What is the difference between pressure trigger and flow trigger (flow-triggering mechanism) in ventilators for patients requiring mechanical ventilation?

Difference Between Pressure Trigger and Flow Trigger in Mechanical Ventilation

Flow triggering is superior to pressure triggering because it reduces patient work of breathing, shortens trigger delay time, and maintains airway pressure at or above end-expiratory levels during the post-trigger phase, though pressure triggering at -0.5 cm H₂O can achieve comparable sensitivity in some patients. 1, 2, 3

Fundamental Mechanism Differences

Pressure Triggering

• Requires the patient to generate negative pressure (typically -1 to -2 cm H₂O below PEEP) to initiate a ventilator breath 1
• The ventilator senses a drop in circuit pressure when the patient begins inspiratory effort 4
• In patients with intrinsic PEEP (PEEPi), the patient must first overcome PEEPi before generating sufficient negative pressure to trigger the ventilator, substantially increasing work of breathing 1
• Adding external PEEP (typically 5 cm H₂O) helps offset PEEPi and reduces triggering effort in obstructive diseases like COPD 1

Flow Triggering

• Detects changes in the machine-produced bias flow through the ventilator circuit 5, 1
• A continuous baseline flow (typically 5-10 L/min) circulates through the circuit; when the patient inhales, the return flow decreases, and this difference triggers the breath 6, 4
• The flow waveform method is more sensitive to patient effort than pressure triggering, resulting in less ineffective effort but potentially more auto-triggering 6

Clinical Performance Differences

Work of Breathing

• Flow triggering reduces patient work of breathing by approximately 50% compared to conventional pressure triggering 2, 7
• Flow triggering significantly decreases the pressure-time product (PTP) and respiratory drive (P0.1) during pressure support ventilation 2
• The mechanism involves maintaining airway pressure at or above end-expiratory pressure levels during the post-trigger phase, unlike demand-flow systems that may allow pressure to drop 3

Trigger Delay and Responsiveness

• Flow triggering provides a relatively shorter time delay between patient effort and ventilator response compared to pressure triggering 3
• The flow waveform method requires considerably less simulated patient effort to trigger the ventilator in bench studies with controlled dynamic hyperinflation 6
• However, pressure triggering set at -0.5 cm H₂O can be significantly more sensitive than flow triggering for some patients, with lower trigger pressure (ΔP), pressure-time product, and trigger time (ΔT) 4

Patient Comfort and Synchrony

• Twelve of fourteen patients reported subjectively better comfort with flow triggering added to pressure support ventilation 2
• Flow triggering improves rapid shallow breathing index (f/Vt) and reduces respiratory rate while maintaining adequate minute ventilation 2
• The British Thoracic Society notes that bi-level ventilators employing flow sensors for triggering have been widely adopted due to improved patient comfort 5

Critical Pitfalls and Practical Considerations

Auto-triggering Risk

• Flow triggering is highly sensitive but may be unstable under certain circumstances, leading to auto-triggering (ventilator-initiated breaths without patient effort) 6
• This occurs when circuit leaks, condensation in tubing, or cardiac oscillations create flow changes that mimic patient effort 6
• Set flow trigger sensitivity carefully to balance responsiveness against auto-triggering risk 1

Patient-Specific Variability

• There is significant inter-patient variability in trigger sensitivity for both methods 4
• In patients with severe COPD where PEEPi may reach 10-15 cm H₂O, flow triggering combined with external PEEP (typically 3-5 cm H₂O) helps offset intrinsic PEEP and improves triggering 5, 1
• For neuromuscular disease patients with insufficient respiratory effort, timed or assist-control modes may be necessary regardless of trigger type 5

Ventilator-Specific Differences

• The American Thoracic Society recommends that ventilators for acute NIV should have "sensitive flow triggers" as an essential feature 5
• Some modern ventilators allow adjustable inspiratory trigger sensitivity, which is listed as a desirable feature for hospital NIV 5
• Portable ventilators may have inferior triggering systems compared to ICU ventilators, impacting patient adaptation and required sedation levels 5

Algorithmic Approach to Trigger Selection

For most mechanically ventilated patients:

1. Start with flow triggering (5 L/min sensitivity) as the default due to reduced work of breathing and improved patient comfort 2, 3
2. Monitor for auto-triggering; if present, reduce flow trigger sensitivity or switch to pressure triggering 6
3. For COPD/asthma patients with air trapping: Use flow triggering with external PEEP (5 cm H₂O) to offset intrinsic PEEP 1
4. If flow triggering unavailable: Set pressure trigger at -0.5 cm H₂O for optimal sensitivity 4
5. If patient-ventilator asynchrony persists despite optimized triggering: Consider switching to timed or assist-control mode 5, 8