Should the flow trigger be increased or reduced for weaning?

Flow Trigger Adjustment for Ventilator Weaning

For ventilator weaning, the flow trigger should be reduced (made more sensitive) rather than increased, as this decreases the work of breathing and improves patient-ventilator synchrony during the weaning process.

Understanding Flow Triggers in Mechanical Ventilation

• Flow triggering refers to the mechanism by which the ventilator detects a patient's inspiratory effort by sensing a change in flow within the circuit 1
• Modern ventilators maintain a continuous base flow through the circuit, and when the patient initiates a breath, they divert some of this flow, which the ventilator detects 2
• The flow trigger setting determines how much flow change is required to trigger a breath - a lower number means greater sensitivity 1

Benefits of Reducing Flow Trigger During Weaning

• Reducing the flow trigger (making it more sensitive) decreases the work of breathing by requiring less patient effort to initiate a breath 3
• Lower flow trigger settings reduce the pressure-time product (PTP), which is a measure of respiratory muscle workload 3
• More sensitive flow triggering results in shorter trigger delay times, allowing for better synchronization between patient effort and ventilator response 1
• Improved patient-ventilator synchrony reduces the risk of trigger asynchrony, which is associated with poor weaning outcomes 4

Clinical Implications for Weaning

• Patient-ventilator trigger asynchrony is associated with significantly lower weaning success rates (16% vs 57% in patients without asynchrony) 4
• Flow triggering reduces inspiratory effort during both mandatory and spontaneous breaths in synchronized intermittent mandatory ventilation (SIMV), which is beneficial during the weaning process 3
• The rapid shallow breathing index (RSBI), an important weaning parameter, is significantly affected by flow trigger settings - more sensitive settings can reduce RSBI values by 12-17% 2

Potential Pitfalls and Considerations

• Setting the flow trigger too sensitive (too low) may cause auto-triggering, where the ventilator delivers breaths without patient effort 1
• In patients with intrinsic PEEP (auto-PEEP), even a sensitive flow trigger may not be sufficient to overcome the inspiratory threshold load 5
• Application of external PEEP may be necessary to counterbalance intrinsic PEEP and improve triggering in patients with conditions like COPD 5
• Flow triggering should be used alongside appropriate ventilator modes for weaning, such as pressure support ventilation (PSV) rather than SIMV, which has shown poorer weaning outcomes 6

Special Considerations for COVID-19 Patients

• In COVID-19 patients with tracheostomy, pressure support mode with an inflated cuff is preferred over spontaneous breathing trials using T-piece during weaning to minimize aerosol generation 5
• When transitioning from mechanical ventilation, an HME with viral filter should be placed to decrease expulsion of viral particles 5

Practical Recommendations

• Start with a flow trigger setting of 1-2 L/min during weaning to minimize work of breathing while avoiding auto-triggering 1
• Monitor for signs of trigger asynchrony such as uncoupling of accessory respiratory muscle efforts and onset of machine breaths 4
• Consider using flow triggering in conjunction with NIV for patients with hypercapnic respiratory failure to facilitate weaning from mechanical ventilation 5
• For patients with high levels of intrinsic PEEP, apply external PEEP (typically up to 85% of measured intrinsic PEEP) to improve triggering sensitivity 5