Three Common Causes of Failed Trigger You Should Know

A failed trigger occurs when a patient's inspiratory effort fails to generate pressure or flow changes required to reach the trigger sensitivity threshold of a ventilator, and as a result, the ventilator does not initiate the delivery of a breath. 

This can happen for various reasons, including:

a) Respiratory muscle weakness: The first example illustrates a failed trigger due to muscle weakness. On the left side of the image, you can observe a pressure-time scalar with a noticeable pressure drop below the baseline, which reaches the trigger sensitivity threshold (pressure trigger), initiating a breath from the ventilator. Conversely, the pressure-time scalar on the right display a trigger sensitivity threshold similar to normal trigger, but the patient's inspiratory effort (pressure drop below the baseline) is insufficient to reach the trigger sensitivity threshold. This situation often arises when a patient's respiratory muscles are too weak to generate a required effort. Respiratory muscle wakness can occur because of excessive sedation, neuromuscular blockers, prolnged mechanical ventilation, electrolyte imbalance.

b) High trigger sensitivity threshold: the second example demonstrates a failed trigger due to an excessively high trigger sensitivity threshold. In this scenario, the patient does generate a pressure drop below the baseline, similar to a normal trigger; however, it fails to reach the trigger sensitivity threshold because the threshold setting is too high. Adjusting the trigger sensitivity threshold to a lower level is necessary to enable the patient's inspiratory effort to trigger the ventilator effectively.

c) Auto PEEP: The third example illustrates failed trigger due to auto PEEP. When there is auto PEEP present, the respiratory muscles must overcome this pressure before generating a pressure drop below the baseline that is equal to trigger sensitivity threshold. Auto PEEP can occur when air becomes trapped inside the lung, making it challenging for the patient's inspiratory effort to initiate a breath. Increased airway resistance (e.g. COPD, asthma, tube block) and inadequate expiratory time can lead to airtrapping. 

Understanding and addressing the causes of failed trigger is essential for providing appropriate mechanical ventilation to patient.