title: Effect of humidifying devices on the measurement of tidal volume by mechanical ventilators.
abstract: OBJECTIVE
  We hypothesized that expiratory tidal volume was underestimated, because a heat-moisture exchanger traps the expired vapor. We, therefore, designed patient and bench studies to investigate the accuracy of tidal volume monitoring.
  
  
  METHODS
  In a patient study, applying two humidifying systems (a heat-moisture exchanger and a heated humidifier) and two tidal volumes (12 and 6 ml x kg(-1)) with a Servo ventilator 300, we recorded the displayed expiratory tidal volume and thoracic volume displacement, measured by respiratory inductive plethysmography. Temperature, relative humidity, and absolute humidity were measured at the airway opening and at the end of the expiratory limb. Using a model lung, we also tested three different ventilators (Puritan-Bennett 7200ae, Evita 4, and Servo ventilator 300) to investigate whether the effects of the heat-moisture exchanger and the heated humidifier on monitored tidal volume varied according to the brand of ventilator.
  
  
  RESULTS
  With the use of the heat-moisture exchanger, the displayed expiratory tidal volume was significantly smaller, by 12%-14%, than that with the heated humidifier, although thoracic volume displacement was identical in the two systems. The temperature and absolute humidity at the end of the expiratory limb were significantly lower with the heat-moisture exchanger than with the heated humidifier. In the model lung study, we investigated the effects of different brands of ventilator on the expiratory tidal volume. A similar degree (8%-14%) of underestimation of tidal volume was observed with the heat-moisture exchanger, regardless of ventilator brand.
  
  
  CONCLUSIONS
  Monitored expiratory tidal volume was underestimated by approximately 10%, when using a heat-moisture exchanger.
authors.affiliation: Intensive Care Unit, Osaka University Hospital, 2-15 Yamadaoka, Suita, 565-0871, Japan.
authors.forename: Yasuki
authors.initials: Y
authors.lastname: Fujita
authors.forename: Hideaki
authors.initials: H
authors.lastname: Imanaka
authors.forename: Yuji
authors.initials: Y
authors.lastname: Fujino
authors.forename: Muneyuki
authors.initials: M
authors.lastname: Takeuchi
authors.forename: Toshiji
authors.initials: T
authors.lastname: Tomita
authors.forename: Takashi
authors.initials: T
authors.lastname: Mashimo
authors.forename: Masaji
authors.initials: M
authors.lastname: Nishimura
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006358:Hot Temperature; D006801:Humans; D006813:Humidity; D008297:Male; D008954:Models, Biological; D008991:Monitoring, Physiologic / Q000379:methods; D009330:Nebulizers and Vaporizers; D015203:Reproducibility of Results; D012121:Respiration, Artificial / Q000295:instrumentation*; D013696:Temperature; D013990:Tidal Volume / Q000502:physiology