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Critical Care

Open Access

Relationship between absolute humidity of inspired and expired gases with an active humidifier

  • P Pelosi1,
  • P Severgnini1,
  • E Bianchi1,
  • R Terzi1,
  • C Lanza1,
  • G Minoja1,
  • D Chiumello2,
  • E Storelli2 and
  • M Chiaranda1
Critical Care20037(Suppl 2):P155

Published: 3 March 2003


Public HealthCatheterRelative HumidityMechanical VentilationEmergency Medicine

Background and goals

To evaluate the relationship between absolute humidity of inspired (AHi) and expired gases (AHe) during mechanical ventilation using a hot water humidifier (ConchathermIII®).

Materials and methods

We measured the absolute humidity (AHi) and relative humidity (RHi) of inspired and expired gases in 40 sedated and paralyzed patients (mean age 60.7 ± 16.9 years). The AHi and AHe was measured using a psycrometer (Yokogawa®). The airflow was separated by a flow separator placed between the Y piece of the ventilator circuit and the catheter mouth. The psycrometer is made of four thermic probes, two of them measure the dry temperature and two of them, covered with a cotton gauze (wet with physiological solution 15 min before every measurement), measure the wet temperature. Dry and wet temperatures were measured on the inspiratory and expiratory lines at the flows separator. We set the active humidifier at 31, 34, 36 and 38°C, and the measurements were made after 30 min for each temperature.


The relationship between AHi and AHe are shown in Fig. 1.

Figure 1


1) AHi is related to AHe (AHi > 38.8 mg/l, positive airway fluid balance; AHi < 38.8 mg/l, negative airway fluid balance). 2) We suggest an AHi of 38.8 mg/l (RHi 100%) with a temperature of 34°C during mechanical ventilation to maintain adequate airway conditioning.

Authors’ Affiliations

Department of Clinical and Biological Sciences, University of Insubria, Varese-Circolo and Fondazione Macchi Hospital, Varese, Italy
Institute of Anesthesia and Intensive Care, Policlinico Hospital – IRCCS, Milan, Italy


© BioMed Central Ltd 2003