Background Adjusting the fraction of inspired oxygen (FiO₂) delivered to preterm infants to keep their oxygen saturation within target range remains challenging. Closed-loop automated FiO₂ control increases the time infants spend within the assigned target range. The delay with which FiO₂ adjustments at the ventilator result in a change in the inspired gas limits the performance of both manual and automated controls.

Objective To evaluate the equilibration time (T_eq) between FiO₂ adjustments and changes in FiO₂ reaching the patient.

Methods In vitro determination of the delay in FiO₂ adjustments at the ventilator at 5 and 8 L/min of gas flow and two different humidifier/ventilator circuit volumes (840 and 432 mL).

Results T_eq values were 31, 23, 20 and 17 s for the volume–flow combinations 840 mL+5 L/min, 840 mL+8 L/min, 432 mL+5 L/min and 432 mL+8 L/min, respectively.

Conclusion The identified delay seems clinically relevant and should be taken into account during manual and automatic control of FiO₂.