Evaluation of Expiratory Humidity With Different Heated Humidifiers Circuits

Abstract

Background:

The optimization of the inspiratory and expiratory circuits used with heated humidification is necessary to avoid condensation and related problems, such as auto-triggering, difficulties to trigger the ventilator, or increase in expiratory resistances¹. The objective of the study was to compare humidity in several expiratory circuits of different heated wire humidifiers.

Methods:

On a bench model simulating humidified expired gas (absolute humidity of 35 mg H₂O/L), we have measured hygrometry of expiratory gases at proximal side (corresponding to the y-piece), and distal side (ventilator inlet) of expiratory circuits. The circuit evaluated were Inspired (ref. 51005683, Vincent medical), FP950 (ref:950A81J, Fisher&Paykel), MR850 (RT210, F&P) and Evaqua2 (RT380, F&P), at different room temperatures (22-24°C and 28-30°C). The ventilator settings were assist control, tidal volume 400 mL; frequency 25 breaths/min; PEEP 5 cm H₂O; FIO₂ 0.21; flow 60 L/min. Hygrometry was measured with the psychrometric method and 3 measurements were performed for each condition. Expiratory tidal volumes in each condition were all recorded from the ventilator screen.

Results:

The absolute humidity was reduced along the expiratory circuit with the FP950 and the Evaqua circuits (RT380). To avoid condensation in the expiratory limb, the VHB20 increases the temperature along the expiratory limb (leading to a reduction of relative humidity with stable water content). A similar functioning was observed with the MR850/RT210 circuit. Main results are shown in the figure. No condensation was found along the expiratory limb for the different tested circuits in the bench study conditions. Results at different ambient temperatures were very closes. Despite high temperatures at the inlet ventilator, the tidal volumes measured by the ventilator were very close.

Conclusions:

Different strategies are used to avoid the occurrence of condensation in the expiratory limb. Two circuits (FP950 and Evaqua2) use ‘’porous’’ materials allowing a reduction of the humidity along the expiratory line. Two circuits (VHB20 and RT210) use a strategy of increased temperature allowing a reduction of the relative humidity to limit the risk of condensation. However, this strategy may alter the tidal volume measurements (which has not been shown with the ventilator used in the study) and may increase the resistance if HME filters are used instead of filters (with low hygrophobic/hygroscopic properties)¹.

Figure: Mean Relative Humidity, Absolute Humidity and gas Temperature measured at the Y-piece and at ventilator inlet with different circuits used with VHB20, FP950 and MR850 (RT210 and RT380, Evaqua2) heated humidifiers.