A 62-yo man admitted to ICU for pneumonia with severe hypoxemia was intubated and ventilated with volume-controlled mode, with VT 6 mL/kg, respiratory rate 30 breaths/min, inspiratory time 0.4 s, FIO2 0.60, and PEEP 10 cm H2O. Pressure-support ventilation was started on day 7, at 20 cm H2O, and gradually lowered, by 5 cm H2O at a time, as long as the respiratory rate was between 26 and 35 breaths/min and pulse oximetry was >88%. PEEP was kept constant at 5 cm H2O. Which of the following changes is primarily responsible for triggering inspiration with pressure support ventilation?
Select one of the following: