The effect of two types of humidifying devices on humidity of inspired gas and tidal volume by ventilator monitoring

ABSTRACT

BACKGROUND: In general anesthesia, a heat and moisture exchanger (HME) is used to prevent hypothermia and to maintain humidity of inspired gas. We hypothesized that after using a HME expiratory tidal volume decreases because a HME traps the expired vapor.The aim of this study was to evaluate the humidity of expired gas and to investigate the accuracy of tidal volume monitoring. METHODS: Forty patients undergoing elective surgery under general anesthesia were randomly allocated into two groups.Group P (n = 20) used Pall Breathing filter (pall BB25, ACE medical, Korea) and Group H (n = 20) used Hygrobac S (Mallinckrodt Dar, Mirandola, Italy) between endotracheal tube and Y-piece.And with same kind of ventilator, we measured expiratory tidal volume, temperature and relative humidity at before and after HEM.And we recorded expiratory tidal volume monitored by ventilator with and without HME. RESULTS: The relative humidity was significantly higher at before using HEM than after using HEM in both group.The temperature in Group H was higher when measured at before HME but not in Group P.The measured expiratory tidal volume at before and after HEM was not significantly different. The tidal volume monitored by ventilator with HEM was significantly smaller than without HEM, by 3.1% in Group P and 5.3% in Group H. CONCLUSIONS: The HMEs improve inspiratory humidity and temperature of anesthetic gas.But monitored expiratory tidal volume was underestimated when using HMEs.