Mechanism of Failed (Ineffective) Trigger

Triggering and Jumping Hurdles: A Clinical Analogy for Failed Triggers (Ineffective triggering) in Mechanical Ventilation.

Imagine the patient's effort to trigger a ventilator breath as a person trying to jump over a hurdle. The height of the hurdle represents the trigger sensitivity threshold, and the person’s strength represents the patient’s inspiratory effort (Pmus).

Scenario 1: Normal trigger- A normal person easily jumps over a normally sized hurdle.

Clinical Parallel: The patient has adequate inspiratory effort, and the ventilator’s trigger sensitivity is appropriately set.

Outcome: The ventilator is triggered effectively and synchrony is maintained.

Scenario 2: Weak Effort — A tired or injured person tries to jump a regular hurdle but lacks the strength.

Clinical Parallel: The trigger sensitivity is normal, but the patient has weak respiratory effort (low Pmus), as seen in neuromuscular weakness, sedation, or fatigue.

Outcome: The patient fails to trigger the ventilator despite trying.

Solution: Enhancing inspiratory effort (e.g., reduce sedation, treat weakness, optimize muscle performance).

Lowering the hurdle → Increase trigger sensitivity (e.g., make flow or pressure trigger more sensitive).

Scenario 3: High Trigger Threshold — A normal person faces an unusually tall hurdle and fails to clear it.

Clinical Parallel: The patient has good inspiratory strength, but the trigger threshold is set too high (e.g., insensitive pressure or flow trigger).

Outcome: The effort is made but does not meet the ventilator’s high trigger requirement.

Solution: Lower the hurdle → Make the ventilator more sensitive by adjusting trigger settings appropriately (e.g., decrease pressure trigger from –4 to –2 cmH₂O, or reduce flow trigger).

Scenario 4: Auto-PEEP — A normal person is placed in a pit and must first climb up to ground level before jumping the hurdle. The deeper the pit, the harder the task.

Clinical Parallel: The pit depth represents auto-PEEP (intrinsic PEEP). The trigger threshold is normal, but the patient must first overcome auto-PEEP just to begin approaching the trigger threshold.

Outcome: High work of breathing, ineffective trigger, or delayed response.

Solution: Raise the person closer to the hurdle → Apply external (extrinsic) PEEP, typically set just below auto-PEEP, which reduces the patient’s effort required to trigger the breath.

Additional strategies: reduce air trapping (increase expiratory time, treat obstruction)