In vitro study of a new vaporization humidifying device: DAR HC 2000^®

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In long-term mechanical ventilation, inspired gas should reach the endotracheal tube with a temperature of 30–32°C, and an absolute humidity (H_A) of approximately 28–30 mg H₂O/l.

We tested a humidifier (DAR HC 2000^® composed of: 1) a vaporising cartridge, containing a hydrophobic membrane that separates the gas flow from the heated water; 2) a heating wire that runs all along the inspiratory line, maintaining the heat and moisture conditions of the inspired gas from the cartridge to the patient; 3) a double temperature control that permits the setting of the desired heating level both at the cartridge (T°c) and at the patient's mouth (T°p).

At first, we studied the system's ability to condition inspired gas at different min ventilation (V_E=6, 10, 14 l/min), tidal volume (V_T=250, 500, 750 ml), and inspiratory flow (V_i=0.5, 1 l/sec) with different settings of T°c and T°p (32–32, 34–34, 36–36, 34–32, 36–32, 36–34, 38–34, 39–34 °C, respectively). We found that increasing V_E from 6 to 10 to 14 l/min led to a progressive decrease of H_A from 33.0 ± 4.4 to 27.5 ± 4.4 to 23.4 ± 3.6 mg/l (P<0.01),while V_T and V_i had no effects on humidification.

Secondly, we tested the humidifier at different V_E (from 4 to 16 l/min, 2 l/min per step) with different T°p (32, 34, 36 °C; T°c initially set at the same level as T°p, and then increased at 1°C at a time until condensate appeared in the thermoregulated tube). Adequate humidification levels (H_A= 30 mg/l) can be obtained for each V_E and T°p by increasing the temperature set at the cartridge enough.

The third part of the study was to verify the decrease of microbiological contamination by simulating a contaminated patient (using a reservoir containing Micrococcus luteus) and analysing the inspiratory line with the heating wire switched first OFF, then ON: keeping the thermoregulated tube OFF, as supposed, a bacterial growth was seen in the inspiratory line, while switching it ON, no M. luteus colonies were observable.

In conclusion, DAR HC 2000^® humidifier allows us to obtain adequate levels of humidity of inspired gas for each ventilatory set, just by increasing the cartridge temperature. Besides, the hydrophobic membrane and the heating wire avoid, respectively, the gas-water contact and the condensate formation in the inspiratory line resulting in a reduction of microbiological contamination of the circuit.