Relative humidity and temperature measures
For both in-vitro and in-vivo experiments, RH and temperature were recorded in the Y-piece just before the endotracheal tube (ETT) with a capacitive thermohygrometer and a sensor 4 mm in diameter (Testo Hygrometer 635-2 with sensor 2135, Forbach, France). The hygrometer was able to record data for a predetermined period of time (4 hours for in-vitro-study and 2 hours for in-vivo study) at a predetermined frequency (every 1 minute). The recordings were converted into two curves: RH (in %) and temperature (in °C) with PC-software (Testo Comfort Software x35 professional, Forbach, France). For analysis, we considered only the second hour of recording in each ventilatory mode. The first hour was considered a stabilization period. AH (in mg/L) was calculated with the following mathematical formula: (287.7 × RH × Psat)/(T+273)12 with the saturated vapor pressure of water (Psat) expressed as mm Hg and temperature (T) as °C.
Beforehand, the introduction of the hygrometer into the Y-piece was tested to make sure that there was no pressure change.
Statistical analysis:
Data were described as means and their standard deviations. We compared relative and absolute humidity during IPPV versus HFOV during both the in-vitro and in-vivo experiments. We compared humidity during IPPV versus HFOV with two different heater-humidifiers (MR-A and MR-B) only during in-vitro experiments. Paired Wilcoxon tests were used in the bench tests to compare HFOV vs IPPV as the comparisons were made at the same time with the same environmental conditions. Comparisons between MR-A and MR-B used Mann-Whitney tests. In the in-vivo study, because the two modes were tested at two potentially separated periods, we used Mann-Whitney tests rather than paired tests. A P -value ≤ 0.05 was considered significant.