Heat and Moisture Exchanger

Reviewed and revised 5 August 2015

OVERVIEW

• Heat and Moisture Exchanger (HME)
• those in current use are generally combined with a microbiological filter, hence they are called HME filters (HMEF)
• when passive humidification using an HME is use, the circuit is termed a “dry circuit” as a opposed to a “wet circuit” using active humidification

USES

• humidification, warming inspired gases and microbiological filtration
• best used in patients with few secretions, who are not hypothermic, do not have large air leaks and do not have high airway resistance

DESCRIPTION

• Generally contains a layer of foam or paper embedded with a hydroscopic salt such as calcium chloride
• Bacterial and viral filters ideally have filtration efficiency of >99.9%
• HME with humidification efficiency >30mg.H2O/L
• connects to a standard 15mm connector on an endotracheal tube

METHOD OF INSERTION AND/OR USE

• placed in line between Y-piece of breathing circuit and ETT

OTHER INFORMATION

Benefits

• ease of use
• light
• can retain their ability to humidify for up to 4 days with minimal change in resistance
• less cumbersome during transport
• lower staff workload
• lower costs
• decreases ventilatory acquired pneumonia (Kola et al, 2005)

Mechanism of heating and humidification

• contains a layer of foam or paper embedded with a hygroscopic salt such as calcium chloride
• expired gas cools as it crosses the membrane, resulting in condensation and release of the mass enthalpy of vaporisation to the HME layer
• on inspiration absorbed heat evaporates the condensate and warms the gas, the hygroscopic salt releases water molecules when the vapor pressure is low
• warming and humidification is thus regulated by the moisture content of the expired gas and patient’s core temperature
• a filter layer is also present, either an electrostatically charged or a pleated hydrophobic layer, the latter helps return moisture to the gas as condensation and evaporation occurs between the pleats

Mechanism of filtration

• Filtration is achieved for larger particles (>0.3 µm) by inertial impaction and interception
• Smaller particles(<0.3 µm) are captured by Brownian diffusion

COMPLICATIONS/DISADVANTAGES

• inability to use with all patients (haemoptysis, tenacious secretions)
• increased airways resistance
• increased dead space
• potential for unrecognized airway obstruction if filter blocks
• less than full humidification and body temperature
• drying of secretions
• not appropriate for patients with large air leaks (e.g. bronchopleural fistulae) due extensive loss of inspired gas and inability to conserve heat and humidity

References and Links

LITFL

• CCC — Humidification
• CCC — Wet circuit

Journal articles and textbooks

• Kelly M, Gillies D, Todd DA, Lockwood C. Heated humidification versus heat and moisture exchangers for ventilated adults and children. Anesth Analg. 2010 Oct;111(4):1072. Review. [pubmed] [free full text pdf]
• Kola A, Eckmanns T, Gastmeier P. Efficacy of heat and moisture exchangers in preventing ventilator-associated pneumonia: meta-analysis of randomized controlled trials. Intensive Care Med. 2005 Jan;31(1):5-11. [pubmed]
• Lawes EG. Hidden hazards and dangers associated with the use of HME/filters in breathing circuits. Their effect on toxic metabolite production, pulse oximetry and airway resistance. Br J Anaesth. 2003;91:(2)249-64. [pubmed]
• Lorente L, Lecuona M, Jiménez A, Mora ML, Sierra A. Ventilator-associated pneumonia using a heated humidifier or a heat and moisture exchanger: a randomized controlled trial [ISRCTN88724583]. Crit Care. 2006;10(4):R116. PMC1750976.
• Wilkes AR. Heat and moisture exchangers and breathing system filters: their use in anaesthesia and intensive care. Part 1 – history, principles and efficiency. Anaesthesia. 2011;66:(1)31-9. [pubmed]
• Wilkes AR. Heat and moisture exchangers and breathing system filters: their use in anaesthesia and intensive care. Part 2 – practical use, including problems, and their use with paediatric patients. Anaesthesia. 2011;66:(1)40-51. [pubmed]