Evaluation of heated humidifiers for use on intubated patients: a comparative study of humidifying efficiency,flow resistance,and alarm functions using a lung model

Objective. Evaluation of humidification efficiency, flow resistance, and alarm functions of heated humidifiers (HH;(Kendall-Aerodyne-delta, Fisher&;Paykel-MR 730; Dräger-Aquapor; Puritan-Bennett-Cascade II) in accordance with ISO/EN-8185:1997 and on a ventilated lung model in accordance with ISO/EN-9360:2000. Methods. Humidification efficiency was evaluated by (a) measuring the water content of the inspiratory air on perfusion with different gas flows, (b) measuring the water loss of a lung model, and (c) simultaneous measurement of the in- and expiratory water content with a capacitive hybrid sensor. The resistance characteristics were measured, the data were compared with a mathematical approximation. The alarm functions were determined. Results. The humidification efficiency of HHs is a function of gas flow and design characteristics. In HHs with tube heating it is possible to make settings at which the inspiratory humidity falls below the minimal value of 33 mgH₂O/l stipulated by ISO/EN-8185:1997. The inspiratory resistances extend from 0.5 to 4.4 cmH₂O l^–1 s^–1; the expiratory flow resistances of the devices are low. The alarm functions of HHs with tube heating are inadequate for cases involving both "dry start" and "running dry." Conclusions. Efficiency data that allow a direct comparison with heat and moisture exchangers data according to ISO/EN-9360:2000 can also be determined for HH. HH do not prevent pulmonary water losses in intubated patients. These losses can exceed the physiological range. The airway resistance of the Cascade II prohibits its use in spontaneously breathing patients. The warning and shut-off features of HH are unacceptable and hazardous.