A new device for 100 per cent humidification of inspired air

Abstract

A new humidifier for use during mechanical ventilation in endotracheally intubated patients is described and tested. The humidifier is based on a heat-moisture exchanger, which absorbs the expired heat and moisture and releases it into the inspired air. External heat and water are then added at the patient side of the heat-moisture exchanger, so that the inspired gas should reach 100% humidity (44 mg/l) at 37°C. In bench tests using constant and decelerating inspiratory flow and minute volumes of 3–25 l the device gave an absolute humidity of 41–44 mg/l, and it reduced the amount of water consumed in eight mechanically ventilated patients compared with a conventional active humidifier. During a 24-h test period there was no water condensation in the ventilator tubing with the new device. Devices for active humidification of the inspired air in mechanically ventilated patients cause water condensation in the ventilator tubing, which may become contaminated or interfere with the function of the ventilator. The present study describes and tests the performance of a new humidifier, which is designed to eliminate water condensation. To test the performance of the new humidifier at different ventilator settings in a lung model, and to compare this new humidifier with a conventional active humidifier in ventilator-treated critically ill patients. O/l air). The external water is delivered to the humidification device via a pump onto a wick and then evaporated into 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 performance characteristics were tested in a lung model ventilated with a constant flow (inspiratory:expiratory ratio 1:2, rate 12–20 breaths/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 minute ventilation of 4.7–16.4 l/min). The device was also tested prospectively and in a 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 period with each device was 24 h. The amount of fluid consumed and the amount of water in the water traps were measured. The number of times that the water traps were emptied, changes of machine filters, the suctions and quality of secretions, nebulizations, and the amount of saline instillations and endotracheal tube obstruction were recorded. In order to evaluate increased expiratory resistance due to the device, the airway pressure was measured at the end of a prolonged end-expiratory pause at 1 h of use and at the end of the test, and was compared with the corresponding pressure before the experiment. The body temperature of the patient was measured before and after the test of each device. < 0.0008). The same relations were found when the water consumption was corrected for differences in minute ventilation. The new humidifier, the Humid-Heat, gave an absolute humidity of 41–44 mg/l at 37°C in the bench tests. The tests in ventilated patients showed that the device was well tolerated and that condensation in the tubing was eliminated. There was no need to empty water traps. The test period was too short to evaluate whether the new device had any other advantages or disadvantages compared with conventional humidifiers.

Introduction:

Devices for active humidification of the inspired air in mechanically ventilated patients cause water condensation in the ventilator tubing, which may become contaminated or interfere with the function of the ventilator. The present study describes and tests the performance of a new humidifier, which is designed to eliminate water condensation.

Objectives:

To test the performance of the new humidifier at different ventilator settings in a lung model, and to compare this new humidifier with a conventional active humidifier in ventilator-treated critically ill patients.

Materials and methods:

O/l air). The external water is delivered to the humidification device via a pump onto a wick and then evaporated into 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 performance characteristics were tested in a lung model ventilated with a constant flow (inspiratory:expiratory ratio 1:2, rate 12–20 breaths/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 minute ventilation of 4.7–16.4 l/min). The device was also tested prospectively and in a 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 period with each device was 24 h. The amount of fluid consumed and the amount of water in the water traps were measured. The number of times that the water traps were emptied, changes of machine filters, the suctions and quality of secretions, nebulizations, and the amount of saline instillations and endotracheal tube obstruction were recorded. In order to evaluate increased expiratory resistance due to the device, the airway pressure was measured at the end of a prolonged end-expiratory pause at 1 h of use and at the end of the test, and was compared with the corresponding pressure before the experiment. The body temperature of the patient was measured before and after the test of each device.

Results:

< 0.0008). The same relations were found when the water consumption was corrected for differences in minute ventilation.

Discussion:

The new humidifier, the Humid-Heat, gave an absolute humidity of 41–44 mg/l at 37°C in the bench tests. The tests in ventilated patients showed that the device was well tolerated and that condensation in the tubing was eliminated. There was no need to empty water traps. The test period was too short to evaluate whether the new device had any other advantages or disadvantages compared with conventional humidifiers.

Introduction

].

]. To avoid this, a new humidifier, which is a hybrid of a hygroscopic HME and an active humidifier, has been developed.

The aims of the present study were to test the performance of this new humidifier at different ventilator settings in a lung model, and to compare this humidifier with a conventional active humidifier in ventilator-treated critically ill patients.

Lung model tests

O, producing an almost constant inspiratory pressure in the ventilator tubing and tidal volumes of 390 and 1370 ml, respectively. The rate was 12 or 15 breaths/min and the minute ventilation was 4.7–16.4l.

As mentioned, the microprocessor of the humidifier uses the patient's minute ventilation for governing the water pump. The user has to enter the minute ventilation into the microprocessor manually. In order to test the tolerance to an unrecognized change in ventilation, we entered the erroneous minute ventilation values of 8 and 12 l/min into the microprocessor, when the actual minute ventilation at both occasions was 10 l with a constant flow pattern.

O/l without condensation occurring; and a change of the amount of water in the lung model during the test period was due to the difference in water content between the inspired and expired air.

is the change of water content in the lung model, Σ VT is the total ventilation during the 90-min test period and Hout is the absolute humidity of the expired air from the lung model (100% saturated at 35.5°C=41 mg/l). The change in the water content was found by weighing the whole model before and after the experiment. To study the reproducibility, the tests at 3, 5, 6, 8, 10 and 15l minute ventilation were made in duplicate. The water consumption was registered. The absolute humidity in the inspiratory and expiratory ventilator tubings was measured continuously by the Humidity Sensor System (Louis Gibeck AB, Upplands Väsby, Sweden) and electronically averaged.

The lung model consists of a box (a) in which the temperature is maintained at 36.0°C, a one-way valve (b), an elastic balloon (c) and a water bath (d). During the inspiratory phase the air passes from the ventilator (e) through the humidifier (f) via the one way-valve to the balloon. During the expiratory phase the air passes from the balloon and bubbles through the water-bath in order to reach 100% humidity at 35.5°C by the time it leaves the lung model. The arrows indicate the direction of the airflow.

Tests in ventilator-treated patients

= 0.40.) The range (mean) of respiratory rate was 11–26 (20) breaths/min. The Humid-Heat was preset to keep the temperature of the inspired gas at 37°C and the MR730 was set at 36°C in the humidifier and 40°C at the Y-piece according to recommendations from the manufacturer (Blyth A, FPCare, personal communication). The room temperature was 21°C.

The temperature of the patient was measured before and after the test of each device. The temperature at the Y-piece (MR730) and in the flex-tube (Humid-Heat) was registered after 30min of use and at the end of the test period. The number of times that the water traps were emptied, changes of machine filters, suctions and quality (thick, normal or thin) of secretions, nebulizations, and the amount of saline instillations and endotracheal tube obstruction were recorded. The amount of fluid consumed was measured by weighing the fluid bags that supplied the humidifiers with water before and after the testing of each device. Likewise, the amount of water in the water-traps was measured by weighing. In order to evaluate increased expiratory resistance due to the device, the airway pressure was measured at the end of a prolonged end-expiratory pause at 1 h of use of the humidifier and at the end of the experiment, and compared with the pressure before the experiment.

Statistics

test. The differences in airway pressures within the two test periods were analyzed using analysis of variance.

Lung model tests

=0.003. The temperature of the inspired air leaving the humidifier was 36.9 ± 0.5°C (mean ± standard deviation). The coefficient of variation of absolute humidity for the double tests was 3.5% (0.3–7.4%). The water consumption was 10.1 ± 1.6 mg/l inspired air (mean ± standard deviation) and the absolute humidity in the expiratory ventilator tubing was 7.5 ± 2.2 mg/l (mean ± standard deviation). This corresponds to 42 ± 12% (mean ± standard deviation) relative humidity at the prevailing temperature (21°C). The absolute humidity in the inspiratory ventilator tubing was 0.14 ± 0.15 mg/l (mean ± standard deviation).

O/l, respectively.

Results from the lung model experiments. The filled and unfilled circles indicate the absolute humidity in the inspired air at different minute volumes during a constant and decelerating flow, respectively.

Tests in ventilator-treated patients

< 0.0008).

The patient test

The numbers are presented as mean ± standard deviation when applicable.

Discussion

This study shows that a new type of humidifier, the Humid-Heat, produced 41–44 mg/l absolute humidity at 37°C when tested in a lung model and that, compared with a conventional active humidifier in patients, it decreased the amount of fluid consumed and eliminated water condensation in the ventilator tubing.

].

].

] that also examined the performance of the Humid-Heat in ventilator-treated patients.

In conclusion, the Humid-Heat provided 41–44 mg/l absolute humidity at 37°C and did not cause any water condensation in the ventilator tubing. The results of this 24 h study are promising, but further tests are needed to evaluate patient tolerance during longer periods of use.

Keywords

• airway humidification
• heated humidifier
• intensive care
• mechanical ventilation