Naoki Miyazaki

Increased airway resistance in the prone position associated with heat and moisture exchangers with integral bacterial/viral filters

the HMEF was increased. The HMEF was replaced and the airway resistance and end-tidal CO2 immediately decreased to the previous levels. The weight of the HMEF was approximately 10 g, 1 g heavier than the new one (dry weight, 9 g). Neither sputum nor any foreign substance was observed in the HMEF. We examined whether as little as 1 ml in volume of water would signifi cantly increase the resistance of an HMEF (DAR Hygrobaby-M; Mallinckrodt Medical; capacity, 8 ml) and a heat and moisture exchanger (HME) without bacterial/viral fi lters (Humid vent; Gibeck, Upplands Väsby, Sweden, capacity, 7 ml). The HMEF or the HME was attached between an anesthesia circuit and a test lung (volume, 500 ml; fl ow resistance, 5 cmH2O · l·min−1). The anesthesia circuit pressure (ACP) was measured with a Datex AS/3 anesthesia monitor (Helsinki, Finland). The ventilator setting was: tidal volume, 300 ml; frequency, 15 cycles · min−1; inspiratory-expiratory (I : E) ratio, 1 : 1.5. After the ACP was measured with or without these devices, 0.5 ml distilled water was dripped into each device. After infi ltration was complete, change in the ACP was evaluated. An additional 0.5 ml distilled water was then infi ltrated and the ACP was measured. The results are shown in Fig. 1. The ACP increased in the HMEF but did not increase in the HME. In the operating manual, the resistance of the dry HMEF to gas fl ow was reported to be 1.9 cmH2O at 10 l·min−1, supporting our observation. These observations indicate that the bacterial/viral fi lter that absorbed water induced high airway resistance. The constituents of the HMEF electrostatic fi lter, made of a non-woven fabric consisting of polypropylene fi bers, and those of the HME, made of a Increased airway resistance in the prone position associated with heat and moisture exchangers with integral bacterial/viral fi lters