Measurement of water vapour pressure in the airways of mechanically ventilated patient using different types of humidifiers.

Aim of the investigation: To evaluate the performance of different types of humidifying systems (heat and moisture exchanger, HME, and heated humidifier, HH) in the tracheo bronchial airway system of intubated mechanically ventilated patients. Methods: A heated and fast responding capacitive sensor with a time constant of 0,23 s was used to measure the water vapor pressure at different locations in the tracheo bronchial airway system of 12 patients after therapeutic bronchoscopy. The sensor was immersed in an airstream of app. 1 ml/s continuously sampled with a bronchoscope in which the working pipe and the handle have been equipped with a heating system to prevent condensation. The sampling positions were 3 cm distal of the bifurcation in the right main bronchus, 2 cm proximal of the bifurcation and at the tube connector respectively. Results: Without any humidifying system there was a dramatic reduction of the climatisation index respectively increase of the pulmonary water loss index, most prominently visible at the tube connector. There were significant differences between different types of HME (Pall BB 100 vs. Medisize Hygrovent S) but no significant differences between the Hygrovent S and the HH Fisher 630 set at 34 C. In consideration of applied tidalvolumens between 550 and 950 ml, isothermic saturation has not been reached in close vicinity to the bifurcation. Conclusions: HME may differ substantially from one type to the other and should not be used as climatisation system without careful consideration of their performance data. Effective HME are equivalent to HH at 34 C and may therefore also be used for long term ventilation.