What are the optimal settings for inspiratory trigger, expiratory trigger, bias flow, Pressure Support (PS) rise time, inspiratory time (Tinsp), and inspiratory pause in mechanical ventilation?

Optimal Mechanical Ventilation Settings: Understanding Key Parameters

The optimal mechanical ventilation settings should prioritize patient-ventilator synchrony by appropriately configuring inspiratory trigger, expiratory trigger, bias flow, PS rise time, inspiratory time, and inspiratory pause based on underlying respiratory mechanics and disease pathophysiology.

Inspiratory Trigger

Inspiratory triggering is critical to successful mechanical ventilation and involves detecting the patient's inspiratory effort to initiate breath delivery 1.

• Flow triggering detects changes in the ventilator's bias flow and generally reduces inspiratory effort compared to pressure triggering in patients with COPD 2
• Pressure triggering requires the patient to generate a negative pressure (typically -1 to -2 cmH2O) to initiate a breath 1
• Sensitivity settings:
  • For flow triggering: 3-5 L/min is typically appropriate 3
  • For pressure triggering: -0.5 to -2 cmH2O 4
• Considerations for specific conditions:
  • In patients with intrinsic PEEP (PEEPi), additional effort is required to trigger the ventilator as PEEPi must be counterbalanced before triggering can occur 1
  • Adding external PEEP (typically 5 cmH2O) can help offset PEEPi and reduce triggering effort in obstructive diseases 1

Expiratory Trigger (Cycling)

Expiratory triggering determines when inspiration ends and expiration begins 1.

• Flow cycling: Ventilator switches to expiration when inspiratory flow decreases to a set percentage (typically 20-80%) of peak inspiratory flow 1
• Time cycling: Inspiration ends after a predetermined inspiratory time 1
• Optimal settings:
  • For obstructive diseases (COPD, asthma): Earlier cycling (25-30% of peak flow) to allow adequate expiratory time and prevent dynamic hyperinflation 1
  • For restrictive diseases: Later cycling (40-50% of peak flow) to optimize inspiratory time and volume delivery 1

Bias Flow

Bias flow is the continuous flow of gas through the ventilator circuit during expiration 5.

• Function: Facilitates triggering by providing a baseline flow that can be detected when the patient initiates inspiration 1, 5
• Optimal settings:
  • Typically 5-10 L/min in adult ventilators 5
  • Higher bias flow can improve trigger sensitivity but may increase triggering delay and work of breathing if set too high 5
• Considerations:
  • Excessive bias flow can lead to auto-triggering and patient-ventilator asynchrony 1, 5
  • Insufficient bias flow may delay breath triggering 5

Pressure Support (PS) Rise Time

PS rise time controls how quickly the ventilator reaches the set inspiratory pressure after triggering 1.

• Function: Determines the rate of pressurization during the initial phase of inspiration 1
• Optimal settings:
  • For obstructive diseases: Slower rise time (200-300 ms) to prevent excessive initial flow and potential discomfort 1
  • For restrictive diseases: Faster rise time (100-150 ms) to meet higher inspiratory demand 1
• Considerations:
  • Too fast rise time can cause pressure overshoot and patient discomfort 1
  • Too slow rise time can increase work of breathing and cause asynchrony 1

Inspiratory Time (Tinsp)

Inspiratory time determines the duration of the inspiratory phase 1.

• Optimal settings:
  • Should be set based on respiratory system mechanics and underlying disease 1
  • Can be calculated based on respiratory rate and desired I:E ratio 1
  • Typical I:E ratios:
    • Normal lungs: 1:2 ratio 1
    • Obstructive disease: 1:3 or greater to allow adequate expiratory time 1
    • Restrictive disease: 1:1 to 1:1.5 to optimize inspiratory time 1
• Formula for calculating inspiratory time:
  • Inspiratory time = (60/respiratory rate) × (I/(I+E)) 1
  • Example: At RR of 15 breaths/min with I:E ratio of 1:2, Tinsp = (60/15) × (1/3) = 1.33 seconds 1

Inspiratory Pause

Inspiratory pause is a brief hold at the end of inspiration before expiration begins 1.

• Function: Allows additional time for gas distribution and alveolar recruitment 1
• Optimal settings:
  • Generally 0.1-0.3 seconds when used 1
  • Consider in restrictive diseases to improve gas distribution 1
• Considerations:
  • Should be avoided or minimized in obstructive diseases to prevent air trapping 1
  • May be beneficial in ARDS to improve oxygenation through recruitment 1

Patient-Ventilator Synchrony

Achieving optimal patient-ventilator synchrony requires proper adjustment of all parameters above 1.

• Common asynchronies:
  • Trigger asynchrony: Ineffective triggering, double triggering, auto-triggering 1
  • Flow asynchrony: Inadequate flow delivery relative to patient demand 1
  • Cycle asynchrony: Premature or delayed cycling 1
• Monitoring for asynchrony:
  • Observe pressure-time and flow-time scalars 1
  • Monitor patient comfort and respiratory rate 1
  • Consider measuring transpulmonary pressure in complex cases 1

Common Pitfalls and Caveats

• Auto-PEEP/Intrinsic PEEP: Can significantly increase triggering effort and cause ineffective triggering; consider adding external PEEP and extending expiratory time 1
• Excessive sensitivity: Can lead to auto-triggering from cardiac oscillations or circuit leaks 1
• Insufficient sensitivity: Increases work of breathing and may lead to patient fatigue 1, 2
• Leaks: Can interfere with proper triggering and cycling, particularly in non-invasive ventilation 1
• Inappropriate rise time: Too fast can cause discomfort; too slow can increase work of breathing 1
• Inappropriate inspiratory time: Too long can lead to air trapping in obstructive disease; too short can cause inadequate ventilation in restrictive disease 1

By understanding and appropriately setting these parameters based on patient pathophysiology, clinicians can optimize mechanical ventilation, minimize work of breathing, and improve patient comfort and outcomes.