basic mechanical ventilation
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Basic mechanical ventilation
Charles GomersallDept of Anaesthesia & Intensive CareThe Chinese University of Hong Kong
Prince of Wales Hospital
Version 1.0May 2003
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Disclaimer
• Although considerable care has been taken in the preparation of this tutorial, the author, the Prince of Wales Hospital and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from its use.
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O2CO2
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Getting oxygen in
• Depends on– PAO2
• FIO2
• PACO2
• Alveolar pressure• Ventilation
– Diffusing capacity– Perfusion– Ventilation-perfusion matching
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Carbon dioxide out
• Respiratory rate• Tidal volume• Deadspace
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Main determinants
• Oxygen in FIO2
mean alveolar pressure
– PEEP• Re-open alveoli
and shunt
• Carbon dioxide out ventilation
RR tidal volume
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Mean airway pressure
Mean airway pressureMean airway pressure
Time
Pre
ssu
re
Time
Pre
ssu
re
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Mean airway pressure
Mean airway pressureMean airway pressure
Time
Pre
ssu
re
Time
Pre
ssu
re
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Inspiratory time
• Set as:– % of respiratory cycle– I:E ratio
• Expiratory time not set– Remaining time after inspiration
before next breath
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Inspiratory time
• Increased inspiratory time– Improved oxygenation– Unnatural pattern of breathing
• Deeper sedation
– Increased risk of gas trapping
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Mean airway pressure
Mean airway pressureMean airway pressure
Time
Pre
ssu
re
Time
Pre
ssu
re
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PEEP
Improves oxygenation mean alveolar pressure shunting
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Other settings
• Trigger sensitivity sensitivity preferable
• Flow triggering general more sensitive than pressure triggering
flow or pressure sensitivity
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Rise time
Inspiratory time
Pre
ssure
Time
Rise time
Inspiratory time
Flow
Time
Other settings
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Respiratory complications• Nosocomial pneumonia• Barotrauma• Gas trapping
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Barotrauma
• High pressures (barotrauma)• High volumes (volutrauma)• Shear injury
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Gas trapping
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Gas trapping
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Gas trapping
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Gas trapping
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Gas trapping
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Gas trapping
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Gas trapping
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Gas trapping
• Predisposing factors:– asthma or COPD– long inspiratory time ( expiratory time
short)– high respiratory rate ( absolute expiratory
time short)
• Effects– progressive hyperinflation of alveoli– progressive rise in end-expiratory pressure
(intrinsic PEEP)
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Intrinsic PEEP (PEEPi)
Time
Pre
ssu
re
PEEPe
PEEPtot
etoti PEEP-PEEP PEEP
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Gas trapping
• Adverse effects– Barotrauma– Cardiovascular compromise
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Cardiovascular effects
• Preload– positive intrathoracic pressure
reduces venous return– exacerbated by
• high inspiratory pressure• prolonged inspiratory time• PEEP
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Cardiovascular effects
• decreased afterload due to decreased LV transmural pressure
2HR x P
T tm
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Cardiovascular effects
• decreased afterload due to decreased LV transmural pressure
2HR x P
T tm
plIC P - P
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Cardiovascular effects
• Overall effect depends on whether ventricular contractility is normal or abnormal contractility
cardiac output
– normal contractility cardiac output
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Hypotension
• Consider– Drug induced– Gas trapping– Tension
pneumothorax
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Modes
• To learn about specific modes of ventilation (eg PRVC) download the appropriate lectures by clicking on the button
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