Mechanical ventilation with heated humidifiers: measurements of condensed water mass within the breathing circuit according to ventilatory settings

Heated wire humidifiers (HWHs) are widely used to heat and humidify gases during mechanical ventilation. The control strategy implemented on commercial HWHs, based on maintaining constant gas temperature at the chamber outlet, shows weaknesses: humidifying performances depend on environmental temperature and ventilatory settings, and often condensation occurs. Herein, we analyzed in vitro HWH performances focusing on the condensation amount according to ventilatory settings. We used a physical model to define the parameters which mainly influence the HWH performances. In order to investigate the influence of minute volume (MV) and frequency rate (f(r)) on condensation, the other influencing parameters were kept constant during experiments, and we introduced a novel approach to estimate the condensation. The method, based on measuring the condensed vapor mass (Delta m), provided more objective information than the visual-based scale used in previous studies. Thanks to both the control of other influencing factors and the accurate Delta m measures, the investigation showed the Delta m increase with MV and f(r). Substantial condensation after 7 h of ventilation and the influence of MV and f(r) on Delta m (i.e., Delta m = 3 g at MV = 1.5 L min(-1) and f(r) = 8 bpm and Delta m = 9.4 g at MV = 8 L min(-1) and f(r) = 20 bpm) confirm the weaknesses of 'single-point temperature' control strategies.