Trigger threshold for auto-PEEP

Effect of auto-PEEP on elevation of the triggering threshold in a mechanically ventilated patient with obstructive airways disease. This figure representation shows airway pressure over time. The solid blue line demonstrates the circuit pressure as measured by the ventilator manometer, and the dashed red line is the alveolar pressure. In the absence of auto-PEEP, the sensitivity setting of –1 cm H₂O would be achieved by the patient making inspiratory efforts at the end of expiration, when airway pressure is at its minimum. In the presence of auto-PEEP, alveolar pressure remains positive. In this setting, the patient's inspiratory effort needs to decrease airway pressure not only by the –1 cm H₂O sensitivity set on the machine, but also by the amount of positive alveolar pressure (auto-PEEP). In this figure, the patient's inspiratory efforts are insufficient to trigger the ventilator, and the patient is "locked out," being unable to get a breath because of an inability to overcome the elevated effective triggering threshold rendered by auto-PEEP.