A new device for 100% humidification of inspired air

Introduction

Passive heat/moisture ex-changers (HME) which are based on a hygroscopic condenser principle usually provide adequate humidity (up to 32 mgH₂O/l air) of the inspired gas during ventilator treatment [1,2]. However, in about 5-10% of the patients with e.g. thick secretions [1,2] active humidifiers that can provide 100% humidity are needed. These devices cause free water condensation in the tubings [3] with risks of contamination and of compromising the ventilator function. To avoid this a new humidifier has been developed. It consists of a supply unit with a microprocessor and a water pump, and a humidification device, which is placed between the Y-piece and the endotracheal tube. The humidification device is based on a hygroscopic HME, which absorbs the expired heat and moisture and releases it to the inspired gas. External heat and water are then added to the patient side of the HME, so the inspired gas reaches 100% humidity at 37 ºC (44 mgH₂O/l air). The external water is delivered via a pump onto a porous membrane and then evaporated in the inspired air by an electrical heater. The microprocessor controls the water pump and the heater by an algorithm using the minute ventilation (which is fed into the microprocessor) and the airway temperature measured by a sensor mounted in the flex tube on the patient side of the humidification device.

The aim of this study was to test the performance of this humidifier at different ventilator settings in a lung model.

Methods

The lung model is based on the ISO 9360 International Standard with the exception of that the water-bath temperature is regulated to have a constant temperature of 35.5 ± 0.5 °C. The model was ventilated with a Siemens 900 B ventilator set a minute ventilation from 5 to 25 l/min, I : E 1 : 2, and a rate of 12, 15 or 20/min during 90 min. The moisture content (MC) in the inspired air was calculated from the water delivered (WD) and the loss of water from the lung model (WL) : MC = WL - WT + (WD-WH),where WT is the water in the tubing between the device and the lung model and WH the water trapped in the HME. WL, WT, WD, and WH were found by weighing before and after the experiment. During the experiment no condensation was found in the flex tube between the device and the lung model.

Results

Figure

In a lung model, ventilated with 5–25 l/min, the new humidifier gave an absolute humidity of 39–45 g/l, with the lower level at the highest ventilation. Thus, the device had the intended performance characteristics

Conclusion

In a lung model, ventilated with 5-25 l/min, the new humidifier gave an absolute humidity of 39-45 g/l. with the lower level at the highest ventilation. Thus, the device had the intended performance characteristics.