What are the different modes of mechanical ventilation and when are they used?

Different Modes of Mechanical Ventilation and Their Clinical Applications

Volume-cycled ventilation using assist-control mode is the most appropriate initial ventilation strategy for most patients with acute respiratory failure, with pressure-targeted modes being beneficial alternatives in specific clinical scenarios. 1

Primary Ventilation Modes

Volume-Controlled Ventilation (VCV)

• Assist-Control Mode (AC/CMV)

  • Delivers a preset tidal volume with each breath
  • Patient can trigger additional breaths, but each breath delivers the same preset volume
  • Recommended as initial mode for most patients with respiratory failure 1
  • Ensures consistent minute ventilation
  • Tidal volumes should be 4-8 ml/kg of predicted body weight to prevent ventilator-induced lung injury 2
  • Plateau pressures should be maintained ≤30 cmH2O 2

• Synchronized Intermittent Mandatory Ventilation (SIMV)

  • Delivers preset number of mandatory breaths but allows spontaneous breathing between them
  • Often combined with pressure support for spontaneous breaths
  • More commonly used in patients with milder oxygenation deficits (PaO₂/FiO₂ 201-300) 3
  • May increase work of breathing compared to assist-control

Pressure-Controlled Ventilation (PCV)

• Pressure Control (PC)

  • Sets inspiratory pressure rather than volume
  • Resulting tidal volume depends on lung compliance and resistance
  • Provides decelerating flow pattern which may improve gas distribution
  • May reduce peak airway pressures but requires careful monitoring of delivered volumes 1
  • Less commonly used (approximately 10% of ventilated patients) 3

• Pressure Support Ventilation (PSV)

  • Patient triggers each breath, ventilator provides preset pressure assistance
  • Patient controls respiratory rate and timing
  • Useful for weaning and spontaneously breathing patients
  • Requires intact respiratory drive
  • May reduce work of breathing in spontaneously breathing patients 4

Continuous Positive Airway Pressure (CPAP)

• Maintains constant positive pressure throughout respiratory cycle
• Not technically ventilation as it provides no inspiratory assistance
• Primary indication is to correct hypoxemia 1
• Recruits underventilated lung areas similar to PEEP
• Can unload inspiratory muscles and reduce work of breathing
• Requires high flow capability (>60 L/min) for patients in respiratory distress 1

Clinical Applications by Patient Condition

Acute Respiratory Distress Syndrome (ARDS)

• Recommended approach based on severity:
  • Use low tidal volumes (4-8 ml/kg PBW) for all ARDS patients 2
  • Maintain plateau pressures ≤30 cmH2O 2
  • Higher PEEP for moderate-severe ARDS 2
  • Consider prone positioning for severe ARDS (PaO₂/FiO₂ <100 mmHg) 2
  • Calculate driving pressure (plateau pressure minus PEEP) and target <15 cmH2O 2

Sepsis-Related Respiratory Failure

• Volume-cycled ventilation using assist-control mode is appropriate initially 1
• Target tidal volumes of 6 ml/kg ideal body weight 1
• Apply adequate PEEP to prevent alveolar collapse 1
• Consider non-invasive ventilation in early/mild cases if patient is alert and hemodynamically stable 1

COPD Exacerbation

• Non-invasive ventilation often preferred initially 1
• If intubated, allow longer expiratory times to prevent air trapping
• Consider permissive hypercapnia to avoid high plateau pressures 1
• Pressure support mode may be beneficial during weaning 1

Practical Considerations and Pitfalls

Dyssynchrony Management

• Patient-ventilator dyssynchrony increases with lower tidal volumes in volume control mode 5
• Consider switching to pressure control mode if significant dyssynchrony occurs with low tidal volumes 5
• Adaptive pressure control modes may reduce dyssynchrony while maintaining lung protection 5

Ventilator-Induced Lung Injury Prevention

• Key strategies:
  • Use low tidal volumes (4-8 ml/kg PBW) 2
  • Maintain plateau pressures ≤30 cmH2O 2
  • Apply appropriate PEEP to prevent cyclic alveolar collapse 2
  • Monitor driving pressure (aim for <15 cmH2O) 2
  • Consider permissive hypercapnia to avoid excessive pressures 1

Hemodynamic Effects

• Positive pressure ventilation can reduce venous return and cardiac output 6
• Higher PEEP levels may further compromise hemodynamics
• Monitor for hypotension when initiating mechanical ventilation or increasing PEEP
• Ensure adequate volume status before applying high PEEP levels

Mode Selection Algorithm

1. Initial Assessment:

   • Assess severity of respiratory failure (PaO₂/FiO₂ ratio)
   • Evaluate patient's respiratory drive and work of breathing
   • Check hemodynamic stability

2. For most acute respiratory failure patients:

   • Start with volume-cycled assist-control ventilation 1
   • Set tidal volume 4-8 ml/kg PBW 2
   • Apply moderate PEEP (5-10 cmH₂O) 2
   • Target plateau pressure ≤30 cmH₂O 2

3. For patients with significant dyssynchrony:

   • Consider switching to pressure-controlled ventilation 5
   • Maintain same tidal volume goals by adjusting pressure settings
   • Monitor delivered volumes closely

4. For weaning:

   • Transition to pressure support as patient improves
   • Conduct daily spontaneous breathing trials when ready 2
   • Use protocolized weaning approach 2

Remember that regardless of mode, the principles of lung-protective ventilation (low tidal volumes, appropriate PEEP, limiting plateau pressures) should be applied to minimize ventilator-induced lung injury and improve outcomes.