Basic mechanical ventilation

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Basic mechanical ventilation

• Charles GomersallDept of Anaesthesia Intensive
  CareThe Chinese University of Hong KongPrince
  of Wales Hospital
• Version 1.0
• May 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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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
Pressure
Pressure
Time
Time
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Mean airway pressure
Pressure
Pressure
Time
Time
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Inspiratory time

• Set as
• of respiratory cycle
• IE 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
Pressure
Pressure
Time
Time
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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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Other settings
Rise time
Rise time
Flow
Inspiratory time
Inspiratory time
Pressure
Time
Time
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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)
Pressure
PEEPtot
PEEPe
Time
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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

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Cardiovascular effects

• decreased afterload due to decreased LV
  transmural pressure

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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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