Wet Circuit / Humidifier

OVERVIEW

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

USES

• humidification of respiratory gases

DESCRIPTION

• 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

METHOD OF INSERTION AND/OR USE

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

Pitfalls

• 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

COMPLICATIONS

• 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

PROS AND  CONS

Advantages

• 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

Disadvantages

• 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)

References and Links

LITFL

• CCC — Humidification
• CCC — Heat and Moisture Exchanger

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

• Respiratory2011 — Wet Circuit Ventilator 1 and Wet Circuit Ventilator 2 (Youtube videos demonstrating set-up)