What are the different modes of invasive ventilation?

Modes of Invasive Mechanical Ventilation

The primary modes of invasive mechanical ventilation include Controlled Mechanical Ventilation (CMV), Assist/Control Ventilation (ACV), Assisted Spontaneous Breathing (Pressure Support), Synchronized Intermittent Mandatory Ventilation (SIMV), and Pressure-Regulated Volume Control (PRVC), each with distinct mechanisms for supporting patient ventilation. 1

Primary Ventilation Modes

1. Controlled Mechanical Ventilation (CMV)

• Provides full ventilatory support with no patient effort required
• Settings include:
  • Inflation pressure or tidal volume
  • Respiratory frequency
  • Timing of each breath
• Two main types:
  • Pressure Control: Set pressure determines delivered volume based on airway resistance and lung compliance
  • Volume Control: Set tidal volume determines required pressure based on patient mechanics 1

2. Assist/Control Ventilation (ACV)

• Delivers preset mandatory breaths if no patient effort detected
• Patient can trigger additional breaths identical to mandatory breaths
• Features:
  • "Lock out" period prevents breath stacking
  • Synchronized with patient effort
  • Often called spontaneous/timed (S/T) or IE mode on some ventilators 1
• Shown to provide shorter duration of ventilation compared to conventional ventilation 2

3. Pressure Support Ventilation (PSV)

• Patient's respiratory effort triggers both inspiration and expiration
• Patient determines respiratory frequency and timing
• If patient makes no effort, no assistance occurs (though many ventilators include backup rates)
• Often used in weaning protocols 3
• Can be combined with SIMV (SIMV+PS) for improved patient comfort 4

4. Synchronized Intermittent Mandatory Ventilation (SIMV)

• Combines mandatory breaths with spontaneous breathing
• Mandatory breaths are synchronized with patient effort
• Patient can breathe spontaneously between mandatory breaths
• Studies show poorer weaning outcomes compared to other modes 3

5. Pressure-Regulated Volume Control (PRVC)

• Hybrid mode that delivers set tidal volumes using variable pressure
• Also known as Volume Control Plus (VC+)
• Attempts to maintain consistent volumes while minimizing pressure 5

Advanced Modes

1. Airway Pressure Release Ventilation (APRV)

• Maintains high continuous positive airway pressure with brief release periods
• Allows spontaneous breathing throughout respiratory cycle
• Used primarily in severe hypoxemic respiratory failure 5

2. Biphasic Ventilation (BiLevel)

• Alternates between two pressure levels
• Allows spontaneous breathing at both pressure levels
• Similar to APRV but with different timing parameters 5

Clinical Considerations

• No single ventilation mode has consistently demonstrated superiority in clinical outcomes 5
• Mode selection should be based on:
  • Underlying pathophysiology
  • Patient-ventilator synchrony
  • Gas exchange requirements
  • Risk of ventilator-induced lung injury

Common Pitfalls

• Patient-ventilator asynchrony: Can occur with any mode if settings don't match patient effort
• Auto-PEEP: Particularly problematic in obstructive lung disease
• Breath stacking: Risk increases with shorter expiratory times
• Over-reliance on SIMV for weaning: Evidence suggests poorer outcomes compared to pressure support or spontaneous breathing trials 3

Mode Selection Algorithm

1. Initial ventilation: Consider ACV or pressure control for patients requiring full support
2. Improving patients: Transition to pressure support as patient effort improves
3. Weaning: Use spontaneous breathing trials rather than SIMV for better outcomes 3

Ventilator settings must be regularly reassessed and adjusted based on patient response, work of breathing, and gas exchange parameters to optimize outcomes and minimize complications.