title: Moisturizing and mechanical characteristics of a new counter-flow type heated humidifier.
abstract: BACKGROUND
  During mechanical ventilation effective conditioning of inspired air is important. In this respect, conventional humidifiers do not perform optimally. By design, a counter-flow-type humidifier should improve humidification and heating, but may increase resistance.
  
  
  METHODS
  We investigated mechanical impedance and work of breathing (using pressure-flow characteristics and additional pressure-time product) of a new counter-flow-type humidifier, a conventional heated humidifier, and a passive heat and moisture exchanger (HME) in physical models of the respiratory system. We investigated moisturizing performance (amount of vaporized water at different air flows and ventilatory frequencies) of the two heated humidifiers. Ease of breathing through both heated humidifiers was investigated in 12 healthy volunteers blinded to the type of humidifier.
  
  
  RESULTS
  Moisturizing performance of the conventional heated humidifier was flow-independent (approximately 32.5 mg vaporized water per breath at inspiratory flow rates of 30-120 litre min (- 1); P > 0.05) but decreased (10%; P < 0.0001) with increasing ventilatory rates (12-20 min (- 1)). In contrast, moisturizing performance of the counter-flow-type humidifier (approximately 33.5 mg vaporized water per breath) was both flow- and rate-independent (P = 0.75). In addition, the counter-flow humidifier caused less physical work (approximately 25%) and resistance (approximately 50%) (both P < 0.05) than the other two devices. The passive HME displayed the least favourable mechanical characteristics. Ten of 12 volunteers felt breathing through the counter-flow humidifier easier than through the heated humidifier (P < 0.05).
  
  
  CONCLUSIONS
  Compared with a conventional humidifier, the new counter-flow-type humidifier displayed improved air conditioning and mechanical characteristics. Its lower resistance, particularly at low airflows, should be of clinical benefit during spontaneous breathing and triggered assisted ventilation.
authors.affiliation: Department of Anaesthesiology and Intensive Care, Section for Experimental Anaesthesiology, University Hospital Freiburg, Hugstetter Strasse 55, 79106 Freiburg, Germany. stefan.schumann@uniklinik-freiburg.de
authors.forename: S
authors.initials: S
authors.lastname: Schumann
authors.forename: C A
authors.initials: CA
authors.lastname: Stahl
authors.forename: K
authors.initials: K
authors.lastname: Möller
authors.forename: H-J
authors.initials: HJ
authors.lastname: Priebe
authors.forename: J
authors.initials: J
authors.lastname: Guttmann
mesh_terms: D000328:Adult; D000403:Airway Resistance; D001294:Attitude to Health; D004867:Equipment Design; D005260:Female; D006361:Heating; D006801:Humans; D006813:Humidity; D008170:Lung Compliance; D008297:Male; D008875:Middle Aged; D008953:Models, Anatomic; D012121:Respiration, Artificial / Q000295:instrumentation*; D016037:Single-Blind Method; D014939:Work of Breathing