title: Relative humidity, not absolute humidity, is of great importance when using a humidifier with a heating wire.
abstract: OBJECTIVE
  Since the introduction of a humidifier with a heating wire, we have frequently experienced severe upper airway obstruction from consolidation of secretions, previously unencountered when a humidifier without a heating wire was used. Such problems led to the suspicion that the heating wire incorporated into the breathing circuit of the heated humidifier might be the cause. Therefore, we scheduled an experiment to assess the hypothesis that relative humidity, rather than absolute humidity, is a dominant factor in the case of drying secretions in the upper airway when using such a humidifier.
  
  
  METHODS
  Three clinical case reports and an experiment with a tracheal model.
  
  
  METHODS
  Intensive care units at Saitama Medical Center, Saitama Medical School, Saitama, Japan.
  
  
  METHODS
  Three intubated patients.
  
  
  RESULTS
  An experiment with a tracheal model showed that gas with a higher temperature and lower relative humidity (35 degrees C, 48%) deprived the tracheal model of significantly more water (5.9 +/- 0.2 [SD] g) than gas with a lower temperature and higher relative humidity (24 degrees C, 87%) (2.9 +/- 0.4 g; p less than .01), even though the gases contained the same amount of water vapor (19 mg H2O/L) minus the same absolute humidity.
  
  
  CONCLUSIONS
  A heated humidifier with a heating wire incorporated into the breathing circuit may be dangerous when only temperature is monitored and controlled. Relative humidity, rather than absolute humidity, is a dominant factor in the case of drying secretions in the upper airway when using such a humidifier.
authors.affiliation: Department of Anesthesiology, Saitama Medical Center, Saitama Medical School, Japan.
authors.forename: H
authors.initials: H
authors.lastname: Miyao
authors.forename: T
authors.initials: T
authors.lastname: Hirokawa
authors.forename: K
authors.initials: K
authors.lastname: Miyasaka
authors.forename: T
authors.initials: T
authors.lastname: Kawazoe
mesh_terms: D000368:Aged; D000402:Airway Obstruction / Q000209:etiology / Q000517:prevention & control*; D005069:Evaluation Studies as Topic; D005260:Female; D006361:Heating / Q000009:adverse effects / Q000592:standards*; D006801:Humans; D006813:Humidity / Q000009:adverse effects / Q000592:standards*; D007442:Intubation, Intratracheal / Q000009:adverse effects / Q000592:standards*; D008297:Male; D008875:Middle Aged; D008991:Monitoring, Physiologic / Q000592:standards; D013696:Temperature