What are the different modes of mechanical ventilation and their applications?

Mechanical Ventilation Modes and Their Clinical Applications

Mechanical ventilation is a life-saving intervention that provides respiratory support for critically ill patients. The most appropriate ventilation mode should be selected based on the patient's underlying pathophysiology, with volume-controlled assist-control ventilation being the preferred initial mode for most critically ill patients requiring full ventilatory support due to its ability to guarantee minute ventilation and reduce mortality. 1

Classification of Ventilation Modes

Mechanical ventilators can be broadly classified based on their control variables:

1. Volume-Targeted Modes

• Volume Assist-Control (AC or ACMV)

  • Preset tidal volume delivered at a backup rate
  • Patient can trigger additional breaths above the set rate
  • Each breath (patient-triggered or machine-triggered) delivers the same preset tidal volume
  • Guarantees minute ventilation
  • Commonly used for initial ventilation in unstable patients 1

• Synchronized Intermittent Mandatory Ventilation (SIMV)

  • Delivers preset mandatory breaths synchronized with patient effort
  • Patient can breathe spontaneously between mandatory breaths
  • Spontaneous breaths depend on patient's inspiratory effort
  • Often combined with pressure support for spontaneous breaths

2. Pressure-Targeted Modes

• Pressure Control Ventilation (PCV)

  • Delivers preset pressure for a set inspiratory time
  • Tidal volume varies based on lung compliance and resistance
  • Time-cycled mode
  • Used in patients with poor lung compliance or high airway pressures

• Pressure Support Ventilation (PSV)

  • Patient-triggered, pressure-limited, flow-cycled mode
  • Patient controls respiratory rate and inspiratory time
  • Ventilator cycles to expiration when flow decreases to 25% of peak flow
  • Requires intact respiratory drive
  • Well-tolerated and comfortable for spontaneously breathing patients 1

3. Other Modes

• Continuous Positive Airway Pressure (CPAP)

  • Maintains positive pressure throughout respiratory cycle
  • Patient performs all work of breathing
  • Improves oxygenation by recruiting collapsed alveoli
  • Not considered ventilatory support but rather oxygenation support 1

• Proportional Assist Ventilation (PAV)

  • Delivers pressure proportional to patient effort
  • Improves patient-ventilator synchrony
  • May improve sleep quality compared to PSV 1

• Neurally Adjusted Ventilatory Assist (NAVA)

  • Uses diaphragmatic electrical activity to trigger and cycle ventilation
  • Improves patient-ventilator synchrony 1

Clinical Applications and Mode Selection

For Initial Ventilation in Critically Ill Patients

• Volume Assist-Control is preferred because:
  • Guarantees minute ventilation
  • Provides full respiratory support
  • Reduces work of breathing
  • Allows respiratory muscles to rest 1

For Acute Respiratory Distress Syndrome (ARDS)

• Volume-controlled ventilation with lung-protective strategy:
  • Low tidal volumes (4-8 ml/kg predicted body weight)
  • Plateau pressure ≤30 cmH₂O
  • Appropriate PEEP based on severity
  • Consider prone positioning for severe ARDS (PaO₂/FiO₂ < 150 mmHg) 2

For COPD Exacerbation

• Pressure Support or Assist-Control with attention to:
  • Adequate expiratory time to prevent air trapping
  • Lower respiratory rates
  • External PEEP to counteract intrinsic PEEP

For Weaning

• SIMV (often with pressure support) or PSV:
  • Gradually decreases ventilatory support
  • Allows respiratory muscles to work progressively
  • Daily assessment for weaning readiness 2

Advantages and Disadvantages of Common Modes

Volume Assist-Control

• Advantages:

  • Guaranteed minute ventilation
  • Full ventilatory support
  • Patient can breathe above set rate
  • Synchronizes with patient effort
  • Rests respiratory muscles

• Disadvantages:

  • Potential for respiratory alkalosis if rate set too high
  • High airway pressures in poor lung mechanics
  • Potential for patient-ventilator asynchrony
  • Often requires sedation
  • Risk of respiratory muscle atrophy with prolonged use 1

SIMV

• Advantages:

  • Synchronizes mandatory breaths with patient effort
  • Patient can breathe at preferred rate between mandatory breaths
  • Maintains respiratory muscle activity
  • Less risk of respiratory muscle atrophy

• Disadvantages:

  • Potential for high work of breathing during spontaneous breaths
  • Risk of hypoventilation if mandatory rate too low
  • Requires pressure support for spontaneous breaths to be effective 1

Pressure Support

• Advantages:

  • Patient controls breath timing and flow
  • Comfortable and well-tolerated
  • Flexible level of support
  • Better patient-ventilator synchrony

• Disadvantages:

  • Does not guarantee tidal volumes
  • Risk of hypoventilation if lung mechanics worsen
  • Requires intact respiratory drive
  • Can cause central apneas during sleep if set too high 1
  • Poor tolerance in active bronchospasm 1

Patient-Ventilator Asynchrony

Patient-ventilator asynchrony occurs when there is a mismatch between neural and ventilator timing, which can lead to:

• Increased work of breathing
• Patient discomfort
• Prolonged mechanical ventilation
• Increased mortality 3

Common types of asynchrony include:

• Trigger asynchrony (ineffective triggering, auto-triggering)
• Flow asynchrony (inadequate flow)
• Cycle asynchrony (premature or delayed cycling)
• Double-triggering

Monitoring for asynchrony is essential, particularly in:

• COPD patients on PSV (delayed cycling due to high flow rates late in inspiration) 1
• Patients with high respiratory drive
• Patients with changing respiratory mechanics

Practical Considerations for Mode Selection

1. Initial Ventilation: Start with Volume AC for unstable patients to ensure adequate ventilation
2. ARDS Management: Use volume-controlled ventilation with lung-protective strategy
3. Patient Comfort: Consider PSV for awake, spontaneously breathing patients with stable respiratory drive
4. Weaning: Transition to SIMV+PS or PSV as patient improves
5. Sleep Quality: Consider assist-control mode during sleep to prevent central apneas that can occur with PSV 1

Pitfalls to Avoid

• Setting excessive pressure support leading to hyperventilation and central apneas during sleep 1
• Inadequate PEEP in COPD patients failing to counteract intrinsic PEEP
• Prolonged full support leading to respiratory muscle atrophy
• Ignoring patient-ventilator asynchrony which can increase work of breathing and prolong ventilation
• One-size-fits-all approach rather than tailoring to the patient's specific pathophysiology 4

By understanding the principles, advantages, and limitations of different ventilation modes, clinicians can optimize respiratory support while minimizing complications, ultimately improving patient outcomes in terms of morbidity, mortality, and quality of life.