synchronized to coincide with patients
spontaneous lung inflations
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Modes of Mechanical Ventilation
Pressure Support (PS)
Analogous to “lift assist” for doing pullups
in a gym. I.E., in gym, you must initiate
effort to do pullup, then “lift assist” helps
you complete pullup (breath)
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Lift assist gym device for pullups
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Modes of Mechanical Ventilation
Pressure Support (PS)
In pure PS mode, patient MUST have
intrinsic rate!!
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Modes of Mechanical Ventilation
Pressure Controlled Ventilation (PCV)
Pressure Cycled
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Modes of Mechanical Ventilation
Pressure Controlled Ventilation (PCV)
Pressure Cycled
Inspiratory flow rate decreases
exponentially during lung inflation to keep
airway pressure at the chosen value
Inflation volumes vary with mechanical
properties of lungs
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PCV
In pure PCV, you don’t set a TV.
TV is dependent on lung compliance!
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PCV
Therefore, you must follow-up what the
resultant TV is
And adjust PC accordingly
Especially to maintain a lung protective
strategy!
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Pressure Regulated Volume
Control ( PRVC )
Volume Ventilation with limitation on
pressure
Automatically adjusts PC so achieve desired
TV without over-distending lung!
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Variables in Mechanical Ventilation
Inverse Ratio Ventilation (IRV)
Usual I:E 1:2 to 1:4
Reverses I:E to 2:1
Thought to prevent alveolar collapse
Tends to produce auto-peep and decrease
C.O.
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HFOV
High Flow Oscillatory Ventilator
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HFOV
High Flow Oscillatory Ventilator
Initial Settings:
Rate 5 Hz (300 bpm)
MAP 3-5 cm > previous PC
Delta P (amplitude) 65 cm H2O
FIO2 100
I time 33%
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HFOV
Unfortunately, recent adult trials have not
shown clear benefit in ARDS
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Modes of Mechanical Ventilation
Jet Ventilation
Rarely, beneficial when other modes not
effective
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BiVent
Covered in a separate lecture.
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THE END