Phase one (n=12): Bench-test study
The ventilator was the VN500 (Drager Lubeck, Germany) in volume guarantee (VG) mode during both IPPV (IPPV-VG) and HFOV (HFOV-VG). The ventilatory settings were the parameters usually used in clinical settings for each mode and repeated at each set of experiments. During IPPV, we targeted a VT of 4 mL with positive end-expiratory pressure set at 4 mbar, frequency at 50/min and an inspiratory time of 0.3 seconds. During HFOV-VG, we targeted a VT of 2 mL, with frequency set at 12 Hz (720/min), mean airway pressure at 12 mbar, and the inspiratory/expiratory time ratio at 1:1 ( i.e., an inspiratory time of 0.042 sec).
The test lung was located in a Giraffe closed incubator (GE Healthcare, USA) with heat set to 37° C. The compliance of the test lung was calibrated around 0.3 mL/mb ± 5%, to be close to the usual lung compliance of ELBW patients.11
To test the influence of heating devices on humidification, we conducted the experiments with 2 heater-humidifiers: the MR850with a RT265 as the ventilator circuit, referred to hereafter as MR-A and the MR950 with the 950-N81 circuit (F&P, Auckland, New Zealand), referred to as MR-B. The heater-humidifiers were in automatic mode without any settings to adjust. For the RT265 circuit, we used a 30 cm extension circuit without wireless heating inside the incubator to leave the temperature sensor outside the incubator. The RT 950-N81 circuit did not necessitate an extension and was directly connected to the Y-piece.
We used the procedure described below to conduct 12 experiments (6 with MR-A and 6 with MR-B). First, the ventilatory circuit was dried until RH < 2% was reached. Then, we started ventilation and humidification in the randomly selected ventilatory mode — HFOV-VG or IPPV-VG — for at least one hour without recording. Humidity and temperature recording started after one hour and lasted for 4 hours: 2 hours in the first ventilatory mode (IPPV-VG or HFOV-VG) and 2 hours in the other mode. The first ventilatory mode was selected randomly, via sealed envelopes.