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.
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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.
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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.
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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