A new device for 100 per cent humidification of inspired air

Devices for active humidification of the inspired air inmechanically ventilated patients cause water condensation in the ventilatortubing, which may become contaminated or interfere with the function of theventilator. The present study describes and tests the performance of a newhumidifier, which is designed to eliminate water condensation. To test the performance of the new humidifier at differentventilator settings in a lung model, and to compare this new humidifier with aconventional active humidifier in ventilator-treated critically illpatients. The humidifier (Humid-Heat; Louis Gibeck AB, Upplands Väsby,Sweden) consists of a supply unit with a microprocessor and a water pump, and ahumidification device, which is placed between the Y-piece and the endotrachealtube. The humidification device is based on a hygroscopic heat-moistureexchanger (HME), which absorbs the expired heat and moisture and releases itinto the inspired gas. External heat and water are then added to the patientside of the HME, so the inspired gas should reach 100% humidity at 37°C(44 mg H 2 O/l air). The external water is delivered to thehumidification device via a pump onto a wick and then evaporated into theinspired air by an electrical heater. The microprocessor controls the waterpump and the heater by an algorithm using the minute ventilation (which is fedinto the microprocessor) and the airway temperature measured by a sensormounted in the flex-tube on the patient side of the humidification device. The performance characteristics were tested in a lung modelventilated with a constant flow (inspiratory:expiratory ratio 1:2, rate 12–20breaths/min and a minute ventilation of 3–25 l/min) or with a decelerating flow(inspiratory:expiratory ratio 1:2, rate 12–15 breaths/min and a minuteventilation of 4.7–16.4 l/min). The device was also tested prospectively and ina randomized order compared with a conventional active humidifier (Fisher &Paykel MR730, Auckland, New Zealand) in eight mechanically ventilated,endotracheally intubated patients in the intensive care unit. The test periodwith each device was 24 h. The amount of fluid consumed and the amount of waterin the water traps were measured. The number of times that the water traps wereemptied, changes of machine filters, the suctions and quality of secretions,nebulizations, and the amount of saline instillations and endotracheal tubeobstruction were recorded. In order to evaluate increased expiratory resistancedue to the device, the airway pressure was measured at the end of a prolongedend-expiratory pause at 1 h of use and at the end of the test, and was comparedwith the corresponding pressure before the experiment. The body temperature ofthe patient was measured before and after the test of each device. Both with constant flow and decelerating flow, the Humid-Heat gavean absolute humidity of 41–44 mgH 2 O/l at 37°C, with the lowerlevel at the highest ventilation. In the patients, both Humid-Heat and theconventional active humidifier (MR730) maintained temperatures, indicating thatthey provided the intended heat and moisture to the inspired air. With bothdevices, the body temperature was maintained during the test period. There wasno difference in the amount of secretions, the quality of the secretions andthe frequency of suctions, saline instillations or nebulizations between thetest periods with the two devices. There was no endotracheal tube obstruction,and after 1 h of use and at the end of the test no increased airway resistancewas found with either device. When the MR730 was used, however, the water trapsneeded to be emptied six to 14 (mean eight) times (total amount of fluid in thetraps was 100–300 ml) and the machine filters were changed two to six (meanfour) times due to an excessive amount of condensed water with flowobstruction. No condensation of water was found in the tubing with theHumid-Heat. The water consumption was 23–65 ml/h (mean 30 ml/h) with the MR730and 4–8 ml/h (mean 6 ml/h) with the Humid-Heat ( P < 0.0008). The samerelations were found when the water consumption was corrected for differencesin minute ventilation. The new humidifier, the Humid-Heat, gave an absolute humidity of41–44 mg/l at 37°C in the bench tests. The tests in ventilated patientsshowed that the device was well tolerated and that condensation in the tubingwas eliminated. There was no need to empty water traps. The test period was tooshort to evaluate whether the new device had any other advantages ordisadvantages compared with conventional humidifiers.