Trigger & Flow

Trigger

• A trigger is what the ventilator senses to give a patient-initiated breath

• There are two types of mechanisms: changes in pressure & changes in flow

• Pressure-triggering generally requires the patient to drop the end-expiratory pressure by about 1-4

  • Pressure-triggering is problematic in patients w/COPD, asthma, or other conditions predisposing to dynamic hyperinflation (auto-PEEP) because they will need to generate a higher inspiratory pressure

  • Ineffective triggering can be recognized on exam as inspiratory efforts that do not lead to a ventilator-initiated breath

• Most ventilators also allow the trigger to be initiated by inspiratory flow, usually in the range of 1-6 L/min

  • If the flow trigger is too low, then it may "auto-cycle" even when the patient is not trying to initiate a breath

  • Oscillations in ventilator tubing (i.e., water, secretions, cardiac contractions, patient movement) may inadvertently trigger a breath

  • The solution is to make the trigger less sensitive or switch to pressure triggering 

Flow

Dynamic Hyperinflation