What is a trigger in a ventilator and how is it used?

Ventilator Triggering: Definition and Clinical Application

A ventilator trigger is a mechanism that detects patient respiratory effort and initiates a mechanical breath, critical for optimizing patient-ventilator synchrony and reducing work of breathing during mechanical ventilation. 1

Types of Ventilator Triggers

Flow Triggers

• Detect changes in the machine-produced bias flow
• Generally more sensitive and responsive than pressure triggers
• Recommended by the American Thoracic Society for reducing work of breathing 2
• Starting settings:
  • Default sensitivity should be adjusted based on patient response
  • Lower numerical values indicate higher sensitivity

Pressure Triggers

• Detect negative pressure generated by patient's inspiratory effort
• May be used when flow triggering is unavailable or ineffective
• Often require more patient effort than flow triggers 3

Clinical Importance of Trigger Settings

Impact on Patient Outcomes

• Proper trigger settings are essential for:
  • Reducing work of breathing
  • Preventing patient-ventilator asynchrony
  • Optimizing ventilation outcomes 2
  • Potentially reducing duration of mechanical ventilation 4

Types of Trigger Asynchrony

1. Auto-triggering: Occurs when ventilator delivers breath without patient effort

   • Often caused by:
     • Excessive trigger sensitivity
     • Cardiogenic oscillations (heart-induced pressure/flow variations) 5
     • Circuit leaks

2. Ineffective triggering/Wasted efforts: Patient effort fails to trigger ventilator

   • Often caused by:
     • Insensitive trigger settings
     • High intrinsic PEEP (PEEPi) in COPD patients
     • Respiratory muscle weakness 6

Optimizing Trigger Settings

General Recommendations

• For patients with obstructive disease (e.g., COPD):

  • Start with E-sens at 25-30% of peak inspiratory flow
  • Consider applying external PEEP to offset intrinsic PEEP 1, 2
  • Use shorter inspiratory time (approximately 30% of total cycle) 2

• For patients with restrictive disease:

  • Start with E-sens at 35-40% of peak inspiratory flow
  • Use longer inspiratory time (approximately 40% of total cycle) 2

Trigger Adjustment Algorithm

1. Assess for trigger asynchrony:

   • Observe for uncoupling between patient effort and ventilator response
   • Look for signs of increased work of breathing or auto-triggering

2. If auto-triggering is present:

   • Decrease trigger sensitivity (increase flow trigger threshold)
   • Consider switching from flow to pressure trigger if persistent
   • Evaluate for and address cardiogenic oscillations 5

3. If ineffective triggering/wasted efforts are present:

   • Increase trigger sensitivity (decrease flow trigger threshold)
   • For COPD patients, apply external PEEP (up to 5 cm H2O typically) 1
   • Consider reducing ventilator support to increase respiratory drive 4

Special Considerations

COPD Patients

• Often have high intrinsic PEEP requiring special attention to trigger settings
• External PEEP helps overcome intrinsic PEEP, improving trigger sensitivity 1
• Persistent trigger asynchrony in COPD is associated with poor weaning outcomes 4

Ventilator-Dependent Patients

• Trigger asynchrony is associated with:
  • Prolonged mechanical ventilation
  • Increased hospital length of stay
  • Lower weaning success rates 4, 7

Practical Tips

• Flow triggering generally performs better with pressure-targeted modes than volume-targeted modes 3
• Cardiogenic oscillations may require less sensitive trigger settings to avoid auto-triggering 5
• Monitor for changes in patient condition that may require trigger adjustments

Common Pitfalls to Avoid

1. Setting trigger sensitivity too high, causing auto-triggering
2. Setting trigger sensitivity too low, increasing work of breathing
3. Failing to recognize and address intrinsic PEEP in COPD patients
4. Overlooking the impact of circuit leaks on trigger function
5. Not adjusting triggers when changing ventilation modes

By optimizing ventilator trigger settings based on patient characteristics and ventilatory needs, clinicians can significantly improve patient comfort, reduce work of breathing, and potentially improve outcomes in mechanically ventilated patients.