Measurement of condensed water mass during mechanical ventilation with heated wire humidifiers: Experiments with and without pre-warming

Heated wire humidifiers (HWHs) are employed in mechanical ventilation with the objective of heating and humidifying the gases delivered to the mechanical ventilator. They use a control based on the adjustment of gas temperature at the chamber outlet. The condensation occurring within the breathing circuit is one of the most important concerns related to this control strategy. In the present study we focused on the measurement of the condensation amount within the breathing circuit during the employment of a commercial HWH (MR850, Fisher & Paykel). The measurement of the condensed vapor mass, performed during 7 h of ventilation, provides more objective information than the visual-based scale used in literature. Moreover, two solutions were proposed to minimize the condensation in the breathing circuit tract downward the heated chamber: i) a flexible insulating pipe was used to cover the mentioned breathing circuit tract, and ii) the air delivered by ventilator was heated before it passes through the chamber at different inlet temperature T_i obtained by employing pre-warming. To assess the improvement obtained by these two solutions, experiments have been carried out with and without their employment at two minute volumes. Results show that: i) insulation and pre-warming allows minimizing the condensation (e.g., at 8 L·min^-1 the mass of condensation after 7 h of ventilation decreases from 9.3 g to 2.5 g by using insulation and T_i=27 °C); ii) the condensation mass decreases with T_i (e.g., at 8 L·min^-1 the mass condensation was 2.5 g at T_i= 27 °C and 1.1 g at T_i= 30 °C); and iii) the amount of condensation linearly increases with time of ventilation.