Wet Circuit / Humidifier


A “wet circuit” is a breathing circuit with active humidification, as opposed to a “dry circuit” that uses a Heat and Moisture Exchanger (HME)


  • humidification of respiratory gases


  • requires power for heating
  • disposable or reusable humidification chamber that holds water +/- a reservoir that adds water as needed
  • heating element to warm water
  • inspiratory tube that transmits heated, humidified gas. Ideally this will be heated  (e.g. by a heated wire along the tube) and insulated. If this tube is not heated, condensation will occur and a water trap will be required.
  • temperature monitor (at the patient end of the circuit) adjusts the amount of heating required


  • inserted into the inspiratory limb of the ventilator circuit
  • provides a minimum of 30mgH2O/L delivered at an appropriate temperature


  • should not be used with a HME
  • must not be placed in the expiratory limb
  • if a filter is used with active humidification it should be upstream to prevent clogging with water
  • ensure the heated humidifier is turned off of there is no flow through the circuit. If not, the absence of flow will cause temperature to fall at the patient end, leading to increased heat output which may case the inspiratory tube to melt


  • condensation (“rain-out”):
    • risks water aspiration
    • potential delivery of excessively hot fluid to the airways
    • may interfere with monitoring (e.g. flows and volumes)
    • may damage equipment (e.g. ventilator)
  • microbiological colonisation and growth (e.g. in reservoir or condensate)
  • overhydration
  • thermal injury
  • increased work of breathing due increased resistance (some cannot be used with spontaneously breathing patients)
  • potential electrical hazard



  • optimal efficiency (more efficient than HME)
  • reliability
  • ability to warm patient
  • proven track record of safety
  • some can be used for spontaneously breathing and tracheotomized patients


  • lack of transportability
  • increased cost and maintenance
  • not all humidifiers are compatible with all circuits
  • increased work for staff (temperature control, refilling the reservoir, draining condensate, cleaning, and sterilization)
  • risk of complications (see above)

CCC Ventilation Series

Journal articles

  • Al Ashry HS, Modrykamien AM. Humidification during mechanical ventilation in the adult patient. Biomed Res Int. 2014;2014:715434. [pubmed]
  • Esquinas Rodriguez AM, Scala R, Soroksky A, et al. Clinical review: humidifiers during non-invasive ventilation–key topics and practical implications. Crit Care. 2012;16:(1)203. [pubmed]
  • Kelly M, Gillies D, Todd DA, Lockwood C. Heated humidification versus heat and moisture exchangers for ventilated adults and children. Cochrane Database Syst Rev. 2010 Apr 14;(4):CD004711. doi: 10.1002/14651858.CD004711.pub2. Review. PubMed PMID: 20393939. [Free Fulltext]
  • Wilkes AR. Humidification: its importance and delivery. BJA CEPD Reviews (2001) 1 (2): 40-43. doi: 10.1093/bjacepd/1.2.40 [Free Fulltext]

FOAM and web resources

Critical Care


Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of three amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC

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